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Cisco Bug: CSCte18258 - Traffic loss in secs on swover from default to data mdt

Last Modified

Jan 27, 2010

Products (1)

  • Cisco IOS

Known Affected Releases

12.2(33)SRD3

Description (partial)

Symptom:
This issue is causing up to 60 sec of traffic loss during default to data switchover. The test
involves multiple default to data MDT switchover by starting and stopping multicast traffic. In the
problem case, the traffic is started just after (S, Gdata) entry has expired and removed from egress
PE. (S, Gdata) entry is still exists in ingress PE.

Conditions:
From the description provided, the problem occurs when the ingress PE is sending traffic on the data mdt and then the vrf traffic stops for a while. At this point the switchback to the default mdt is made. However the (S,G) for the data mdt is still present and its expiration timer is counting down. In sparsemode the expiration timer is updated by traffic and not by PIM joins from downstream routers. 

On the 7600 platform, the stats are sent by the HW to the Router processor every 25 sec. PIM keeps a track of the number of packets switched and based on the periodic stats it receives from the HW, it determines the average traffic rate. If at least one packet was seen in that 25 sec interval, the (S,G) expiration timer is updated back to 3mins.

So in this case, by the time we switch back to the data mdt, the (S,G) expiration timer is almost expired. In the debug logs, we notice for the data mdt mroute entry, the PIM join for the mroute entry comes in when only 3 sec were left on the expiration timer. This time is not enough for the next periodic stats update to come in and allow PIM to refresh the (S,G) expiration timer. Therefore after the timer counts down to 0, the (S,G) mroute entry is deleted. After this, the ingress PE will simply drop all packets till the next periodic (S,G) joins (default 60 seconds) comes in from the downstream P router to recreate the (S,G) mroute forwarding entry.
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