Hands-On Lab: EIGRP Automatic Summarization

Lab Objective

Enable EIGRP's automatic summarization feature on a router connecting two discontiguous subnets of the same classful network, observe the resulting routing failure, then disable auto-summary to restore correct connectivity.

Lab Purpose

Earlier labs in this series have consistently disabled auto-summary as a matter of course. This lab deliberately enables it to make concrete exactly why that practice exists — auto-summary can cause serious, hard-to-diagnose routing failures in any topology with discontiguous subnets of the same major network.

Lab Topology

R1 ---- Serial0/0/0 ---- R2 ---- Serial0/0/1 ---- R3

R1's LAN: 10.1.1.0/24
R2-R3 link: 10.99.99.0/30 (a different major
             network entirely, to isolate the
             discontiguous-subnet effect)
R3's LAN: 10.2.2.0/24

Both R1's LAN and R3's LAN belong to the same
classful network (10.0.0.0/8), but are
separated by the 10.99.99.0/30 link in between

Task 1: Configure Basic Addressing and EIGRP

Configure all interfaces and enable EIGRP AS 100 on all three routers, leaving auto-summary at its default enabled state.

Task 2: Observe the Resulting Routing Table

Examine R1's routing table and identify the problem caused by auto-summary.

Task 3: Test Connectivity

Attempt to ping from R1's LAN to R3's LAN and observe the failure.

Task 4: Disable Auto-Summary

Disable automatic summarization on all three routers.

Task 5: Verify Connectivity Is Restored

Confirm the routing table now shows specific routes and connectivity works correctly.

Solution and Verification

R1(config)# router eigrp 100
R1(config-router)# network 10.0.0.0
-- auto-summary remains at its default
-- enabled state (not explicitly disabled)

R1# show ip route eigrp

D    10.0.0.0/8 is a summary, 00:01:12, Null0
-- R1 receives only a single summarized
-- 10.0.0.0/8 entry pointing to Null0 --
-- the specific /24 subnets for R2-R3's link
-- and R3's actual LAN are completely hidden

R1# ping 10.2.2.1

.....
Success rate is 0 percent (0/5)
-- Traffic destined for R3's LAN is discarded
-- by R1's own summarized Null0 route, since
-- 10.2.2.1 falls within the summarized
-- 10.0.0.0/8 range but R1 has no more
-- specific route to actually reach it

R1(config)# router eigrp 100
R1(config-router)# no auto-summary

R2(config)# router eigrp 100
R2(config-router)# no auto-summary

R3(config)# router eigrp 100
R3(config-router)# no auto-summary

R1# show ip route eigrp

D    10.99.99.0/30 [90/2681856] via ..., Serial0/0/0
D    10.2.2.0/24 [90/2707456] via ..., Serial0/0/0
-- Specific routes now appear instead of the
-- single misleading summary

R1# ping 10.2.2.1

!!!!!
Success rate is 100 percent (5/5)

Key Takeaway

Auto-summary's Null0 route is a deliberate loop-prevention mechanism, discussed earlier in this series regarding EIGRP summarization, but it becomes actively harmful in a discontiguous topology like this one, where it silently discards legitimate traffic to a real destination that happens to fall within the summarized range. This is precisely why no auto-summary has been standard practice throughout every EIGRP lab in this series, and remains essential in virtually all modern networks.