Establishing static IP routing

[Dwight Stephenson]

We have a 4108gl switch acting as our backbone. Right now we're using 2 IP subnets in our network; one for admin stuff and one for process stuff. The admin IP subnet is 10.36.12.0/23 and the other is 10.36.58.0/24. Right now our main CISCO 3600 router is handling the layer 3 routing to keep the traffic flowing between these two subnets. We're going to have the router down for replacement in a few days and I'd like to not hinder operations. So is it possible for me to establish some internal layer 3 IP routing on the 4108 and keep that job off the router? I've never done this... so I need some good info.

[Terhorst]

Hi Dwight,

See: http://www.hp.com/rnd/support/manuals/4108gl.htm

There is one chapter about routing. Should not be a problem I suppose.

Regards,
Alexander

[Dwight Stephenson]

Yes, I've seen the manual. But it is not very clear on how to accomplish my situation. I was looking for info from someone who might have done this before. I have called HP and they gave me instructions on how to accomplish my goal. But I'd still be interested in hearing other ideas.

[Kell van Daal]

I assume you have both subnets in different VLAN's on the switch.
Enabling routing is as easy as:
switch(config)#ip routing

The VLAN's need to have ip-addresses assigned to them.

Things to consider:
Your clients and servers have default gateways assigned. I assume they have the Cisco 3600 as the default gateway (for both subnets).
Only enabling ip routing on the switch will not be enough to establish communications between the two subnets (from the clients perspective).
You can do two things:
- Assign the VLAN's on the switch unused ip-addresses, and on the day change the default gateway of all nodes (clients/servers/printers etc).
- Right after the 3600 is disconnected from the network, assign the VLAN's the ip-addresses that 3600 had, and enable routing.

I would choose for the second option. The first option is a lot of work (changing DHCP option for the clients isn't too much work, manually changing the def gw on the servers and printers is. (and changing it back again after maintenance)).
If you do the second option, and some nodes aren't working immediately, clear their arp-cache (arp -d *) or reboot them.

A final note:
If you use the 3600 only for connecting the subnets, you can consider using the switch for a router as a permanent solution (not bringing back the 3600). This has two distinct advantages:
- The performance is better.
- One less single point of failure