Routing/Switching Query

[alienbob]

Hi,

I've got a setup where I have a ADSL router on a LAN as a default gateway and a Cisco router setup to provide point to point access over a serial link to another network (10.0.0.x).

There is a static route setup on the default gateway that points to the router for all traffic to the remote LAN. This all works hunkey dorey but a colleague and I are in disagreement if this is an efficient way to redirect the traffic.

At the moment a host on the local LAN will have a packet destined for the 10.0.0.x network, send this to the default gateway based on the fact that that it's a non local network and be told to look at the interface of the Cisco router, off it goes, happy days.

When the next packet is sent will it have to go through the same process?

Am I correct in thinking that the switch will have learnt that the remote host's MAC address is associated with the switches interface that the Cisco is connected too as a result of the initial redirect from the default gateway and just bypass the router?

Thanks

James

[lammle]

Default gatways are important to understand as they are a large Cisco CCNA objective, and based on your post they are correct in doing so.

Unfortantly, there probably is not one answer here, but from the router connected to the Internet I'd have this:

ip route 0.0.0.0 0.0.0.0 s0/0 (assuming s0/0 goes to the internet)
ip route 10.0.0.0 255.255.255.0 f0/0 (assuming f0/0 goes towards your inside networks)
ip classless

I would NOT put a default-route from your gateway to the inside network, you are just asking for loop issues, although it may apear that it is working just fine, and it may be, but it is not a good design.

The switch is irrelevant in this case and just a pass-through and would have no say in how these packets are routed or how they are passed between routers.

I wish I could answer this question better, but without more knowledge of your network and a little more detail, I can't provide a better answer.

Cheers!
Todd Lammle

[alienbob]

I've been mulling this over and I think that my conclusion that the switch would remember what is destined for the remote network is wrong, tried this is a simulator and it's not showing up in the mac-address table and also, all switches would start to build up huge tables of hardware addresses for devices which are on the internet on the interface that is connected to your network gateway! Not really practical and what you have said has confirmed that!

There is only a static route pointing from the default gateway to the 10 network so, if my interpretation your response is correct there shouldn't be any routing loops.

Knowing what I do about the network and how they work it (and what the "Cisco answer" would be ;)) it would be better to point the clients at the Cisco router, then set routes along the principles you have suggested as the majority of the traffic on the network is to and from the remote site, web traffic is only light.