Discontiguous Network with Classful Routing — Routes Missing
Presenting Symptom
Pings from PC1 to PC2 fail intermittently, and some routes are missing from the routing table.
Network Context
A small branch office network with two routers (R1 and R2) connected via a serial link. R1 has a subnet 172.16.1.0/24 on its LAN, and R2 has 172.16.2.0/24 on its LAN. Both routers are running RIP version 1 (classful routing). The network is discontiguous because the 172.16.0.0/16 major network is split by a different major network (e.g., 10.0.0.0/8) on the serial link. IOS version 15.x.
Diagnostic Steps
Check routing table on R1
show ip routeR1# show ip route
Codes: C - connected, S - static, R - RIP, ...
172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
C 172.16.1.0/24 is directly connected, GigabitEthernet0/0
R 172.16.0.0/16 [120/1] via 10.0.0.2, 00:00:12, Serial0/0/0Notice that R1 sees a route to 172.16.0.0/16 (classful) instead of 172.16.2.0/24. This indicates RIP v1 is summarizing at the classful boundary, causing loss of the specific subnet.
Check RIP configuration on R1
show running-config | section router riprouter rip version 1 network 172.16.0.0 network 10.0.0.0
Confirms RIP version 1 is in use. Version 1 does not support VLSM or discontiguous networks; it automatically summarizes to classful boundaries.
Check RIP updates received on R1
debug ip ripRIP: received v1 update from 10.0.0.2 on Serial0/0/0
172.16.0.0 in 1 hopsThe update shows 172.16.0.0 (classful) instead of 172.16.2.0/24. This confirms R2 is also summarizing at the classful boundary.
Check routing table on R2
show ip routeR2# show ip route
172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
C 172.16.2.0/24 is directly connected, GigabitEthernet0/0
R 172.16.0.0/16 [120/1] via 10.0.0.1, 00:00:15, Serial0/0/0R2 also sees only the classful summary, confirming the problem is symmetric.
Root Cause
RIP version 1 is a classful routing protocol that automatically summarizes routes at major network boundaries. In a discontiguous network (where subnets of the same major network are separated by a different major network), this summarization causes loss of specific subnet routes, leading to incomplete routing and connectivity failures.
Resolution
Verification
After applying the fix, run 'show ip route' on R1. Expected output: R1# show ip route 172.16.0.0/24 is subnetted, 2 subnets C 172.16.1.0/24 is directly connected, GigabitEthernet0/0 R 172.16.2.0/24 [120/1] via 10.0.0.2, 00:00:09, Serial0/0/0 Ping from PC1 to PC2 should now succeed.
Prevention
1. Use classless routing protocols (RIPv2, EIGRP, OSPF) that support VLSM and discontiguous networks. 2. Always disable auto-summary when using RIPv2 or EIGRP. 3. Design IP addressing to avoid discontiguous subnets if using classful protocols.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario tests understanding of classful vs classless routing protocols, VLSM, and discontiguous networks. Questions may present a troubleshooting scenario where pings fail and ask to identify the root cause (RIPv1 auto-summary) or the solution (upgrade to RIPv2 or disable auto-summary).
Exam Tips
Remember that RIPv1 and IGRP are classful; they summarize at major network boundaries automatically.
Discontiguous networks require classless routing protocols or manual summarization to avoid route loss.
The command 'no auto-summary' is critical for RIPv2 and EIGRP to prevent automatic summarization.
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 route
Displays the current IP routing table on a Cisco router, used to verify routes, check next-hop addresses, and troubleshoot connectivity issues.
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.
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