PE Aggregate Routed Default vs Label Switched Non-Aggregate Default

[ipotts]

Hello,

In MPLS and VPN Architectures, in Chapter 12 p310, Association of the Global Routing Table with a VRF, it describes the following. If PEa is connected to two ISPs, and contains the full Internet routing table in a VRF from both ISPs, and it injects a default to remote PEs using a next hop of the ISPs (i.e. a default to the next hop of ISPa and another to the next hop of ISPb), then packets towards the internet will be label switched at PEa, so the BGP best path will not be consulted.

However, if the default is generated by having a next hop of a loopback on PEa, the default is injected as an aggregate, and now packets following the default to the internet via PEa will be routed by the BGP best path, not label switched.

How is this represented in the LFIB, since the packet entering PEa, destined for the Internet must have the default label on it, so must initially consult the LFIB, but it appears that the LFIB somehow directs the packet to consult the main routing table?

I assume that pen label pop could not be used, since this would cause the remote PE to inject the route without a VPN label, so PEa could not then associate it with a VRF.

Thanks in advance,

Ian

[Harold Ritter]

The aggregate label is marked in the LFIB and causes the router to lookup the RIB to forward the packet.

The phb removes the IGP label and leaves the VPN label as the only label. When the PE receives the MPLS frame, it only sees the VPN label, which is the aggregate label in your case.

Hope this helps,