RIP Split Horizon Blocking Routes on Hub-and-Spoke Frame Relay
Presenting Symptom
Remote branch routers cannot reach each other via the hub router, but each branch can ping the hub and the hub can ping all branches.
Network Context
Hub-and-spoke Frame Relay topology with a central hub router (R1) and two spoke routers (R2, R3). All routers run RIP version 2. The hub has a single physical serial interface with multiple point-to-point subinterfaces or a multipoint interface. The spokes have point-to-point subinterfaces. IOS version 15.x. Small branch office scenario.
Diagnostic Steps
Check RIP routes on a spoke router
show ip route ripR 10.1.1.0/24 [120/1] via 192.168.1.1, 00:00:17, Serial0/0/0 R 10.2.2.0/24 [120/2] via 192.168.1.1, 00:00:17, Serial0/0/0
If the spoke sees routes for other spokes (e.g., R2 sees R3's LAN) with metric 2, the problem is not route propagation from hub. If missing, routes are not being advertised properly.
Check RIP routes on the hub router
show ip route ripR 10.1.1.0/24 [120/1] via 192.168.1.2, 00:00:23, Serial0/0/0.1 R 10.2.2.0/24 [120/1] via 192.168.1.3, 00:00:18, Serial0/0/0.2
Hub should have routes to all spoke LANs. If missing, check RIP neighbor relationships.
Examine RIP updates on the hub using debug
debug ip ripRIP: sending v2 update to 224.0.0.9 via Serial0/0/0 (192.168.1.1) RIP: build update entries network 10.1.1.0 metric 1 network 10.2.2.0 metric 1 RIP: received v2 update from 192.168.1.2 on Serial0/0/0 10.1.1.0 in 1 hops
If the hub sends updates containing all routes (including those learned from other spokes), split horizon is not blocking. If the update sent to a spoke only contains the hub's directly connected routes and not routes learned from other spokes, split horizon is active.
Check split horizon setting on the hub interface
show ip interface serial 0/0/0Serial0/0/0 is up, line protocol is up Internet address is 192.168.1.1/24 Split horizon is enabled
If split horizon is enabled on a multipoint interface, it prevents routes learned on one spoke from being advertised back out the same interface to other spokes. This is the root cause.
Root Cause
Split horizon is enabled on the hub router's Frame Relay multipoint interface. RIP's split horizon rule prevents routes learned on an interface from being advertised back out that same interface. In a hub-and-spoke topology, the hub learns routes from each spoke on the same physical interface (or multipoint subinterface), so it cannot advertise those routes to other spokes, leaving them unreachable from each other.
Resolution
Verification
After applying the fix, verify that spoke routers now see each other's routes. On R2: ``` show ip route rip ``` Expected output should include route to R3's LAN (e.g., 10.2.2.0/24) with metric 2. Also verify on hub: ``` show ip interface serial 0/0/0 | include split horizon ``` Expected: 'Split horizon is disabled'
Prevention
1. Use point-to-point subinterfaces for each Frame Relay PVC instead of a single multipoint interface. This avoids split horizon issues because each subinterface is treated as a separate interface. 2. If multipoint is required, explicitly disable split horizon on the interface. 3. Consider using a routing protocol that is not affected by split horizon, such as EIGRP with the 'no ip split-horizon' command or OSPF which uses different loop prevention.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario appears in troubleshooting questions, often as a drag-and-drop or multiple-choice question where you must identify why spoke-to-spoke connectivity fails in a hub-and-spoke Frame Relay with RIP. The exam tests understanding of split horizon behavior and the command 'show ip interface' to verify split horizon status. Key fact: Split horizon prevents routes from being advertised out the interface they were learned on.
Exam Tips
Remember that split horizon is enabled by default on all interfaces; you must explicitly disable it on multipoint Frame Relay interfaces for hub-and-spoke RIP.
The exam may show a 'show ip route' output where spoke routes are missing; look for the hub's interface configuration to see if split horizon is enabled.
Know the command 'show ip interface [interface]' to check split horizon status; the output line 'Split horizon is enabled/disabled' is key.
Commands Used in This Scenario
debug ip rip
Enables real-time debugging of RIP routing updates to troubleshoot routing issues by displaying sent and received RIP updates.
show ip interface
Displays the status and configuration of all IP interfaces on a Cisco router, including IP address, protocol status, and interface statistics, used for verifying interface IP configuration and troubleshooting connectivity issues.
show ip route
Displays the current IP routing table on a Cisco router, used to verify routes, check next-hop addresses, and troubleshoot connectivity issues.
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