Routing Q - Diff. bt routing out an interface and routing to an ip address

[andysaykao]

This is a bit of a weird one.

Scenario...

subnet 1: 203.x.x.56/29

subnet 2: 210.x.x.80/29

203.x.x.57 is on the Service Provider's end.

203.x.x.58 is customer's WAN IP.

We route subnet 2 down subnet 1.

Customer has two subnets with us. I work for a service provider and when we route the customer's second subnet out their respective ATM sub-interface, the can't establish data flow to the machines on the second subnet.

Eg:

#sh ip route | inc 210.x.x.80

S 210.x.x.80/29 is directly connected, ATM1/0.xxxxxx

However, when we instead route the second subnet directly to the WAN IP of the first subnet, data flow to the second subnet is established.

Eg:

#sh ip route | inc 210.x.x.80

S 210.x.x.80/29 [1/0] via 203.x.x.58

Why might one way to add a static route work and another not work? And what is the subtle difference between routing the second subnet out the ATM sub-interface as opposed to routing it directly to the customers WAN IP of their first subnet?

Thanks in advance.

[Richard Burts]

Andy

The subtle difference here has to do with mapping layer 3 addresses to layer 2 addresses. When you configure a static route which specifies the next hop address it is easy to determine which layer 2 address (which virtual circuit in the case of ATM) to use and the static route works. When you configure the static route which specifies the outbound interface the router needs to determine which layer 2 address to use and there is not a way to resolve this, so the static route fails. If you were to run debug ip packet for this situation I am pretty sure that you will see an error message about encapsulation failed because the router could not determine the proper layer 2 address to use.

HTH

Rick

[andysaykao]

Rick,

Thank you for your reply.

So...is this more of an issue on the Service Provider's end or is it an issue with the end user's cpe?

And would best practice be to route the second subnet to the end user ip or to the outbound interface?

F.Y.I, we've found through trial and error that if one way doesn't work, the other way will work, hence my confusion as to why one way might work and the other not work, and is this an issue the Service Provider needs to address or does the problem arise depending on what type of cpe the end user is using?

Cheers.

Andy

[Richard Burts]

Andy

This is an issue on the router which configures the static route. So in your frame of reference it is the cpe and not an issue for the Service Provider. (If the Service Provider is configuring static routes to direct traffic to you, then they also face this issue. But it is independent of what you configure.)

If you want a general approach which should always work, then I suggest routing to the next hop address. That should always work. In situations where the outbound interface is a true point to point connection, then routing to the outbound interface should work well. In situations where the outbound inteface is multipoint (or potentially multipoint) such as LAN interfaces, Frame Relay or ATM, then routing to the outbound interface may work or may not work depending on how mapping of layer 3 to layer 2 addresses can be done. (In the case of a LAN interface if the next hop router supports proxy ARP then routing to the outbound interface should work, but if it does not do proxy ARP then routing to the outbound interface will not work. So in this case you would be dependent of how the Service Provider is configured.)

So routing to the next hop address should always work. Routing to the outbound interface may or may not work.

HTH

Rick

[andysaykao]

Rick.

Thank you for your clear and precise explaination.

Cheers.

Andy

[Richard Burts]

Andy

I am glad that my explanations helped. Thanks for rating the posts.

I encourage you to continue active participation in the NetPro forums.

HTH

Rick