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Easy Question of the Day???

mattsthe2

If we have 1 network using 10.0.0.0/255.255.0.0 and another using 10.0.11.0/255.255.255.0, is
this going to cause a routing problem?

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Comments

Claymoore

Nope, as long as they aren't connected to each other

mattsthe2

so basically thats a no.

xwesleyxwillisx

The only way you would run into an issue is if you are using a classful routing protocol i. e. RIPv1. If you were, RIP would see both networks as 10.0.0.0 /8 and cause a routing issue.

Claymoore

Sorry, but my humor can be an acquired taste.

Assuming that the routes have an equal administrative cost (both are static or both were learned by the same routing protocol) the router will choose the route based on the most specific subnet mask and you won't have a problem. Routes learned by different routing protocols can complicate things though.

However, if the two networks are joined together and a host from each is trying to talk to the other you will have a problem. Host A on the 10.0.0.0/16 network will think Host B (on the 10.0.11.0 /24 network) is on the same network and won't send packets destined to Host B to the router interface. That's what I meant by 'as long as they aren't connected'.

xwesleyxwillisx

Holy crap I just realized they are 2 different subnet masks.

What I said still applies, but you will definitely have issues since the 10.0.11.0/24 subnet is part of the supernet 10.0.0.0/16.

mattsthe2

Claymore. They would have IP connectivity and we run EIGRP. They are not directly connected.

So i guess we're ok

mattsthe2

xwesleyxwillisx - So it wont work ?

Bit of a conflicting answer from you both.

lol.

dtlokee

Since they' different prefixes they will both end up in the routing table, anything that has a 10.0.x.x will be sent via the 10.0.0.0/16 route and anything with the 10.0.11.x will be sent via the 10.0.11.0 route. Remember the router will use longest match to determine what route to use.

so 10.0.1.1 will be routed vai 10.0.0.0/16
10.0.2.1 will be routed via 10.0.0.0/16
.
.
.
10.0.10.1 will be routed via 10.0.0.0/16
10.0.11.1 will be routed via 10.0.11.0/24
10.0.12.1 will be routed via 10.0.0.0/16
.
.
.
.
.
10.0.255.1 will be routed via 10.0.0.0/16

mikej412

dtlokee wrote:

Remember the router will use longest match to determine what route to use.

+1

And remember to use the no auto-summary command under the eigrp router process on any intervening routers that may need to advertise that more specific route.

mattsthe2

DT - cool.

As long as the 10.0.0.0/16 network never uses the 10.0.11.0/24 network we are good.