Route summarization problem.

I've been working through the CCNA exam cram third edition its been a great read for the most part and I'm almost confident enough to take my exam. However I am struggling with one of the examples in the book it just simply does not make sense, if anyone can help explaining it I would much appreciate it:

The longest prefix match is the best

Simplified IP route table
172.16.8.0/24 via 192.168.0.1, Ethernet 0
172.16.10.0/30 via 192.168.0.1, Ethernet 0
172.16.10.64/26 via 192.168.1.1, Ethernet 1
172.16.10.0/24 via 192.168.2.1, Ethernet 2
0.0.0.0/0 via 24.16.5.65, Serial 0

Assume a packet has arrived at router with destination IP 172.16.10.131.

Would it be possible for someone to give me a detailed explanation on the right route and why the other summarised routes are not correect. I have tried to work this out with the example in the book and it just doesn't match up. Thanks oin advance.

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Comments

0Day

Route Selection in Cisco Routers - Cisco Systems

I found this at Cisco site, the section, "Making Forwarding Decisions"

Is what you need, I think...

Oh and what answer are you getting for this question?

Camster187

0Day wrote: »

Route Selection in Cisco Routers - Cisco Systems

I found this at Cisco site, the section, "Making Forwarding Decisions"

Is what you need, I think...

Oh and what answer are you getting for this question?

Thanks for the link the example on their regarding forwarding decisions makes perfect sense, however here is what the book says regarding my original question:

The second,third and fourth all match for their respective prefix length of /28,/26 and /24. The fifth entry is a default route that matches any address but with a /0 prefix.

All entries are valid but the one with the longest prefix length match - 172.16.10.128/28 is chosen as best. The router switches the packet out of ethernet 0.

Apparently it the packet should be leaving ethernet 0 from a /28 network? I simply cannot work out how. I don't think it is leaving from the 172.16.8.0/24 address because that is not a summarised network (i think). However on working out the 172.16.10.0/30 I can only find the addresses 172.16.10.1 - 172.16.10.4.

tech-airman

Camster187 wrote: »

I've been working through the CCNA exam cram third edition its been a great read for the most part and I'm almost confident enough to take my exam. However I am struggling with one of the examples in the book it just simply does not make sense, if anyone can help explaining it I would much appreciate it:

The longest prefix match is the best

Simplified IP route table
172.16.8.0/24 via 192.168.0.1, Ethernet 0
172.16.10.0/30 via 192.168.0.1, Ethernet 0
172.16.10.64/26 via 192.168.1.1, Ethernet 1
172.16.10.0/24 via 192.168.2.1, Ethernet 2
0.0.0.0/0 via 24.16.5.65, Serial 0

Assume a packet has arrived at router with destination IP 172.16.10.131.

Would it be possible for someone to give me a detailed explanation on the right route and why the other summarised routes are not correect. I have tried to work this out with the example in the book and it just doesn't match up. Thanks oin advance.

Camster187,

For the destination IP address above, what class is the IP address?

bermovick

Camster187 wrote: »

Thanks for the link the example on their regarding forwarding decisions makes perfect sense, however here is what the book says regarding my original question:

The second,third and fourth all match for their respective prefix length of /28,/26 and /24. The fifth entry is a default route that matches any address but with a /0 prefix.

All entries are valid but the one with the longest prefix length match - 172.16.10.128/28 is chosen as best. The router switches the packet out of ethernet 0.

Apparently it the packet should be leaving ethernet 0 from a /28 network? I simply cannot work out how. I don't think it is leaving from the 172.16.8.0/24 address because that is not a summarised network (i think). However on working out the 172.16.10.0/30 I can only find the addresses 172.16.10.1 - 172.16.10.4.

Can you double-check this? Your original question didn't have a /28 prefix length, but the answer lists it as a match. Perhaps it's a typo in the book or I'm missing something, but that looks strange! Also I'm a bit confused, as in the original problem, only 1 route (and the default route) even match at all. Route summarization was never my strongest point, but the KISS rule can apply here.

chmorin

I'm going to break this down...

Router gets a packet with a destination IP address of 172.16.10.131. The router checks its routing table for matching routes.

From a routing table of

172.16.8.0/24 via 192.168.0.1, Ethernet 0
172.16.10.0/30 via 192.168.0.1, Ethernet 0
172.16.10.64/26 via 192.168.1.1, Ethernet 1
172.16.10.0/24 via 192.168.2.1, Ethernet 2

only

172.16.10.0/24 via 192.168.2.1, Ethernet 2

is a matching route for that host address.

172.16.8.0/24 has a host range of 8.1-8.254 which does not match 10.131.
172.16.10.0/30 has a host range of 10.1-10.2 which does not match 10.131
172.16.10.64/26 has a host range of 10.65-127 which does not match 10.131.

172.16.10.0/24 has a host range of 10.1-10.255 which does match 10.131.

That being said, you contradicted yourself in the proper solution.

If 172.16.10.128/28 was part of the routing table, it was a range of 10.129-142, which does match 10.131 and would be a better pick than the 172.16.10.0/24 match on the basis of longer prefix length.

Please create a situation that does not involve contradiction if you want a better answer than that.

Camster187

chmorin wrote: »
I'm going to break this down...

Router gets a packet with a destination IP address of 172.16.10.131. The router checks its routing table for matching routes.

From a routing table of
172.16.8.0/24 via 192.168.0.1, Ethernet 0
172.16.10.0/30 via 192.168.0.1, Ethernet 0
172.16.10.64/26 via 192.168.1.1, Ethernet 1
172.16.10.0/24 via 192.168.2.1, Ethernet 2
only
172.16.10.0/24 via 192.168.2.1, Ethernet 2
is a matching route for that host address.

172.16.8.0/24 has a host range of 8.1-8.254 which does not match 10.131.
172.16.10.0/30 has a host range of 10.1-10.2 which does not match 10.131
172.16.10.64/26 has a host range of 10.65-127 which does not match 10.131.

172.16.10.0/24 has a host range of 10.1-10.255 which does match 10.131.

That being said, you contradicted yourself in the proper solution.

If 172.16.10.128/28 was part of the routing table, it was a range of 10.129-142, which does match 10.131 and would be a better pick than the 172.16.10.0/24 match on the basis of longer prefix length.

Please create a situation that does not involve contradiction if you want a better answer than that.

I took this solution directly from the book, so as has been previously said(and what was confusing me) it was probably a typo in the book itself which threw me off. Thanks for the breakdown and alkl input.