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Route Summarization
- 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.
10.2.1.0
10.2.2.0
10.2.3.0
10.2.4.0
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
RT1 10.1.2.0/30 RT2 10.1.3.0/30 RT3
S0/0
S0/0
S0/0
10.1.1.0/24
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#
113
S0/1
10.3.4.0
10.3.5.0
10.3.6.0
10.3.7.0
Copyright © 2008 Yap Chin Hoong
yapchinhoong@hotmail.com