Cisco CCNA Exam Tutorial: Route Summarization

Preparing to pass the CCNA exam and earn thisthe summary route. To arrive at the summary
important Cisco certification? Route summarizationroute, write out a mask in binary with a "1" for
is just one of the many skills you'll have toevery bit to the left of the line we drew
master in order to earn your CCNA. Whether it'spreviously, and a "0" for every bit to the right.
RIP version 2, OSPF, or EIGRP, the CCNA examThat gives us the following string:
will demand that you can flawlessly configure11111111 11111100 00000000 00000000
route summarization.Converting that to dotted decimal, we arrive at
Route summarization isn't just important for thethe summary mask 255.252.0.0. The correct
CCNA exam. It's a valuable skill to have in the realsummary network and mask to advertise are
world as well. Correctly summarizing routes can100.16.0.0 252.0.0.0.
lead to smaller routing tables that are still able toFor the CCNA exam, emphasis is put on knowing
route packets accurately - what I like to callhow to advertise these summary routes in RIPv2
"concise and complete" routing tables.and EIGRP. For both of these protocols, route
The first skill you've got to have in order to worksummarization happens at the interface level - it's
with route summarization is binary math; morenot configured under the protocol. On the
specifically, you must be able to take multipleinterface that should advertise the summary
routes and come up with both a summary routeroute, use the command "ip summary-address".
and mask to advertise to downstream routers.Here are examples of how the above summary
Given the networks 100.16.0.0 /16, 100.17.0.0 /16,route would be configured on ethernet0 in both
100.18.0.0 /16, and 100.19.0.0 /16, could you quicklyRIPv2 and EIGRP.
come up with both the summary address andR1(config-if)#ip summary-address rip 100.16.0.0
mask? All you need to do is break the four255.252.0.0
network numbers down into binary strings. WeR1(config-if)#ip summary-address eigrp 100
know the last two octets will all convert to the100.16.0.0 255.252.0.0
binary string 00000000, so in this article we'll onlyThe main difference between the two is that the
illustrate how to convert the first and secondEIGRP command must specify the AS number -
octet from decimal to binary.that's what the "100" is in the middle of the
100 16 = 01100100 00010000EIGRP command. Since RIPv2 does not use AS
100 17 = 01100100 00010001numbers, there's no additional value needed in the
100 18 = 01100100 00010010configuration.
100 19 = 01100100 00010011For OSPF, the commands differ. If you're
To come up with the summary route, just workconfiguring inter-area route summarization, use
from left to right and draw a line where the fourthe "area range" command; if you are
networks no longer have a bit in common. Forsummarizing routes that are being redistributed
these four networks, that point comes betweeninto OSPF, use the summary-address command
the 14th and 15th bits. This leaves us with thisunder the OSPF routing process on the ASBR.
string: 01100100 000100xx. All you need to do isNeither of these are interface-level commands.
convert that string back to decimal, which givesI speak from experience when I tell you that
us 100 for the first octet and 16 for the second.practice makes perfect on the CCNA exam,
(The two x values are bits on the right side ofespecially with binary and summarization questions.
the line, which aren't used in calculating theThe great thing about these questions is that
summary route.) Since we know that zero is thethere are no grey areas with these questions -
value for the last two octets, the resultingyou either know how to do it or you don't. And
summary network number is 100.16.0.0.with practice and an eye for detail, you can
But we're not done! We now have to come upmaster these skills, pass the exam, and become
with the summary mask to advertise along witha CCNA. Here's to your success!