Hands-On Lab: Summarizing Routes with EIGRP

Lab Objective

Configure manual EIGRP route summarization on a router's interface to advertise a single summarized route representing several specific subnets, and verify the automatically created Null0 route that accompanies it.

Lab Purpose

Unlike EIGRP's legacy auto-summary feature, which summarizes only at classful boundaries and caused problems in an earlier lab, manual summarization using ip summary-address can be applied at any interface and any prefix length the administrator chooses, offering precise, deliberate control over the routing table's size.

Lab Topology

R1 has four /24 LANs behind it, all falling
within a contiguous /22 block:

192.168.4.0/24
192.168.5.0/24
192.168.6.0/24
192.168.7.0/24

R1 ---- Serial0/0/0 ---- R2 (should receive
                          only one summarized
                          route for all four)

Task 1: Verify R2 Currently Receives Four Separate Routes

Before summarization, confirm R2's routing table shows all four individual /24 routes via EIGRP.

Task 2: Configure Manual Summarization on R1

On R1's serial interface facing R2, configure a summary address covering all four subnets as a single /22 block.

Task 3: Verify R2 Now Receives Only the Summary

Confirm R2's routing table now shows a single summarized /22 route instead of four individual entries.

Task 4: Verify the Automatic Null0 Route on R1

Confirm R1 itself has automatically gained a discard route for the summarized range.

Solution and Verification

R2# show ip route eigrp

D    192.168.4.0/24 [90/2681856] via ...
D    192.168.5.0/24 [90/2681856] via ...
D    192.168.6.0/24 [90/2681856] via ...
D    192.168.7.0/24 [90/2681856] via ...
-- Four separate entries, before summarization

R1(config)# interface serial0/0/0
R1(config-if)# ip summary-address eigrp 100 192.168.4.0 255.255.252.0

-- 255.255.252.0 is the /22 mask covering
-- exactly 192.168.4.0 through 192.168.7.255,
-- the smallest single block containing all
-- four /24 subnets

R2# show ip route eigrp

D    192.168.4.0/22 [90/2681856] via ...
-- A single summarized route now replaces
-- all four specific entries, reducing R2's
-- routing table size while still providing
-- reachability to every subnet within the
-- summarized range

R1# show ip route

D    192.168.4.0/22 is a summary, 00:02:15, Null0
-- R1 automatically gained this discard route
-- the moment summarization was configured --
-- it prevents a routing loop for any address
-- within the summarized range that does not
-- correspond to an actual specific subnet

Key Takeaway

Manual summarization with ip summary-address can be configured at essentially any interface on any router in an EIGRP topology, unlike OSPF's summarization which is restricted to Area Border Routers, discussed earlier in this series — this flexibility lets an administrator summarize at exactly the topological point that makes the most sense, rather than being constrained to a specific router role.