How do you summarize in EIGRP, and what does the router do when you configure a summary?
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On the interface facing the neighbors that should receive the summary: ip summary-address eigrp 100 10.20.0.0 255.255.252.0 in classic mode, or summary-address under the af-interface in named mode.
The router stops sending the component routes out that interface, sends the summary with the metric of
the best component, and installs a Null0 discard route with AD 5 so traffic for unused space inside
the summary is dropped rather than looped. The summary stays as long as at least one component exists.
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Example
DSW1 has 10.20.0.0/24 to 10.20.3.0/24 from two access switches and summarizes toward CORE1 on Gi1/0/1.
interface GigabitEthernet1/0/1
ip summary-address eigrp 100 10.20.0.0 255.255.252.0On DSW1:
DSW1# show ip route eigrp | include 10.20
D 10.20.0.0/22 is a summary, 00:03:52, Null0
D 10.20.0.0/24 [90/3072] via 10.0.1.2, 00:03:52, GigabitEthernet1/0/2
D 10.20.1.0/24 [90/3072] via 10.0.1.2, 00:03:52, GigabitEthernet1/0/2
D 10.20.2.0/24 [90/3072] via 10.0.1.6, 00:03:50, GigabitEthernet1/0/3
D 10.20.3.0/24 [90/3072] via 10.0.1.6, 00:03:50, GigabitEthernet1/0/3On CORE1, only the summary arrives, with DSW1's best component metric 3072 plus one gigabit hop:
CORE1# show ip route eigrp | include 10.20
D 10.20.0.0/22 [90/3328] via 10.0.0.2, 00:03:40, GigabitEthernet1/0/1Why the Null0 route exists
Suppose VLAN 21 on ASW2 goes down and 10.20.3.0/24 disappears, while CORE1 still sees the /22. Without the Null0 route, DSW1 would forward a packet for 10.20.3.50 along its default route up to CORE1, and CORE1 would send it straight back down because of the /22. The packet would bounce until its TTL ran out. With the Null0 route, DSW1 drops it at once.
Leaking a specific
ip summary-address eigrp 100 10.20.0.0 255.255.252.0 leak-map LEAK also advertises any component
that the route map LEAK permits, for example one /24 that should take a specific path.
They will ask next
Can EIGRP summarize anywhere, unlike OSPF?
Yes. EIGRP summarizes per interface on any router. OSPF only summarizes at area borders and at the ASBR. That freedom is one of EIGRP's design strengths.
What happens if you summarize 0.0.0.0/0?
Neighbors on that interface receive only a default route, which is a quick way to shrink a stub's table. But the summarizing router installs 0.0.0.0/0 to Null0 with AD 5, which can override its own learned default route and black-hole internet traffic. Check the routing table on that router after.
Traps
- Configuring the summary on the downlink instead of the uplink. It goes on the interface toward the routers that should see only the summary.