ECMP with RSVP-TE

Kireeti KompellaJuniper Networks

Outline

• Current situation• LDP vs. TE ECMP• NxTE LSPs vs. TE ECMP MLSP• Signaling• Next steps

Current Situation

• An MPLS network relies on an underlying LSP mesh connecting all edge devices– In principle, this could be IP tunnels

• This may be based on LDP or RSVP-TE, or sometimes a combination of LDP on the edge and RSVP-TE in the core– There are a number of attributes that dictate the

design

LSP Design Attributes

Attribute RSVP-TE LDP

Ease of configuration ✖ ✔

FRR ✔ ✖

TE, CAC, BW guarantees ✔ ✖

ECMP ✖ ✔

Either/or … Why Not Both?

• Can one create an underlying mesh of tunnels that has both TE and ECMP?– Yes!

• Introduce the notion of a “multi-path” TE LSP signaled using RSVP– Several “sub-LSPs” under one container tunnel

• Note that the state required for RSVP-TE is still higher (O(N^2)) than that for LDP (O(N))

Illustration (LDP vs. TE ECMP)

100

100

25

100

200 Mbps LSP from A to B

A B

160

40

40

40

NxTE LSPs vs. TE ECMP MLSP200 Mbps MLSP from A to B vs. 5 40Mbps TE LSPs

A B

Discussion

• NxLSPs requires N things to provision and manage; an MLSP is a single object with N sub-objects

• An MLSP with N sub-LSPs may (depending on the topology) have significantly less state than N LSPs– The use of equi-bandwidth sub-LSPs can also

significantly reduce state

Discussion

• Computing the NxLSPs is done pretty much independent– The number and placement of the LSPs may not

be optimized for the purpose of ECMP• Failure of one of the N LSPs means that the

overall bandwidth drops (e.g., by 40Mbps)– Failure of one sub-LSP can be compensated by the

head end bumping up the bandwidths of the rest

Signaling

• The current draft talks a little bit about signaling– Basic idea: signal N sub-LSPs, and tie them

together via a Session object to form an MLSP– Introduce the idea of “equi-bandwidth” sub-LSPs

• If this draft is of interest, the signaling piece would probably need more work

• Have to work out how FRR and DiffServ-awareness work in the context of MLSPs

Next Steps

• Get a sense of how useful this is