EIGRP Route Summarization – Automatic And Manual
EIGRP and RIPv2 automatically summarize routes when they are advertised across classful network boundaries. When you have discontiguous networks (subnets of the same major network number that are separated by another major network number), this behavior can result in suboptimal routing at its worst. *Already covered in previous EIGRP posts.
Where to disable auto summary?
The best place to turn this feature off is normally at the spoke routers, as they are advertising networks and performing summarization facing towards the hub router.
Configuring any kind of manual summarization does NOT enable or disable auto-summarization — they’re two separate operations.
Reasons to use Autosummarization
- The routing tables are smaller, making the entire routing process faster.
- Since the tables are smaller, the load on the CPU from the routing process is lessened.
- The more-specific network numbers are hidden.
- Routing updates are smaller.
- The impact of flapping routes on the rest of the network is lessened, and correct summarization helps to limit the number of EIGRP queries.
R3#show ip route eigrp
184.108.40.206/16 is subnetted, 7 subnets
D 220.127.116.11 [90/2297856] via 18.104.22.168, 00:00:00, Serial0
D 22.214.171.124 [90/2297856] via 126.96.36.199, 00:00:00, Serial0
D 188.8.131.52 [90/2297856] via 184.108.40.206, 00:00:00, Serial0
D 220.127.116.11 [90/2297856] via 18.104.22.168, 00:00:00, Serial0
D 22.214.171.124 [90/2297856] via 126.96.36.199, 00:00:00, Serial0
D 188.8.131.52 [90/2297856] via 184.108.40.206, 00:00:00, Serial0
D 220.127.116.11 [90/2297856] via 18.104.22.168, 00:00:00, Serial0
With the help of manual route summarization, we can knock this table down to one line. First, we use binary math to convert the subnet numbers to binary strings.
22.214.171.124 01100100 00000001 00000000 00000000
126.96.36.199 01100100 00000010 00000000 00000000
188.8.131.52 01100100 00000011 00000000 00000000
184.108.40.206 01100100 00000100 00000000 00000000
220.127.116.11 01100100 00000101 00000000 00000000
18.104.22.168 01100100 00000110 00000000 00000000
22.214.171.124 01100100 00000111 00000000 00000000
Believe it or not, that’s the hard part of summarizing routes. 😉
The next step in route summarization is to work from left to right and identify the common bits. From there, it’s easy:
- The decimal value of these bits is the summary route.
- The number of common bits yields the summary mask.
Here, the value of the common bits (bolded above) is 126.96.36.199. There are 13 common bits, expressed as /13 in prefix notation and 255.248.0.0 in dotted decimal.
Where to configure the manual summarization?
Your manual summary statement should be configured on the physical interface that connects to the adjacent router:
R1(config-if)#ip summary-address eigrp 100 188.8.131.52 255.248.0.0
2d11h: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor 184.108.40.206 (Serial0) is down: summary configured
2d11h: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor 220.127.116.11 (Serial0) is down: summary configured
It’s important to keep in mind that when you configure EIGRP summary addresses, your neighbor relationships will be lost. When they come back up, R3 will have a single summary route instead of the seven more specific routes it previously had.
After a manual summary is performed, the neighbours route table will look like this:
R1#show ip route
< code table removed for clarity >
18.104.22.168/8 is variably subnetted, 8 subnets, 2 masks
C 22.214.171.124/16 is directly connected, Loopback4
C 126.96.36.199/16 is directly connected, Loopback5
C 188.8.131.52/16 is directly connected, Loopback6
C 184.108.40.206/16 is directly connected, Loopback7
D 220.127.116.11/13 is a summary, 00:07:32, Null0
C 18.104.22.168/16 is directly connected, Loopback0
C 22.214.171.124/16 is directly connected, Loopback2
C 126.96.36.199/16 is directly connected, Loopback3
The summary route is seen as a route to Null0, which is basically a route to the trash can. If a packet comes into this router that doesn’t match one of the seven more-specific routes, it will be “black-holed” – dropped by the router. This default behavior of EIGRP route summarization helps to prevent routing loops. This null route will only be seen on the router performing the manual summarization.
How do we find the AD value of 5 for this summary route?
R1#show ip route 188.8.131.52 255.248.0.0
Routing entry for 184.108.40.206/13
Known via "eigrp 100", distance 5, metric 128256, type internal
Redistributing via eigrp 100
Routing Descriptor Blocks:
* directly connected, via Null0
Route metric is 128256, traffic share count is 1
Total delay is 5000 microseconds, minimum bandwidth is 10000000 Kbit
Reliability 255/255, minimum MTU 1514 bytes
Loading 1/255, Hops 0
Route summarization is more efficient when it’s configured on the ASBR. (OSPF concept, but also referenced in EIGRP)