- By default, EIGRP has auto summarization enabled which summarizes routes to classful boundaries, but this can cause issues, so EIGRP allows manual summarization at arbitrary boundaries using the ip summary-address command. - Manual summarization can be configured on any router or interface in an EIGRP domain. A summary route will exist as long as there is at least one more specific route; if the last specific route disappears, the summary will also disappear. - The document provides an example configuration of EIGRP routing between four routers (R1, R2, R3, R4) with the configuration of manual summarization on R2's S0/0 interface to summarize routes

1. EIGRP ROUTE
SUMMARIZATION
WWW.NETPROTOCOLXPERT.IN

2. • BY DEFAULT EIGRP HAS AUTO SUMMARIZATION ENABLED. THIS
MEANS THAT IT WILL SUMMARIZE TO CLASSFUL ADDRESSES AT
NETWORK BOUNDARIES.
• LEAVING AUTO SUMMARIZATION ENABLED COULD CAUSE A LOT
OF TROUBLES IN OUR NETWORK. EIGRP HAS THE ABILITY TO
CREATE SUMMARY ROUTES AT ARBITRARY BOUNDARIES BY USING
MANUAL SUMMARIZATION.
• MANUAL SUMMARIZATION CAN BE APPLIED ANYWHERE IN EIGRP
DOMAIN, ON EVERY ROUTER, ON EVERY INTERFACE.
• SUMMARY ROUTE WILL EXIST IN ROUTING TABLE AS LONG AS AT
LEAST ONE MORE SPECIFIC ROUTE WILL EXIST. IF THE LAST
SPECIFIC ROUTE WILL DISAPPEAR, SUMMARY ROUTE ALSO WILL

3. • IF YOU WANT TO USE AUTO-SUMMARIZATION YOU HAVE TO DO
NOTHING BECAUSE IT IS ENABLED BY DEFAULT, OTHERWISE USE NO
AUTO-SUMMARY EIGRP COMMAND TO DISABLE IT.
• TO DO MANUAL SUMMARIZATION GO UNDER INTERFACE AND
USE IP SUMMARY-ADDRESS EIGRP AS-NUMBER ADDRESS MASK
[ADMINISTRATIVE-DISTANCE ] COMMAND.

5. • UNLIKE OSPF WHERE WE CAN SUMMARIZE ONLY ON ABR OR ASBR IN
EIGRP WE CAN SUMMARIZE ANYWHERE. WE WILL DO THIS ON R2’S S0/0
INTERFACE, SO A SUMMARY ROUTE WILL BE SENT TO R1. FIRST WILL
CONFIGURE EIGRP TO RUN IN THIS NETWORK.
R1
• R1(CONFIG)#INTERFACE SERIAL 0/0
• R1(CONFIG-IF)#IP ADDRESS 192.168.0.2 255.255.255.0
• R1(CONFIG-IF)#NO SHUTDOWN
• R1(CONFIG)#ROUTER EIGRP 1
• R1(CONFIG-ROUTER)#NO AUTO-SUMMARY
• R1(CONFIG-ROUTER)#NETWORK 192.168.0.0

6. R2
• R2(CONFIG)#INTERFACE SERIAL 0/0
• R2(CONFIG-IF)#IP ADDRESS 192.168.0.1 255.255.255.0
• R2(CONFIG-IF)#NO SHUTDOWN
• R2(CONFIG-IF)#INTERFACE FA0/0
• R2(CONFIG-IF)#IP ADDRESS 192.168.10.1 255.255.255.0
• R2(CONFIG-IF)#NO SHUTDOWN
• R2(CONFIG-IF)#INTERFACE FA0/1
• R2(CONFIG-IF)#IP ADDRESS 192.168.8.1 255.255.255.0
• R2(CONFIG-IF)#NO SHUTDOWN
• R2(CONFIG)#ROUTER EIGRP 1
• R2(CONFIG-ROUTER)#NO AUTO-SUMMARY
• R2(CONFIG-ROUTER)#NETWORK 192.168.0.0
• R2(CONFIG-ROUTER)#NETWORK 192.168.8.0
• R2(CONFIG-ROUTER)#NETWORK 192.168.10.0

7. R3
• R3(CONFIG)#INTERFACE FASTETHERNET 0/0
• R3(CONFIG-IF)#IP ADDRESS 192.168.10.2 255.255.255.0
• R3(CONFIG-IF)#NO SHUTDOWN
• R3(CONFIG-IF)#INTERFACE LOOPBACK 0
• R3(CONFIG-IF)#IP ADDRESS 192.168.11.1 255.255.255.0
• R3(CONFIG)#ROUTER EIGRP 1
• R3(CONFIG-ROUTER)#NO AUTO-SUMMARY
• R3(CONFIG-ROUTER)#NETWORK 192.168.10.0
• R3(CONFIG-ROUTER)#NETWORK 192.168.11.0

8. R4
• R4(CONFIG)#INTERFACE FASTETHERNET 0/0
• R4(CONFIG-IF)#IP ADDRESS 192.168.8.2 255.255.255.0
• R4(CONFIG-IF)#NO SHUTDOWN
• R4(CONFIG-IF)#INTERFACE LOOPBACK 0
• R4(CONFIG-IF)#IP ADDRESS 192.168.9.1 255.255.255.0
• R4(CONFIG)#ROUTER EIGRP 1
• R4(CONFIG-ROUTER)#NO AUTO-SUMMARY
• R4(CONFIG-ROUTER)#NETWORK 192.168.8.0
• R4(CONFIG-ROUTER)#NETWORK 192.168.9.0

9. LET’S CHECK NOW R1’S ROUTING TABLE
• R1#SH IP ROUTE
• ... OUTPUT OMMITED ...
• GATEWAY OF LAST RESORT IS NOT SET
• D 192.168.8.0/24 [90/2195456] VIA 192.168.0.1, 00:00:12,
SERIAL0/0
• D 192.168.9.0/24 [90/2323456] VIA 192.168.0.1, 00:00:12,
SERIAL0/0
• D 192.168.10.0/24 [90/2195456] VIA 192.168.0.1, 00:00:12,
SERIAL0/0
• D 192.168.11.0/24 [90/2323456] VIA 192.168.0.1, 00:00:12,
SERIAL0/0

10. • LOOK TO THE METRICS OF LEARNED ROUTES, AND AFTER WE DO
SUMMARIZATION, WILL COMPARE THIS ONE WITH THAT SUMMARY
METRIC. NOW WE WILL DO SUMMARIZATION ON R2’S S0/0
INTERAFCE.
• R2(CONFIG-IF)#IP SUMMARY-ADDRESS EIGRP 1 192.168.8.0
255.255.248.0

11. CHECK ROUTING TABLE ON R1AGAIN
• R1#SH IP ROUTE
• ... OUTPUT OMMITED ...
• GATEWAY OF LAST RESORT IS NOT SET
• C 192.168.0.0/24 IS DIRECTLY CONNECTED, SERIAL0/0
• D 192.168.8.0/21 [90/2195456] VIA 192.168.0.1, 00:01:42,
SERIAL0/0
• R1#
• ALL ROUTES LEARNED FROM R1 ARE NOW SUMMARIZED AND THE
METRIC IS 2195456, THE SMALLEST METRIC OF THE SPECIFIC
ROUTES

12. • R2#SHOW IP ROUTE
• ... OUTPUT OMMITED ...
• GATEWAY OF LAST RESORT IS NOT SET
• C 192.168.8.0/24 IS DIRECTLY CONNECTED,
FASTETHERNET0/1
• D 192.168.9.0/24 [90/409600] VIA 192.168.8.2, 01:04:43,
FASTETHERNET0/1
• C 192.168.10.0/24 IS DIRECTLY CONNECTED,
FASTETHERNET0/0
• D 192.168.11.0/24 [90/409600] VIA 192.168.10.2, 01:04:43,
FASTETHERNET0/0
• C 192.168.0.0/24 IS DIRECTLY CONNECTED, SERIAL0/0

13. • WHEN YOU CONFIGURE AN EIGRP SUMMARY ROUTE, THAT ROUTE WILL
APPEAR ON GENERATING ROUTER ALSO, BUT WITH NEXT
HOP NULL0 WHICH IS A SOFTWARE INTERFACE.
• NULL0 INTERFACE IS USED TO PREVENT PACKETS FROM BEING SENT
FURTHER IF THE ROUTER DOESN'T HAVE A SPECIFIC ROUTE, THUS WILL
DENY PACKETS TO GO THROUGH THE NETWORK.

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