EIGRP supports automatic and manual summarisation. Automatic summarisation is the process of advertising a classful network. However in today’s networks summarising at classful boundaries has little use since the implementation of Classless Inter-Domain Routing (CIDR). Therefore as of IOS 15.0(1)M auto-summary was switched off by default. IOS releases prior to this should have the no auto-summary command configured.
Manual summarisation is configured at the interface level. The example below shows R1 advertising the 1.1.1.1/32 subnet, which is summarised by R2 to a /24 and R3 to a /16.
R2#show run | s eigrp
router eigrp ccie
!
address-family ipv4 unicast autonomous-system 100
!
af-interface FastEthernet0/1
summary-address 1.1.1.0 255.255.255.0
exit-af-interface
!
topology base
exit-af-topology
network 0.0.0.0
exit-address-family
R3#show run | s eigrp
router eigrp ccie
!
address-family ipv4 unicast autonomous-system 100
!
af-interface FastEthernet0/1
summary-address 1.1.0.0 255.255.0.0
exit-af-interface
!
topology base
exit-af-topology
network 0.0.0.0
exit-address-family
As a result it is possible to manually influence the routing path. From the perspective of R4 the route via R2 will be preferred due to being a more specific prefix. Should R2 fail the route via R3 will come into effect.
R4#show ip route | i 1.1
D 1.1.0.0/16 [90/154240] via 3.4.3.3, 00:23:19, FastEthernet0/1
D 1.1.1.0/24 [90/154240] via 2.4.2.2, 00:24:05, FastEthernet0/0
The optional leak-map <route-map> command allows you to also advertise more specific prefixes that comprised the summary route. In the below example we leak the 1.1.1.1/32 subnet via R3 which now makes this the preferred route. This can be useful in some networks to avoid suboptimal routing.
R3#show run | s eigrp
router eigrp ccie
!
address-family ipv4 unicast autonomous-system 100
!
af-interface FastEthernet0/1
summary-address 1.1.0.0 255.255.0.0 leak-map leak-map1
exit-af-interface
!
topology base
exit-af-topology
network 0.0.0.0
exit-address-family
R3#show run | b ^route-map leak-map1
route-map leak-map1 permit 10
match ip address prefix-list leak-map1
R3#show run | i ip prefix-list leak-map1
ip prefix-list leak-map1 seq 5 permit 1.1.1.1/32
ip prefix-list leak-map1 seq 10 deny 0.0.0.0/0 le 32
R4#show ip route | i 1.1
D 1.1.0.0/16 [90/154240] via 3.4.3.3, 00:33:49, FastEthernet0/1
D 1.1.1.0/24 [90/154240] via 2.4.2.2, 00:34:35, FastEthernet0/0
D 1.1.1.1/32 [90/154240] via 3.4.3.3, 00:01:30, FastEthernet0/1
The Discard Route
Whenever a summary route is configured EIGRP installs a null route, so-called the “discard route”. This route discards packets when there is not a more specific routing match in the routing table.
R2#show ip route | i Null D 1.1.1.0/24 is a summary, 00:43:24, Null0 R3#show ip route | i Null D 1.1.0.0/16 is a summary, 00:43:17, Null0
Note that the discard route has an administrative distance of 5 by default.
R2#show ip route 1.1.1.0
Routing entry for 1.1.1.0/24
Known via "eigrp 100", distance 5, metric 103040, type internal
Redistributing via eigrp 100
Routing Descriptor Blocks:
* directly connected, via Null0
Route metric is 103040, traffic share count is 1
Total delay is 102 microseconds, minimum bandwidth is 100000 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 0
Occasionally it may be necessary to change the administrative distance. This can be achieved by using the admin-distance optional argument.
R2#show run | s eigrp
router eigrp ccie
!
address-family ipv4 unicast autonomous-system 100
!
af-interface FastEthernet0/1
summary-address 1.1.1.0 255.255.255.0
exit-af-interface
!
topology base
summary-metric 1.1.1.0/24 distance 10
exit-af-topology
network 0.0.0.0
exit-address-family
R2#show ip route 1.1.1.0
Routing entry for 1.1.1.0/24
Known via "eigrp 100", distance 10, metric 103040, type internal
Redistributing via eigrp 100
Routing Descriptor Blocks:
* directly connected, via Null0
Route metric is 103040, traffic share count is 1
Total delay is 102 microseconds, minimum bandwidth is 100000 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 0
Note Avoid setting the administrative distance to 255. In earlier releases of IOS this prevented the discard route being installed in the routing table, but still advertised the summary route. Later revisions of IOS didn’t install the discard route and didn’t advertise the summary route. The below example sets the distance to 255 on R2. R4 can now only see routes from R3.
R2#show run | s eigrp
router eigrp ccie
!
address-family ipv4 unicast autonomous-system 100
!
af-interface FastEthernet0/1
summary-address 1.1.1.0 255.255.255.0
exit-af-interface
!
topology base
summary-metric 1.1.1.0/24 distance 255
exit-af-topology
network 0.0.0.0
exit-address-family
R4#show ip route | i 1.1 D 1.1.0.0/16 [90/154240] via 3.4.3.3, 01:06:25, FastEthernet0/1 D 1.1.1.1/32 [90/154240] via 3.4.3.3, 00:34:06, FastEthernet0/1
Note By default the summary EIGRP route uses the lowest metric from all the more specific routes contained in the summary. In networks with hundreds of thousands of more specific routes, it can be CPU intensive to go through all the routes to identify the lowest metric. It therefore makes sense to manually configure this, as illustrated below:
R2#show run | s eigrp
router eigrp ccie
!
address-family ipv4 unicast autonomous-system 100
!
af-interface FastEthernet0/1
summary-address 1.1.1.0 255.255.255.0
exit-af-interface
!
topology base
summary-metric 1.1.1.0/24 1000 100 255 0 1500 distance 10
exit-af-topology
network 0.0.0.0
exit-address-family
References:
The Traceroute Blog 