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Cisco CCNP / BSCI Exam Tutorial: Using OSPF's "Summary-Address" Command |
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Your BSCI and CCNP exam success depends on knowing the details,
and one such detail is knowing the proper way to summarize
routes in OSPF. Route summarization is not just a test of
your binary conversion abilities, but knowing where and when
to summarize routes. It will not surprise any CCNA or CCNP
certification candidate that OSPF gives us the most options
for route summarization, and therefore more details to know!
OSPF offers us two options for route summarization configurations.
In a previous tutorial, we looked at the "summary-address"
command, and today we'll look at the proper use of the "area
range" command.
The "area range" command should be used on an Area
Border Router (ABR) to summarize routes being advertised from
one OSPF area to another. In this tutorial, R1 is acting as
an ABR, with interfaces in both Area 0 and Area 1. Four loopbacks
have been placed into R1's Area 1.
R1(config)#router ospf 1
R1(config-router)#network 12.0.0.0 0.255.255.255 a 1
R1(config-router)#network 13.0.0.0 0.255.255.255 a 1
R1(config-router)#network 14.0.0.0 0.255.255.255 a 1
R1(config-router)#network 15.0.0.0 0.255.255.255 a 1
The routing table of an OSPF neighbor, R2, shows all four
routes.
R2#show ip route ospf
12.0.0.0/32 is subnetted, 1 subnets
O IA 12.12.12.12 [110/65] via 172.12.123.1, 00:18:52, Serial0
13.0.0.0/32 is subnetted, 1 subnets
O IA 13.13.13.13 [110/65] via 172.12.123.1, 00:18:42, Serial0
14.0.0.0/32 is subnetted, 1 subnets
O IA 14.14.14.14 [110/65] via 172.12.123.1, 00:18:32, Serial0
15.0.0.0/32 is subnetted, 1 subnets
O IA 15.15.15.15 [110/65] via 172.12.123.1, 00:18:32, Serial0
To keep the routing tables of downstream routers smaller
but still have the desired IP connectivity, we can use the
area range command on R1 to summarize these four routes. The
key to keep in mind with the area range command is that the
area number given in the command is the area containing the
destinations, NOT the area that will receive the summary route.
R1(config)#router ospf 1
R1(config-router)#area 1 range 12.0.0.0 252.0.0.0
R2 now shows a single summary route that can be used to reach
all four remote networks.
R2#show ip route ospf
O IA 12.0.0.0/6 [110/65] via 172.12.123.1, 00:00:21, Serial0
Interestingly enough, there's now an additional route in
R1's routing table.
R1#show ip route ospf O 12.0.0.0/6 is a summary, 00:07:53,
Null0
When you configure summary routes in OSPF, a route to null0
will be installed into the OSPF routing table of the router
performing the summarization. This helps to prevent routing
loops. Any packets destined for the routes that have been
summarized will have a longer match in the routing table,
and packets that do not match one of the summarized routes
but do match the summary route will be dropped.
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage,
home of free CCNA and CCNP 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”, visit
the website and download your free copies. You can also get
FREE CCNA and CCNP exam questions every day! Pass the CCNA
exam with The Bryant Advantage!
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