[j-nsp] Longest Match for LDP (RFC5283)

[James Bensley]

Hi All,

Like my other post about Egress Protection on Juniper, is anyone using
what Juniper call "Longest Match for LDP" - their implementation of
RFC5283 LDP Extension for Inter-Area Label Switched Paths (LSPs) ?

The Juniper documentation is available here:

https://www.juniper.net/documentation/en_US/junos/topics/concept/longest-match-support-for-ldp-overview.html

https://www.juniper.net/documentation/en_US/junos/topics/task/configuration/configuring-longest-match-ldp.html

As before, as far as I can tell only Juniper have implemented this:
- Is anyone use this?
- Are you using it in a mixed vendor network?
- What is your use case for using it?

I'm looking at IGP/MPLS scaling issues where some smaller access layer
boxes that run MPLS (e.g. Cisco ME3600X, ASR920 etc.) and have limited
TCAM. We do see TCAM exhaustion issues with these boxes however the
biggest culprit of this is Inter-AS MPLS Option B connections. This is
because Inter-AS OptB double allocates labels, which means label TCAM
can run out before we run out of IP v4/v6 TCAM due to n*2 growth of
labels vs prefixes.

I'm struggling to see the use case for the feature linked above that
has been implemented by Juniper. When running LDP the label space TCAM
usage increments pretty much linearly with IP prefix TCAM space usage.
If you're running the BGP VPNv4/VPNv6 address family and per-prefix
labeling (the default on Cisco IOS/IOS-XE) then again label TCAM usage
increases pretty much linearly with IP prefix TCAM usage. If you're
using per-vrf/per-table labels or per-ce labels then label TCAM usage
increases in a logarithmic fashion in relation to IP Prefix usage, and
in this scenario we run out of IP prefix TCAM long before we run out
of label TCAM.

My point here is that label TCAM runs out because of BGP/RSVP/SR
usage, not because of LDP usage.

So who is using this feature/RFC on low end MPLS access boxes (QFX5100
or ACX5048 etc.)?
How is it helping you?
Who's running out of MPLS TCAM space (on a Juniper device) before they
run out of IP prefix space when using LDP (and not RSVP/SR/BGP)?

Cheers,
James.