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Community Member

Multicast Storms

I have 2 x RSMs on 6509s running HSRP servicing a VLAN on which I'm trying to install two servers acting as cluster ie. same IP address and same MAC 03-xx-xx-xx-xx-xx.

access from the LAN is OK , but I'm struggling with WAN access. The routers do not recognise the ARP reply from the cluster .

I tried : arp 10.yyy.yyy.yyy 03xx.xxxx.xxxx arpa on both RSMs , but this produced a multicast storm .

thanks in anticipation

Community Member

Re: Multicast Storms


What do you mean by "multicast storm"? You mean that every port on that VLAN receives the packet?

I'm just wondering if the servers source packets with the same MAC address on which they listen to (03xx.xxxx.xxxx)...

Try to statically map the 03xx.xxxx.xxxx to the ports where the servers connect to.

Hope this helps.


Community Member

Re: Multicast Storms

NM - More alarming than flooding to each port on the VLAN I'm afraid .

1. Router "A" receives a PING request ,(meant to be unicast but flooded to all due to 03XX Mac). The static entry leads to "A" putting the packet back to the VLAN (decrementing the TTL)

2.This is then received by Router "B" . And the cycle continues with "B" bouncing the packet and decrementing the TTL , until we hit 0.

3.Of course the initial Ping request has also gone to router "B" so the process is also initiated by B.

Good fun ??

Earlier discussions on this problem suggested config-if > ip multicast ttl-threshold 1 as a get round , but it made no difference.

Putting a static CAM entry on the 6509s works in so far as stopping the erroneous traffic , and thanks for the tip - BUT I can't use it - The cluster is supposed to load balance requests as well as provide resilience - The two servers forming the cluster are on separate 6509s The static entry will prevent either .

I may have to resort to putting the two servers into a hub , then onto a single swith port, but it's a "messy get round".

Thanks .

Community Member

Re: Multicast Storms


If you have the static ARP entry and the only two devices that are interested in the 03-xx-xx addresses are off of two ports, you could simply create a filter that allows only those two ports to receive traffic destined for the servers. Or you could simply stop the routers receiving the traffic (ie if the packet has a local source address forward to the router; if it's a remote source address, do not forward to the router). Either method would work.

As an aside the use of multicast MAC addresses is explicitly disallowed in RFC1812 Section 3.3.2 states a router MUST not believe an ARP response that claims a link layer broadcast or multicast address. So the application breaks the rules a bit!

HTH - if you need to discuss further email

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