To pass your BSCI exam and earn your CCNP certification,
you've got to master route summarization. When you get to
the BSCI level, actually breaking the routes down into binary
strings and performing summarization is second nature to you.
(If it isn't, get some more practice!) What makes CCNP / BSCI
route summarization more difficult is just keeping the different
protocol summarization commands straight!
RIP and EIGRP both perform route summarization at the interface
level with the ip summary-address command. In the following
example, R2 is running RIP and was sending four routes to
R3, R3's table looked like this before summarization:
R3#show ip route rip
172.16.0.0/24 is subnetted, 4 subnets
R 172.16.8.0 [120/1] via 172.23.23.2, 00:00:02, Ethernet0
R 172.16.9.0 [120/1] via 172.23.23.2, 00:00:02, Ethernet0
R 172.16.10.0 [120/1] via 172.23.23.2, 00:00:02, Ethernet0
R 172.16.11.0 [120/1] via 172.23.23.2, 00:00:02, Ethernet0
By summarizing the routes and using the ip summary-address
command, RIP advertises only the summary route to the downstream
neighbor.
R2(config)#int ethernet0
R2(config-if)#ip summary-address rip 172.16.8.0 255.255.252.0
R3#clear ip route *
R3#show ip route rip
172.16.0.0/22 is subnetted, 1 subnets
R 172.16.8.0 [120/1] via 172.23.23.2, 00:01:24, Ethernet0
EIGRP works much the same way, except that the EIGRP AS number
must be named in the ip summary-address command.
In the following example, R2 was advertising four separate
routes to R3 via EIGRP 100: 100.0.0.0, 101.0.0.0, 102.0.0.0,
and 103.0.0.0, all with an eight-bit mask. What summary route
can be used here?
The summary is 100.0.0.0 252.0.0.0. To send that route to
downstream routers, configure the following on R2:
R2(config)#interface ethernet0
R2(config-if)#ip summary-address eigrp 100 100.0.0.0 252.0.0.0
R3 will then have only one route in its EIGRP table - the
summary route.
R3#show ip route eigrp
D 100.0.0.0/6 [90/2297856] via 172.23.23.2, 00:02:33, Ethernet0
By mastering basic binary skills and keeping in mind that
RIP and EIGRP perform route summarization at the interface
level, you're one step closer to passing your BSCI exam and
earning your CCNP certification!
In the next part of this tutorial, we'll take a detailed
look at the different methods OSPF uses for route summarization.
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage,
home of free CCNP and CCNA tutorials, The Ultimate CCNA Study
Package, and Ultimate CCNP Study Packages. For a FREE copy
of his latest e-books, “How To Pass The CCNA” and
“How To Pass The CCNP”, just visit the website!
You can also get FREE CCNA and CCNP exam questions every day!
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