EIGRP Auto-Summary Creating Black Holes with Discontiguous Networks
Presenting Symptom
Users in one branch office cannot reach servers in another branch office, even though both are in the same EIGRP autonomous system and have connectivity to the core.
Network Context
The network consists of two branch offices (Branch A and Branch B) connected via a WAN link to a central core router. Each branch has its own subnet: 10.1.1.0/24 in Branch A and 10.2.2.0/24 in Branch B. The core router connects to both branches and also has a loopback 0 with IP 172.16.1.1/24. All routers run EIGRP AS 100 on Cisco IOS 15.x. The problem occurs because EIGRP auto-summary is enabled by default, causing the core router to summarize the 10.0.0.0/8 network, which creates a black hole for traffic between the discontiguous 10.x.x.x subnets.
Diagnostic Steps
Check EIGRP topology table on the core router
show ip eigrp topologyP 10.0.0.0/8, 1 successors, FD is 28160
via 10.1.1.1 (28160/2560), GigabitEthernet0/0
via 10.2.2.1 (28160/2560), GigabitEthernet0/1If you see a single summarized route 10.0.0.0/8 instead of the specific /24 subnets, auto-summary is causing the issue. The core router is advertising a summary route to both branches, which can cause black holes if the branches are discontiguous.
Check EIGRP interfaces for auto-summary status
show ip protocolsRouting Protocol is "eigrp 100"
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
EIGRP-IPv4 Protocol for AS(100)
Metric weight K1=1, K2=0, K3=1, K4=0, K5=0
NSF-aware route hold timer is 240
Router-ID: 172.16.1.1
Topology : 0 (base)
Active Timer: 3 min
Distance: internal 90 external 170
Maximum path: 4
Maximum hopcount 100
Maximum metric variance 1
Automatic Summarization: enabledIf 'Automatic Summarization: enabled' is shown, auto-summary is on. This confirms the root cause.
Check routing table on a branch router for the remote subnet
show ip route eigrpD 10.0.0.0/8 [90/28160] via 172.16.1.1, 00:00:05, GigabitEthernet0/0
If you see only a summary route 10.0.0.0/8 pointing to the core, but no specific route to the other branch's subnet (e.g., 10.2.2.0/24), the branch router is relying on the summary, which may point to the core but the core may not have a specific route back, causing a black hole.
Trace route from Branch A to Branch B server
traceroute 10.2.2.10Type escape sequence to abort. Tracing the route to 10.2.2.10 1 10.1.1.254 4 msec 4 msec 4 msec 2 * * * 3 * * *
If the trace stops at the core router (next hop) and then times out, the core router is likely dropping the packet because it has a summary route but no specific route to the destination, or it is sending the packet back to the source due to the summary.
Root Cause
EIGRP auto-summary is enabled by default on the core router, causing it to summarize the 10.0.0.0/8 network. Since the branch subnets (10.1.1.0/24 and 10.2.2.0/24) are discontiguous (not part of a contiguous block), the core router advertises the summary route to both branches. Each branch then installs the summary route pointing to the core, but the core router itself does not have a specific route to the remote branch's subnet (it only has the summary pointing to itself), creating a routing black hole. Traffic from Branch A to Branch B reaches the core, but the core has no specific route to 10.2.2.0/24 and may drop the packet or send it back to Branch A.
Resolution
Verification
After applying the fix, run the following commands: 1. `show ip protocols` - Verify 'Automatic Summarization: disabled' 2. `show ip route eigrp` - Should now see specific routes like: D 10.1.1.0/24 [90/28160] via 172.16.1.1, 00:00:05, GigabitEthernet0/0 D 10.2.2.0/24 [90/28160] via 172.16.1.1, 00:00:05, GigabitEthernet0/1 3. `traceroute 10.2.2.10` from Branch A - Should now complete successfully to the destination.
Prevention
["Disable EIGRP auto-summary on all routers as a best practice, especially in networks with discontiguous subnets or VLSM.","Use manual summarization with the `ip summary-address eigrp` command on specific interfaces to control route advertisement.","Design IP addressing to be contiguous where possible to avoid discontiguous network issues."]
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario may appear as a troubleshooting question where you must identify why hosts in one subnet cannot reach hosts in another subnet when both are in the same EIGRP domain. The exam tests understanding of EIGRP auto-summary behavior and its impact on discontiguous networks. A typical question might present a topology with two branches using 10.x.x.x subnets and a core router, and ask why traffic fails. The key fact is that auto-summary is enabled by default and causes classful summarization, which can create black holes for discontiguous networks.
Exam Tips
Remember that EIGRP auto-summary is enabled by default on Cisco IOS; you must explicitly disable it with `no auto-summary` under the EIGRP process.
In the exam, if you see a routing table with a summary route (e.g., 10.0.0.0/8) instead of specific subnets, suspect auto-summary as the cause.
Know that discontiguous networks (subnets of the same classful network separated by another network) require auto-summary to be disabled for proper routing.
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 protocols
Displays the current state of all IP routing protocols running on the router, including timers, filters, and network advertisements.
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