show ip protocols – Routing Protocol Summary and Troubleshooting

1. What Does show ip protocols Do?

show ip protocols is a Cisco IOS privileged EXEC command that displays a real-time summary of every active IPv4 routing protocol on the device. It is the single most efficient first step in routing troubleshooting — rather than digging through show ip route looking for missing routes or checking neighbour tables separately, this one command reveals the complete operational configuration of every running protocol in one place.

In a single pass, show ip protocols answers: which protocols are running; what networks each protocol is advertising; what the router ID is; who the known neighbours are and when they last sent an update; which interfaces are passive; whether any route filters are active; what the administrative distance is; whether redistribution is configured; and what the Hello and Dead timer values are. Each of these is a potential failure point when routing breaks.

  Routing troubleshooting decision tree:

  Routes missing from routing table?
       │
       ▼
  show ip protocols   ← START HERE
       │
       ├─ Is the expected protocol listed?          No  → protocol not configured
       ├─ Is the missing network in "Routing for Networks"? No → add network statement
       ├─ Is the interface passive when it should not be?  Yes → remove passive-interface
       ├─ Is a distribute-list filter blocking it?         Yes → remove or adjust filter
       ├─ Is the Router ID duplicated?                     Yes → fix duplicate RID
       ├─ Is the neighbour present in "Routing Info Sources"? No → check timers / adjacency
       └─ AD conflict with a static or connected route?    Yes → adjust AD or remove route

2. OSPF Output — Field-by-Field Annotation

The following is a fully annotated show ip protocols output for a router running OSPF. Every line is explained.

Router# show ip protocols

Routing Protocol is "ospf 1"
│ │
│ └─ Process ID (1) — local significance only; does NOT
│ need to match neighbours. Two routers with OSPF
│ process 1 and OSPF process 2 will still form adjacency.
└─ Confirms OSPF is active on this device

Outgoing update filter list for all interfaces is not set
│
└─ No distribute-list is filtering OSPF LSA advertisements outbound.
If a filter IS listed here, it may be suppressing routes you expect to see.

Incoming update filter list for all interfaces is not set
│
└─ No distribute-list is filtering inbound OSPF updates.
A misconfigured inbound filter is a common cause of missing routes.

Router ID 1.1.1.1
│
└─ The OSPF Router ID for this device. Must be UNIQUE in the OSPF domain.
A duplicate RID causes adjacency failures and unpredictable routing.
Selection order: (1) manually set → (2) highest loopback IP →
(3) highest physical interface IP.

Number of areas in this router is 1. 1 normal 0 stub 0 nssa
│
└─ How many OSPF areas this router participates in, and their types.
A router in multiple areas is an ABR (Area Border Router).

Maximum path: 4
│
└─ Maximum number of equal-cost paths OSPF will install in the routing table
for load balancing. Default is 4. Change with: maximum-paths

Routing for Networks:
10.10.10.0 0.0.0.255 area 0
│
└─ The network statements configured under "router ospf 1". These define
which interfaces OSPF activates on (and therefore which networks are
advertised). If a subnet is missing here, it will NOT be advertised
and OSPF will NOT form adjacency on that interface.

Passive Interface(s):
GigabitEthernet0/0
│
└─ This interface is PASSIVE — OSPF will advertise its connected network
but will NOT send Hello packets. No adjacency will form on this interface.
CORRECT use: LAN ports facing end hosts (prevents unnecessary OSPF traffic).
WRONG use: accidentally set on a port facing another router — adjacency fails.

Routing Information Sources:
Gateway Distance Last Update
10.10.10.2 110 00:00:10
│ │ │
│ │ └─ Time since last update received from this
│ │ neighbour. Should be recent (seconds to a
│ │ few minutes). A stale value means the
│ │ neighbour may have gone down.
│ └─ Administrative Distance this router assigned to routes
│ received from this neighbour (110 = OSPF default).
└─ The IP address of a known OSPF neighbour that is sending updates.
If an expected neighbour is missing here, the adjacency has not formed.

Distance: internal 110 external 150
│
└─ Administrative Distance values used for routes from this protocol.
Internal OSPF routes = AD 110. External OSPF routes (redistributed in) = AD 150.

Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
│ │ │ │ │
│ │ │ │ └─ Retransmit interval (s):
│ │ │ │ how long to wait before
│ │ │ │ retransmitting unacked LSAs
│ │ │ └─ Wait interval (= Dead timer by default)
│ │ └─ Dead interval (s): if no Hello received in this
│ │ window, the neighbour is declared dead.
│ │ MUST match on both ends of the link.
│ └─ Hello interval (s): how often Hello packets are sent.
│ MUST match on both ends. Default: 10s Ethernet, 30s serial.
└─ Timer mismatches are the #1 cause of OSPF adjacency failures!

3. EIGRP Output — Field-by-Field Annotation

  Routing Protocol is "eigrp 10"
  │                        │
  │                        └─ Autonomous System (AS) number (10). MUST match on
  │                           all routers that need to form EIGRP neighbours.
  │                           Unlike OSPF process IDs, EIGRP AS numbers ARE
  │                           significant — mismatched AS = no adjacency.
  └─ Confirms EIGRP is active

    Outgoing update filter list for all interfaces is not set
    Incoming update filter list for all interfaces is not set
    │
    └─ Same as OSPF — distribute-lists that filter which routes are sent/received.

    Default networks flagged in outgoing updates
    │
    └─ EIGRP will flag default network routes in its updates so neighbours can
       use them as a gateway of last resort.

    EIGRP-IPv4 Protocol for AS(10)
      Metric weight K1=1, K2=0, K3=1, K4=0, K5=0
      │
      └─ K-values determine how EIGRP calculates its composite metric.
         Default: K1 (bandwidth) and K3 (delay) are used.
         ALL routers in the EIGRP AS must have IDENTICAL K-values.
         A K-value mismatch prevents EIGRP adjacency from forming.

    Routing for Networks:
      192.168.1.0
      10.1.1.0
    │
    └─ Networks covered by this EIGRP process. Interfaces whose IP addresses
       fall within these classful networks will participate in EIGRP.
       Note: EIGRP network statements are classful by default unless a wildcard
       is specified.

    Passive Interface(s):
      GigabitEthernet0/1
    │
    └─ Same semantics as OSPF passive interface — connected network is advertised
       but no EIGRP Hello packets are sent on this interface.

    Routing Information Sources:
      Gateway         Distance      Last Update
      192.168.1.2     90            00:00:22
    │
    └─ EIGRP neighbour 192.168.1.2, AD=90 (EIGRP default), last heard 22s ago.

    Distance: internal 90 external 170
    │
    └─ Internal EIGRP routes = AD 90. External (redistributed into EIGRP) = AD 170.

4. RIP Output — Field-by-Field Annotation

  Routing Protocol is "rip"
  │
  └─ RIP (Routing Information Protocol). No process ID or AS number.

    Sending updates every 30 seconds, next due in 14 seconds
    │                  │                   │
    │                  │                   └─ Countdown to next scheduled update.
    │                  └─ RIP sends its full routing table every 30 seconds
    │                     regardless of whether anything has changed — wasteful
    │                     on large networks.
    └─ Update timer (30s by default).

    Invalid after 180 seconds, hold down 180, flushed after 240
    │                          │               │
    │                          │               └─ Route flushed from table after 240s
    │                          │                  without an update (garbage collection).
    │                          └─ Hold-down timer: once a route is marked unreachable,
    │                             RIP ignores better-metric updates for 180s to
    │                             prevent routing loops (split horizon, poison reverse).
    └─ Invalid timer: a route not refreshed within 180s is marked as possibly down.

    Routing for Networks:
      172.16.0.0
      192.168.1.0
    │
    └─ Classful network statements. Interfaces matching these classful networks
       will send and receive RIP updates.

    Routing Information Sources:
      Gateway         Distance      Last Update
      192.168.1.1     120           00:00:18
    │
    └─ RIP neighbour; AD = 120 (RIP default); last update 18 seconds ago.

    Distance: (default is 120)
    │
    └─ RIP administrative distance = 120.

    Default version control: send version 2, receive version 2
    │
    └─ Running RIPv2 (supports classless routing with subnet masks).
       If one router sends v1 and the other only accepts v2, updates are silently
       discarded — a common misconfiguration to check here.

5. All Output Fields — Master Reference Table

Field	Protocol(s)	What It Shows	Troubleshooting Significance
Routing Protocol is	All	Protocol name and identifier (OSPF process ID, EIGRP AS number, RIP)	If an expected protocol is missing, it has not been configured or has been removed; EIGRP AS numbers must match between neighbours
Router ID	OSPF, EIGRP	The unique 32-bit identifier for this router in the protocol domain	Duplicate RIDs cause adjacency instability and unpredictable SPF results; selection order: manual config > highest loopback > highest active interface IP
Routing for Networks	All	Network statements configured under the routing process — determines which interfaces participate and which subnets are advertised	If a network is missing here, it will not be advertised and OSPF/EIGRP will not send Hellos on that interface — most common cause of missing routes
Passive Interface(s)	All	Interfaces on which Hello packets are suppressed — the connected network is still advertised but no neighbour adjacency can form	Accidentally setting a router-facing interface as passive is the single most common OSPF/EIGRP misconfiguration; correct use is on LAN ports facing end hosts
Routing Information Sources	All	IP addresses of neighbours currently sending updates, along with their AD and the time since their last update was received	A missing neighbour means no adjacency has formed; a stale "Last Update" time suggests the neighbour has gone down or the link is flapping
Distance	All	The Administrative Distance applied to routes learned from this protocol — OSPF internal=110, OSPF external=150, EIGRP internal=90, EIGRP external=170, RIP=120	When two protocols know the same prefix, lower AD wins; an unexpectedly absent route may be suppressed by a lower-AD source (static or connected route)
Timer intervals (Hello/Dead)	OSPF, EIGRP	Hello: frequency of keepalive packets; Dead: how long without a Hello before declaring the neighbour down; both must match on adjacent interfaces	Timer mismatches are the most common cause of OSPF adjacency failure; OSPF Hello/Dead must match; EIGRP timers are negotiated and do not need to match
Outgoing/Incoming filter lists	All	Distribute-lists, route-maps, or prefix-lists filtering which routes are sent or received by the protocol	"Not set" = no filter active. If a filter IS listed, it may be intentionally or accidentally blocking routes; check the referenced ACL or prefix-list
Maximum path	OSPF, EIGRP, RIP	Maximum number of equal-cost (OSPF/RIP) or unequal-cost (EIGRP with variance) paths installed in the routing table for load balancing	Default is 4 for OSPF/EIGRP, 4 for RIP; increase with `maximum-paths <n>` under the routing process
Redistribution	All	Indicates if routes from another protocol are being imported (redistributed) into this protocol	Missing redistribution config, wrong seed metric, or absent route-map tag are common causes of routes not appearing after redistribution
Default originate	OSPF, EIGRP, RIP	Whether a default route (0.0.0.0/0) is being advertised to neighbours	If edge routers cannot reach the internet, verify the border router is originating the default; OSPF: `default-information originate`
K-values	EIGRP only	The metric weighting coefficients (K1–K5) used in EIGRP's composite metric formula	All routers in the EIGRP AS must have identical K-values; a mismatch prevents adjacency; default is K1=1, K2=0, K3=1, K4=0, K5=0

6. Router ID — Selection, Importance, and Configuration

The Router ID (RID) is a 32-bit value formatted as a dotted-decimal IP address (e.g., 1.1.1.1) that uniquely identifies a router within an OSPF or EIGRP domain. Every router in the domain must have a unique RID. A duplicate RID causes OSPF LSA conflicts, unpredictable SPF calculations, and neighbour instability.

  Router ID selection — checked in this order (first match wins):

  Priority 1: Manually configured RID (always wins):
  Router(config)# router ospf 1
  Router(config-router)# router-id 1.1.1.1

  Priority 2: Highest IP address on any loopback interface:
  (loopbacks are always up — preferred for stability)
  Router has Lo0=1.1.1.1 and Lo1=2.2.2.2 → RID = 2.2.2.2

  Priority 3: Highest IP address on any active physical interface:
  Router has Gi0/0=192.168.1.1 and Gi0/1=10.0.0.1 → RID = 192.168.1.1

  IMPORTANT: RID is selected ONCE at OSPF process startup.
  After changing a loopback or physical IP, the RID does NOT
  change automatically. Force it with:
  Router# clear ip ospf process
  (Warning: this resets all OSPF adjacencies temporarily)

  Best practice: Always configure the RID explicitly with router-id
  on a loopback address. Relying on automatic selection can cause
  unexpected RID changes if interfaces are added or removed.

7. Administrative Distance in show ip protocols Output

Administrative Distance (AD) is the measure of trustworthiness assigned to a routing information source. When the same prefix is known from multiple sources, the router installs only the route with the lowest AD into the routing table. show ip protocols shows the AD for each running protocol under the Distance line.

Route Source	Default AD	Shown In show ip protocols As	Notes
Directly connected	0	Not shown (implicit)	Always wins; cannot be changed
Static route	1	Not shown (not a protocol)	Wins over all dynamic protocols by default
EIGRP (internal)	90	`Distance: internal 90 external 170`	Internal = routes from within the same AS
EIGRP (external)	170	`Distance: internal 90 external 170`	External = redistributed into EIGRP from another protocol
OSPF (internal)	110	`Distance: internal 110 external 150`	Intra-area and inter-area OSPF routes
OSPF (external)	150	`Distance: internal 110 external 150`	E1 and E2 OSPF external routes (redistributed in)
RIP	120	`Distance: (default is 120)`	All RIP routes use the same AD value

See: Administrative Distance | show ip route

8. Passive Interfaces — What They Are and When to Use Them

A passive interface is an interface on which the routing protocol's outgoing Hello packets (OSPF) or update packets (RIP/EIGRP) are suppressed. The connected network is still included in the routing protocol's advertisements to other routers — but no adjacency can form on the passive interface itself.

  Normal interface (routing-active):
  Router ── Gi0/1 ──→  sends Hello/Updates ──→  Neighbour Router
                  ←──  receives Hello/Updates ←──
  Adjacency forms; routes exchanged ✓

  Passive interface:
  Router ── Gi0/0 ──→  NO Hellos sent ──→  PC / End hosts
                       Connected subnet still advertised to other routers ✓
                       No adjacency will form on Gi0/0 ✗

  Correct uses of passive-interface:
  ✓ LAN interfaces facing end hosts (PCs, printers, servers)
    — prevents unnecessary OSPF/EIGRP traffic toward devices that
      are not routers
  ✓ Loopback interfaces (always advertised; never need adjacency)

  WRONG uses (very common exam misconfiguration):
  ✗ Interface facing another router
    — adjacency will never form; routes will not be exchanged

  Configuration:
  ! Make one interface passive:
  Router(config)# router ospf 1
  Router(config-router)# passive-interface GigabitEthernet0/0

  ! Make ALL interfaces passive by default, then re-enable specific ones:
  Router(config-router)# passive-interface default
  Router(config-router)# no passive-interface GigabitEthernet0/1
  ! (only Gi0/1 will send Hellos — all others are suppressed)

  Verification:
  Router# show ip protocols
  → Look for "Passive Interface(s):" section — lists every passive interface.

9. Hello and Dead Timers — Why They Must Match

Hello packets are the keepalive mechanism for OSPF and EIGRP — they are the means by which routers confirm their neighbours are still reachable. The Dead timer (OSPF) defines how long a router waits without receiving a Hello before declaring the neighbour down. If Hello or Dead timer values do not match between adjacent OSPF routers, the adjacency will never form.

  OSPF timers — both ends of a link MUST match:
  Default (Ethernet): Hello = 10s, Dead = 40s (4× Hello)
  Default (Serial):   Hello = 30s, Dead = 120s (4× Hello)

  Timer mismatch scenario:
  Router A: Hello=10, Dead=40     (default)
  Router B: Hello=5,  Dead=20     (customised)

  What happens:
  Router A sends Hello every 10s.
  Router B declares A dead after 20s — then resets.
  Router A declares B dead after 40s — then resets.
  Adjacency flaps continuously; routing is unstable.

  Verify timer values:
  Router# show ip protocols
  → Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

  Router# show ip ospf interface GigabitEthernet0/1
  → Shows per-interface timers including the neighbour's Hello timer

  Fix a timer mismatch:
  Router(config)# interface GigabitEthernet0/1
  Router(config-if)# ip ospf hello-interval 10
  Router(config-if)# ip ospf dead-interval 40

  EIGRP timers: EIGRP negotiates hold timers — they DO NOT need to match.
  A higher hold time on one side simply means that router will wait longer
  before declaring the neighbour dead. EIGRP adjacency forms even with
  mismatched timers (unlike OSPF).

10. Routing Information Sources — Reading the Neighbour Table

The Routing Information Sources section lists every router that is currently sending routing updates to this device. This is the quick check for whether expected neighbours have formed adjacencies and are actively exchanging routes.

  Routing Information Sources:
    Gateway         Distance      Last Update
    10.10.10.2      110          00:00:10
    10.10.10.3      110          00:01:45
    192.168.1.2     90           2d03h

  Column meanings:
  Gateway     = Neighbour's IP address (the source of the routing updates)
  Distance    = Administrative Distance applied to routes from this source
  Last Update = Time since the last routing update was received

  Reading the output:
  00:00:10  → 10 seconds ago — very recent; neighbour is healthy
  00:01:45  → 1 min 45s ago  — slightly aged; may be normal or may indicate
                                a slow link; watch over time
  2d03h     → 2 days 3 hours — extremely stale for a dynamic protocol;
                                this neighbour has likely gone down; investigate

  Absent neighbour:
  If a router you expect to see as a neighbour is NOT listed here at all,
  the adjacency has not formed. Check:
  1. Is the correct interface included under "Routing for Networks"?
  2. Is that interface listed as passive when it should not be?
  3. Do Hello/Dead timers match (OSPF)?
  4. Do EIGRP K-values and AS numbers match?
  5. Is there an ACL blocking OSPF multicast (224.0.0.5/6) or EIGRP
     multicast (224.0.0.10) on the interface?

11. Step-by-Step Troubleshooting Scenarios

Scenario A — OSPF Route Missing from Routing Table

  Problem: Network 10.10.30.0/24 should appear in the routing table via OSPF
           but is missing from "show ip route".

  Step 1: Run show ip protocols
          Is "ospf 1" listed? → YES
          Is 10.10.30.0 under "Routing for Networks"? → NO
  Diagnosis: The network statement is missing.
  Fix: Router(config)# router ospf 1
       Router(config-router)# network 10.10.30.0 0.0.0.255 area 0

  Step 2 (if Step 1 network is present): Check Passive Interface(s)
          Is the interface facing R2 listed as passive? → YES
  Diagnosis: Passive interface on a router-facing port — OSPF Hello
             packets are not being sent; adjacency never formed.
  Fix: Router(config-router)# no passive-interface GigabitEthernet0/1

  Step 3 (if Step 2 OK): Check Routing Information Sources
          Is R2 (10.10.10.2) listed? → NO (not present at all)
  Diagnosis: No OSPF adjacency with R2. Check timers.
             Router# show ip ospf interface Gi0/1 → Hello=10, Dead=40
             Check R2's interface → Hello=5, Dead=20  ← MISMATCH
  Fix: Match timers on both routers.

  Step 4 (if neighbour IS present): Check outgoing filter list
          "Outgoing update filter list for GigabitEthernet0/1 is set to X"
  Diagnosis: A distribute-list is blocking the advertisement of 10.10.30.0.
  Fix: Review and remove or adjust the distribute-list.

Scenario B — EIGRP Neighbour Not Forming

  Problem: EIGRP neighbour 192.168.1.2 is not appearing in
           "Routing Information Sources" despite the link being up.

  Step 1: Run show ip protocols
          Is "eigrp 10" listed? → YES on R1; check R2
          On R2: "eigrp 20" → AS MISMATCH (R1=10, R2=20)
  Fix: Router2(config)# no router eigrp 20
       Router2(config)# router eigrp 10   ← match R1's AS

  Step 2 (if AS matches): Check K-values
          R1: K1=1, K2=0, K3=1, K4=0, K5=0
          R2: K1=1, K2=1, K3=1, K4=0, K5=0  ← K2 mismatch
  Fix: R2(config-router)# metric weights 0 1 0 1 0 0

  Step 3 (if K-values match): Check Routing for Networks
          Is 192.168.1.0 listed under R1's EIGRP networks? → NO
  Fix: R1(config-router)# network 192.168.1.0

  Step 4 (if networks OK): Check passive-interface
          Is Gi0/1 (facing R2) listed as passive on R1? → YES
  Fix: R1(config-router)# no passive-interface GigabitEthernet0/1

Scenario C — RIP Routes Disappearing Intermittently

  Problem: Routes learned via RIP randomly disappear and reappear.

  Step 1: Run show ip protocols — check "Sending updates every 30 seconds"
          Check "Invalid after 180 seconds" — if updates are not arriving,
          the route is marked invalid after 180s and flushed after 240s.

  Step 2: Check "Routing Information Sources" — is the RIP neighbour listed?
          Last Update = 2m30s → approaching the 3-minute invalid threshold
  Diagnosis: RIP updates are arriving but very slowly — link congestion,
             high CPU, or a slow interface is causing update delays.

  Step 3: Check RIP version:
          "Default version control: send version 1, receive version 1"
          Neighbour is using version 2. RIPv1 and RIPv2 updates may not
          be accepted depending on configuration.
  Fix: Router(config-router)# version 2

See: OSPF Neighbor States | OSPF Areas & LSAs | show ip route

12. Quick Reference — show ip protocols Troubleshooting Checklist

What to Check	What to Look For	Fix if Wrong
Protocol listed?	Expected protocol name and ID appear in output	Configure the routing process: `router ospf 1` or `router eigrp 10`
EIGRP AS number match?	Both routers show the same AS number (e.g., eigrp 10)	Reconfigure EIGRP with matching AS on both routers
Missing network in "Routing for Networks"?	All expected subnets and their areas are listed	Add: `network <net> <wildcard> area <n>`
Router-facing interface passive?	Passive Interface list — should only contain LAN/loopback ports	`no passive-interface <int>`
Neighbour in Routing Information Sources?	Expected neighbour IPs appear with recent Last Update times	Check timers, K-values, passive interface, ACLs
OSPF Hello/Dead timers match?	"Hello 10, Dead 40" matches neighbour's interface timer	`ip ospf hello-interval` / `ip ospf dead-interval`
Filter list active?	"Outgoing/incoming update filter list ... is not set"	Review the ACL/prefix-list referenced; remove if incorrect
AD causing route suppression?	Distance value matches expected protocol preference	Check for static or connected routes suppressing the dynamic route; use `distance` command to adjust
EIGRP K-values match?	K1=1, K2=0, K3=1, K4=0, K5=0 on both routers	`metric weights` command to align K-values
RIP version consistent?	"send version 2, receive version 2" on all routers	`version 2` under `router rip`

13. Exam Tips & Key Points

show ip protocols is the first command to run when routing is broken — it shows every active protocol's configuration in one output without modifying anything.
The Passive Interface check is the single most important item in the output. An interface accidentally set as passive on a router-facing port prevents all adjacency formation on that link.
OSPF timers must match on both ends of a link — Hello and Dead intervals must be identical. EIGRP timers do not need to match.
The EIGRP AS number must match between routers — mismatched AS numbers mean no adjacency regardless of all other configuration. OSPF process IDs are locally significant and do not need to match.
Router ID must be unique in the OSPF domain. Manually configure it with router-id on a loopback address for stability.
The Routing Information Sources section shows actual neighbours. If an expected neighbour is missing, the adjacency has not formed — use the checklist in Section 12 to diagnose why.
Know all AD values: Connected=0, Static=1, EIGRP internal=90, OSPF=110, RIP=120, EIGRP external=170, OSPF external=150.
If a filter list is shown as "set", it means a distribute-list, prefix-list, or route-map is actively filtering routing updates — this is a common source of missing routes.
For OSPF, default-information originate must be configured on the border router for other OSPF routers to receive the default route.

14. Summary Reference Table

Topic	Key Detail
Command mode	Privileged EXEC (`Router#`)
What it shows	All active IPv4 routing protocol configurations and status
OSPF process ID in output	Local significance only — does not need to match neighbours
EIGRP AS number in output	Must match between all EIGRP neighbours
Router ID selection order	Manual config > highest loopback > highest active interface IP
OSPF Hello/Dead timers	Must match on both ends of a link; default 10s/40s on Ethernet
EIGRP timers	Do NOT need to match — negotiated automatically
Passive interface effect	Suppresses Hello/update packets; no adjacency can form; subnet still advertised
AD — EIGRP internal / external	90 / 170
AD — OSPF internal / external	110 / 150
AD — RIP	120
Filter list "not set"	No distribute-list filtering active — all routes are being exchanged
Stale "Last Update" value	Neighbour may be down or link flapping — investigate adjacency

show ip protocols Quiz

1. What is the primary purpose of the show ip protocols command, and when should it be the first command run during troubleshooting?

A. To display the MAC address table — it helps identify which devices are connected to which switch ports B. To display a real-time summary of all active IPv4 routing protocols including their networks, neighbours, timers, passive interfaces, and filters — it should be run first whenever routes are missing or adjacencies are not forming C. To configure routing protocols — it opens an interactive mode for entering routing protocol commands D. To show interface status and input/output error counters — equivalent to show interfaces

Correct answer is B.

show ip
                  protocols

is a read-only privileged EXEC command that provides a complete snapshot of every IPv4 routing protocol's operational state without making any changes. In a single output it reveals: which protocols are running, what networks each protocol is advertising, the Router ID, all known neighbours and when they last sent updates, which interfaces are passive (the most commonly missed misconfiguration), whether any route filters are active, the administrative distance for each protocol, and the Hello and Dead timer values. This makes it the single best starting point for routing troubleshooting — it directly answers most "why is the route missing?" questions without requiring multiple separate commands.

2. What is the Router ID shown in show ip protocols output, and what is the correct order of precedence for how it is selected?

A. The MAC address of the router's primary interface — it is selected automatically and cannot be changed B. The IP address of the interface that connects to the OSPF backbone area — highest priority goes to area 0 interfaces C. The subnet mask of the first network statement — it is derived automatically from the routing configuration D. A unique 32-bit dotted-decimal identifier for the router in OSPF/EIGRP; selected in this order: (1) manually configured with router-id, (2) highest IP on any loopback interface, (3) highest IP on any active physical interface

Correct answer is D. The Router ID (RID) is a 32-bit value formatted like an IP address (e.g., 1.1.1.1) that uniquely identifies this router within the OSPF or EIGRP domain. The selection process is evaluated once when the routing process starts and follows a strict priority: (1) if a router-id statement is configured manually, it always wins; (2) if not, the highest IP address across all loopback interfaces is chosen — loopbacks are preferred because they never go down; (3) if no loopbacks exist, the highest IP address across all active physical interfaces is chosen. The RID must be unique — duplicate RIDs in an OSPF domain cause LSA database conflicts, SPF instability, and unreliable routing. Best practice: always manually configure the RID on a loopback address.

3. In show ip protocols output for OSPF, what do the Hello and Dead timer values represent, and what happens when they do not match between adjacent routers?

A. Hello is how often OSPF keepalive packets are sent; Dead is how long without a Hello before declaring the neighbour down — both MUST match on both ends of a link or the OSPF adjacency will never form B. Hello is the time between routing table updates; Dead is the holddown timer that prevents routing loops — these apply to RIP, not OSPF C. Hello and Dead are the time intervals for DHCP address renewal — they are unrelated to OSPF neighbour relationships D. Hello is how long the router waits before resetting; Dead is the duration of the routing process restart — timers do not need to match between OSPF neighbours

Correct answer is A. OSPF Hello packets are sent at the Hello interval (default 10 seconds on Ethernet, 30 seconds on serial links) to tell neighbours the router is still alive. The Dead interval (default 40 seconds on Ethernet = 4× Hello) defines how long to wait without receiving a Hello before declaring the neighbour dead and removing its routes. Both the Hello and Dead intervals are part of the OSPF Hello packet itself — if routers on the same link have different values, each router rejects the other's Hello packets because the values do not match, and the adjacency never progresses past the Init state. This is one of the most common OSPF misconfiguration causes. Note that EIGRP handles timers differently — EIGRP timers are negotiated and do not need to match between adjacent routers.

4. What does the "Routing for Networks" section in show ip protocols output indicate, and what is the consequence of a subnet being absent from this list?

A. The current routing table — it lists all routes currently installed and being used for packet forwarding B. Interfaces that are in a down state — the protocol stops advertising them until they recover C. The network statements configured under the routing process that determine which interfaces participate — a missing subnet means that interface will not send Hellos and its connected network will not be advertised D. VLAN assignments for each interface — it shows which VLAN each routed port belongs to

Correct answer is C. The "Routing for Networks" section lists the network statements that were configured under the routing process with commands like network 10.10.10.0 0.0.0.255 area 0 (OSPF) or network 192.168.1.0 (EIGRP/RIP). These statements serve two purposes: (1) they activate the routing protocol on any interface whose IP address falls within the specified range — so OSPF will send Hello packets on that interface and attempt to form adjacency; (2) they cause the connected subnet to be included in the routing protocol's advertisements to neighbours. If a subnet is missing from this list, the corresponding interface will not participate in the routing protocol at all — no Hellos, no adjacency, and the connected network will not be advertised to neighbours. This is the most common single cause of "missing OSPF routes".

5. Which administrative distance value corresponds to OSPF internal routes by default, and what does this mean when OSPF and a static route both know the same prefix?

A. 90 — OSPF routes are preferred over all other dynamic routing protocols because 90 is the lowest dynamic routing AD B. 110 — when both OSPF (AD=110) and a static route (AD=1) know the same prefix, the static route wins and the OSPF route is not installed in the routing table C. 120 — OSPF has the same AD as RIP; when both are present for the same prefix, the route with the better metric wins D. 0 — OSPF routes have the highest trust value, equivalent to connected routes

Correct answer is B. OSPF internal routes have a default administrative distance of 110 (shown in show ip protocols as Distance: internal 110). Administrative distance is the router's preference mechanism when multiple sources know a route to the same destination — lower AD wins. A static route has AD=1, which is lower than OSPF's 110. Therefore, if a static route and an OSPF route both point to the same destination prefix, the static route is installed in the routing table and the OSPF route is held as a backup. This is a common source of confusion in troubleshooting: an engineer configures OSPF correctly but a leftover static route silently overrides it. The AD values to memorise: connected=0, static=1, EIGRP internal=90, OSPF=110, RIP=120, EIGRP external=170.

6. What does a passive interface entry in show ip protocols mean, and when is it correctly and incorrectly applied?

A. The interface participates in routing — its connected network is still advertised to other routers — but does not send Hello or update packets; correct on LAN ports facing end hosts, incorrect on ports facing other routers where adjacency is needed B. The interface is administratively shut down and will not forward any traffic C. The interface is a trunk port carrying multiple VLANs — passive in this context means the VLAN tagging is automatic D. The interface is completely disabled for routing — its connected network is removed from all routing advertisements

Correct answer is A. A passive interface is a nuanced concept that is frequently misunderstood. The interface is not "down" or "disabled" — it is fully operational for data traffic. The only thing suppressed is the outgoing routing protocol Hello or update packets. The connected subnet is still advertised to other routers via the interfaces that are not passive. Correct use: apply passive-interface to LAN ports facing PCs, servers, and printers — these devices are not routers, so sending Hello packets toward them is unnecessary noise. Incorrect use: applying passive-interface to a port that connects to another router — this silently prevents adjacency from forming. Because the interface looks active in show interfaces and the network appears in show ip route as connected, this misconfiguration is very easy to overlook without specifically checking show ip protocols.

7. If routes from one routing protocol are advertised into another in show ip protocols output, which feature is this and what must be carefully configured to prevent routing loops?

A. Route summarization — aggregating multiple specific routes into a single summary advertisement to reduce routing table size B. Default routing — configuring a 0.0.0.0/0 default route and advertising it to neighbours through the default-information originate command C. Redistribution — importing routes from one protocol into another; requires a seed metric to be set (routes with infinite metric are not redistributed) and route-maps or tags to prevent feedback loops where redistributed routes are re-imported back into the original protocol D. Route filtering — using distribute-lists or prefix-lists to prevent specific routes from being advertised

Correct answer is C. Redistribution is the process of importing routes learned by one routing protocol into another. For example, redistributing EIGRP routes into OSPF: redistribute eigrp 10 subnets under the OSPF process. Two critical requirements: (1) a seed metric must be specified for protocols that require it (RIP and EIGRP will not redistribute routes without an explicit metric); (2) routing loops must be prevented. A loop occurs when Router A redistributes OSPF into EIGRP, and Router B then redistributes those same routes back into OSPF — the routes bounce back into the protocol they came from. Prevention: use route-maps with tags — tag routes as they enter the second protocol, then filter out any routes with that tag to prevent re-redistribution.

8. An engineer runs show ip protocols and sees "Outgoing update filter list for all interfaces is set to X". What does this indicate and how should it be investigated?

A. It indicates the MAC address table is configured with a filter — check the CAM table with show mac address-table B. It indicates the router is filtering VLAN trunk traffic — check switchport configuration with show interfaces trunk C. It indicates interfaces are being reset — run clear interface to remove the filter D. A distribute-list, prefix-list, or route-map named "X" is actively filtering which routing updates are sent outbound — this filter may be intentionally or accidentally suppressing routes; investigate with show ip access-lists X or show ip prefix-list X

Correct answer is D. The filter list line in show ip protocols indicates whether a distribute-list, prefix-list, or route-map is configured to filter routing updates. When the output reads "is not set", no filtering is active and all routes are being exchanged normally. When it reads "is set to <name/number>", an active filter is applied — routing updates are being selectively suppressed based on the referenced ACL or prefix-list. To investigate: identify the filter name or number shown, then run show ip access-lists <name> or show ip prefix-list <name> to see its contents and determine which prefixes it permits or denies. If routes are mysteriously missing even though the network statement is correct and the adjacency is up, an active outbound or inbound filter is one of the most likely causes.

9. An OSPF adjacency between two routers is not forming despite both interfaces being up and in the same network statement. show ip protocols on both routers shows: R1 Hello=10, Dead=40 and R2 Hello=5, Dead=20. What is the problem and how is it fixed?

A. The routers will forward traffic incorrectly because the different timer values cause different routing table calculations B. OSPF Hello and Dead timers must match on both ends of a link — the mismatch means R1 rejects R2's Hello packets and vice versa; fix by matching both timers on the relevant interface of one router to match the other C. The routing protocols will redistribute routes incorrectly into each other's topology tables D. The interfaces will go administratively down until the timer values are resolved

Correct answer is B. OSPF embeds the Hello interval and Dead interval values directly inside every Hello packet. When a router receives an OSPF Hello from a neighbour, it compares the timer values in the packet against its own configured values. If they do not match, the Hello is silently rejected and the adjacency never progresses. In this scenario, R1 sends Hellos with Hello=10 and Dead=40; R2 sees those values and rejects them because its own values are 5 and 20. Neither router ever accepts the other's Hellos. Fix: change one router's timers to match the other using interface-level commands: ip ospf hello-interval 10 and ip ospf dead-interval 40. Note that EIGRP timers do NOT need to match — EIGRP negotiates a hold time per neighbour and adjacency forms even with different timer values on each side.

10. A network engineer cannot reach 10.10.30.0/24 which should be in the OSPF routing table. show ip protocols shows OSPF is running, the network 10.10.30.0 appears in "Routing for Networks", and the neighbour 10.10.10.2 is present in "Routing Information Sources". What should be checked next?

A. Whether the interface connecting to the neighbour (or the interface in the 10.10.30.0 subnet) appears in the "Passive Interface(s)" list — an accidentally passive router-facing interface prevents adjacency and route exchange even when the network statement is correct B. Whether the MAC address table needs to be cleared — stale MAC entries can prevent IP routing from working correctly C. Whether VLAN trunking is enabled — if trunk ports are misconfigured, OSPF cannot send its updates across the link D. Whether static routes are configured for this subnet — static routes always supersede OSPF regardless of network statements

Correct answer is A. The systematic show ip protocols troubleshooting checklist is: (1) Is the protocol running? ✓ (2) Is the network in "Routing for Networks"? ✓ (3) Is the neighbour present in "Routing Information Sources"? ✓ The next item in the checklist is: Is the interface listed in "Passive Interface(s)"? If the interface facing the neighbour that should be advertising 10.10.30.0 is accidentally marked passive, it will not send Hello packets on that link — which means that although the neighbour entry may appear in Routing Information Sources from Hello packets received through another interface, the route itself may not be exchanged correctly. After the passive interface check, the next step is to examine the outgoing/incoming filter list — if a distribute-list is "set", it may be suppressing the specific prefix 10.10.30.0 from being advertised even though the adjacency is up.

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