On 12/16/2012 11:59 AM, Robert Williams wrote:
Hi, I'll try to go into some additional detail on the traffic and
other router config elements now.
The traffic is basically made up of a randomly generated packet which
is almost identical to the below.
Yuck.
Essentially, this is a malformed packet and I'm not surprised it's
hitting the CPU ("it's multicast" / "it's not multicast") and causing
problems.
Basically "don't do that".
As such there is no multicast configuration. In fact, if anything it
would be ideal if the box dropped all multicast traffic apart from
the HSRP/VRRP to be honest.
You might be able to write a MAC ACL which drops all multicast frames.
The reason I think this may be causing issues is because it is
destined to a non-multicast IP, but with a multicast MAC....?
Very likely.
The situation arose in the real world when a Windows NLB cluster went
Wait, so this isn't the "real" problem traffic? You're testing?
offline and there was a load of traffic heading to its shared IPv4
address ('not' a multicast IP, but 'is' a multicast MAC) - so the
switch flooded to all ports, including the 6500 upstream, triggering
high CPU.
Well known problem. Microsoft NLB is broken by design. Don't use it.
Not the most helpful answer I know, but honestly, any work you do
"fixing" this is better spent on supplanting MS NLB (e.g. install a
couple of Linux boxes using haproxy as SLB-on-the-cheap).
IIRC people who *do* use MS NLB often have to install static ARP / FDB
entries to "stick" the output frames and get around just these sorts of
problems.
That said - I'm sympathetic to the general problem of "device collapsed
under external load and I want to secure it from happening again". But
bear in mind the age of the sup720/Earl7 architecture and its
limitations (e.g. CoPP and multicast/ipv6). You might find a sup2t can
be made more resistant to this kind of thing - but then again, you might
not. State of the art in control-plane resilience to unusual traffic is
not great IME :o(
Cheers,
Phil
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