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Layer 3 Interfaces for VLANs

Layer 3 Interfaces for VLANs

Layer 3 Interfaces for VLANs

I have a question regarding VLANs and associated layer 3 interfaces.

I have say VLAN10 on a core switch. There are 3 switches connected to this via fibre for edge devices. VLAN10 exists on all switches, and the switch to switch links are set as trunks and allow vlans 1 and 10 (3com switches, all layer 3).

Now, I only have an IP interface for VLAN10 on the core switch. Everything works as I would expect. Is it necessary to have an IP address for VLAN10 on each switch? I can't see why but just thought I would ask in case I am missing something obvious. Naturally the VLAN stuff is happening at layer 2 anyway and IP wise devices on VLAN10 use the IP of the core interface as their default gateway.


'When all else fails.......read the manual'

RE: Layer 3 Interfaces for VLANs

If it was Notrel gear, only the core switch would have an IP for a given VLAN*, and that would be the default gateway.**

*each switch has an IP in the management VLAN, to manage the switch.

** Nortel allows backup Default gateways, but they are invisible until the core switch fails.

I tried to remain child-like, all I acheived was childish.

RE: Layer 3 Interfaces for VLANs

Each switch should have one IP address (and only one), but that IP address is not used by the switch to perform its switching functions, it is only used by you to manage the switch.

RE: Layer 3 Interfaces for VLANs

Thanks for that chaps, had a feeling that would be the case. Didn't seem logical to me.

Jimbo, these are 3Com switches and operate in a similar manner to what you have described.


'When all else fails.......read the manual'

RE: Layer 3 Interfaces for VLANs

You can have as many IP address assignements as required on a switch - not just 1.

1 assignement for each requirement - yes - but not 1 IP per switch.

If you have 20 VLANs you may want an IP assignment as a gateway for each VLAN - therefore 20 IP addresses.

We utilise Cisco and Extreme and for security reasons run the IP Gateway for each VLAN on each switch.

Rmemebr with switches that the default VLAN on ALL switches is VLAN 1. A common security mistake is to leave VLAN 1 as the management. Always create a seperate management VLAN and then disable VLAN1.


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