- The larger the network, more routes to advertise, larger routing update packets, more bandwidth the updates take, and hence reducing the bandwidth available for data transmission. Besides that, large IP networks also produce large routing tables, which consume more router memory, and take more time and CPU cycles to perform routing table lookup when routing a packet. - Route summarization reduces the size of routing updates and tables while still maintain routing
information to all destination networks. It is also known as route aggregation or supernetting. - Route summarization also able to reduce convergences. Upstream routers that received the
summary route do not have to reconverge whenever there is a status change in the component subnets, which can effectively insulate upstream routers from problems such as route flapping. Route flapping is when a network goes up and down on a router, causing it to constantly advertise the status about the network.
- A requirement for route summarization is a classless routing protocol (eg: RIPv2, EIGRP, OSPF, IS-IS, and BGP) must be running, as they support variable-length subnet masks (VLSMs) and carry subnet mask information along with the routing updates.
- A summary route substitutes multiple original component routes. Once configured, the routing protocol advertises only the single summary route instead of multiple specific component routes. - Route summarization works great in contiguous networks that were designed and planned for it.
Figure 15-3: A Network Designed and Planned for Route Summarization
- Below shows the routing table on RT2 before route summarization, with EIGRP as the routing protocol. It shows 4 routes to 10.2.x.x subnets out its Serial0/0 interface to RT1, and 4 routes to
10.3.x.x subnets out its Serial0/1 interface to RT3:
RT2#sh ip route
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 11 subnets, 2 masks C 10.1.1.0/24 is directly connected, Ethernet1/0 C 10.1.2.0/30 is directly connected, Serial0/0 C 10.1.3.0/30 is directly connected, Serial0/1
D 10.2.1.0/24 [90/2195456] via 10.1.2.1, 00:02:13, Serial0/0 D 10.2.2.0/24 [90/2195456] via 10.1.2.1, 00:02:13, Serial0/0 D 10.2.3.0/24 [90/2195456] via 10.1.2.1, 00:02:13, Serial0/0 D 10.2.4.0/24 [90/2195456] via 10.1.2.1, 00:02:13, Serial0/0 D 10.3.4.0/24 [90/2195456] via 10.1.3.2, 00:00:18, Serial0/1 D 10.3.5.0/24 [90/2195456] via 10.1.3.2, 00:00:18, Serial0/1 D 10.3.6.0/24 [90/2195456] via 10.1.3.2, 00:00:19, Serial0/1 D 10.3.7.0/24 [90/2195456] via 10.1.3.2, 00:00:19, Serial0/1 RT2# 10.1.1.0/24 10.1.2.0/30 10.1.3.0/30 10.2.1.0 10.2.2.0 10.2.3.0 10.2.4.0 10.3.4.0 10.3.5.0 10.3.6.0 10.3.7.0 RT1 RT2 RT3 S0/0 S0/1 S0/0 S0/0
- RT1 and RT3 are summarizing routers, which advertise summary routes to other routers.
- Route summarization configuration differs with different routing protocols. The ip summary- address eigrp {as-num} {address} {mask} interface subcommand is used to create a summary route that is to be advertised out an interface for an EIGRP autonomous system.
Note: EIGRP route summarization takes effect immediately upon the issuance of the
manual summarization command and would tear down EIGRP neighbor relationship; this ensure neighbors remove previous topology information, and accept to the new topology
information upon the neighborship is recovered. - Route Summarization configuration on RT1 and RT3:
- Below shows the routing tables on RT1 and RT3 after route summarization:
RT1#conf t
Enter configuration commands, one per line. End with CNTL/Z. RT1(config)#int s0/0
RT1(config-if)#ip summary-address eigrp 100 10.2.0.0 255.255.0.0
RT1(config-if)#
--- RT3#conf t
Enter configuration commands, one per line. End with CNTL/Z. RT3(config)#int s0/0
RT3(config-if)#ip summary-address eigrp 100 10.3.0.0 255.255.0.0
RT3(config-if)
RT1#sh ip route
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 9 subnets, 2 masks
D 10.1.1.0/24 [90/2195456] via 10.1.2.2, 00:04:36, Serial0/0 C 10.1.2.0/30 is directly connected, Serial0/0
D 10.1.3.0/30 [90/2681856] via 10.1.2.2, 00:04:36, Serial0/0 D 10.2.0.0/16 is a summary, 00:04:41, Null0
C 10.2.1.0/24 is directly connected, Ethernet1/0 C 10.2.2.0/24 is directly connected, Ethernet1/1 C 10.2.3.0/24 is directly connected, Ethernet1/2 C 10.2.4.0/24 is directly connected, Ethernet1/3
D 10.3.0.0/16 [90/2707456] via 10.1.2.2, 00:03:05, Serial0/0 RT1#
--- RT3#sh ip route
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 9 subnets, 2 masks
D 10.1.1.0/24 [90/2195456] via 10.1.3.1, 00:05:32, Serial0/0 D 10.1.2.0/30 [90/2681856] via 10.1.3.1, 00:05:34, Serial0/0 C 10.1.3.0/30 is directly connected, Serial0/0
D 10.2.0.0/16 [90/2707456] via 10.1.3.1, 00:05:32, Serial0/0 D 10.3.0.0/16 is a summary, 00:05:37, Null0
C 10.3.4.0/24 is directly connected, Ethernet1/0 C 10.3.5.0/24 is directly connected, Ethernet1/1 C 10.3.6.0/24 is directly connected, Ethernet1/2 C 10.3.7.0/24 is directly connected, Ethernet1/3 RT3#
- Static routes can be said to support VLSM, as they can be used to define summary routes for routing protocols that do not support VLSM, eg: RIP and IGRP.
- Below shows the routing table on RT2 after route summarization:
- RT1 no longer seeing the 4 10.3.x.x routes, and RT3 no longer seeing the 4 10.2.x.x routes.
With route summarization, both routers advertise only the summary routes.
Route summarization conserve great amount of bandwidth (smaller routing update packets) and minimizes processing for routing updates.
- EIGRP route summarization configuration introduces a route destined to the Null0 interface, where packets matching the route will be discarded. Whenever the summarizing router receives a packet destined for the summary route but the specific route does not exist in the routing table, the packet will be matched to the null route and being discarded. This prevents the summarizing router from forwarding the packet to its default route which would possibly create a routing loop. The null route will only be seen on the summarizing router.
- To find the best summary route, locate the bits of the subnet numbers that have the common bit pattern, from left to right (highest-order bits) – this is the subnet mask for the summary route. - Best summary route for RT1:
00001010 00000010 00000001 00000000 – 10.2.1.0 00001010 00000010 00000010 00000000 – 10.2.2.0 00001010 00000010 00000011 00000000 – 10.2.3.0 00001010 00000010 00000100 00000000 – 10.2.4.0
The best summary route for RT1 is 10.2.0.0, subnet mask 255.255.248.0. - Best summary route for RT3:
00001010 00000011 00000100 00000000 – 10.3.4.0 00001010 00000011 00000101 00000000 – 10.3.5.0 00001010 00000011 00000110 00000000 – 10.3.6.0 00001010 00000011 00000111 00000000 – 10.3.7.0
The best summary route for RT3 is 10.3.4.0, subnet mask 255.255.252.0.
RT2#sh ip route
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks
D 10.2.0.0/16 [90/2195456] via 10.1.2.1, 00:03:34, Serial0/0 D 10.3.0.0/16 [90/2195456] via 10.1.3.2, 00:02:06, Serial0/1 C 10.1.1.0/24 is directly connected, Ethernet1/0
C 10.1.2.0/30 is directly connected, Serial0/0 C 10.1.3.0/30 is directly connected, Serial0/1 RT2#