LDP Troubleshooting Steps

‎10-29-2019 09:25 PM


R2 - R4 - R6 - R8

 |        |       |       |

R1 - R3 - R5 - R7


I have a very simple LDP-over-RSVP situation

R1 & R8 are LDP-only, 3,4,6,7 area RSVP/LDP

The middle 6 have full RSVP-based LSP reachability



set protocols mpls explicit-null
set protocols mpls interface ae0.0
set protocols mpls interface ge-0/0/2.0
set protocols mpls interface lo0.0

set protocols ldp explicit-null
set protocols ldp interface ge-0/0/2.0
set protocols ldp interface ae0.0
set protocols ldp interface lo0.0


(all routers have similar config - mpls has all interfaces + lo0, ldp has all ldp-speaking interfaces + lo0

as well explicit-null on mpls & ldp, core routers have ldp-tunneling on their rsvp lsp - all in up status)


I am having trouble getting an LSP between R1 to R3 & R8 to R6 (both directly connected)

fam mpls is on all relevant interfaces - fyi this works great in an all-RSVP but LDP has always given me issues

These seem to boil down to neighbor discovery issues?

R1 has R3 as a neighbor and the session is Operational and Open

Where R8 & R6 are neighbor but the session is down

sessions extensive on R6/8 shows "connection error" - not sure what to do with that

I have cleared processes, bounced protocols & rebooted machines - no avail


I am looking for some general troubleshooting steps

What can i check when sessions dont form?

what about when sessions do form but the inet.3 route doesnt show up?

and how can i troubleshoot an LDP over RSVP LSP that wont reach a remote end?


thanks so much all for all the help you've given me


Re: LDP Troubleshooting Steps

‎10-30-2019 12:55 AM

Ok, would have been easier to have the full configuration of R6 and R8 to see what's going on there and not a partial one of a router that works. Anyway.


Regarding the connection errors between R6 and R8: In general, there is not much that can go wrong with LDP session setup. LDP uses UDP/TCP port 646, so first of all make sure that is not blocked by firewall filters on the physical interfaces or lo0. Second, check the transport address.


As of the missing routes in inet.3: LDP is not a routing protocol, but only a label binding protocol. You did not mention the configuration of an IGP (OSPF or IS-IS), but that would be a prerequisite of populating the inet.3.