[c-nsp] LDP sessions established to non directly connected routers, when new link is activated

[CiscoNSP List]

Hi Adam - Long time no speak...how have you been?

Yes - Checked all routers, no tLDP sessions configured(To the routers the "new" LDP sessions establish), we have xconnects from the ME3600's in POPB -> ME3600's in POPA, but none from POPA -> POPC ASR1006 - And as you mention, they would come come up over the existing path, but only come up when this new link is activated.

Once BGP fails to the three routers(Which is almost immediately after the "new" LDP sessions establish), it stays down, and only recovers once I re-apply the high OSPF cost to the new link...mtu's are definitely different(As we purchase them from different carriers)....but as soon as the LDP sessions establish, I am not even able to ping the routers that lose BGP.

POPC(ASR1006) -> POPB (ME3600) mtu 1552
POPB(ME3600)->POPA(ASR1001) mtu 3000
POPA(ASR1001-2) -> POPD(ASR1006) mtu 1950


Very strange one indeed :)



________________________________________
From: Adam Vitkovsky <Adam.Vitkovsky at gamma.co.uk>
Sent: Wednesday, 23 September 2015 6:11 PM
To: CiscoNSP List; cisco-nsp at puck.nether.net
Subject: RE: LDP sessions established to non directly connected routers, when new link is activated

Hi,

Have you checked both ends if either end has a tLDP session configured it would come up.
Also xconnects are using targeted LDP sessions, isn't there some forgotten xconnect pointing to these routers somewhere?
But then the question is why the session wouldn't be formed over the existing path.

With regards to BGP failing are the sessions then re-established or they remain down please?
I suspect it could be an MTU problem that is some of the links on the new path ASR2(POPA) to ASR1(POPD) to ASR1(POPC) -have lower MTU than the links on the existing path via POPB.


adam
>

        Adam Vitkovsky
        IP Engineer

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-----Original Message-----
> From: cisco-nsp [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of
> CiscoNSP List
> Sent: 22 September 2015 23:59
> To: cisco-nsp at puck.nether.net
> Subject: [c-nsp] LDP sessions established to non directly connected routers,
> when new link is activated
>
> Hi Everyone,
>
>
>
> Bit of a strange one.
>
>
>
> 2 x ASR1001's (ASR1 and ASR2) directly connected (POPA), with OSPF, MPLS
> and BGP established
>
>
>
> ASR1 connects to POPB (ME36001), also with OSPF, MPLS and BGP
> established
>
>
>
> ME36001 connects to ME36002 at POPB, also with OSPF, MPLS and BGP
> established
>
>
>
> ME36002 connects to an ASR1006 at POPC, also with OSPF, MPLS and BGP
> established
>
>
>
> We have a new link connecting to ASR2(POPA) that goes to POPD, also with
> OSPF, MPLS and BGP established - but OSPF cost set high so that it is not
> used...due to a strange outage it causes when OSPF is set to no cost.
>
>
>
> As soon as the new link on ASR2(POPA) and ASR1006(POPD) OSPF is set to no
> cost, ASR2(POPA) for some reason establishes LDP to ME36002(POPC) - but it
> has no direct connection to this device, nor is targetted LDP enabled to this
> device, and ASR1(POPA) establishes LDP to ASR1006(POPC) - but it also has
> no direct connection to this device, nor is targetted LDP enabled to this
> device....result of these LDP sessions establishing is ASR1(POPA) losing BGP
> neighbourships to three other routers that are connected via the
> ASR1006(POPC).
>
>
>
> Ive opened a TAC case, but if anyone can explain how/why the LDP sessions
> would establish when this new link is "activated" it would be greatly
> appreciated!
>
>
>
> Cheers.
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