EIGRP Stub Router Not Advertising Necessary Routes
Presenting Symptom
Remote branch users report that they cannot reach a specific subnet (192.168.10.0/24) located behind the branch EIGRP stub router, although other subnets are reachable.
Network Context
A small branch office connects to the corporate headquarters via a single WAN link. The branch router (R2) is configured as an EIGRP stub router to reduce query scope. The branch LAN includes two subnets: 192.168.10.0/24 (server VLAN) and 192.168.20.0/24 (user VLAN). The HQ router (R1) learns the 192.168.20.0/24 route but not 192.168.10.0/24. Both routers run IOS 15.x and EIGRP is configured with AS number 100.
Diagnostic Steps
Check EIGRP neighbors and topology on the HQ router
show ip eigrp neighborsEIGRP-IPv4 Neighbors for AS(100)
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
0 10.1.1.2 GigabitEthernet0/0 13 00:12:34 1 100 0 45Verify that the neighbor relationship is established. If no neighbor is listed, the problem is at Layer 2 or EIGRP authentication. If neighbor is up, proceed to topology.
Check EIGRP topology on the HQ router for the missing subnet
show ip eigrp topology 192.168.10.0/24%EIGRP-IPv4: Route not in topology table
The route is not present in the topology table, meaning it was not advertised by the stub router. This confirms the stub router is not sending the route.
Examine the EIGRP stub configuration on the branch router
show running-config | section router eigrprouter eigrp 100 network 10.1.1.0 0.0.0.3 network 192.168.10.0 0.0.0.255 network 192.168.20.0 0.0.0.255 eigrp stub connected summary
The 'eigrp stub connected summary' command restricts the stub router to advertise only connected and summary routes. However, the missing subnet (192.168.10.0/24) is connected and should be advertised. If the subnet is not in the routing table as a connected route, it will not be advertised.
Check the routing table on the branch router for the missing subnet
show ip route 192.168.10.0Routing entry for 192.168.10.0/24
Known via "connected", distance 0, metric 0 (connected, via interface)
Redistributing via eigrp 100
Advertised by eigrp 100
Last update from 192.168.10.1 on GigabitEthernet0/1, 00:12:34 ago
Routing Descriptor Blocks:
* directly connected, via GigabitEthernet0/1
Route metric is 0, traffic share count is 1The route is present as a connected route. If it were missing, the issue would be a Layer 1/2 problem on the branch LAN. Since it is present, the stub configuration should advertise it. The problem may be that the network statement is missing or the interface is passive.
Verify EIGRP interface participation on the branch router
show ip eigrp interfacesEIGRP-IPv4 Interfaces for AS(100)
Xmit Queue Mean Pacing Time Multicast Pending
Interface Peers Un/Reliable SRTT Un/Reliable Flow Timer Routes
Gi0/0 1 0/0 1 0/10 50 0
Gi0/1 0 0/0 0 0/10 0 0Notice that Gi0/1 (the LAN interface for 192.168.10.0/24) has 0 peers and 0 pending routes. This indicates that EIGRP is not running on that interface, possibly because it is configured as passive-interface.
Check for passive-interface configuration on the branch router
show running-config | include passive-interfacepassive-interface GigabitEthernet0/1
The interface Gi0/1 is configured as passive-interface. In EIGRP, a passive interface does not form neighbors but still advertises its connected subnet. However, if the interface is passive, the stub router should still advertise the connected route. But if the network statement is missing for that subnet, it won't be advertised. Check the network statement.
Verify network statements on the branch router
show running-config | section router eigrprouter eigrp 100 network 10.1.1.0 0.0.0.3 network 192.168.20.0 0.0.0.255 eigrp stub connected summary
The network statement for 192.168.10.0/24 is missing! The stub router only advertises connected routes that are also covered by a network statement. Without the network statement, the route is not injected into EIGRP, even though it is connected. This is the root cause.
Root Cause
The branch router's EIGRP configuration is missing the network statement for the 192.168.10.0/24 subnet. Although the interface is connected and the stub router is configured with 'eigrp stub connected', EIGRP only advertises connected routes that are explicitly enabled via a network command (or redistribution). The missing network statement prevents the route from being advertised to the HQ router.
Resolution
Verification
On the HQ router (R1), run: ``` show ip eigrp topology 192.168.10.0/24 ``` Expected output: ``` EIGRP-IPv4 Topology Entry for AS(100)/ID(192.168.10.0/24) State: Passive, Reply status: 0, Originating router: 10.1.1.2 Metric (minimum/feasible): 256000/256000 FD is 256000, S successors: 1 Routes: 10.1.1.2 (GigabitEthernet0/0), from 10.1.1.2, Send flag: 0x0 Composite metric: (256000/0) Vector metric: Min bandwidth is 100000 Kbit Total delay is 100 microseconds Reliability is 255/255 Load is 1/255 Minimum MTU is 1500 Hop count is 1 ``` Also verify the routing table: ``` show ip route 192.168.10.0 ``` Expected output: ``` Routing entry for 192.168.10.0/24 Known via "eigrp 100", distance 90, metric 256000, type internal Redistributing via eigrp 100 Last update from 10.1.1.2 on GigabitEthernet0/0, 00:00:15 ago Routing Descriptor Blocks: * 10.1.1.2, from 10.1.1.2, via GigabitEthernet0/0 Route metric is 256000, traffic share count is 1 ```
Prevention
1. Always ensure that all connected subnets that need to be advertised are covered by a network statement under the EIGRP process. 2. Use the 'eigrp stub connected summary' command carefully; remember that 'connected' only advertises routes that are both connected and enabled via network statement. 3. Implement a configuration review process that verifies network statements match all desired subnets, especially when adding new VLANs or interfaces.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario tests your understanding of EIGRP stub router behavior and the interaction between network statements and stub advertising. Expect a troubleshooting question where you must identify why a specific route is missing from the routing table. The exam may present a topology and partial configurations, asking you to determine the missing network statement or the effect of passive-interface on stub advertising.
Exam Tips
Remember that 'eigrp stub connected' only advertises connected routes that are also enabled via a network command under the EIGRP process.
A passive-interface in EIGRP still allows the connected subnet to be advertised; it only prevents neighbor formation. Do not assume passive-interface blocks route advertisement.
Use 'show ip eigrp topology' to quickly see if a route is missing from the topology table; if it's missing, the issue is on the advertising router.
Commands Used in This Scenario
show ip eigrp interfaces
Displays detailed information about interfaces on which EIGRP is enabled, including neighbor status, pending routes, and interface statistics, used to verify EIGRP adjacency and interface participation.
show ip eigrp neighbors
Displays the neighbor table for EIGRP, showing all directly connected EIGRP routers and their status, used to verify EIGRP adjacencies and troubleshoot neighbor relationships.
show ip eigrp topology
Displays the EIGRP topology table, showing all learned routes and their feasible successors, used to verify EIGRP convergence and path selection.
show running-config
Displays the current active configuration in DRAM, showing all non-default settings.
Test Your CCNA Knowledge
Practice with scenario-based questions to prepare for the CCNA 200-301 exam.
Practice CCNA Questions