Routing table question?

rapyoke

Hey guys, I've been doing a combination of reading and test-taking in preparation for the CCNA, and I ran across this:



Could somebody tell me why the answer was 10.1.3.3? And possibly an explanation? Thanks in advance

martell1000

Router takes the most specific entry in his routing table.

The other two possible routes are considered as summerized routes because a subnet mask of /29 is more specific.

jmc012

rapyoke wrote: »

Hey guys, I've been doing a combination of reading and test-taking in preparation for the CCNA, and I ran across this:



Could somebody tell me why the answer was 10.1.3.3? And possibly an explanation? Thanks in advance

Here is a good description from Cisco's website;

Prefix Lengths
Let's look at another scenario to see how the router handles another common situation: varying prefix lengths.
Assume, again, that a router has four routing processes running on it, and each process has received these routes:
· EIGRP (internal): 192.168.32.0/26
· RIP: 192.168.32.0/24
· OSPF: 192.168.32.0/19
Which of these routes will be installed in the routing table? Since EIGRP internal routes have the best administrative distance, it's tempting to assume the first one will be installed. However, since each of these routes has a different prefix length (subnet mask), they're considered different destinations, and they will all be installed in the routing table.
Let's see how the forwarding engine uses the information from the routing table to make forwarding decisions.
Making Forwarding Decisions
Let's look at the three routes we just installed in the routing table, and see how they look on the router.
router# show ip route
....
D 192.168.32.0/26 [90/25789217] via 10.1.1.1
R 192.168.32.0/24 [120/4] via 10.1.1.2
O 192.168.32.0/19 [110/229840] via 10.1.1.3
....
If a packet arrives on a router interface destined for 192.168.32.1, which route would the router choose? It depends on the prefix length, or the number of bits set in the subnet mask. Longer prefixes are always preferred over shorter ones when forwarding a packet.
In this case, a packet destined to 192.168.32.1 is directed toward 10.1.1.1, because 192.168.32.1 falls within
the 192.168.32.0/26 network (192.168.32.0 to 192.168.32.63). It also falls within the other two routes available, but the 192.168.32.0/26 has the longest prefix within the routing table (26 bits verses 24 or 19 bits).
Likewise, if a packet destined for 192.168.32.100 arrives on one of the router's interfaces, it's forwarded to 10.1.1.2, because 192.168.32.100 doesn't fall within 192.168.32.0/26 (192.168.32.0 through 192.168.32.63),
but it does fall within the 192.168.32.0/24 destination (192.168.32.0 through 192.168.32.255). Again, it also falls into the range covered by 192.168.32.0/19, but 192.168.32.0/24 has a longer prefix length.

cisco_trooper

Most specific route will always win. /29 is the most specific.

MrXpert

Very helpful posts
I learnt a lot from it as I wasn't understanding the concept of longest prefix length until now. Thanks