RIP Auto-Summary Summarizing to Classful Boundary — Route Missing
Presenting Symptom
A router in a discontiguous network cannot reach a remote subnet, and show ip route shows only the classful summary route instead of the specific prefix.
Network Context
A small enterprise network with two routers (R1 and R2) connected via a serial link. R1 has subnets 172.16.1.0/24 and 172.16.2.0/24 on its LAN, and R2 has subnet 172.16.3.0/24 on its LAN. Both routers run RIP version 1 with default auto-summary enabled. The network is discontiguous because the 172.16.0.0/16 major net is split by a different major net (e.g., 10.0.0.0/8) on the serial link. IOS version 15.x.
Diagnostic Steps
Check the routing table on the router missing the route
show ip routeR 172.16.0.0/16 [120/1] via 10.0.0.2, 00:00:17, Serial0/0/0
If you see only the classful summary (172.16.0.0/16) instead of the specific /24 subnet, auto-summary is summarizing at the classful boundary. The specific route is missing.
Verify RIP configuration and auto-summary status
show ip protocolsRouting Protocol is "rip"
Sending updates every 30 seconds, next due in 16 seconds
Invalid after 180 seconds, hold down 180, flushed after 240
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
RIP version 1
Redistributing: rip
Default version control: send version 1, receive version 1
Interface Send Recv Key-chain
Serial0/0/0 1 1
GigabitEthernet0/0 1 1
Routing for Networks:
10.0.0.0
172.16.0.0
Routing Information Sources:
Gateway Distance Last Update
10.0.0.2 120 00:00:17
Distance: (default is 120)
Note: Auto-summary is enabled by default for RIP version 1.The output confirms RIP version 1 is running and auto-summary is enabled (no 'no auto-summary' line). The router is summarizing 172.16.0.0/16 at the classful boundary.
Check the routing table on the other router to see if it has the specific routes
show ip routeR 172.16.0.0/16 [120/1] via 10.0.0.1, 00:00:12, Serial0/0/0
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 10.0.0.0/30 is directly connected, Serial0/0/0
L 10.0.0.2/32 is directly connected, Serial0/0/0
172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
C 172.16.3.0/24 is directly connected, GigabitEthernet0/0
L 172.16.3.1/32 is directly connected, GigabitEthernet0/0The other router also only sees the summary route. Both routers are summarizing, so specific subnets are hidden from each other.
Examine the RIP database to see what routes are being advertised
show ip rip database10.0.0.0/8 auto-summary 10.0.0.0/30 directly connected, Serial0/0/0 172.16.0.0/16 auto-summary 172.16.1.0/24 directly connected, GigabitEthernet0/0 172.16.2.0/24 directly connected, GigabitEthernet0/0
The RIP database shows that the router is advertising the auto-summary route 172.16.0.0/16 instead of the individual /24 subnets. This confirms auto-summary is causing the issue.
Root Cause
RIP version 1 with default auto-summary enabled summarizes subnets to their classful boundary (e.g., 172.16.1.0/24 and 172.16.2.0/24 become 172.16.0.0/16) when advertising across a different major network boundary. In a discontiguous network, this causes loss of specific route information, leading to unreachable subnets.
Resolution
Verification
After applying the fix, verify the routing table on both routers: show ip route Expected output on R1: R 172.16.3.0/24 [120/1] via 10.0.0.2, 00:00:05, Serial0/0/0 10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks C 10.0.0.0/30 is directly connected, Serial0/0/0 L 10.0.0.1/32 is directly connected, Serial0/0/0 172.16.0.0/16 is variably subnetted, 3 subnets, 2 masks C 172.16.1.0/24 is directly connected, GigabitEthernet0/0 C 172.16.2.0/24 is directly connected, GigabitEthernet0/0 R 172.16.3.0/24 [120/1] via 10.0.0.2, 00:00:05, Serial0/0/0 Now the specific /24 route to 172.16.3.0 appears. Also verify with ping from R1 to 172.16.3.1.
Prevention
1. Use RIP version 2, which supports classless routing and disables auto-summary by default. 2. In discontiguous networks, always disable auto-summary (no auto-summary) regardless of RIP version. 3. Plan IP addressing to avoid discontiguous subnets when using classful routing protocols.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario appears in troubleshooting questions where a route is missing due to auto-summary. The exam may present a drag-and-drop or multiple-choice question asking to identify the cause and solution. Key fact: RIP version 1 always auto-summarizes; RIP version 2 has auto-summary enabled by default but can be disabled with 'no auto-summary'.
Exam Tips
Remember that RIP version 1 is classful and always summarizes; RIP version 2 is classless but has auto-summary enabled by default.
In discontiguous networks, auto-summary causes loss of specific routes; the fix is 'no auto-summary' under router rip.
Use 'show ip protocols' to verify if auto-summary is enabled; look for the absence of 'no auto-summary' in the output.
Commands Used in This Scenario
show ip protocols
Displays the current state of all IP routing protocols running on the router, including timers, filters, and network advertisements.
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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