VLSM Subnets Overlapping After Route Summarization
Presenting Symptom
After configuring route summarization on a router, some remote networks become unreachable due to overlapping VLSM subnets.
Network Context
A medium-sized enterprise campus network uses VLSM with subnets like 10.1.0.0/24, 10.1.1.0/24, and 10.1.2.0/24. A distribution router (Cisco 4321, IOS 16.9) summarizes these as 10.1.0.0/22, but the summary also covers a non-contiguous subnet 10.1.4.0/24 used elsewhere, causing routing black holes.
Diagnostic Steps
Check routing table for missing routes
show ip routeCodes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 5 subnets, 3 masks
C 10.1.0.0/24 is directly connected, GigabitEthernet0/0
C 10.1.1.0/24 is directly connected, GigabitEthernet0/1
C 10.1.2.0/24 is directly connected, GigabitEthernet0/2
D 10.1.0.0/22 [90/2172416] via 10.2.0.1, 00:00:12, GigabitEthernet0/3
D 10.1.4.0/24 [90/2172416] via 10.2.0.1, 00:00:12, GigabitEthernet0/3The routing table shows a summary route 10.1.0.0/22 and a more specific route 10.1.4.0/24. The summary covers 10.1.0.0-10.1.3.255, but 10.1.4.0/24 is outside that range, so no overlap yet. However, if the summary were 10.1.0.0/21, it would cover 10.1.4.0/24, causing overlap.
Examine the summarization configuration on the distribution router
show running-config | section router eigrprouter eigrp 100 network 10.0.0.0 auto-summary !
Auto-summary is enabled, which summarizes at classful boundaries (10.0.0.0/8). This can cause overlapping if VLSM subnets are not contiguous. The output shows no manual summary, but auto-summary is on.
Check for overlapping subnets using a subnet calculator or manual analysis
show ip route 10.1.4.0 255.255.255.0 longer-prefixesRouting entry for 10.1.4.0/24
Known via "connected", distance 0, metric 0 (connected)
Redistributing via eigrp 100
Advertised by eigrp 100
Last update from 0.0.0.0 on GigabitEthernet0/4, 00:00:05 ago
Routing Descriptor Blocks:
* directly connected, via GigabitEthernet0/4
Route metric is 0, traffic share count is 1The subnet 10.1.4.0/24 is directly connected on a different interface. If a summary route like 10.1.0.0/21 is advertised, it will include 10.1.4.0/24, causing the remote router to prefer the summary over the more specific route, leading to black hole.
Verify the summary route being advertised to neighbors
show ip eigrp topology 10.1.0.0 255.255.248.0P 10.1.0.0/21, 1 successors, FD is 2172416
via Summary (2172416/0), Null0The summary route 10.1.0.0/21 is present in the topology table, pointing to Null0. This indicates that the router is summarizing and discarding packets for subnets within that range that are not directly connected, including 10.1.4.0/24 if it falls within the summary.
Root Cause
Auto-summary is enabled on the EIGRP process, causing the router to summarize at the classful boundary (10.0.0.0/8). This summary overlaps with non-contiguous VLSM subnets (e.g., 10.1.4.0/24), causing the router to install a summary route via Null0 and drop traffic to those subnets.
Resolution
Verification
Router# show ip route 10.1.4.0 255.255.255.0 Routing entry for 10.1.4.0/24 Known via "connected", distance 0, metric 0 (connected) Redistributing via eigrp 100 Advertised by eigrp 100 Last update from 0.0.0.0 on GigabitEthernet0/4, 00:00:05 ago Routing Descriptor Blocks: * directly connected, via GigabitEthernet0/4 Also verify that the summary route 10.1.0.0/22 does not include 10.1.4.0/24: Router# show ip route 10.1.0.0 255.255.252.0 longer-prefixes Should show only subnets within 10.1.0.0/22.
Prevention
1. Always disable auto-summary when using VLSM in EIGRP or RIPv2. 2. Plan IP addressing with contiguous blocks to allow summarization without overlap. 3. Use manual summarization with precise prefix lengths to avoid covering unintended subnets.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario appears in troubleshooting questions where a router's summary route causes reachability issues. The exam tests understanding of VLSM, route summarization, and the impact of auto-summary. Candidates must know that auto-summary is disabled by default in EIGRP for IPv4 in recent IOS versions, but may be enabled in older configurations.
Exam Tips
Memorize that auto-summary is disabled by default in EIGRP for IPv4 starting from IOS 15.0(1)M, but may be enabled in older versions.
Remember that a summary route pointing to Null0 indicates that the router is summarizing and will drop packets for subnets not in its routing table within the summary range.
Practice identifying overlapping subnets by converting to binary and comparing prefix lengths.
Commands Used in This Scenario
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 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.
Practice CCNA Questions