Router config using L3 switches

colink24

Chaps

I have a question that for some reason I cant seem to get my head around

Assuming I have am a SMB with, say, 5 vlans setup with SVI's using say 3560's. Now obviously all inter-vlan routing will take place inside my switches. But what would my config look like on my router, assuming I need to route my vlans out to the web/WAN? Do I still need to create my subinterfaces in the same way I would for router-on-a-stick?

Forsaken_GA

No, you just need to put on ip routing and configure the SVI's as gateways. That will cause the switch to see the SVI's as directly connected interfaces and puts them in the routing table, so it can do all of it's intervlan routing that way.

For routing outside, it all depends on how you want to do it. You can pick a port and issues the no switchport command to it, that turns it into a layer 3 port, and you treat it exactly as you would a router interface.

My setup is like this -

Internet --- 2611XM --- 3550

The cable modem is hooked to one interface of the 2611XM, the 3550 is hooked to the other. The 3550 has 5 SVI's on it, one vlan which is my management vlan, which has one port on it. That port is hooked to the 2611XM, and I run opsf between the two (the 2611 is generating my default route and passing it, my switch is redistributing connected), The other 4 SVI's are for my home network (wired host access, wireless host access, server access, and wireless guest access), and I drop ports into the vlan as necessary if i need more ports.

I chose to use an SVI for my ospf interface because I can easily drop another router into the routing domain by just putting another port in that vlan and then configuring ospf on the new router (it's essentially treating it as a broadcast medium instead of point to point). This lets me get easy access to my tftp server for stuff like backups and image upgrades and such

Forsaken_GA

Actually, I think I may have misread your question. Easiest way is to just bring up an IGP between your switches so that they're sharing routes, then introduce the router into it as well and have it propagate default. Hooking a bunch of layer 3 switches together is no different than hooking a bunch of routers together (within reason, don't try to run a full bgp table on a 3560!), you just have much bigger port density

colink24

Thanks for the response, I suspected as much. The reason I ask is that we are doing a new office implementation at work and my colleague is insistent that our ISP (managed service) will need to configure sub interffaces on the router.

I figure the config to be

5 vlans with SVI's created on the L3 switches, each of the hosts in the vlans having their gateway pointing to the vlan interface address

an ip default gateway specified on the switch pointing to our ISP router FA0/x interface

We checked out old router config and the ISP does have sub interfaces setup on the router for our vlans at offices where we use L3 switches with SVI's.

Could it be that the hosts were pointing their default gateway to the subinterface address, and therefore the router was handling the routing of the intervlan traffic?

Forsaken_GA

colink24 wrote: »

Thanks for the response, I suspected as much. The reason I ask is that we are doing a new office implementation at work and my colleague is insistent that our ISP (managed service) will need to configure sub interffaces on the router.

I figure the config to be

5 vlans with SVI's created on the L3 switches, each of the hosts in the vlans having their gateway pointing to the vlan interface address

an ip default gateway specified on the switch pointing to our ISP router FA0/x interface

We checked out old router config and the ISP does have sub interfaces setup on the router for our vlans at offices where we use L3 switches with SVI's.

Could it be that the hosts were pointing their default gateway to the subinterface address, and therefore the router was handling the routing of the intervlan traffic?

Sure, it's possible to trunk up to the router and use it's subinterfaces as gateways. The only thing that's really required to use a host as a gateway is ip connectivity, what the host with the gateway IP does with the packet afterwards is up to whoever configured it.

I think that's kind of retarded though. I'd put in a call to the provider and see if they're willing to be a little more sane, or at least give you a good reason for doing it like that. Personally, the only thing I want a provider to do is give me a drop, and my own gateway into their network, and I'll handle the rest.

colink24

Forsaken_GA wrote: »

Sure, it's possible to trunk up to the router and use it's subinterfaces as gateways. The only thing that's really required to use a host as a gateway is ip connectivity, what the host with the gateway IP does with the packet afterwards is up to whoever configured it.

I think that's kind of retarded though. I'd put in a call to the provider and see if they're willing to be a little more sane, or at least give you a good reason for doing it like that. Personally, the only thing I want a provider to do is give me a drop, and my own gateway into their network, and I'll handle the rest.

If this was the case, then I could essentially just create a trunk between my interface connecting to the router, create the vlans with (no SVI config) and im done (once the subinterfaces are created on the router). This is essentially my setup now, except a whole bunch of svi's are created and not used. I just dont see the need to create sub interfaces on a router when intervlan routing is/can being handled by the L3 switches!

I will speak to the ISP too - I guess for all they know we are not using L3 switches