InterVLAN Routing and Multiple Switches

Thank you. I'll give this a shot tomorrow and I'll be sure to post my results. This was and is still a major learning experience for me, and I'd like it to be out there and documented for others.

Much better... you need a layer 2 trunk between the two switches. The traffic needs to be switched at layer 2 and if you don't have a trunk, that traffic isn't gonna get to the other switch.


switchport mode trunk
switchport trunk allow vlan <vlanid> {or you can omit this, by default it allows all VLANs defined to span the trunk}

Today, I tried setting up a trunk link between the 3550 and the 4507R switch.

I set up dot1q as the encapsulation, and allowed both VLAN100 and VLAN900.

I did this on both ends of the link, and connected the two switches with a crossover cable.

If I have a DHCP server on VLAN 100 on the 4507R, and I want a PC on VLAN900 of the 3550 (which is connected over a trunk link) to be able to pull an address from it, what needs to be done? Is this where an ip helper address is going to come into play? If so, how does that work?

Also important to note, I have VLAN100 and VLAN900 set to route between each other on the 3550... That means that VLAN100 has an IP address, as does VLAN900... How does this affect DHCP (or does it at all)?

Something else peculiar that I noticed was that I couldn't get ip routing to "stick" on the 4507R, when I tried to enable it. If I typed in no ip routing, it would show in the running config, but if I typed ip routing, nothing showed up in the running config at all... In other words, I would imagine that if I typed "ip routing" in at the configure terminal prompt, it would then "stick" and show up somewhere at the top of the running-config when a "sh run" command is issued. However, this didn't happen.

Thanks again for all of your help.

I believe that 4507R does layer 3 by default so you don't need to tell it. You need DHCP relay agent to point to the IP address of the DHCP server. DHCP relay is unicast so you have to have a layer 3 route to the DHCP server. You have to have a scope for the subnetwork that you want to assign IPs out. When the DHCP server gets the request, it will match the source address to a scope, if it doesn't have a scope for that source address, then your workstations won't get an IP.

Today, I tried setting up a trunk link between the 3550 and the 4507R switch.

I set up dot1q as the encapsulation, and allowed both VLAN100 and VLAN900.

I did this on both ends of the link, and connected the two switches with a crossover cable.

If I have a DHCP server on VLAN 100 on the 4507R, and I want a PC on VLAN900 of the 3550 (which is connected over a trunk link) to be able to pull an address from it, what needs to be done? Is this where an ip helper address is going to come into play? If so, how does that work?

Also important to note, I have VLAN100 and VLAN900 set to route between each other on the 3550... That means that VLAN100 has an IP address, as does VLAN900... How does this affect DHCP (or does it at all)?

Something else peculiar that I noticed was that I couldn't get ip routing to "stick" on the 4507R, when I tried to enable it. If I typed in no ip routing, it would show in the running config, but if I typed ip routing, nothing showed up in the running config at all... In other words, I would imagine that if I typed "ip routing" in at the configure terminal prompt, it would then "stick" and show up somewhere at the top of the running-config when a "sh run" command is issued. However, this didn't happen.

Thanks again for all of your help.

I believe that 4507R does layer 3 by default so you don't need to tell it.

I have tried to ping from hosts on VLAN100 to hosts on VLAN900 and I get no response. I shouldn't get a response, because by definition, I'm using VLANs to separate the switch into two virtual LANs. The question is now, how do I route between these LANs using only the 4507R? (No router on a stick possible a)because it's extra hardware, b) because this functionality should already exist somewhere/somehow in the 4507R. I'm at a loss here, but this is critical to the task at hand.

Okay...I was wrong... the 4500 series aren't layer 3 by default. Once you assign an ip address to vlan 900 and vlan 100 on the 4507 and 3550 they should be able to reach each other without any additional routing because they are directly connected interfaces and your trunk between the two switch will switch the traffic.

I believe that 4507R does layer 3 by default so you don't need to tell it.

I have tried to ping from hosts on VLAN100 to hosts on VLAN900 and I get no response. I shouldn't get a response, because by definition, I'm using VLANs to separate the switch into two virtual LANs. The question is now, how do I route between these LANs using only the 4507R? (No router on a stick possible a)because it's extra hardware, b) because this functionality should already exist somewhere/somehow in the 4507R. I'm at a loss here, but this is critical to the task at hand.

Okay, let me make sure I'm clear... You're saying that the routing would be offloaded to the 3550 if someone on VLAN100 on the 4507R sent something to someone on VLAN900 on the 4507R? So it would go from person A on 100 to the interface they're plugged into, over the trunk link, get routed over to VLAN900 on the 3550, go back over the trunk link, into the 4507R again, and finally out the interface and to the destination? I guess the next question is, this happens automatically? How does the 4507R know to use the 3550 as a router on a stick? (Though I think I just answered my own question... the 3550 becomes a router on a stick as well as a switch that stuff can get plugged into... correct me if I'm wrong)...

Okay...I was wrong... the 4500 series aren't layer 3 by default. Once you assign an ip address to vlan 900 and vlan 100 on the 4507 and 3550 they should be able to reach each other without any additional routing because they are directly connected interfaces and your trunk between the two switch will switch the traffic.