I recently researched this and could find any reference in the RFCs or common TCP/IP books on using multicast addresses in ARP replies. The ARP RFC (RFC826) does not say one way or the other. > My personal opinion is that ARP should be fixed on all IP stacks (well.. > ARP "stack") so that they won't accept multicasts addresses.. I can't > think of any reason why they should. One thing that can be configured to use multicast Ethernet addresses for unicast IP addresses is Microsoft's WLBS (Windows Load Balancing Server/Service). Briefly: - a set of machines appear to have a single IP address and the machines somehow load balance incoming requests It does this by - when the cluster's IP address is ARP'd for the cluster responds with a made up MAC address - all the machines participating in the cluster are expected to see the packets to the cluster MAC address and then agree among themselves who is handling it - the response (TCP ACK or whatever) comes out with a different MAC address from one cluster member. It relies on all cluster hosts seeing the inbound packets. Works wonderfully on a hub. If the cluster hosts are connected to a switch it requires the switch to flood the unknown cluster MAC address to all ports. This will happen because the MAC address in the ARP reply never appears as a source address. Some older switches will only flood to a backbone port, so this does not work at all. Clever switches have flood limits that choke it off, viewing it as broadcast storm that needs to be controlled. So WLBS works until the traffic load goes high enough to kick in flood limits. WLBS lets you use a multicast Ethernet address for the cluster MAC address. Presumably so you could configure a modern Ethernet switches to send that multicast to minimal set of ports. More likely as a gross hack around limits of some switches ;) This is off by default because some routers do not like it; the help file does not say which ones. Russell (The people who installed this onto our network only discovered all this after the network team read the help file to them... over shouts of "this network stinks" and "we need more bandwidth!")
This archive was generated by hypermail 2b30 : Fri Apr 13 2001 - 14:46:43 PDT