Overlapping subnets and routing protocols

Magic Johnson

Oh how I love these questions (Boson, this time).

I remembered from the CISCO Press horrific question to check on connectivity problems if there is an overlap in the VLSM internetwork, and there is, but only in the router's routing table.

A LAN host has an IP of 200.120.45.180 /27
The router LAN int has an IP of 200.120.45.161 /27

So that network is 200.120.45.160-191

Ping does not work between the host and the LAN int of the router.

The router has a EIGRP route to 200.120.45.176 /28 out of a Serial interface, because of an overlapping 176-191 network. Why does it put this route in because it is already 'summarised' with the 160 /27 route that is directly connected?



So I guess my question is why is the router preferencing this route when EIGRP is AD 90 and directly connected is 0? Is it because the more specific one is matched? Damn, I thought the larger route is preferred!

EDIT: Here is the answer to this part of the lab:

A ping to a directly connected host could fail if an erroneous route in the routing table diverts the traffic to an invalid destination, where the traffic is ultimately dropped.

I got this correct, but I don't understand why, I only got it correct because that is the only thing I could logically come to, but that totally isn't satisfying at all.

fredrikjj

When the router is attempting to return traffic to 200.120.45.180 it matches the /28 route to the serial interface, and not the directly connected /27.

So I guess my question is why is the router preferencing this route when EIGRP is AD 90 and directly connected is 0?

Because AD is only compared when there is an exact match of prefix length. In this case, the .176/28 network is a "longer match" than the 160/27 network. Longer in this case refers to the fact that the network portion of the address contain more bits.

The order of the logic is:

Prefix-length
If the same prefix-length, then AD
If the same AD, then Metric

Magic Johnson

fredrikjj wrote: »

When the router is attempting to return traffic to 200.120.45.180 it matches the /28 route to the serial interface, and not the directly connected /27.



Because AD is only compared when there is an exact match of prefix length. In this case, the .176/28 network is a "longer match" than the 160/27 network. Longer in this case refers to the fact that the network portion of the address contain more bits.

The order of the logic is:

Prefix-length
If the same prefix-length, then AD
If the same AD, then Metric

That's a superb summary. Thanks very much, repped. I don't remember this in the books! Or maybe I've just forgotten...!

Magic Johnson

Also it is interesting to note that whilst the host cannot ping its own default gateway, it can ping other devices on the internetwork.

That could be a mind-blown for someone who doesn't have access to their managed network!