LDP-IGP Sync

[vibhorpach]

Set-Up

GSR1(12410) POS link connected to another GSR2(12410) POS Link (Primary Link)

GSR1(12410) GigE link connected to another GSR2(12410) GigE Link (Secondary Link)

Software Version: 12.0(32)SY4

Have enabled "mpls ldp sync" under my OSPF Process on both GSR's and have configured a hold-down time of 60000 msec.

I have the following doubts:

1) When I remove the mpls ip command from POS interface to make the LDP down, the traffic doesn't switches to secondary link.What may be the issue?

2) Can someone please explain when the max-metric will be advertised by OSPF. Will it be advertised after the holdtime expires or before that and what will happen if sync is not achieved.

3) Also what will happen, if I am having a single link between my routers. HOW will sync behave then ?

[mheusing]

Hi,

Regarding your doubts:

1) When I remove the mpls ip command from POS interface to make the LDP down, the traffic doesn't switches to secondary link.What may be the issue?

The issue is that LDP-IGP synchronization is only enabled for MPLS enabled interfaces. once you remove "mpls ip" you remove MPLS and hence the whole feature. Have a look at

http://www.cisco.com/en/US/docs/ios/mpls/configuration/guide/mp_ldp_igp_synch_ps6922_TSD_Products_Configuration_Guide_Chapter.html

"When the MPLS LDP-IGP Synchronization feature is enabled on an interface, LDP determines if any peer connected by the interface is reachable by looking up the peer's transport address in routing table. If a routing entry (including longest match and/or default routing entry) for the peer exists, LDP assumes that LDP-IGP Synchronization is required for the interface and notifies the IGP to wait for LDP convergence. "

So LDP is the starting point. No LDP means no LDP-IGP sync.

To test the feature, shutdown and then "no shut" the link.

2) Can someone please explain when the max-metric will be advertised by OSPF. Will it be advertised after the holdtime expires or before that and what will happen if sync is not achieved.

From the document above:

"If the LDP peer is reachable, the IGP waits indefinitely (by default) for synchronization to be achieved. To limit the length of time the IGP session must wait, enter the mpls ldp igp sync holddown command. If the LDP peer is not reachable, the IGP establishes the adjacency to enable the LDP session to be established.

When an IGP adjacency is established on a link but LDP-IGP Synchronization is not yet achieved or is lost, the IGP advertises the max-metric on that link. "

So LDP informs OSPF, which announces maximum metric for that link until either LDP announces synchronisation or the holddown timer expires (if configured).

3) Also what will happen, if I am having a single link between my routers. HOW will sync behave then ?

This depends on what you mean. If there is IP connectivity between the routers through other parts of your network, then LDP-IGP sync works as described above.

If the two routers are not reachable, also from the document above:

"If the LDP peer is not reachable, the IGP establishes the adjacency to enable the LDP session to be established. "

So the routing table is checked first.

If the LDP peer is reachable, the IGP is informed to wait for LDP and to announce max-metric.

If the LDP peer is not reachable (f.e. after a router reload), the IGP will establish an adjacency to learn the Loopback IP. Once this has happened, the peer is reachable and LDP-IGP sync kicks in.

Hope this helps! Please use the rating system.

Regards, Martin

[vibhorpach]

Thanks for replying...but again, some doubts....

>>>>>>(FROM ABOVE)When sync is lost,LDP informs OSPF, which announces maximum metric for that link until either LDP announces synchronisation or the holddown timer expires (if configured).

>>>>>>But, if in case, sync is not achieved and the holddown time expires, will OSPF still announce max-metric for the link, if so, whats the point of configuring holddown time OR will it starts advertising links original metric?

[prakashj]

HI,

I have the network which does not have ldp-igp sync enabled. But if I would like to enable it now (router by router) does it have any impact on the live traffic?

We have point to point POS links in the backbone (Core-Dist) and DPT/RPR at the access (Dist-Access). There are almost 75 routers where sync has to be enabled.

Regards,

Prakash

[mheusing]

Hi Prakash,

LDP-IGP-sync is only performed, if the link was down. Thus in a stable network I would not assume any impact on your traffic when configuring the feature.

To be absolutely sure, however, it is best practice to test the migration AND the correct operation of the feature in a test lab with your IOS version and hardware before rollout into production.

Hope this helps! Please use the rating system.

Regards, Martin

[mheusing]

Hi,

holddown timer expiring means for OSPF "send normal metric" after the timer expires. This way you would use the link after a specified amount of time no matter if LDP is working or not.

Regards, Martin

[vibhorpach]

Hi Martin...

Again Thanks for reply....

correct me if I am wrong

but as per your reply, if OSPF starts sending normal metric after the holdtime expires, then, all the "mpls traffic " which was earlier going through the secondary link will start droping, as IGP will make this as the preffered path....

[mheusing]

Hi,

exactly correct, and this is why the default is that the IGP will wait indefinitely for LDP synchronization to be achieved.

Have a look at

http://www.cisco.com/en/US/docs/ios/12_0s/feature/guide/fsldpsyn.htm

explaining this aspect and all other details of LDP-IGP synchronization.

Regards, Martin

[shivlu jain]

simply it will send the max metric i.e 65536 to all its peer.

regards

shivlu

[dara111983]

Scratching my head !!!!

May be 65535 !!! :) :) :)

[vibhorpach]

Hi Shivlu

Thanks for the reply......but I have a doubt...... if it will advertise max-metric after the holdtime expires then can you please explain what will OSPF advertise after interface comes up and holdtime has not expired and the IGP(OSPF) is waiting for sync to be achieved.

[mheusing]

Hi,

OSPF will FIRST advertise max-metric. If no holddown timer is configured, this will be done indefinitely or up to the point in time, when LDP is converged.

Once LDP is converged or the holddown timer (if configured) is expired, the normal link metric will be advertised.

Hope this helps!

Regards, Martin

[viveksrivastava]

Hi Guys,

I was searching for "Wrong TE metric advertisements after ospf/ldp sync". Can some one help me to rectify this problem.

Setup -> 1. MPLS network with OSPF using auto-tunnels for TE

             2.  below is already configured

mpls ldp igp sync holddown 250

mpls ldp holdtime 300

mpls ldp sync

mpls ldp autoconfig

Problem - On a perticular link, I am getting highest metric-65535 and also link is not showing ldp igp sync not achived. When I am disabling "ldp igp sync" on that interface, I am getting correct metric. I have already verifyed the configuration even discussing with Cisco TAC on the same issue. In the mean time pls suggest.

Regards,

Vivek