PE router aggregation

Unanswered Question

Hi all,

I'm trying to find a way to aggregate the PE routers to a P router. I've read somewhere that this could be done by putting a LAN switch in the middle and run dot1q trunk to the P router. This would cause the PE to PE traffic on the same switch to hair-pin. Is there a better way to do this?

Thanks in advance,


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Giuseppe Larosa Tue, 09/16/2008 - 12:26

Hello John,

you can also use a single Vlan and a single IP subnet:

a 10 GE uplink on the L2 switch to the P router and a GE link on each PE and you can aggregate up to 10 PE without oversubscribing in this case PE to PE communication is direct without going through the P router.

MPLS is supported in a LAN environment you are not constrained to point-to-point links.

Be aware that the L2 switch requires specific configuration to deal with MPLS MTU issues.

Hope to help


Giuseppe Larosa Wed, 09/17/2008 - 01:44

Hello John,

MPLS VPN forwarding uses a two levels label stack:

the label that is first after the L2 header (including 802.1Q header if present) is the one chosen by LDP to reach the remote PE loopback.

the second label is the one advertised by the remote PE within vpnv4 address-family.

the second never changes, the first label changes at each router hop.

A special case for two directly connected PE is that instead of a label the remote PE loopback is reached by an implicit null label that is signaled by LDP.

This case is supported the frame will have only one label and that label is the VPNv4 label, the remote PE in receiving the frame understands it is for the right VRF.

So the scenario I've suggested works for MPLS VPN too

Hope to help


Giuseppe Larosa Thu, 09/18/2008 - 05:47

Hello John,

the switch needs to support two levels of labels because may be in the future you will need it.

depending on switch platform look for

system mtu


enabling jumbo frames

this allow to introduce the switch without effects

Hope to help



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