extend a subnet across a layer 3 interface

[Steve Coady]

All

I have a 6509 with a subnet created which is used for VoIP. This subnet is trunked to the access switches. HOWEVER,

another switch was added off this 6509 that has a point-to-point connection. There will be  1 - 3 VoIP phones on this switch.

Not sure why this newer switch has the L3 connection.

It seems a waste to create a new subnet for 3 phones.

The logic fails me on how to extend that VoIP subnet across this P-2-P link.

I guess I could create an SVI using an ip within that subnet range and also define DHCP server in the SVI.

But the DHCP server will define the default gateway on the 6509 instead of the new SVI so that's not really going to work.

Eigrp is used between the 6509 and new switch advertising the 10.0.0.0

Probably easiest to define new subnets but any guidance would be appreciated.

[Jon Marshall · Accepted solution]

Steve

L2TPv3 can be used to do but I'm not sure this is supported on 6500s.

But to be totally honest if the new 6500 is connected direct with a L3 link even if it was supported it's more bother than it's worth.

Basically you have two options -

1) if the link has to be L3 just create a new vlan/IP subnet on the new switch and route between the switches.

2) if the link doesn't have to be L3 then you can still route all vlans between the switches except for the VoIP vlan ie.

you make the link a trunk link that only allows the VoIP vlan and one new vlan.

You create an SVI on each 6500 for the new vlan and you just treat it as a P2P link between the switches.

No end devices are in that vlan and it used for peering with EIGRP between the switches. Then because you are only allowing this vlan and the VoIP vlan across the trunk link all other vlans are routed across the new vlan.

Up to you really but personally if there is no need for the VoIP clients on the new switch to be in the same vlan/IP subnet I would probably just create a new one.

Jon

[Steve Coady]

Jon

Thank you for the response. Think I'll just create the new vlan!

[Jon Marshall]

Steve

Wise decision :-)

Jon