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Arp issue with MSCS on different VLAN's

 
 
Dan Pearson
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      01-02-2004
Ok I ve seen this issue posted although can't find a fix for it.

The basic situation is: I have a Ms cluster server in one vlan and
clients in another, clients can connect fine until you fail over the
cluster server and obviously it changes arp mapping to the other nics
MAC address, clients on the other side of the switch then cannot
connect until i clear the arp cache on the cisco switch (a 3750)

Ok so I m guessing this is an ARP broadcast thing and I have to enable
it on the switch to allow ARP traffic to the other vlan but am a bit
unsure of how to do this? would it be ARP Proxy'ing?

Any pointers including the relevent commands and at which level they
need to be applied would be gratefully received.

Thanks in advance

Dan
 
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shope
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      01-02-2004
"Dan Pearson" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) om...
> Ok I ve seen this issue posted although can't find a fix for it.
>
> The basic situation is: I have a Ms cluster server in one vlan and
> clients in another, clients can connect fine until you fail over the
> cluster server and obviously it changes arp mapping to the other nics
> MAC address, clients on the other side of the switch then cannot
> connect until i clear the arp cache on the cisco switch (a 3750)


i havent done this on 3750, but it sounds more like a cluster setup problem.

sounds like your cluster is set up to use a unicast MAC for the cluster
address.

last time i set this up (on Alcaltel kit), we set the cluster to use a
multicast MAC - this means every server request gets copied to all machines
in the cluster (which only makes sense if that isnt a big chunk of
bandwidth - typical of a web server where most traffic is read from the
server).

To support this you need the 3750 to support multicast ARP entries in its
ARP table.

microsoft have a white paper on their web site which gives the various
config options for a cluster and explains how they work and what you need
from the network for each.
>
> Ok so I m guessing this is an ARP broadcast thing and I have to enable
> it on the switch to allow ARP traffic to the other vlan but am a bit
> unsure of how to do this? would it be ARP Proxy'ing?


No - ARP is local to the VLAN where the ip address has to be resolved.

If the IP is changing to another machine, then that server should send out a
broadcast ARP to update ARP entries in all the other IP devices on the
subnet.

If you are testing by physically pulling the server cable, then maybe
spanning tree is blocking the gratuitous ARP - try setting the server ports
on the switch to host mode
>
> Any pointers including the relevent commands and at which level they
> need to be applied would be gratefully received.


All the config stuff needs exec level access.
>
> Thanks in advance
>
> Dan

--
Regards

Stephen Hope - remove xx from email to reply


 
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Greg Reaume
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      01-03-2004

"Dan Pearson" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) om...
>
> The basic situation is: I have a Ms cluster server in one vlan and
> clients in another, clients can connect fine until you fail over the
> cluster server and obviously it changes arp mapping to the other nics
> MAC address, clients on the other side of the switch then cannot
> connect until i clear the arp cache on the cisco switch (a 3750)
>


I can't speak for how MSCS handles MAC swapping, but from a switch
perspective, as soon as a frame comes in with a new MAC (one that already
isn't in the CAM for that interface) it is added to the table. If this
means bumping it from another interface table then so be it.

It sounds like your switch is not doing this, as I can't imagine that MSCS
is staying THAT quiet after a failover. You may want to set up a
sniffer and turn on SPAN to see what is REALLY going on. Once you
understand that, watch what your switch is doing with its CAM as the
failover events happen and the MAC swap takes place. One workaround may be
to lower the caching time.

Let us know what you turn up. HTH, good luck.

Greg


 
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