[c-nsp] Unicast traffic being sent to every port? Aging issue?

[Jay Hennigan]

On 3/22/10 7:03 PM, Ray Van Dolson wrote:
> We have two Dell PowerConnect M6220 switches (A1 and B1).  They are not
> cross-connected, but both have uplinks to the same subnet:
> 
>                       zfs1
>                      /
>                    +----+
>                    | A1 |---------|
>                    +----+     +-------+
>                               | Cisco |------- linux1
>                    +----+     +-------+
>                    | B1 |---------|
>                    +----+
>                     / \
>                   esx1 esx2
> 
> There's a host hanging off of A1 (zfs1) and several ESX hosts hanging
> off of B1 (esx1, esx2, etc).  There's a host linux1 hanging off the
> Cisco as well (actually many hosts, but for the sake of description
> 
> What's happening is, esx1/2 beging talking to zfs1.  All is well for a
> while... but at some point, zfs1's MAC address expires from the CAM on
> the switch (I guess that is what is happening).
> 
> At that point, the Cisco begins forwarding the unicast packets to all
> its ports.  The result -- linux1, and all other hosts see the packets.
> Occasionally, when we're dealing with a lot of traffic, this seriously
> impacts performance.

Is the Cisco a router or a layer 2 switch?  All hosts in the same IP
subnet?  Subnet masks all match?  Nothing doing proxy-arp?

> My question here is.. what is the _right_ way to deal with this?  This
> "flooding" can continue for many minutes at a time.. it isn't until an
> ARP reply eminates from zfs1 that the CAM table is populated again and
> the broadcasting stops.

If these are layer 2 switches, ARP won't have anything to do with it.

If zfs1's MAC expires from the MAC address table on the cisco, it will
flood the next packet for that MAC.  A1 will forward it to zfs1 or flood
if it too has expired the MAC.

When zfs1 replies, A1 forwards the reply to the cisco.  At that point,
the cisco should re-install the MAC into its address table and the
flooding cease.

This should happen with a single packet.

Does this happen with any other hosts behind A1?  Any interface errors
on any of the devices?

> I wonder if zfs1 would send back an ARP response quicker were it not
> behind an additional switch (the PowerConnect)... 

If layer 2 switches, ARP doesn't have anything to do with it.

--
Jay Hennigan - CCIE #7880 - Network Engineering - jay at impulse.net
Impulse Internet Service  -  http://www.impulse.net/
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