Imagine a router that simply drops packets for a destination it should know about. In the CCNA 200-301 exam, objective 3.1 tests your ability to systematically diagnose why a route is missing from the routing table. This skill is the backbone of real-world network troubleshooting: you must distinguish between problems in neighbor adjacency, routing protocol configuration, prefix advertisement, administrative distance, and route summarization. Without this ability, even a simple network becomes unmanageable. This chapter gives you a repeatable, exam-focused methodology to find the root cause quickly.
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Think of a router's routing table as an airline's departure board at a major airport. Each row on the board lists a destination city (the network prefix), the flight number (the route source, like OSPF or EIGRP), the gate (the next-hop IP), and the departure time (the metric). Now imagine that a new destination, say Tokyo, should appear but doesn't. The airline's operations center (the router's control plane) follows a strict process: first, it must have a valid agreement with Tokyo's airport (a neighbor adjacency formed via a routing protocol). Second, the flight must be approved by the route originator (the route must be in the routing protocol's database). Third, the schedule must pass through the central scheduling system (the routing table calculation). A missing flight could be because the agreement with Tokyo never formed (neighbor down), or the flight was never filed (route not in topology database), or the schedule was overwritten by a higher-priority flight (administrative distance conflict), or the flight was cancelled due to weather (prefix filtering or route summarization). The dispatcher (network engineer) checks each layer: verify the agreement (show ip ospf neighbor), check the filed flights (show ip ospf database), review the schedule (show ip route), and look for conflicting flights (show ip route | include /24). This systematic approach ensures you don't waste time blaming the wrong cause.
What Does "Route Missing from Routing Table" Mean?
A router's routing table (RIB – Routing Information Base) contains the best routes to known destinations. When a route is missing, the router cannot forward packets to that network. The cause is not always a routing protocol failure; it can be anything from a down interface to a misconfigured prefix list. The CCNA exam tests your ability to follow a structured troubleshooting model: check physical connectivity, verify neighbor adjacencies, confirm the route exists in the protocol database, and then check why it wasn't installed in the RIB.
The Route Installation Decision Process
For a route to appear in the routing table, it must pass three gates:
Feasibility: The route must be learned from a routing protocol or a static configuration. For dynamic protocols, the route must be in the protocol's topology database (e.g., OSPF LSDB, EIGRP topology table).
Administrative Distance (AD): If multiple routing sources know the same prefix, the one with the lowest AD wins. The losing route is not installed (but may be kept as a candidate in the routing table's database). Default AD values: Connected 0, Static 1, EIGRP summary 5, External BGP 20, Internal EIGRP 90, IGRP 100, OSPF 110, IS-IS 115, RIP 120, External EIGRP 170, Internal BGP 200, Unknown 255.
Metric: If the same routing protocol learns multiple paths to the same prefix, the path with the best metric is installed. For OSPF, the metric is cost (10^8 / bandwidth). For EIGRP, the metric is a composite of bandwidth, delay, load, reliability (by default bandwidth and delay).
Step-by-Step Troubleshooting Flow
Step 1: Verify the destination network exists in the routing protocol's database.
Use the appropriate show command for the protocol:
OSPF: show ip ospf database
EIGRP: show ip eigrp topology
RIP: show ip rip database
If the prefix is not in the database, the problem is upstream: the neighbor is not advertising it, or inbound filtering is blocking it.
Example output for OSPF:
Router# show ip ospf database
OSPF Router with ID (10.0.0.1) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
10.0.0.1 10.0.0.1 123 0x80000004 0x00A0B0 2
10.0.0.2 10.0.0.2 456 0x80000003 0x00C0D0 3
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
192.168.1.0 10.0.0.2 789 0x80000001 0x00E0F0If the prefix 192.168.2.0/24 is missing, check if the advertising router has it in its database.
Step 2: Verify neighbor adjacency is established.
If the database is empty or missing expected entries, check neighbor state:
OSPF: show ip ospf neighbor
EIGRP: show ip eigrp neighbors
RIP: show ip rip database (RIP has no neighbor table; it relies on broadcast updates)
OSPF neighbor states: DOWN, ATTEMPT, INIT, 2WAY, EXSTART, EXCHANGE, LOADING, FULL. The expected state is FULL (for DR/BDR) or 2WAY (for other neighbors).
Example output:
Router# show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
10.0.0.2 1 FULL/DR 00:00:34 10.1.1.2 GigabitEthernet0/0If the state is DOWN or INIT, troubleshoot Layer 1/2 or OSPF configuration (area mismatch, passive interface, authentication).
Step 3: Verify the interface is up/up.
Use show ip interface brief to confirm the interface connecting to the neighbor is up/up. If the interface is down/down or administratively down, fix cabling or issue no shutdown.
Step 4: Check for filtering or summarization.
Even if the route is in the database, it may be filtered out before installation. Common filters:
Distribute-lists under the routing protocol: distribute-list <acl> in or out
Prefix-lists: ip prefix-list BLOCK seq 5 deny 192.168.2.0/24
Route-maps applied to redistribution or neighbor updates
Use show ip protocols to see if any distribute-lists are configured.
Step 5: Check administrative distance.
If the route is in the database but not in the routing table, another route with a lower AD might be installed. For example, a static route to 0.0.0.0/0 with AD 1 will override an OSPF-learned default route (AD 110). Use show ip route <prefix> to see if a different source is present.
Command:
Router# show ip route 192.168.2.0
Routing entry for 192.168.2.0/24
Known via "static", distance 1, metric 0
Redistributing via static
Last update from 10.0.0.1 on GigabitEthernet0/1
Routing Descriptor Blocks:
* 10.0.0.1, via GigabitEthernet0/1
Route metric is 0, traffic share count is 1If the expected OSPF route is missing because of a static route, you have an AD conflict.
Step 6: Verify the subnet mask and prefix length.
Sometimes the route is present but with a different mask than expected. For example, a summary route 192.168.0.0/16 might cover 192.168.2.0/24. Use show ip route | include 192.168 to see all routes containing that prefix.
Common IOS Commands for Troubleshooting
show ip route [prefix] – displays the best route to a prefix
show ip route connected – lists directly connected networks
show ip route static – lists static routes
show ip route ospf – lists OSPF-learned routes
show ip route eigrp – lists EIGRP-learned routes
show ip protocols – shows routing protocol parameters and filters
debug ip routing – logs route changes (use with caution in production)
Interaction with Related Protocols
Missing routes can also be caused by issues in other protocols: - FHRP (HSRP/VRRP/GLBP): If the next-hop IP is a virtual IP, ensure the FHRP is active. - BGP: Missing routes may be due to missing network statements or prefix filtering. - Policy-based routing (PBR): PBR can override the routing table; check if PBR is applied.
Summary of Causes
Physical/Layer 2: Interface down, cable fault, VLAN mismatch
Neighbor adjacency: OSPF area mismatch, EIGRP AS mismatch, authentication failure, passive interface
Route advertisement: Network statement missing, redistribution not configured, prefix filtering
Route installation: Administrative distance conflict, metric not best, maximum paths exceeded
Summarization: Auto-summary (RIP, EIGRP) or manual summary hiding more specific routes
The CCNA exam expects you to use a systematic approach: start with show ip route, then drill down to the protocol database, then neighbor, then interface.
1. Identify the missing prefix
Use `show ip route | include <prefix>` or `show ip route <prefix>` to confirm the route is absent. For example, if the destination is 192.168.2.0/24, run `show ip route 192.168.2.0`. If the output says '% Network not in table', the route is missing. Also check for a default route that might cover it: `show ip route 0.0.0.0`. Document the expected next-hop IP and outgoing interface.
2. Check the routing protocol database
For OSPF, run `show ip ospf database` and look for the prefix in the Type 1 or Type 3 LSAs. For EIGRP, run `show ip eigrp topology all-links` to see all routes, not just the successors. If the prefix appears in the database, the problem is in route selection (AD/metric). If it does not, the problem is upstream: the neighbor is not advertising it, or inbound filtering is blocking it.
3. Verify neighbor adjacency
Run `show ip ospf neighbor` (OSPF) or `show ip eigrp neighbors` (EIGRP). For OSPF, the state should be FULL or 2WAY. For EIGRP, the state should be UP. If the neighbor is not listed or is in a state like DOWN/INIT, check the interface status (`show ip interface brief`) and OSPF/EIGRP configuration (area ID, AS number, authentication, passive interface, network type).
4. Check interface and Layer 1/2 status
Run `show ip interface brief` to verify the interface connecting to the neighbor is up/up. If it is down/down, check cabling and the remote device. If it is administratively down, issue `no shutdown`. For VLAN interfaces, ensure the VLAN exists and is active (`show vlan brief`). Also check for duplex mismatch or speed mismatch using `show interfaces`.
5. Examine administrative distance and metric
If the route is in the database but not in the RIB, run `show ip route <prefix>` to see if another route (static, connected, or from another protocol) is installed. Compare AD values: a static route (AD 1) will override OSPF (AD 110). If the same protocol, compare metrics. For OSPF, use `show ip ospf interface` to check cost. For EIGRP, use `show ip eigrp topology <prefix>` to see the metric components.
6. Look for filtering or summarization
Use `show ip protocols` to see if any distribute-lists or route-maps are applied. For example, if a distribute-list in OSPF denies the prefix, it will not be installed. Also check for manual summarization on the advertising router: `show ip route summary` or `show running-config | section router ospf`. Auto-summarization (RIP, EIGRP) can cause missing subnets if not disabled with `no auto-summary`.
7. Verify redistribution and network statements
If the route is learned via redistribution, ensure redistribution is properly configured and that the route is not being filtered by a route-map during redistribution. For OSPF, check that the network statement covers the correct interface: `show running-config | section router ospf`. For EIGRP, the network statement must match the interface IP. Use `show ip eigrp interfaces` to see which interfaces are participating.
In a large enterprise network, missing routes can cause widespread outages. Consider a three-tier topology with core, distribution, and access layers running OSPF Area 0. A distribution switch stops seeing a specific /24 VLAN subnet from an access switch. The troubleshooting engineer starts at the access switch: show ip route shows the VLAN subnet as connected, so the route exists. But the distribution switch's routing table does not have it. The engineer checks show ip ospf neighbor on the distribution switch: the access switch is listed as FULL. Next, show ip ospf database on the distribution switch reveals the Type 1 LSA from the access switch, but the specific prefix is missing. This indicates that the access switch is not advertising that subnet. A quick show ip ospf interface vlan100 on the access switch shows that the interface is passive under OSPF. The network statement was added after the interface was made passive, so the subnet is not advertised. Removing the passive-interface default or explicitly enabling OSPF on that VLAN fixes the issue.
Another common scenario: an MPLS VPN environment with BGP as the PE-CE protocol. A customer route is missing from the VRF routing table. The engineer checks show ip route vrf CUSTOMER and sees no route. Then show bgp vpnv4 unicast all shows the route is present in BGP but not installed. The reason is that the next-hop IP is unreachable: show ip route <next-hop> returns nothing. The problem is that the core network is not advertising the loopback of the remote PE. Fixing the IGP (OSPF) in the core resolves the issue.
Scale considerations: In large OSPF networks with hundreds of routers, missing routes often result from area boundary summarization. A misconfigured area 0 range can hide more specific prefixes. Performance-wise, using distribute-lists on every router is inefficient; prefix-lists are more scalable. Misconfiguration of route filters can cause partial blackholing, where traffic for some subnets fails while others work. Always verify both directions: a missing route on Router A may be due to a filter on Router B that blocks the advertisement.
The CCNA 200-301 exam objective 3.1 includes troubleshooting missing routes. Expect scenario-based questions where you must determine why a route is absent. The most common wrong answers:
"The route is not in the routing protocol database because the neighbor adjacency is down." While this is possible, candidates often jump to this conclusion without checking the database first. The correct approach is to check the database; if the route is there, the adjacency is fine.
"The administrative distance is too high." Candidates forget that AD is only compared between different routing sources. For the same protocol, metric decides. Also, a lower AD always wins; if the missing route has a lower AD than the installed route, it should be installed unless filtered.
"The route is being filtered by a distribute-list." This is often true, but candidates fail to verify with show ip protocols. The exam expects you to know the command to see distribute-lists.
"The interface is down." While this can cause a missing connected route, for dynamic routes the neighbor adjacency would also be affected. Candidates often stop at interface status without checking neighbor state.
Specific values to memorize: OSPF AD 110, EIGRP internal AD 90, static AD 1. OSPF neighbor states: FULL is expected for DR/BDR, 2WAY for other neighbors. EIGRP neighbor state: UP. For RIP, there is no neighbor table; routes are received via broadcast every 30 seconds.
Calculation traps: In OSPF, the cost is based on bandwidth; a mismatched reference bandwidth (auto-cost reference-bandwidth) can cause suboptimal path selection but not missing routes. In EIGRP, the metric formula uses bandwidth and delay; a missing route might be due to variance or maximum-paths settings.
Decision rule for scenario questions: First, determine if the route is in the protocol database. If yes, the issue is in route installation (AD/metric/filtering). If no, the issue is in route advertisement (neighbor/network/filtering). Always start with show ip route then show ip ospf database (or equivalent).
A route missing from the routing table means the router cannot forward packets to that network.
The three gates for route installation: feasibility (in protocol database), administrative distance (lowest wins), metric (best path).
Default AD values: Connected 0, Static 1, OSPF 110, EIGRP internal 90, RIP 120.
OSPF neighbor state FULL indicates adjacency is up; 2WAY is also normal for non-DR/BDR routers.
Use 'show ip ospf database' to check if a prefix exists in OSPF's LSDB.
Use 'show ip protocols' to see distribute-lists and route-maps applied to a routing process.
Auto-summarization in RIP and EIGRP (enabled by default) can hide subnets; use 'no auto-summary' to disable.
These come up on the exam all the time. Here's how to tell them apart.
OSPF
Link-state protocol; uses LSDB and SPF algorithm
AD internal 110; external 110 (by default)
Neighbor state machine: DOWN, INIT, 2WAY, EXSTART, EXCHANGE, LOADING, FULL
Network statement matches interface IP; must be in same area
Metric is cost (10^8/bandwidth); can be manipulated with 'ip ospf cost'
EIGRP
Advanced distance-vector; uses topology table and DUAL algorithm
AD internal 90; external 170
Neighbor state: UP (simpler; uses hello/hold timers)
Network statement matches interface IP; must be in same AS
Metric is composite (bandwidth, delay, load, reliability); default uses bandwidth and delay
Mistake
If a route is missing from the routing table, the neighbor adjacency must be down.
Correct
A neighbor can be FULL while the specific route is missing due to filtering, summarization, or the route not being advertised.
Candidates often assume the neighbor relationship is the only cause, ignoring filters and network statements.
Mistake
Administrative distance is compared between routes of the same routing protocol.
Correct
AD is compared between different routing sources (e.g., OSPF vs static). For the same protocol, metric determines the best path.
Candidates confuse AD with metric; both influence route selection but at different levels.
Mistake
If a route is in the OSPF database, it must appear in the routing table.
Correct
A route in the database may not be installed if a route with a lower AD exists, or if it is filtered by a distribute-list.
Candidates think the database is the final step, but route installation is a separate process.
Mistake
The 'show ip route' command always shows all routes known to the router.
Correct
'show ip route' shows only the best routes installed in the RIB. Use 'show ip route all' or protocol-specific commands to see all candidates.
Candidates assume 'show ip route' is exhaustive, missing the fact that backup routes are not displayed.
Reveal each answer, then mark whether you got it right. Score 60%+ to unlock the next chapter.
The neighbor being FULL means the adjacency is established, but the specific prefix may not be advertised. Check the OSPF database on the local router: if the prefix is not there, the advertising router is not including it. Common reasons: the network statement on the advertising router does not cover that interface, or the interface is passive under OSPF. If the prefix is in the database but not in the routing table, check for administrative distance conflicts or distribute-lists. Use 'show ip ospf database' to verify presence, then 'show ip route' to see if another route is overriding.
'show ip route' displays the Routing Information Base (RIB) – the best routes installed for forwarding. 'show ip ospf database' displays the Link-State Database (LSDB) – all OSPF link-state advertisements (LSAs) known to the router. A route can be in the LSDB but not in the RIB if it is filtered or if a better route exists. Troubleshooting missing routes often starts with checking the LSDB to see if the route is known at all.
Use 'show ip protocols' to see if a distribute-list is configured under the routing protocol. For example, 'show ip protocols' might show 'Incoming filter list for GigabitEthernet0/0 is 10'. Then examine access-list 10 with 'show access-lists 10' to see if it denies the prefix. Alternatively, use 'show ip route <prefix>' and look for the route; if it's missing and the database has it, a distribute-list is a likely cause.
In EIGRP, the topology table contains all learned routes, but only the best route (successor) is installed in the routing table. If the route is not the successor, it may be a feasible successor or a non-feasible successor. Use 'show ip eigrp topology all-links' to see all routes. The route may also be filtered by a distribute-list or route-map. Check 'show ip protocols' for filters.
Yes, if the route is learned over a trunk link, a VLAN mismatch (native VLAN mismatch or allowed VLAN misconfiguration) can prevent Layer 2 connectivity, causing the OSPF/EIGRP neighbor adjacency to fail. This would result in missing routes. Check 'show interfaces trunk' and 'show vlan brief' on both switches.
When two different routing protocols (or a static route and a dynamic route) advertise the same prefix, the route with the lowest administrative distance is installed in the routing table. The other route remains in the protocol's database but is not used for forwarding. For example, a static route with AD 1 will override an OSPF route with AD 110 for the same prefix.
The 'network' statement under 'router ospf' tells OSPF which interfaces to include in the OSPF process and in which area. If an interface's IP address matches the network statement, OSPF will try to form adjacencies over that interface and advertise its connected subnet. If the network statement is missing for an interface, OSPF will not run on that interface, and the connected subnet will not be advertised.
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