very easy LAN question

[n.titchener]

Hi all,

I have a very easy question regarding using two fast ethernet interfaces across a single LAN. I'm sure I have got it right but just want to be absolutely positive before I leave this for others to implement.

I have five 3550-48 switches (switches 1 to 5) with a single full duplex link between each switch. Switches 1 and 5 connect to two fast ethernet interfaces on a 3640 router, each having a unique ip address in the same subnet. So the setup looks something like this

router1 int1--swi1--swi3-----------|

router1 int2--swi2--swi4--swi5---|

Note: Switch 3 is connected to switch 5

The aim of the design is to give full redundancy.

The router is running EIGRP and the switches will run RSTP

I'm sure this is OK but I just wanted to sound it out really.

The reason for this setup is provide two default gateways, one for devices on switches 2,4 and 5 and the other for devices on switches 1 and 3. This will reduce the number of hops that data has to traverse if only one default gateway was available.

This is a very simplistic design, the actual design is more complex with multiple routers, vlans and MHSRP running but it's the concept of using the two fast ethernet interfaces on a single router that I need comfirming.

Any comments welcome.

Thanks

[peter.mainwaring]

Have you managed to configure the 3640 in this way yet? I can see what you are trying to achieve, but I think you might have problems configuring the router with two interfaces in the same subnet.

Pete

[scottosan]

I agree that the 3640 having 2 interfaces on the same router in the same subnet both active at the same time will cause issues. I think a safer solution would be to make fa0/0 on the router active and use fa0/1 as a backup(using same subnet). Configure the fa0/0 with the following statement "backup interface FastEthernet0/1". I also think you should mesh your 3550's using spanning tree and have a topology similar to the diagram below. Just make sure to set the 3550 directly connected to the router as the root bridge

[n.titchener]

I thought that might be the case but I didn't want to say it. The backup command dosen't work on Fast ethernet interfaces. I'm trying to create a situation where, if the primary switch fails it dosen't cut the connection to the primary router and thus force the backup router to take over. I haven't included the full layout so you probably wont understand what I'm talking about.

Anyway thanks for your input.

[scottosan]

The backup interface command will work on the fast ethernet interfaces of a router, I am currently running this similar scenario and it has been tested in production. If the primary switch fails, either by having the physical link go down, or physical link being up but protocol down, it will fail over the the backup link because it is no longer receiving routing updates on the switch. In the later case, the failover will not be fast, because it has to wait to miss a routing update before it will fail over.