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Cisco Bug: CSCup46329 - LDP sends unlabel to FIB if LDP GR races behind RIB after RPFO

Last Modified

Sep 20, 2018

Products (1)

  • Cisco Carrier Routing System

Known Affected Releases

5.2.0.BASE

Description (partial)

Symptom:
With LDP NSR/GR configured, if there is ICCP session, during RPFO LDP might program no label as outgoing label for the route to iccp peer, which causes LSP flap and traffic loss during RPFO.

Conditions:
The problem is a timing related issue and the following conditions has to reach to see this issue:
1. There must be ICCP session, which will flap during RPFO, as LDP does not support NSR for ICCP session;

2. The route updates post FO from rib must arrive before GR timer kicks off. If GR timer is running when route updates coming into LDP, even there is no LDP session, LDP will install UNKNOWN label, instead of no label. From FIB prospective, with UNKNOWN label, it will use the old label that LDP used to install and the forwarding info could be maintained. In this case, no traffic will be impacted;

3. The ICCP session must come up very late post RPFO. If the session comes up very quickly, LDP will program the proper outgoing to FIB as soon as the session is up and no traffic drop will happen;
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