MPLS : IGP route aggregation and broken LSPs.

[mpugliese]

If I do (IGP) route aggregation inside a MPLS domain it would break LSPs created between end-points, I mean ingress-egress points (PEs), that have the node deploying route aggregation is its path.

Also, LSP creation is topology driven, not traffic driven; and LSPs are created

among all possible ingress/egress points/options inside a MPLS Domain.

However as far as I think I know, LSPs are always established between PEs´

(Edge-LSRs) router-ids, and so the egress PE does Layer 3 forwarding (as result of penultimate hop popping) when routing toward a CE.

01) Is that right ?

If so I understand that, if I allocate a CIDR to a PE and all its customers' wans & lans prefixes (or if you prefer, all PE´s connected and static routes) are subnets of this CIDR; the PE IGP could

advertise to other PEs only one route regarding the whole CIDR instead of all its subnets, without break any LSP that has this PE as one of its endpoints.

02) Did I make myself clear ?

03) Is that right ?

Yours Truly.

Murilo Pugliese.

[mpugliese]

I'm asking that ´cause I'd like to deploy IGP route aggregation at PEs nodes, not P nodes, looking

toward decrease the number of IGP prefixes propagated inside my MPLS Domain, and so decreasing

the FIB size and its memory consumption; without break connectivity among MPLS-based VPN (VPNv4) sites.

[Harold Ritter · Accepted solution]

Murilio,

You can do IGP route aggregation in your MPLS core as long as you don't aggrgate the loopback address of the PEs. Just don't allocate the loopback address as part of the block that you will summarize. Any summarization in the core of your PEs loopback address will indeed break the LSP between the ingress and egress PE.

Hope this helps,

[mpugliese]

Harold.

Thanks a lot for your reply.

That answers my doubts.

Yours Truly.

Murilo Pugliese.

[jsangh2002]

Hi,

Can you please elaborate why aggregating loopback interfaces on PE would break LSP? I am just not getting it.

Thanks!

[attrgautam]

Say you are summarising on Router1 and Router 2 receives the summarized route and router 3 is the loopback you are trying to reach. Router 1 will generate a label for the Summarized route. When the packet comes to router 2 , it will do an IP lookup for router 3 as the label is for the summarised route and hence the LSP breaks.

Iam sure someone will correct me if iam wrong

[sachins_shinde]

Aggregating loopback ips of PE in MPLS will generate new label for aggreagted route which causes break in LSP. please find the attach pdf. I hope it will help u to understand.