Frame with FCS not being dropped!

Unanswered Question
Sep 2nd, 2009


I have a scenario where the ospf is flapping and it is configured as point-to-point network i.e the ospf communication between neighboer is multicast. We have found that when the neigh tries to exchange database (of size 1500 routes approx.)the ospf relationship flaps.

We did some furthet testing and found out that the only multicast packets with size 1497 and above do not go through. But also found out that for all the multicast packets under size 1497 have FCS header attched with wrong checksum.

If we change ospf network type to non-broadcast i.e unicast the ospf flapping stops.

I would like to know why i am seeing those packets with FCS in the sniffer plugged to transit device , ideally packets with wrong checksum should have been dropped.

I have this problem too.
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Giuseppe Larosa Wed, 09/02/2009 - 09:20

Hello Saju,

>> ideally packets with wrong checksum should have been dropped.

if there is a lan switch in the middle this happens only if it uses store and forward.

FCS cannot be checked if the lan switch used cut through or modified cut cut through:

in these cases the frame is replicated to the destination port before receiving the whole frame so FCS cannot be checked.

Hope to help


singhsaju Wed, 09/02/2009 - 09:40

Hi Giuseppe,

Thanks for the reply.I would certainly check about that.

If its service provider L2 cloud with Layer-2 SP switches of different make (maynot be cisco)do you think they would use "store and forward or Cut Through"?



Giuseppe Larosa Wed, 09/02/2009 - 13:40

Hello Saju,

most modern switches use cut through or modified cut through for the lower latency they can provide in this way.

Also if using a L2 transport service I'm not sure what they does.

However, I remember that testing EoMPLS we found that the device (a GSR with Engine 2 3GE linecard) had disabled any MAC address learning and it was sending over the EoMPLS pipe frames with destination mac = interface MAC address.

Hope to help



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