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Overlapping Routes Question - Need help!

ChickenNuggetz

Hey guys, I'm stuck with this particular question and I'm not entirely sure how to work out the answer. Here's the question:

Predict which route this router would match for packets destined to the following IP addresses:
172.16.1.1
172.16.1.2
172.16.2.2
172.16.4.3

Here's the show ip route command output

172.16.0.0/16 is variably subnetted, 5 subnets, 4 masks
R 172.16.1.1/32 [120/1] via 172.16.25.2, 00:00:04, Serial0/1/1
R 172.16.1.0/24 [120/2] via 172.16.25.129, 00:00:09, Serial0/1/0
R 172.16.0.0/22 [120/1] via 172.16.25.2, 00:00:04, Serial0/1/1
R 172.16.1.0/16 [120/2] via 172.16.25.129, 00:00:09, Serial0/1/0
R 0.0.0.0/0 [120/3] via 172.16.25.129, 00:00:09, Serial0/1/0

Any advice/tips?

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Comments

Forsaken_GA

Longest match on the prefix wins.

ie, /32 routes will be taken over /24's which will be taken over /16's.

So look at the ip, figure out which routes in the routing table it matches. If it can match more than one, look to see who has the longest prefix. That's the route it will take.

SubnetZero

Forsaken_GA wrote: »

Longest match on the prefix wins.
ie, /32 routes will be taken over /24's which will be taken over /16's.

Agreed. More specific routing (longest prefix match) always wins. In his example the following is true:

A packet destined to 172.16.1.1 is forwarded to 172.16.1.1 (longest prefix match)
A packet destined to 172.16.1.2 is forwarded to 172.16.25.129 (longest prefix match)
A packet destined to 172.16.2.2 is forwarded to 172.16.25.2 (longest prefix match)
A packet destined to 172.16.4.3 is forwarded to 172.16.25.129 (longest prefix match)

Had the /16 not existed the 172.16.4.2 would have been forwarded using the default (0/0) route which still would have been 172.16.25.129.

ChickenNuggetz

Forsaken_GA wrote: »

Longest match on the prefix wins.

ie, /32 routes will be taken over /24's which will be taken over /16's.

So look at the ip, figure out which routes in the routing table it matches. If it can match more than one, look to see who has the longest prefix. That's the route it will take.

Oh ok I think I got it now! I wasnt realizing I needed to figure out which of subnets each IP address fits into (derp derp).

So I think the answer should be:
172.16.1.1 = 172.16.1.1/32 (all five routes are applicable, but this one is the best)
172.16.1.2 = 172.16.1.0/24 (bottom 4 routes are applicable, but this one is the best)
172.16.2.2 = 172.16.0.0/22 (bottom 3 routes are applicable, but this one is the best)
172.16.4.3 = 172.16.1.0/16 (bottom 2 routes are applicable, but this one is the best)

Does this sound right?

Forsaken_GA

ChickenNuggetz wrote: »

172.16.4.3 = 172.16.1.0/16 (bottom 2 routes are applicable, but this one is the best)

Does this sound right?

Actually, I think that ones a typo, as 172.16.1.0/16 isn't a valid prefix, that should be 172.16.0.0/16.

But otherwise, yes, you're correct. If an IP can match multiple routes, then the prefix length is the tiebreaker, and longest match always wins.

4_lom

Forsaken_GA wrote: »

Longest match on the prefix wins.

ie, /32 routes will be taken over /24's which will be taken over /16's.

So look at the ip, figure out which routes in the routing table it matches. If it can match more than one, look to see who has the longest prefix. That's the route it will take.

'nough said.

ChickenNuggetz

Forsaken_GA wrote: »

Actually, I think that ones a typo, as 172.16.1.0/16 isn't a valid prefix, that should be 172.16.0.0/16.

But otherwise, yes, you're correct. If an IP can match multiple routes, then the prefix length is the tiebreaker, and longest match always wins.

Haha, yeah that was a typo, thanks for the help!