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Cisco CCNP / BSCI Exam Tutorial: Route Summarization |
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Preparing to pass the BSCI exam and earn your Cisco CCNP?
Route summarization is just one of the many skills you'll
have to master in order to earn your CCNP. Whether it's RIP
version 2, OSPF, or EIGRP, the BSCI exam will demand that
you can flawlessly configure route summarization.
Route summarization isn't just important for the BSCI 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 BSCI exam, emphasis is put on knowing how to advertise
these summary routes in RIPv2, EIGRP, and OSPF. For RIP v2
and EIGRP, 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.
R1(config-if)#ip summary-address rip 100.16.0.0 255.252.0.0
R1(config-if)#ip summary-address eigrp 100 100.16.0.0 255.252.0.0
The main difference between the two is that the EIGRP command
must specify the AS number - that's what the "100"
is in the middle of the EIGRP command. Since RIPv2 does not
use AS numbers, there's no additional value needed in the
configuration.
For OSPF, the commands differ. If you're configuring inter-area
route summarization, use the "area range" command.
The number following "area" is the area containing
the routes being summarized, not the area receiving the summary.
R1(config)#router ospf 1
R1(config-router)#area 1 range 100.16.0.0 255.252.0.0
If you are summarizing routes that are being redistributed
into OSPF, use the summary-address command under the OSPF
routing process on the ASBR.
R1(config)#router ospf 1
R1(config-router)#summary-address 100.16.0.0 255.252.0.0
I speak from experience when I tell you that practice makes
perfect on the BSCI exam, especially with binary and summarization
questions. The great thing about these questions is that there
are no grey areas with these questions - you either know how
to do it or you don't. And with practice and an eye for detail,
you can master these skills, pass the exam, and become a CCNP.
Here's to your success on these tough Cisco certification
exams!
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.
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!
Pass the CCNP exam with The Bryant Advantage!
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