GLBP Configuration on a Campus LAN

[robward]

Hello, I am in the process of implementing GLBP on a large Campus LAN.

I understand the protocol uses multicast to communicate between peers.

The Network topology is a standard three tier architecture and the GLBP implementation is to provide resiliency for common VLAN's routed at the distribution layer which is divided into logical areas i.e. 10 Voice 20 Wireless 30 Building Management 40 Network Management etc.

As these VLANs will be duplicated in each logical area my question is will each GLBP group number need to be unique or is it possible to re-use numbers in each logical area?

I suspect the answer is no as all routers will join the multicast group but I'm wondering if there's a possible workaround to help simplify the configuration.

Thanks in advance for any help!

[Chad Peterson · Accepted solution]

Hi Rob,

The GLBP gorup number needs to match for each pair.  Each vlan can use the same number as they are logically seperate.

The main reason for having multiple group numbers, if you wanted to have two different pairs in the same vlan you would then need two groups.

All GLBP packets will remain local the the VLAN they are in...so if you re-use your VLAN numbers but they are in different locations seperated by L3, then you can use the same numbers.

I hope that clears it up a little.

Chad

[robward]

Thanks for the answer Chad but that adds to the confusion slightly.

The understanding I had was that GLBP packets used Multicast and as the network is multicast enabled and the multicast address remains consistent I expected that the packets would find their way across the layer 3 network outside of the local VLAN?

[Jon Marshall · Accepted solution]

GLBP uses 224.0.0.102 as it's mutlicast address. The 224.0.0.0/24 range is reserved in multicast for use on the local subnet. Packets in this range are always sent with a TTL of 1 so they cannot be routed across vlans so even with multicast routing enabled on your network the GLBP multicast packets sourced within a vlan will stay in that vlan ie. they will not be seen by any other vlans.

Jon

[robward]

Thanks both, that's answered my questions!