Targeted LDP sessions

[Alexey Stytsenko]

I have a question about targeted LDP sessions -  as I have red in MPLS Fundamentals book we can configure mpls ldp discovery targeted-hello parameter. What makes me doubt here is - why we need LDP discovery for targeted session?

As I understand LDP discovery function is to discover LDP neighbor and get from him information such as transport ip to establish LDP neighborship. But for targeted session we can statically define neighbor. So from my point I can`t see reason to send discovery messages between targeted neighbors - why not immediately establish neighborship?

Regards,

[Sudeep Valengattil]

Hi Alexey,

Targeted LDP is used when we have non-directly connected routers and we need to have LDP neighborship between them.  We commenly see this in cases of traffic-engineering tunnels and L2VPN scenarios.  Another reason for targeted LDP is to improve the convergence.  For example, if we have a directly connected neighbor and the link flaps between them, then we nave to re-negotiate the neighborship and get converged.  If we use targeted LDP here, we can avoid from from flapping and hence the convergence would be better.

"mpls ldp discovery targeted-hello accept" is confgured to accept targeted hello requests coming from other routers.  If you dont have this in your config, the default behaviour is to reject it.

You can go through below link to get more details on it.

http://www.cisco.com/en/US/docs/ios/12_4t/12_4t2/ftldp41.html

Do let us know if you have any more quieries.

Regards,

Sudeep

[Harold Ritter]

Hi Alexey,

When LDP is configured over an MPLS TE tunnel, an LDP targeted session needs to be established between the tunnel head end and the tunnel tail end. If there is no corresponding tunnel in the opposite direction, the tail end has no idea it has to run an LDP targeted session. This is why you need to configure this command to tell the tail end to establish an LDP targeted session.

Regards

[Alexey Stytsenko]

Sirs, tnx for responses, but I already red MPLS Fundamentals LDP section, so have some grasp about targeted sessions.

My Question not why we need LDP targeted sessions, but questions is - Why we need LDP DISCOVERY for targeted sessions?

If  we can manually define LDP neighbors, I can`t see reason why we need  send UDP LDP messages instead immediately establish TCP session.

Regards,

[Sudeep Valengattil]

Hello Alexey,

If you just have the targeted LDP enabled, but have not enabled MPLS on interface, then the LSP is broken.  We need the interface to MPLS enabled (mpls ip) so mpls traffic can pass through it. When we enable "mpls ip" on interface, it starts sending hello packets out.

So now if you see "show mpls ldp discovery, you would see both targeted and link hello information.  And as said earlier, having multiple reachability information, would help us in convergence (if one fails)

regards,

sudeep

[Alexey Stytsenko]

I don’t ask why we need discovery enabled on link-local layer. I ask why we need discovery enable between remote peers.

As I understand mpls ldp discovery targeted-hello {holdtime|interval} seconds command allows us configure options for ldp discovery between remote neighbors. This is not standard hellos sent to 224.0.0.2 address with multicast. This unicast LDP Discovery messages sent to addresses we specified when configure mpls ldp neighbor ip-address targeted {ldp|tdp} command. And I can`t get why we need sent those UDP messages between those remote neighbors, instead of bringing up TCP session immediately.

[Nagendra Kumar Nainar · Accepted solution]

Alexey,

LSR that trigger tLDP session first need to understand if or not the remote side supports a targeted LDP session. In order to do this, LSR will unicast the hello with R bit set. If it receives a reply from remote node, it goes to the next step of creating the session.

Per my understanding, this is a kind of security check that unintented node doesnt establish tLDP session and to avoid try creating unsuccessful TCP session if neighbor is not interested

HTH,

Nagendra

[Alexey Stytsenko]

Tnx for clarification, but such security drived behavior looks a bit strange for me. Taking in consideration that for TCP packets can be authenticated and TCP sessions LDP have throttling mechanism (backoff) but for UDP packets not. So transmission will be used more actively by UDP hellos compare to TCP in case if remote end will not accept targeted session.

Regards,

[gubhulla]

Hi, 

Why UDP is used for LDP discovery and TCP for creating sessions.

Thanks

[Harold Ritter]

Hi Gurbir,

It simply makes sense to use UDP for the discovery function, which is connection less in nature and to use TCP for label information exchange.

Hope this helps,

Harold