[c-nsp] Weird Multicast microburst amplification issue

Chuck Church chuckchurch at gmail.com
Fri Dec 9 14:10:38 EST 2011

Are there multiple streams passing through the switch?  A spanning tree
recalculation will cause IGMP to flush associations, and flood all streams
out all ports until they're relearned.  Portfast will fix it, as will a
multicast-specific interface command, would need to look it up.


-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Matthew Huff
Sent: Friday, December 09, 2011 1:49 PM
To: 'cisco-nsp (cisco-nsp at puck.nether.net)'
Subject: [c-nsp] Weird Multicast microburst amplification issue

We have a multicast data stream (real-time ticker data) that by its nature
is very bursty.

When we connect a source server via gigabit Ethernet to our 6500/sup720
switch via a 6748 module and a destination server via gigabit to  the same
or different module in the same switch, everything works fine. If the
destination server is on a different switch connected by a layer3 10GB
connection then we have significant output drops on the Ethernet connected
to the destination server.

All switches are 6509/sup720 with 6748 line cards. QoS is disabled globally.
The servers are identical. The output drops only occur on the Ethernet drop
connected to the server.

The only thing I can think is happening is that by routing the traffic via
the 10gb L3 interface, something is causing the traffic burst to amplify,
overrunning the output port. Has anyone seen this, and does anyone know how
to mitigate this?

Matthew Huff             | 1 Manhattanville Rd
Director of Operations   | Purchase, NY 10577
OTA Management LLC       | Phone: 914-460-4039
aim: matthewbhuff        | Fax:   914-460-4139

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