Cisco Switches dropping fragmented Multicast Traffic over L3 Links

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Oct 19th, 2008
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Hi,

is anyone there facing the following Problem.

I have the problem that different Cisco MLS switches (3650E,3750) are dropping multicast traffic over L3 links when they are connected over a gigabit(fiber or cooper) and the multicast traffic have a size > 32000 Bytes so the udp packets have to be fragmented. To rebuild the problem you need two Layer3 switches (ipbase or advance) connected over a L3 Link and a multicast sender and receiver (for exampel wsend,wlisten). Each in different networks so the traffic have to be routed over the transfer L3 Link and then in the according vlan where the receiver is connected. If the Sender or the link between the switches are 100 Mbit no problem occurs. But if both links are 1Gig and the payload is >30000Byte udp packets are dropped on the receiver side. There is no counter showing this drops on the switch but you can see it in the wlisten tool or if you trace the packets. I have tested it with different configuration of multicast. Using dense or sparse, pim v1 or v2 the problem still exists. The only solution is to connect the sender with 100Mbit or reduce the Uplink between the switches to 100Mbit. But this could not be the solution.

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