Route Summarization Creating Black Hole for Specific Subnets
Presenting Symptom
Users in subnet 10.1.2.0/24 cannot reach servers in subnet 10.1.3.0/24, while other subnets communicate normally.
Network Context
A small enterprise campus with three routers (R1, R2, R3) running EIGRP. R1 connects to the 10.1.0.0/16 network, which is summarized as 10.1.0.0/16 on R1's interface toward R2. R2 and R3 are in the same EIGRP domain. The network uses Cisco IOS 15.x. The issue occurs after a new subnet 10.1.3.0/24 was added behind R3, but R1's summary route 10.1.0.0/16 already exists, causing traffic to be dropped at R2.
Diagnostic Steps
Check routing table on the router where traffic is lost (R2)
show ip route 10.1.3.0Routing entry for 10.1.3.0/24 Known via "eigrp 100", distance 90, metric 28160, type internal Last update from 10.1.1.3 on GigabitEthernet0/1 * 10.1.1.3, via GigabitEthernet0/1 OR Routing entry for 10.1.0.0/16 Known via "eigrp 100", distance 90, metric 28160, type internal Last update from 10.1.1.1 on GigabitEthernet0/0 * 10.1.1.1, via GigabitEthernet0/0
If the output shows a route to 10.1.3.0/24, the problem is elsewhere. If it shows only the summary route 10.1.0.0/16, the summary is hiding the more specific route.
Check EIGRP topology table for the missing subnet
show ip eigrp topology 10.1.3.0/24IP-EIGRP topology entry for 10.1.3.0/24
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 28160
Routing Descriptor Blocks:
10.1.1.3 (GigabitEthernet0/1), from 10.1.1.3, Send flag is 0x0
Composite metric is (28160/2560), Route is Internal
OR
% EIGRP: Route not in topology tableIf the route is in the topology table but not in the routing table, the summary route is suppressing it. If not in topology, the route is not being advertised.
Check for summary routes configured on R1
show running-config | section router eigrprouter eigrp 100 network 10.0.0.0 summary-address 10.1.0.0 255.255.0.0
The presence of 'summary-address' indicates manual summarization. This summary route will be advertised to R2, covering all subnets within 10.1.0.0/16, including 10.1.3.0/24.
Verify that R3 is advertising the specific subnet
show ip route 10.1.3.0 on R3Routing entry for 10.1.3.0/24 Known via "connected", distance 0, metric 0 (connected) * directly connected via GigabitEthernet0/2
Confirm that R3 has the route and is advertising it via EIGRP. If not, check R3's EIGRP configuration.
Root Cause
R1 is configured with a manual summary route 'summary-address 10.1.0.0 255.255.0.0' under EIGRP. This summary route is advertised to R2, which installs it as a candidate route. Because the summary route has a lower administrative distance (5 for summary) than the specific route learned from R3 (90 for EIGRP internal), R2 prefers the summary route and sends traffic for 10.1.3.0/24 to R1, which does not have a route for that subnet, creating a black hole.
Resolution
Verification
On R2, run: ``` show ip route 10.1.3.0 ``` Expected output: ``` Routing entry for 10.1.3.0/24 Known via "eigrp 100", distance 90, metric 28160, type internal Last update from 10.1.1.3 on GigabitEthernet0/1 * 10.1.1.3, via GigabitEthernet0/1 ``` Also verify end-to-end connectivity with ping from a host in 10.1.2.0/24 to a host in 10.1.3.0/24.
Prevention
["Ensure that manual summary routes only cover subnets that are actually present in the routing table of the summarizing router.","Use automatic summarization with caution; disable it if not all subnets are contiguous.","Implement route filtering or use more specific summaries to avoid black holes."]
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario appears in troubleshooting questions where a summary route causes reachability issues. Candidates must understand that manual summary routes have an administrative distance of 5 and can override more specific routes. The exam may present a drag-and-drop or multiple-choice question asking to identify the cause and solution.
Exam Tips
Remember that manual summary routes have AD 5, which is lower than any dynamic routing protocol (EIGRP internal 90, OSPF 110).
When troubleshooting reachability, always check for summary routes that might be hiding more specific prefixes.
The command 'show ip route' and 'show ip protocols' are key to identifying summary routes.
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.
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