Route Summarization & Aggregation — OSPF and EIGRP Summary Address Configuration

Every router in a network maintains a routing table — a list of destinations and next-hops. As networks grow, routing tables grow with them. A large enterprise may have hundreds of individual subnets across dozens of sites. Without summarization, every router in the network must store, process, and react to every individual prefix. When a host subnet goes down in a remote site, every router in the OSPF or EIGRP domain runs the SPF algorithm or diffusing computation to recalculate routes — burning CPU, consuming memory, and generating unnecessary topology churn across the entire network.

Route summarization (also called route aggregation or supernetting) solves this by replacing multiple specific prefixes with a single summary prefix that covers the same address space. Instead of advertising ten /24 subnets from a branch site, the ABR advertises a single /20 summary. The rest of the network stores one route instead of ten. When any individual /24 flaps, the summary remains stable — no SPF re-run, no routing table churn in the core. The performance benefits compound at scale: a network with 500 subnets reduced to 50 summaries experiences 10x fewer routing table lookups, 10x less memory consumption, and dramatically reduced convergence events.

This lab covers the complete summarization workflow: binary summary address calculation (the skill that underpins all summarization work), OSPF ABR and ASBR summarization using area range and summary-address, EIGRP interface-level summarization using ip summary-address, and full verification. For the OSPF and EIGRP protocols these summaries build on see OSPF Single-Area Configuration, OSPF Multi-Area Configuration, and EIGRP Configuration. For filtering routes rather than summarizing them see Policy-Based Routing and BGP Basics (eBGP) which uses a different aggregation model.

1. Core Concepts — Why Summarize and How It Works

Benefits and Trade-offs of Route Summarization

Benefit	Explanation
Smaller routing tables	One summary prefix replaces N specific prefixes. Lookup is faster; memory consumption is lower.
Topology change isolation	When a specific subnet within the summary flaps, the summary itself stays stable. SPF/DUAL is only triggered in the local area — not domain-wide.
Reduced SPF/DUAL computation	Fewer LSAs/topology table entries means faster convergence and lower CPU during topology changes.
Bandwidth conservation	Fewer routing protocol advertisements — especially beneficial on slow WAN links.
Simplified troubleshooting	Fewer routes to analyse during outages. Clear hierarchical structure.

Trade-off / Risk	Explanation
Summary covers supernet space	A summary like 10.1.0.0/20 covers 10.1.0.0–10.1.15.255. If only 10.1.0.0–10.1.9.255 is in use, the summary advertises reachability to 10.1.10.0–10.1.15.255 as well — traffic to unused addresses may be blackholed.
Null0 route (black hole)	OSPF and EIGRP automatically install a Null0 route for the summary prefix to prevent routing loops. Traffic to addresses inside the summary but not in any specific subnet is dropped at Null0.
Requires contiguous addressing	Summarization only works when subnets are contiguous and share a common bit prefix. Discontiguous address space cannot be cleanly summarized without covering gaps.
Loss of specific path visibility	Downstream routers only see the summary — they cannot make routing decisions based on individual subnets within the summary. All traffic to the summarized range uses the same path.

The Null0 Black Hole Route

  When a router summarizes routes and advertises a summary prefix,
  IOS automatically installs a Null0 (discard) route for that prefix
  in the local routing table. This prevents routing loops:

  WITHOUT Null0:
    Summary 10.1.0.0/20 advertised to core.
    Packet arrives for 10.1.14.5 (not a real subnet).
    No specific /24 route exists locally.
    Router looks for a less-specific match — finds default route.
    Default route points back to core (which sent the packet here).
    Packet loops between core and summarizing router indefinitely. ← LOOP

  WITH Null0 (automatic):
    Summary 10.1.0.0/20 advertised to core.
    Packet arrives for 10.1.14.5 (not a real subnet).
    No specific /24 route exists locally.
    Null0 route for 10.1.0.0/20 is the next-best match.
    Packet is silently discarded (dropped). ← No loop, no forwarding

  This Null0 route appears in the routing table as:
    O     10.1.0.0/20 is a summary, 00:01:23, Null0    (OSPF)
    D     10.1.0.0/20 is a summary, 00:01:23, Null0    (EIGRP)

Where Summarization Can Be Configured

Protocol	Router Role	IOS Command	What Is Summarized
OSPF	ABR (Area Border Router)	`area [id] range [prefix] [mask]`	Inter-area summary — summarizes routes from one area advertised into another area (Type 3 LSA)
OSPF	ASBR (AS Boundary Router)	`summary-address [prefix] [mask]`	External summary — summarizes redistributed external routes (Type 5/Type 7 LSA)
EIGRP	Any router	`ip summary-address eigrp [asn] [prefix] [mask]`	Summarizes all EIGRP routes behind this interface — sent to neighbors on that interface only
Static	Any router	`ip route [summary] [mask] [next-hop]`	Manual summary — a static route covering a range, redistributed or used as default. See Static Route Configuration
BGP	Any BGP router	`aggregate-address [prefix] [mask]`	BGP aggregate — summarizes BGP prefixes in the BGP table. Used extensively in service provider networks alongside MPLS (covered separately)

OSPF summarization only works at boundary routers. Unlike EIGRP (which can summarize at any router on any interface), OSPF summarization is strictly confined to boundary points: inter-area summarization at ABRs and external summarization at ASBRs. You cannot configure area range on an internal router that is not an ABR — IOS will accept the command but it has no effect. Always verify the router's OSPF role with show ip ospf before configuring area range.

2. Summary Address Calculation — Binary Method

Correct summary address calculation is the most critical skill in route summarization. An incorrect summary address either fails to cover all subnets (traffic leaks around the summary) or covers too much address space (creating black holes for unused addresses). The binary method is the only reliable approach for all cases. A firm grasp of subnet masks is essential before working through these calculations.

Step-by-Step Method

  RULE: The summary address is formed by the COMMON BIT PREFIX
        shared by ALL networks being summarized.

  STEP 1: Write all network addresses in binary (focus on the octets that differ)
  STEP 2: Align the bits column by column from left to right
  STEP 3: Find the last bit position where ALL networks have the SAME value
  STEP 4: Everything to the left of that position = summary network bits
  STEP 5: Everything to the right = host bits (set to 0 for the network address)
  STEP 6: Count the matching bit positions = prefix length (subnet mask)

Example 1 — Four /24 Networks into a Single Summary

  Networks to summarize:
    10.1.0.0/24
    10.1.1.0/24
    10.1.2.0/24
    10.1.3.0/24

  STEP 1-2: Write the third octet in binary (first three octets are identical):
    10.1. [0] .0    →  0 0 0 0 0 0 0 0
    10.1. [1] .0    →  0 0 0 0 0 0 0 1
    10.1. [2] .0    →  0 0 0 0 0 0 1 0
    10.1. [3] .0    →  0 0 0 0 0 0 1 1
                       ─────────────────
                       ↑ ↑ ↑ ↑ ↑ ↑ ↕ ↕
    STEP 3:            All same up to bit 6  ← last matching bit position

  STEP 4-5: Summary network = common prefix + remaining bits set to 0
    Common bits: 0 0 0 0 0 0 _ _  → 0 0 0 0 0 0 0 0 = 0 (third octet)
    Summary address: 10.1.0.0

  STEP 6: Count matching bit positions
    First octet:   8 bits match (all 10s)
    Second octet:  8 bits match (all 1s)
    Third octet:   6 bits match (0 0 0 0 0 0)
    Fourth octet:  0 bits (don't count fourth octet — it's all host bits)
    Total: 8 + 8 + 6 = /22

  ✓ SUMMARY: 10.1.0.0/22
    Covers: 10.1.0.0 → 10.1.3.255 (1024 addresses, 4 × /24 networks)

Example 2 — Non-Consecutive Third Octet Values

  Networks to summarize:
    172.16.64.0/24
    172.16.65.0/24
    172.16.66.0/24
    172.16.67.0/24
    172.16.68.0/24
    172.16.69.0/24
    172.16.70.0/24
    172.16.71.0/24

  STEP 1-2: Third octet in binary:
    172.16. [64] .0   →  0 1 0 0 0 0 0 0
    172.16. [65] .0   →  0 1 0 0 0 0 0 1
    172.16. [66] .0   →  0 1 0 0 0 0 1 0
    172.16. [67] .0   →  0 1 0 0 0 0 1 1
    172.16. [68] .0   →  0 1 0 0 0 1 0 0
    172.16. [69] .0   →  0 1 0 0 0 1 0 1
    172.16. [70] .0   →  0 1 0 0 0 1 1 0
    172.16. [71] .0   →  0 1 0 0 0 1 1 1
                          ─────────────────
                          ↑ ↑ ↑ ↑ ↑ ↕ ↕ ↕
    STEP 3:               All same up to bit 5 (value = 0 1 0 0 0)

  STEP 4-5: Summary
    Common bits: 0 1 0 0 0 _ _ _  → 0 1 0 0 0 0 0 0 = 64 (third octet)
    Summary address: 172.16.64.0

  STEP 6: Prefix length
    8 + 8 + 5 = /21

  ✓ SUMMARY: 172.16.64.0/21
    Covers: 172.16.64.0 → 172.16.71.255 (2048 addresses, 8 × /24 networks)

Example 3 — Summarizing Across Octet Boundaries

  Networks to summarize (different second AND third octets):
    192.168.0.0/24
    192.168.1.0/24
    192.168.2.0/24
    192.168.3.0/24

  STEP 1-2: Write BOTH second and third octets in binary:
    192.168. [0.0]  →  second: 1010 1000  third: 0000 0000
    192.168. [1.0]  →  second: 1010 1000  third: 0000 0001
    192.168. [2.0]  →  second: 1010 1000  third: 0000 0010
    192.168. [3.0]  →  second: 1010 1000  third: 0000 0011

  NOTE: second octet = 168 for all → fully matches (8 bits)
  Third octet in binary:
    0 0 0 0 0 0 0 0
    0 0 0 0 0 0 0 1
    0 0 0 0 0 0 1 0
    0 0 0 0 0 0 1 1
    ─────────────────
    ↑ ↑ ↑ ↑ ↑ ↑ ↕ ↕
    6 bits match in third octet

  STEP 6: 8 (first) + 8 (second) + 6 (third) = /22

  ✓ SUMMARY: 192.168.0.0/22
    Covers: 192.168.0.0 → 192.168.3.255

  QUICK TRICK — Power of 2 check:
    Number of subnets being summarized must be a POWER OF 2:
    2 networks → /25 one level up from /26, or /24 up from two /25s
    4 networks → move 2 bits left from original prefix
    8 networks → move 3 bits left
    16 networks → move 4 bits left

    Example: 4 × /24 networks → /24 - 2 = /22 ✓
             8 × /24 networks → /24 - 3 = /21 ✓
             16 × /24 networks → /24 - 4 = /20 ✓
    WARNING: This trick ONLY works when the starting networks are
    perfectly aligned. Always verify with binary when in doubt.

Verification: Does the Summary Cover All Subnets?

  FORMULA TO VERIFY:
    Summary network:  apply the summary mask to each individual subnet
    Result must equal: the summary network address for every subnet

  Example: Verify 10.1.0.0/22 covers 10.1.0.0–10.1.3.0

    Summary mask /22 = 255.255.252.0 = 1111 1111.1111 1111.1111 1100.0000 0000

    10.1.0.0 AND 255.255.252.0 = 10.1.0.0  ✓ matches summary
    10.1.1.0 AND 255.255.252.0 = 10.1.0.0  ✓ matches summary
    10.1.2.0 AND 255.255.252.0 = 10.1.0.0  ✓ matches summary
    10.1.3.0 AND 255.255.252.0 = 10.1.0.0  ✓ matches summary

  All four subnets AND the summary mask = the summary address. ✓
  The summary is correct.

  RANGE CHECK:
    10.1.0.0/22 covers 10.1.0.0 → 10.1.3.255
    All four /24 networks fall within this range. ✓

3. Lab Topology — Multi-Area OSPF + EIGRP Network

                           ┌──────────────────┐
                           │   OSPF Area 0    │
                           │   (Backbone)     │
                           │  NetsTuts_R1     │
                           │  10.255.0.1/30   │ ← ABR between Area 0 / Area 1
                           └───────┬──────────┘
                                   │ 10.255.0.0/30
                      ┌────────────┴─────────────────────────┐
             OSPF Area 1                            OSPF Area 2
        NetsTuts_R2 (ABR)                       NetsTuts_R3 (ABR)
          10.255.0.2                              10.255.0.6
         ┌───────────┐                           ┌───────────┐
         │ Branch-A  │                           │ Branch-B  │
         │ Loopbacks │                           │ Loopbacks │
         │10.1.0.0/24│                           │10.2.0.0/24│
         │10.1.1.0/24│                           │10.2.1.0/24│
         │10.1.2.0/24│                           │10.2.2.0/24│
         │10.1.3.0/24│                           │10.2.3.0/24│
         └─────┬─────┘                           └─────┬─────┘
               │                                       │
               └──────────── EIGRP AS 100 ─────────────┘
                          NetsTuts_R4 (EIGRP)
                        ┌──────────────────────┐
                        │  EIGRP AS 100        │
                        │  10.4.0.0/24         │
                        │  10.4.1.0/24         │
                        │  10.4.2.0/24         │
                        │  10.4.3.0/24         │
                        │  10.4.4.0/24         │
                        │  10.4.5.0/24         │
                        │  10.4.6.0/24         │
                        │  10.4.7.0/24         │
                        └──────────────────────┘

  SUMMARIZATION TASKS:
    Task A: OSPF ABR R2 — summarize Area 1 routes (10.1.0.0–10.1.3.0/24)
            into Area 0 as single summary → area 1 range 10.1.0.0 255.255.252.0

    Task B: OSPF ABR R3 — summarize Area 2 routes (10.2.0.0–10.2.3.0/24)
            into Area 0 as single summary → area 2 range 10.2.0.0 255.255.252.0

    Task C: EIGRP R4 — summarize 8 routes (10.4.0.0–10.4.7.0/24)
            toward R2/R3 as single summary → ip summary-address eigrp 100 10.4.0.0 255.255.248.0

    Task D: OSPF ASBR — summarize redistributed EIGRP routes entering OSPF
            → summary-address 10.4.0.0 255.255.248.0

4. Step 1 — OSPF ABR Summarization (`area range`)

The area range command is configured on an ABR (a router that belongs to both Area 0 and another area). It suppresses individual Type 3 LSAs for subnets within the range and replaces them with a single Type 3 summary LSA. The syntax specifies which area's routes are being summarized and the summary prefix/mask.

Verify Current OSPF State Before Summarization (R2)

! ── Check that R2 is an ABR ──────────────────────────────────────
NetsTuts_R2#show ip ospf
 Routing Process "ospf 1" with ID 2.2.2.2
 ...
 It is an area border router   ← ✓ R2 is ABR (connected to Area 0 + Area 1)
 ...
 Area 1
   Number of interfaces in this area is 4
   Area has no authentication
   Number of full virtual adjacencies through this area is 0

! ── Verify routes being advertised into Area 0 (before summary) ──
NetsTuts_R1#show ip route ospf
      10.0.0.0/8 is variably subnetted
O IA  10.1.0.0/24 [110/2] via 10.255.0.2, 00:15:43, Gi0/0  ← 4 individual /24s
O IA  10.1.1.0/24 [110/2] via 10.255.0.2, 00:15:43, Gi0/0  ← flooding the backbone
O IA  10.1.2.0/24 [110/2] via 10.255.0.2, 00:15:43, Gi0/0
O IA  10.1.3.0/24 [110/2] via 10.255.0.2, 00:15:43, Gi0/0
! ── 4 Type 3 LSAs sent from Area 1 into Area 0 — goal: replace with 1 ──

Configure area range on R2 (ABR between Area 0 and Area 1)

! ── OSPF ABR inter-area summarization ────────────────────────────
NetsTuts_R2(config)#router ospf 1
NetsTuts_R2(config-router)#area 1 range 10.1.0.0 255.255.252.0
!                           ^^^^^^ ^^^^^ ^^^^^^^^^ ^^^^^^^^^^^^^
!                           area   cmd   summary    summary mask
!                           being       address     (/22)
!                           summarized

! ── area 1 range means: "summarize routes FROM area 1 as they are
!    advertised into other areas (Area 0 in this case)" ───────────

! ── Optional: suppress the summary entirely (instead of advertising it)
NetsTuts_R2(config-router)#area 1 range 10.1.0.0 255.255.252.0 not-advertise
! ← Use this when you want to prevent area 1 routes from leaking into
!   backbone without advertising any alternative. Rare in production.

! ── Optional: set a specific cost for the summary LSA ────────────
NetsTuts_R2(config-router)#area 1 range 10.1.0.0 255.255.252.0 cost 100
! ← Overrides the automatically calculated cost for the summary
!   (default: highest cost among all specific routes in the summary)

NetsTuts_R2(config-router)#exit

The area [id] range command tells the ABR to summarize routes from that area as they cross into the backbone or other areas. The area ID in the command identifies the source area — not the destination area. R2 is connected to Area 1 and Area 0. area 1 range summarizes Area 1 routes advertised into Area 0 (and any other area R2 is connected to). The OSPF process automatically installs a Null0 discard route locally for 10.1.0.0/22 to prevent routing loops.

Configure area range on R3 (ABR between Area 0 and Area 2)

NetsTuts_R3(config)#router ospf 1
NetsTuts_R3(config-router)#area 2 range 10.2.0.0 255.255.252.0
! ← Summary: 4 × /24 (10.2.0–10.2.3) into one /22 ────────────────
NetsTuts_R3(config-router)#exit

Verify OSPF Area Range on R2

! ── Verify the summary is configured ────────────────────────────
NetsTuts_R2#show ip ospf
 Routing Process "ospf 1" with ID 2.2.2.2
 ...
 Area 1
   Number of interfaces in this area is 4
   Summary Link States (Type 3)
     Area range:  10.1.0.0/22 Advertise    ← ✓ summary configured + active
     ...

! ── Verify R1 now sees the summary instead of 4 individual routes
NetsTuts_R1#show ip route ospf
      10.0.0.0/8 is variably subnetted
O IA  10.1.0.0/22 [110/2] via 10.255.0.2, 00:00:33, Gi0/0  ← ✓ one summary
O IA  10.2.0.0/22 [110/2] via 10.255.0.6, 00:00:28, Gi0/1  ← ✓ Area 2 summary
! ── 8 individual /24 routes reduced to 2 summary /22 routes ─────

! ── Verify Null0 installed on the summarizing ABR ────────────────
NetsTuts_R2#show ip route 10.1.0.0
Routing entry for 10.1.0.0/22
  Known via "ospf 1", distance 110, metric 0
  Routing Descriptor Blocks:
  * directly connected, via Null0   ← ✓ Null0 black hole route installed

5. Step 2 — OSPF ASBR External Summarization (`summary-address`)

When external routes are redistributed into OSPF (from EIGRP, static, or another routing protocol), each redistributed prefix generates a Type 5 (or Type 7 in NSSA areas) External LSA. These external LSAs flood throughout the entire OSPF domain. The summary-address command on the ASBR suppresses individual external LSAs and replaces them with a single summarized external LSA — reducing flooding and routing table size across all OSPF routers.

Scenario: R2 Redistributes EIGRP into OSPF

! ── R2 is both ABR and ASBR — redistributing EIGRP routes ────────
! ── Without summarization, 8 individual EIGRP routes flood as ─────
! ── 8 Type 5 External LSAs into the OSPF domain ──────────────────

! ── Verify external routes before summarization ──────────────────
NetsTuts_R1#show ip route ospf | include OE2
O E2  10.4.0.0/24 [110/20] via 10.255.0.2, Gi0/0  ← 8 individual external routes
O E2  10.4.1.0/24 [110/20] via 10.255.0.2, Gi0/0
O E2  10.4.2.0/24 [110/20] via 10.255.0.2, Gi0/0
O E2  10.4.3.0/24 [110/20] via 10.255.0.2, Gi0/0
O E2  10.4.4.0/24 [110/20] via 10.255.0.2, Gi0/0
O E2  10.4.5.0/24 [110/20] via 10.255.0.2, Gi0/0
O E2  10.4.6.0/24 [110/20] via 10.255.0.2, Gi0/0
O E2  10.4.7.0/24 [110/20] via 10.255.0.2, Gi0/0
! ── Goal: replace with one O E2 10.4.0.0/21 ─────────────────────

! ── Configure ASBR external summarization ────────────────────────
NetsTuts_R2(config)#router ospf 1
NetsTuts_R2(config-router)#redistribute eigrp 100 subnets
! ← First ensure redistribution is configured ─────────────────────

NetsTuts_R2(config-router)#summary-address 10.4.0.0 255.255.248.0
!                           ^^^^^^^^^^^^^^^^ ^^^^^^^^^ ^^^^^^^^^^^^^
!                           command          summary   mask (/21)
!                                            address

! ── Optional: suppress entirely ─────────────────────────────────
NetsTuts_R2(config-router)#summary-address 10.4.0.0 255.255.248.0 not-advertise
! ← Suppresses ALL matching external routes without sending any summary

NetsTuts_R2(config-router)#exit

The summary-address command (without an area keyword) applies to external routes being redistributed into OSPF — it is the ASBR counterpart to area range. The command must be configured on the same router that is performing the redistribution. A Null0 route is automatically installed for the summary prefix on the ASBR, just as with area range. Note the syntax difference: area [id] range uses a prefix and mask, and the area ID is mandatory. summary-address also uses a prefix and mask but has no area ID — it applies to all external routes that fall within the specified range.

Verify ASBR External Summary

! ── Verify external summary on R1 ────────────────────────────────
NetsTuts_R1#show ip route ospf | include OE2
O E2  10.4.0.0/21 [110/20] via 10.255.0.2, 00:00:15, Gi0/0  ← ✓ one summary

! ── Verify LSA database — only one Type 5 LSA for the summary ────
NetsTuts_R1#show ip ospf database external
            OSPF Router with ID (1.1.1.1) (Process ID 1)

                Type-5 AS External Link States

  LS age: 47
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 10.4.0.0 (External Network Number)
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000001
  Network Mask: /21                                ← ✓ /21 summary in LSA DB
  ...

! ── Verify Null0 on ASBR ─────────────────────────────────────────
NetsTuts_R2#show ip route 10.4.0.0
Routing entry for 10.4.0.0/21
  Known via "ospf 1", distance 110, metric 0, type extern 2
  * directly connected, via Null0                  ← ✓ Null0 installed

6. Step 3 — EIGRP Interface-Level Summarization

EIGRP summarization is configured per-interface — on the specific interface through which the summary will be advertised to neighbors. Unlike OSPF which restricts summarization to ABR/ASBR roles, any EIGRP router can summarize routes on any interface. EIGRP summarization suppresses individual component routes in EIGRP updates sent out that interface and replaces them with the summary.

Understand the Summary to Configure

  R4's routes to summarize:
    10.4.0.0/24, 10.4.1.0/24, 10.4.2.0/24, 10.4.3.0/24
    10.4.4.0/24, 10.4.5.0/24, 10.4.6.0/24, 10.4.7.0/24

  Binary third octet:
    0:  0 0 0 0 0 0 0 0
    1:  0 0 0 0 0 0 0 1
    2:  0 0 0 0 0 0 1 0
    3:  0 0 0 0 0 0 1 1
    4:  0 0 0 0 0 1 0 0
    5:  0 0 0 0 0 1 0 1
    6:  0 0 0 0 0 1 1 0
    7:  0 0 0 0 0 1 1 1
        ─────────────────
        ↑ ↑ ↑ ↑ ↑ ↕ ↕ ↕     ← 5 bits match in third octet

  Summary: 8 + 8 + 5 = /21
  Summary address: 10.4.0.0/21  (mask 255.255.248.0)
  Covers: 10.4.0.0 → 10.4.7.255

Configure EIGRP Summarization on R4

! ── Verify current EIGRP neighbors and routes advertised ─────────
NetsTuts_R4#show ip eigrp neighbors
EIGRP-IPv4 Neighbors for AS(100)
H   Address         Interface       Hold  Uptime   SRTT  RTO  Q  Seq
0   10.3.0.1        Gi0/0           14    00:22:45  12   200   0  147
! ── Neighbor via Gi0/0 — this is the interface to summarize on ───

! ── Check routes before summarization ────────────────────────────
NetsTuts_R4#show ip eigrp topology | include P 10.4
P 10.4.0.0/24, 1 successors, FD is 128256
P 10.4.1.0/24, 1 successors, FD is 128256
P 10.4.2.0/24, 1 successors, FD is 128256
P 10.4.3.0/24, 1 successors, FD is 128256
P 10.4.4.0/24, 1 successors, FD is 128256
P 10.4.5.0/24, 1 successors, FD is 128256
P 10.4.6.0/24, 1 successors, FD is 128256
P 10.4.7.0/24, 1 successors, FD is 128256

! ── Configure summarization on the interface facing R2/R3 ────────
NetsTuts_R4(config)#interface GigabitEthernet0/0
NetsTuts_R4(config-if)#ip summary-address eigrp 100 10.4.0.0 255.255.248.0
!                       ^^^^^^^^^^^^^^^^^^^ ^^^^ ^^^^^^^^^ ^^^^^^^^^^^^^
!                       command             ASN  summary   mask (/21)
!                                                address
NetsTuts_R4(config-if)#exit

The ip summary-address eigrp command is applied under the interface configuration — not inside the EIGRP router process. This is a key difference from OSPF summarization. The interface specified should be the one facing the neighbor(s) to whom you want to advertise the summary. When this command is entered, EIGRP immediately withdraws the 8 individual /24 routes from updates sent out Gi0/0 and sends a single update advertising 10.4.0.0/21 instead. A Null0 route is installed locally. The individual routes remain in R4's local routing table and EIGRP topology table — they are simply suppressed in advertisements out this specific interface.

EIGRP Auto-Summary vs Manual Summary

! ── IOS 15.x+ has auto-summary DISABLED by default ───────────────
! ── Verify it is off (it should be) ─────────────────────────────
NetsTuts_R4#show ip protocols | include auto-summary
  Automatic Summarization: disabled   ← ✓ auto-summary off

! ── If auto-summary is enabled (IOS 12.x default — legacy) ───────
! ── Disable it globally ──────────────────────────────────────────
NetsTuts_R4(config)#router eigrp 100
NetsTuts_R4(config-router)#no auto-summary
NetsTuts_R4(config-router)#exit

! ── DIFFERENCE: ─────────────────────────────────────────────────
! auto-summary: summarizes at classful boundary automatically
!   10.4.0.0/24, 10.4.1.0/24 → summarized to 10.0.0.0/8 (Class A!)
!   This is almost never desirable in modern networks.

! manual summary (ip summary-address): you control the prefix/mask
!   10.4.0.0–10.4.7.0/24 → 10.4.0.0/21 (precise, correct)
! Always use manual summarization and keep auto-summary disabled.

Verify EIGRP Summarization

! ── Verify on R4: Null0 installed for the summary ────────────────
NetsTuts_R4#show ip route 10.4.0.0 255.255.248.0
Routing entry for 10.4.0.0/21
  Known via "eigrp 100", distance 5, metric 128256, type internal
  Redistributing via eigrp 100
  Routing Descriptor Blocks:
  * directly connected, via Null0                 ← ✓ Null0 installed

! ── Verify on R4: summary-address configured on interface ────────
NetsTuts_R4#show ip eigrp interfaces detail GigabitEthernet0/0
EIGRP-IPv4 Interfaces for AS(100)
                       Xmit Queue   PeerQ        Mean   Pacing Time   Multicast    Pending
Interface       Peers  Un/Reliable  Un/Reliable  SRTT   Un/Reliable   Flow Timer   Routes
Gi0/0             1        0/0       0/0          12    0/0          50            0

  Summary address: 10.4.0.0/21 distance 5   ← ✓ summary configured on Gi0/0

! ── Verify on NEIGHBOR: individual routes replaced by summary ─────
NetsTuts_R2#show ip route eigrp
      10.0.0.0/8 is variably subnetted
D     10.4.0.0/21 [90/130816] via 10.3.0.2, 00:00:47, Gi0/1  ← ✓ one summary
! ── 8 individual D routes replaced by single D 10.4.0.0/21 ──────

! ── Verify summary not split into individual routes ──────────────
NetsTuts_R2#show ip route | include 10.4
D     10.4.0.0/21 [90/130816] via 10.3.0.2            ← only the summary, no /24s

7. Intra-Area Summarization Limitation and Workarounds

A common exam question and real-world confusion: OSPF cannot summarize routes within an area. The area range command only summarizes routes between areas (at the ABR). If all routers are in Area 0, there is no ABR and therefore no summarization point. Understanding this limitation and its workarounds is important for OSPF design.

  SCENARIO: All routers in OSPF Area 0 only.
  Routers R5, R6, R7, R8 each advertise /24 subnets.
  R1 wants to see only summary routes, not individual /24s.

  PROBLEM: There is no ABR — all routers are internal Area 0 routers.
  'area 0 range' has no effect on an internal router (not an ABR).

  WORKAROUND OPTIONS:

  Option 1 — Redesign into multi-area OSPF:
    Create a new area (Area 1) for the /24 subnets.
    R5 becomes ABR between Area 0 and Area 1.
    R5 uses 'area 1 range' to summarize Area 1 into Area 0.
    ✓ Correct OSPF design — preferred for large networks.

  Option 2 — Use EIGRP for the stub area and redistribute:
    Run EIGRP between stub routers.
    Redistribute EIGRP into OSPF at the boundary.
    Use 'summary-address' (ASBR) to summarize external routes.
    ✓ Works but creates redistribution complexity.

  Option 3 — OSPF Stub/NSSA area:
    Stub areas block Type 5 External LSAs.
    NSSA areas generate Type 7 LSAs summarized at ABR.
    ✓ Reduces LSA flooding but doesn't create custom summaries.

  BOTTOM LINE: Plan your IP address space so that subnets that
  need to be summarized are grouped into separate OSPF areas.
  Summarization works best with hierarchical addressing design.

8. Step 4 — Full Verification Across the Network

show ip route — Before and After Summary

! ═══════════════════════ BEFORE SUMMARIZATION ════════════════════
NetsTuts_R1#show ip route ospf
O IA  10.1.0.0/24 [110/2] via 10.255.0.2, Gi0/0
O IA  10.1.1.0/24 [110/2] via 10.255.0.2, Gi0/0
O IA  10.1.2.0/24 [110/2] via 10.255.0.2, Gi0/0
O IA  10.1.3.0/24 [110/2] via 10.255.0.2, Gi0/0
O IA  10.2.0.0/24 [110/2] via 10.255.0.6, Gi0/1
O IA  10.2.1.0/24 [110/2] via 10.255.0.6, Gi0/1
O IA  10.2.2.0/24 [110/2] via 10.255.0.6, Gi0/1
O IA  10.2.3.0/24 [110/2] via 10.255.0.6, Gi0/1
O E2  10.4.0.0/24 [110/20] via 10.255.0.2, Gi0/0
O E2  10.4.1.0/24 [110/20] via 10.255.0.2, Gi0/0
O E2  10.4.2.0/24 [110/20] via 10.255.0.2, Gi0/0
O E2  10.4.3.0/24 [110/20] via 10.255.0.2, Gi0/0
O E2  10.4.4.0/24 [110/20] via 10.255.0.2, Gi0/0
O E2  10.4.5.0/24 [110/20] via 10.255.0.2, Gi0/0
O E2  10.4.6.0/24 [110/20] via 10.255.0.2, Gi0/0
O E2  10.4.7.0/24 [110/20] via 10.255.0.2, Gi0/0
! ── 16 individual entries ← before summarization ─────────────────

! ═══════════════════════ AFTER SUMMARIZATION ═════════════════════
NetsTuts_R1#show ip route ospf
O IA  10.1.0.0/22 [110/2] via 10.255.0.2, 00:01:14, Gi0/0  ← ✓ Area 1 summary
O IA  10.2.0.0/22 [110/2] via 10.255.0.6, 00:00:58, Gi0/1  ← ✓ Area 2 summary
O E2  10.4.0.0/21 [110/20] via 10.255.0.2, 00:00:47, Gi0/0 ← ✓ EIGRP summary
! ── 3 entries instead of 16 ← 81% routing table reduction ────────

show ip ospf database — LSA Reduction Confirmation

! ── Before: 16 LSAs (8 Type3 inter-area + 8 Type5 external) ──────
! ── After: 3 LSAs (2 Type3 summaries + 1 Type5 summary) ──────────

NetsTuts_R1#show ip ospf database summary
            OSPF Router with ID (1.1.1.1) (Process ID 1)

                Summary Net Link States (Area 0)

  LS age: 73
  Link State ID: 10.1.0.0      ← Type 3 LSA — Area 1 inter-area summary
  Advertising Router: 2.2.2.2
  Network Mask: /22             ← /22 summary

  LS age: 61
  Link State ID: 10.2.0.0      ← Type 3 LSA — Area 2 inter-area summary
  Advertising Router: 3.3.3.3
  Network Mask: /22

NetsTuts_R1#show ip ospf database external
  LS age: 47
  Link State ID: 10.4.0.0      ← Type 5 LSA — ASBR external summary
  Advertising Router: 2.2.2.2
  Network Mask: /21             ← /21 summary

show ip ospf border-routers — ABR/ASBR Identification

NetsTuts_R1#show ip ospf border-routers
OSPF Process 1 internal Routing Table

Codes: i - Intra-area route, I - Inter-area route

i 2.2.2.2 [10] via 10.255.0.2, GigabitEthernet0/0, ABR/ASBR, Area 0
i 3.3.3.3 [10] via 10.255.0.6, GigabitEthernet0/1, ABR, Area 0

! ── R2 shown as ABR/ASBR (performing both inter-area and external summary)
! ── R3 shown as ABR only (inter-area summary only)

show ip protocols — EIGRP Summary Confirmation

NetsTuts_R4#show ip protocols
*** IP Routing is NSF aware ***

Routing Protocol is "eigrp 100"
  Outgoing update filter list for all interfaces is not set
  Incoming update filter list for all interfaces is not set
  Default networks flagged in outgoing updates
  Default networks accepted from incoming updates
  EIGRP-IPv4 Protocol for AS(100)
    ...
  Routing for Networks:
    10.3.0.0/24
    10.4.0.0/8     ← loopback networks advertised
  Routing Information Sources:
    Gateway         Distance  Last Update
    10.3.0.1        90        00:25:14
  Automatic Summarization: disabled   ← ✓ auto-summary off

NetsTuts_R4#show ip eigrp interfaces detail Gi0/0 | include Summary
  Summary address: 10.4.0.0/21 distance 5   ← ✓ manual summary active

Verification Command Summary

Command	Router	What It Confirms
`show ip ospf`	ABR	Confirms router is ABR; shows "Area range: X.X.X.X/YY Advertise"
`show ip route ospf`	Core/downstream router	Summary routes (O IA or O E2) replace individual /24 entries — see show ip route reference
`show ip route [summary-prefix] [mask]`	Summarizing router	Shows Null0 route for the summary — confirms loop prevention
`show ip ospf database summary`	Any OSPF router	Type 3 LSAs — confirms one summary LSA per area range, not multiple /24 LSAs
`show ip ospf database external`	Any OSPF router	Type 5 LSAs — confirms one external summary LSA, not individual external LSAs
`show ip eigrp interfaces detail [int]`	EIGRP summarizing router	Shows "Summary address: X.X.X.X/YY" confirming summary on interface
`show ip route eigrp`	EIGRP neighbor	Summary D route replaces individual D routes — confirms correct advertisement
`show ip protocols`	EIGRP router	Confirms "Automatic Summarization: disabled" — verifies auto-summary is off

9. Complete Configuration Reference & Key Points

Full Summarization Configuration

! ════════════════ R2 — OSPF ABR + ASBR ══════════════════════════
router ospf 1
 router-id 2.2.2.2
 network 10.1.0.0 0.0.3.255 area 1      ! Area 1 interfaces
 network 10.255.0.0 0.0.0.3 area 0      ! Area 0 uplink
 area 1 range 10.1.0.0 255.255.252.0    ! Inter-area summary → /22
 redistribute eigrp 100 subnets         ! Redistribute EIGRP into OSPF
 summary-address 10.4.0.0 255.255.248.0 ! External summary → /21
!
! ════════════════ R3 — OSPF ABR ══════════════════════════════════
router ospf 1
 router-id 3.3.3.3
 network 10.2.0.0 0.0.3.255 area 2
 network 10.255.0.4 0.0.0.3 area 0
 area 2 range 10.2.0.0 255.255.252.0    ! Inter-area summary → /22
!
! ════════════════ R4 — EIGRP Summarization ═══════════════════════
router eigrp 100
 network 10.3.0.0 0.0.0.255
 network 10.4.0.0 0.0.7.255
 no auto-summary
!
interface GigabitEthernet0/0
 description Link-to-R2-R3
 ip address 10.3.0.2 255.255.255.0
 ip summary-address eigrp 100 10.4.0.0 255.255.248.0   ! EIGRP summary → /21

Summary Address Command Reference

Command	Mode	Protocol	Purpose
`area [id] range [prefix] [mask]`	Router OSPF config	OSPF ABR	Summarize routes from the specified area into other areas (inter-area). Installs Null0 locally.
`area [id] range [prefix] [mask] not-advertise`	Router OSPF config	OSPF ABR	Suppress all routes from the area matching the prefix — no summary LSA sent.
`area [id] range [prefix] [mask] cost [n]`	Router OSPF config	OSPF ABR	Override the automatically calculated metric for the summary LSA.
`summary-address [prefix] [mask]`	Router OSPF config	OSPF ASBR	Summarize external (redistributed) routes before advertising as Type 5/7 LSAs. Installs Null0.
`summary-address [prefix] [mask] not-advertise`	Router OSPF config	OSPF ASBR	Suppress all external routes matching the prefix.
`ip summary-address eigrp [asn] [prefix] [mask]`	Interface config	EIGRP	Advertise summary to neighbors on this interface. Individual routes suppressed in updates out this interface. Installs Null0.
`no auto-summary`	Router EIGRP config	EIGRP	Disable automatic classful summarization. Always configure this on IOS 12.x — default off in IOS 15.x+.
`show ip ospf`	Privileged exec	OSPF	Verify ABR/ASBR role and show configured area ranges.
`show ip route ospf`	Privileged exec	OSPF	Confirm summary routes (O IA = inter-area, O E2 = external) in routing table.
`show ip ospf database summary`	Privileged exec	OSPF	View Type 3 summary LSAs — confirms single summary LSA per area range.
`show ip eigrp interfaces detail [int]`	Privileged exec	EIGRP	Show EIGRP interface details including active summary addresses.

Key Points & Exam Tips

Always use binary to calculate summary addresses. The power-of-2 shortcut works only for perfectly aligned contiguous subnets — in any other case, binary is the only reliable method. Write out the differing octets in binary, find the common prefix, and count the matching bits to determine the prefix length.
Null0 routes are installed automatically for all summaries. Both OSPF (area range and summary-address) and EIGRP (ip summary-address) automatically install a Null0 discard route for the summary prefix when the command is configured. This prevents routing loops — the Null0 route has an administrative distance that makes it a valid tie-breaker against the default route. If any specific routes within the summary are removed, the Null0 ensures traffic to those addresses is dropped locally rather than looped back to the advertising router.
OSPF summarization is ABR/ASBR-only. area range only has effect on a router that is genuinely an ABR (connected to at least two OSPF areas). Configuring it on an internal router in a single area is accepted by IOS but has no operational effect. Verify the router is an ABR with show ip ospf before configuring area range.
EIGRP summarization is per-interface, not per-process. The ip summary-address eigrp command is configured under the interface facing the neighbor(s) who should receive the summary. The same router can advertise different summaries (or no summary) on different interfaces — giving precise control over which neighbors see the summary.
area range summarizes the area ID in the command, not the destination area. area 1 range 10.1.0.0 255.255.252.0 on an ABR means "summarize routes from Area 1" — not "summarize routes that are going into Area 1." The area ID identifies the source area of the routes being aggregated.
Auto-summary in EIGRP is dangerous and must be disabled. EIGRP auto-summary summarizes at classful network boundaries (Class A/B/C). In discontiguous networks (same major network number on multiple non-adjacent interfaces), auto-summary causes black holes — the classful summary covers both sides but only one is reachable from any given point. Always disable with no auto-summary.
A summary covering unused address space creates a black hole. 10.1.0.0/22 covers 10.1.0.0–10.1.3.255. If 10.1.2.0/24 and 10.1.3.0/24 are not real subnets, traffic destined for those addresses will match the summary at the ABR, be forwarded toward Area 1, match the Null0 at the ABR, and be dropped. Plan your summaries to cover only address space that is actually in use.
Summary cost in OSPF area range. By default, the summary LSA cost equals the maximum cost among all component routes within the summary. This can be overridden with area range ... cost [n]. A fixed cost ensures the summary metric doesn't fluctuate as individual component route costs change.
On the CCNP exam: know the binary calculation method, the difference between area range (ABR, inter-area) and summary-address (ASBR, external), the fact that OSPF cannot summarize within a single area, the Null0 black hole route behaviour, and the per-interface nature of EIGRP summarization.

Related Labs: Route summarization is most effective when combined with a well-designed multi-area OSPF topology. See OSPF Multi-Area Configuration for area design fundamentals. For EIGRP topology and DUAL algorithm context see EIGRP Configuration. In BGP environments, the equivalent feature is aggregate-address — see BGP Basics (eBGP). Route summarization combined with Policy-Based Routing (PBR) allows fine-grained traffic engineering at summarization boundaries. For OSPFv3 and IPv6 summarization the same concepts apply — see OSPFv3 Configuration and IPv6 Basic Configuration.

TEST WHAT YOU LEARNED

1. Calculate the correct summary address for the following four networks: 192.168.16.0/24, 192.168.17.0/24, 192.168.18.0/24, 192.168.19.0/24. Show your work.

A. 192.168.16.0/23 — covers only .16 and .17 B. 192.168.0.0/16 — classful Class C summarization C. 192.168.16.0/23 and 192.168.18.0/23 — two summaries needed D. 192.168.16.0/22. Binary: third octet — 16=00010000, 17=00010001, 18=00010010, 19=00010011. Common bits (left to right): 0-0-0-1-0-0 = 6 bits match. Prefix = 8+8+6 = /22. Summary network = set remaining bits to 0 = 00010000 = 16. Answer: 192.168.16.0/22, covers 192.168.16.0–192.168.19.255

Correct answer is D. Walking through the binary calculation: the third octet values are 16, 17, 18, 19. In binary: 16 = 0001 0000, 17 = 0001 0001, 18 = 0001 0010, 19 = 0001 0011. Aligning these four values and comparing bit by bit from left: bit 8 (value 128) → 0,0,0,0 — all same. Bit 7 (64) → 0,0,0,0 — all same. Bit 6 (32) → 0,0,0,0 — all same. Bit 5 (16) → 1,1,1,1 — all same. Bit 4 (8) → 0,0,0,0 — all same. Bit 3 (4) → 0,0,0,0 — all same. Bit 2 (2) → 0,0,1,1 — DIFFERENT. Matching stops at 6 bits in the third octet. Total prefix = 8 (first) + 8 (second) + 6 (third) = /22. Summary address = set all non-matching bits to zero: 0001 0000 = 16. Summary = 192.168.16.0/22. Verification: /22 mask = 255.255.252.0. 192.168.19.0 AND 255.255.252.0 = 192.168.16.0 ✓. The summary covers 192.168.16.0 through 192.168.19.255 — all four /24 networks are within this range. Option A (/23) is wrong because /23 only covers two /24 networks — 192.168.16.0 and 192.168.17.0, not 18 and 19. You would need to verify this: 192.168.18.0 AND 255.255.254.0 = 192.168.18.0 ≠ 192.168.16.0 — fails the verification test.

2. What is the difference between `area 1 range 10.1.0.0 255.255.252.0` and `summary-address 10.1.0.0 255.255.252.0` in OSPF? When is each used?

A. They are functionally identical — both commands produce the same result when configured on any OSPF router B. area range is configured on an ABR to summarize inter-area routes — it suppresses individual Type 3 Summary LSAs for subnets within the range and replaces them with a single Type 3 LSA as the ABR advertises into other areas. summary-address is configured on an ASBR to summarize external (redistributed) routes — it suppresses individual Type 5 (or Type 7) External LSAs and replaces them with a single external LSA. Use area range for native OSPF routes crossing area boundaries; use summary-address for routes redistributed from another protocol (EIGRP, BGP, static) entering OSPF C. area range applies to all interfaces in the area; summary-address applies only to the interface where redistribution occurs D. summary-address is the modern replacement for the deprecated area range command in IOS 15.x and later

Correct answer is B. The two commands operate at different OSPF boundaries and on different LSA types. area range is the ABR mechanism: when an ABR learns specific routes from one area (say Area 1) via Type 1 and Type 2 LSAs within that area, it normally generates individual Type 3 Summary LSAs for each prefix as it advertises into other areas. area range intercepts this process — instead of generating 10 Type 3 LSAs for ten /24 networks, it generates one Type 3 LSA for the summary /22. The command identifies the source area (Area 1) and the summary range. summary-address is the ASBR mechanism: when routes are redistributed into OSPF from an external source, the ASBR generates Type 5 (or Type 7 in NSSA areas) External LSAs. Without summarization, one Type 5 LSA per prefix is generated and flooded domain-wide. summary-address merges matching prefixes into one Type 5 LSA before flooding. A router can be both ABR and ASBR simultaneously (as in this lab's R2), in which case both commands may be configured. They operate independently — area range on the inter-area paths, summary-address on the external paths.

3. Why does IOS automatically install a Null0 route when a summary address is configured in OSPF or EIGRP? What routing problem does it prevent?

A. The Null0 route prevents a routing loop. When a router advertises a summary (e.g., 10.1.0.0/22) to its neighbors, those neighbors send traffic for any address in that range to the summarizing router. If a packet arrives for an address within the summary range but no specific subnet route exists (e.g., 10.1.14.5 — an unused address), the summarizing router would have no specific route and would use its default route, which might point back toward the upstream router — creating a loop. The Null0 route (10.1.0.0/22 → Null0) is a more specific match than the default route, so the packet is dropped locally rather than looped. Without Null0: traffic loops. With Null0: traffic is discarded at the source with no loop. B. The Null0 route is installed as a backup path — if the summary's component routes all fail, traffic is sent to Null0 to trigger an SNMP trap C. The Null0 route improves CEF performance by reserving a FIB slot for the summary prefix D. Null0 routes are installed to allow ping testing — traffic to Null0 generates ICMP unreachable messages that can be used for connectivity verification

Correct answer is A. The Null0 route is a classic loop-prevention mechanism. Consider the scenario without it: R2 (ABR) advertises 10.1.0.0/22 to the backbone. R1 (backbone router) receives traffic destined for 10.1.14.5 (which is within the 10.1.0.0/22 range but is not an actual subnet). R1's routing table shows 10.1.0.0/22 via R2 — so R1 forwards to R2. R2 receives the packet, looks in its routing table for 10.1.14.5 — no specific /24 exists for this address. R2 uses its default route, which points back to R1. R2 forwards to R1. R1 forwards back to R2. Infinite loop until TTL reaches 0. With the Null0 route: R2 receives the packet for 10.1.14.5. Routing table lookup: 10.1.0.0/22 → Null0 (longest match, more specific than default route). Packet is dropped at R2. No loop. The Null0 route is automatically installed by OSPF (for both area range and summary-address) and EIGRP (for ip summary-address) whenever the summarization feature creates a risk of this type of loop. It appears in the routing table as "10.1.0.0/22 is a summary, Null0" and is removed if all component routes within the summary are withdrawn.

4. An engineer configures `area 1 range 10.1.0.0 255.255.252.0` on R2 but notices that R1 still sees all four individual /24 routes from Area 1. What are the two most likely causes?

A. The command syntax is wrong — OSPF summarization requires a wildcard mask, not a subnet mask B. The OSPF process needs to be restarted after adding an area range command — summarization is not dynamic C. Cause 1: R2 is NOT actually an ABR — it may be configured in only one OSPF area (not both Area 0 and Area 1). On a non-ABR router, area range is accepted by IOS but has no effect. Verify with show ip ospf — it must say "It is an area border router." Cause 2: The summary range does not cover all four /24 networks — one of the subnets may fall outside 10.1.0.0/22 (e.g., 10.1.4.0/24 is NOT covered by /22 and would still be advertised individually). Verify the summary calculation covers all subnets. D. The area range command only works on virtual links — a physical ABR connection requires a different command

Correct answer is C. These are the two most common real-world causes when area range appears to have no effect. Cause 1 — Router is not an ABR: OSPF only allows ABRs to perform inter-area summarization. An ABR is a router with at least one interface in Area 0 and at least one interface in a non-backbone area. If R2 has all interfaces in Area 1 (no Area 0 connection), or all interfaces in Area 0 (no Area 1 connection), it is not an ABR and the area range command is silently ignored. Check with show ip ospf — if it says "It is an internal router" or does not list "It is an area border router," the command will not work. The fix is to ensure the router has interfaces in both areas and OSPF adjacencies in both areas. Cause 2 — Summary range mismatch: if one of the /24 networks falls outside the configured summary range, that specific route continues to be advertised individually. Example: if Area 1 contains 10.1.0.0/24, 10.1.1.0/24, 10.1.2.0/24, 10.1.3.0/24, and 10.1.4.0/24 (five networks), configuring area 1 range 10.1.0.0 255.255.252.0 (/22) covers only the first four. 10.1.4.0/24 (outside the /22 range) is still advertised as an individual Type 3 LSA. The summary only suppresses routes that fall within the configured range.

5. Why is EIGRP auto-summary dangerous in discontiguous networks, and what specific problem does it cause? How do you fix it?

A. Auto-summary is only dangerous on Class C networks — Class A and B networks are not affected B. Auto-summary causes EIGRP neighbors to form adjacencies only with routers in the same classful network, blocking cross-network communication C. Auto-summary advertises the host mask instead of the network mask, causing routing table corruption D. A discontiguous network exists when the same major network number (e.g., 10.0.0.0/8) appears on two or more routers that are not directly connected — separated by a different major network. With auto-summary enabled, EIGRP summarizes at the classful boundary when routes cross a major network boundary. Router A advertises 10.1.0.0/24 summarized to 10.0.0.0/8. Router B also advertises 10.2.0.0/24 summarized to 10.0.0.0/8. Both summaries point to different locations. Any router receiving both summaries cannot distinguish between them — the routing table installs both, load-balancing (or picking one), causing half the traffic to reach the wrong destination. The fix: configure no auto-summary under the EIGRP process on all routers. This disables classful summarization — specific routes (/24s) are advertised as-is. Manual summaries can still be configured with ip summary-address eigrp where needed.

Correct answer is D. Discontiguous networks with EIGRP auto-summary is a classic problem that was extremely common in the IOS 12.x era when auto-summary was enabled by default. The scenario: a company has 10.1.0.0/24 in Site A and 10.2.0.0/24 in Site B, connected via 172.16.0.0/24 WAN links. With auto-summary: Site A's router advertises 10.1.0.0/24 out its WAN interface — auto-summary converts this to 10.0.0.0/8 at the class boundary crossing. Site B's router similarly advertises 10.0.0.0/8. The WAN router receives two routes to 10.0.0.0/8 from two directions. It load-balances between them. Traffic from 172.16.x.x destined for 10.1.0.5 has a 50% chance of going to Site B instead of Site A — where it finds no matching specific route and is dropped. The symptom is intermittent connectivity: pings succeed every other packet. The diagnosis: show ip route 10.0.0.0 shows two equal-cost paths (via two different next-hops), both pointing to classful summaries. The fix is simple: no auto-summary on all EIGRP routers causes them to advertise specific /24 routes instead of classful summaries. Modern IOS (15.x+) has auto-summary disabled by default precisely because of this well-known problem.

6. A router is configured with `ip summary-address eigrp 100 10.4.0.0 255.255.248.0` on Gi0/0. The summary covers 10.4.0.0/24 through 10.4.7.0/24, but only routes 10.4.0.0/24 through 10.4.5.0/24 actually exist in the routing table. What happens to traffic destined for 10.4.6.5?

A. Traffic is forwarded normally — EIGRP automatically informs neighbors that 10.4.6.0/24 is unreachable within the summary B. The summarizing router still advertises 10.4.0.0/21 (the full summary) even though 10.4.6.0/24 and 10.4.7.0/24 don't exist. Neighbors receive the summary and forward traffic for 10.4.6.5 to this router. The router looks up 10.4.6.5 — no specific /24 route exists. The Null0 route for 10.4.0.0/21 is the best match. Traffic is dropped (black hole). The summary overadvertises reachability to addresses that don't exist. C. EIGRP automatically splits the summary into two parts to exclude the missing subnets D. The summarizing router withdraws the entire /21 summary when any component /24 is missing, and re-advertises all remaining /24s individually

Correct answer is B. This scenario demonstrates the black hole risk of over-summarization. The EIGRP summary 10.4.0.0/21 is advertised as long as at least one component route within the summary range exists in the routing table. The summary does not dynamically shrink to exclude missing subnets — it covers the entire configured range (10.4.0.0 through 10.4.7.255) regardless of which specific subnets are present. Neighbors see the summary and assume the summarizing router can reach any address in 10.4.0.0/21. When a packet arrives for 10.4.6.5: the summarizing router receives it (forwarded here because of the /21 summary advertisement), looks for 10.4.6.0/24 in the routing table — not found. The Null0 route 10.4.0.0/21 is the longest matching prefix (more specific than any default route). Packet is dropped. The sender never receives an explicit error — the packet is silently discarded at the Null0. This is the "black hole" trade-off of summarization with unused address space. The engineering solution is to only configure summaries that cover address ranges where all subnets are actually in use, or to carefully plan IP address allocation so the summarized range is contiguous and fully allocated.

7. What is the correct summary address for these eight subnets, and what prefix length does it use? 172.16.32.0/24, 172.16.33.0/24, 172.16.34.0/24, 172.16.35.0/24, 172.16.36.0/24, 172.16.37.0/24, 172.16.38.0/24, 172.16.39.0/24

A. 172.16.32.0/22 — covers only .32 through .35 B. 172.16.0.0/20 — overly broad, includes 172.16.0–172.16.15 as well C. 172.16.32.0/21. Binary third octet: 32=00100000, 33=00100001, 34=00100010, 35=00100011, 36=00100100, 37=00100101, 38=00100110, 39=00100111. Common bits: 0-0-1-0-0 = 5 bits match. Prefix = 8+8+5 = /21. Summary = 172.16.32.0/21, covers 172.16.32.0–172.16.39.255 D. 172.16.32.0/20 — this is the nearest classful boundary

Correct answer is C. Binary calculation for the third octet: 32 = 0010 0000, 33 = 0010 0001, 34 = 0010 0010, 35 = 0010 0011, 36 = 0010 0100, 37 = 0010 0101, 38 = 0010 0110, 39 = 0010 0111. Comparing column by column: bit 8 (128) → 0,0,0,0,0,0,0,0 — same. Bit 7 (64) → 0,0,0,0,0,0,0,0 — same. Bit 6 (32) → 1,1,1,1,1,1,1,1 — same. Bit 5 (16) → 0,0,0,0,0,0,0,0 — same. Bit 4 (8) → 0,0,0,0,0,0,0,0 — same. Bit 3 (4) → 0,0,0,0,1,1,1,1 — DIFFERENT. 5 matching bits in the third octet. Prefix = 8 + 8 + 5 = /21. Summary address: set non-matching bits to 0 → 0010 0000 = 32. Summary = 172.16.32.0/21. Mask = 255.255.248.0. Range = 172.16.32.0 → 172.16.39.255. Verification: 172.16.39.0 AND 255.255.248.0 = 172.16.32.0 ✓ (because 39 AND 248 = 32). The power-of-2 shortcut confirms: 8 networks, /24 original, /24 - 3 = /21 ✓ (2³ = 8). Option A (/22) only covers 4 networks (.32 to .35 — the first four). Option B (/20) is too broad — it covers .16 through .31 as well as .32 through .47, which is more than needed.

8. An OSPF network has all routers in a single area (Area 0). A network engineer wants to reduce routing table size by summarizing groups of /24 subnets. They attempt to configure `area 0 range` on R1. Will this work? What are the alternatives?

A. No — area range only works on ABRs, and in a single-area network there are no ABRs by definition. All routers are internal Area 0 routers. Configuring area 0 range on any of them will be accepted by IOS but will have no operational effect — no summarization occurs. Alternatives: (1) Redesign into multi-area OSPF — create Area 1, Area 2, etc. for groups of subnets with an ABR for each; use area range on the ABRs. (2) Redistribute routes into OSPF at a boundary router and use summary-address (ASBR summarization). (3) Accept the full routing table in a single-area design — single area is appropriate for smaller networks where summarization is less critical. B. Yes — area 0 range works on any router that has interfaces in Area 0, regardless of whether it is an ABR C. Yes — but only if the router has the highest OSPF router ID in the area, which makes it the de facto summarization point D. No — but it can be fixed by configuring area 0 range on the DR/BDR router instead, which acts as the summarization point on multi-access segments

Correct answer is A. This is one of the most fundamental OSPF summarization limitations and a frequent source of frustration for engineers who are new to OSPF multi-area design. The reason area range has no effect on internal routers is architectural: OSPF summarization works by controlling what information crosses area boundaries. An ABR sits at the boundary between areas and can filter/aggregate the Type 3 LSAs it generates when translating intra-area information (Type 1/2 LSAs) into inter-area information (Type 3 LSAs). An internal router has no such boundary to control — it simply originates Type 1 LSAs for its own prefixes and floods all Type 1 and Type 2 LSAs throughout the single area. There is no mechanism to aggregate within an area. This is why proper IP address planning is essential for OSPF: subnets that will be summarized must be grouped into separate areas. The practical consequence: if a network starts with all routers in Area 0 and later needs summarization, a redesign to multi-area OSPF is required. This involves creating new areas, reconfiguring router network statements, and establishing new adjacencies — it is not a trivial change in production. Starting with multi-area OSPF design (even with only two areas initially) preserves the ability to summarize as the network grows.

9. The `show ip ospf` output on R2 shows: "Area range: 10.1.0.0/22 Advertise." But `show ip route` on R1 shows both the /22 summary AND individual /24 routes from Area 1. What does this indicate?

A. This is normal OSPF behaviour — summaries and specific routes always coexist in the routing table B. R1 is running a different OSPF process ID from R2 — summaries are only effective within the same process C. R2's area range is misconfigured — the mask should be specified in wildcard format, not subnet mask format D. Another ABR exists between Area 1 and Area 0 that does NOT have area range configured. In OSPF, if multiple ABRs connect the same area to the backbone, each ABR independently advertises inter-area routes. R2 advertises the summary (suppressing its /24 LSAs), but the second ABR advertises all individual /24 LSAs without any summarization. R1 receives both: the summary from R2 and individual /24s from the other ABR. The fix: configure area 1 range 10.1.0.0 255.255.252.0 on ALL ABRs connecting Area 1 to Area 0.

Correct answer is D. This scenario exposes an important operational consideration for multi-ABR OSPF designs. When multiple ABRs connect the same non-backbone area to Area 0, each ABR independently advertises Type 3 Summary LSAs into Area 0. The area range command only affects the ABR it is configured on — it has no influence on other ABRs. If R2 has area range configured but R3 (also an ABR for Area 1) does not, then: R2 advertises 10.1.0.0/22 (one summary Type 3 LSA). R3 advertises 10.1.0.0/24, 10.1.1.0/24, 10.1.2.0/24, 10.1.3.0/24 (four individual Type 3 LSAs). R1 receives all five LSAs, installs all five routes (the summary + four specifics). The intent of summarization is defeated. The fix requires configuring the same area range on all ABRs for that area. This is an important operational checklist item: whenever you configure summarization, verify that all ABRs for the area have consistent area range statements. If they use different summary ranges or one is missing, the routing table on backbone routers will contain a mix of summaries and specific routes.

10. An EIGRP router has `ip summary-address eigrp 100 10.4.0.0 255.255.248.0` configured on Gi0/0 and Gi0/1 (two different neighbors). All eight component /24 routes exist. If the last remaining component route (10.4.7.0/24) fails, what happens to the Null0 route and the summary advertisement?

A. The Null0 route stays permanently — it is never removed once configured B. The summary advertisement stays up on both interfaces but the Null0 is removed — traffic is now passed to the default route C. When the last component route within the summary range is removed from the routing table, EIGRP withdraws the summary advertisement from all interfaces where it was configured and removes the Null0 route. Neighbors receive an EIGRP update withdrawing 10.4.0.0/21. The summary only exists in the routing table and is only advertised as long as at least one component route within the summary range is present. As soon as all component routes are gone, the summary disappears entirely — both locally (Null0 removed) and from neighbor advertisements (summary withdrawn). D. The summary is split into smaller summaries (/22s) to continue covering the remaining routes

Correct answer is C. EIGRP's summary behaviour is dynamic and tied to the presence of component routes in the routing table. The summary is not a static advertisement — it is generated automatically when component routes are present and withdrawn automatically when they are all gone. The lifecycle is: one or more component routes present → summary advertised + Null0 installed. All component routes removed → summary withdrawn from all interfaces + Null0 removed. One component route restored → summary re-advertised + Null0 re-installed. This dynamic behaviour is by design: advertising a summary when no component routes exist would create a black hole with no path to the destination (not even the Null0 would help — there is nowhere to forward, just a drop). The summary advertisement accurately reflects reachability — if at least one subnet within the range is reachable, the summary is advertised. Contrast with OSPF area range behaviour: OSPF area range is slightly different — if all component routes within the range are removed, OSPF also withdraws the summary. But if some are present and some are absent, the summary covers the full configured range (potentially advertising unreachable space). The EIGRP behaviour of "remove summary when last component is gone" is an important verification point: show ip eigrp topology all-links can show whether the summary is active and which component routes are contributing to it.

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