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03.08.06
Cisco CCNA Exam Tutorial
 By
Chris Bryant
Preparing to pass the CCNA exam and earn this important Cisco
certification? Route summarization is just one of the many skills you'll have
to master in order to earn your CCNA. Whether it's RIP version 2, OSPF, or EIGRP,
the CCNA exam will demand that you can flawlessly configure route summarization.
Route summarization isn't just important for the CCNA exam. It's a valuable skill
to have in the real world as well. Correctly summarizing routes can lead to smaller
routing tables that are still able to route packets accurately - what I like to
call "concise and complete" routing tables.
The first skill you've got to have in order to work with route summarization is
binary math; more specifically, you must be able to take multiple routes and come
up with both a summary route and mask to advertise to downstream routers. Given
the networks 100.16.0.0 /16, 100.17.0.0 /16, 100.18.0.0 /16, and 100.19.0.0 /16,
could you quickly come up with both the summary address and mask? All you need
to do is break the four network numbers down into binary strings. We know the
last two octets will all convert to the binary string 00000000, so in this article
we'll only illustrate how to convert the first and second octet from decimal to
binary.
100 16 = 01100100 00010000
100 17 = 01100100 00010001
100 18 = 01100100 00010010
100 19 = 01100100 00010011
 |
To come up with the summary route, just work from left to right
and draw a line where the four networks no longer have a bit in common. For these
four networks, that point comes between the 14th and 15th bits. This leaves us
with this string: 01100100 000100xx. All you need to do is convert that string
back to decimal, which gives us 100 for the first octet and 16 for the second.
(The two x values are bits on the right side of the line, which aren't used in
calculating the summary route.) Since we know that zero is the value for the last
two octets, the resulting summary network number is 100.16.0.0.
But we're not done! We now have to come up with the
summary mask to advertise along with the summary route. To arrive at the summary
route, write out a mask in binary with a "1" for every bit to the left of the
line we drew previously, and a "0" for every bit to the right. That gives us the
following string:
11111111 11111100 00000000 00000000
Converting that to dotted decimal, we arrive at the summary mask 255.252.0.0.
The correct summary network and mask to advertise are 100.16.0.0 252.0.0.0.
For the CCNA exam, emphasis is put on knowing how to advertise these summary routes
in RIPv2 and EIGRP. For both of these protocols, route summarization happens at
the interface level - it's not configured under the protocol. On the interface
that should advertise the summary route, use the command "ip summary-address".
Here are examples of how the above summary route would be configured on ethernet0
in both RIPv2 and EIGRP.
Read
the Full Article
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage
(www.thebryantadvantage.com),
home of FREE CCNA and CCNP tutorials and daily exam questions, as well as The
Ultimate CCNA and CCNP Study Packages. |
