On 08/21/2012 03:28 PM, Lang, David wrote: > Remember that I was responding to the statement that it was > impossible to have an IP address on two machines.
Right. For those without a networking/sysprog major out there: it's certainly possible, no-one can stop you. Under normal circumstances arp will map one ip to one mac address, so the other mac address won't get any traffic. Until you update all arp caches, at which point the first mac address stops getting traffic. The fun starts when they don't update all at once -- they normally don't. For that reason ifconfigs and dhcpds (or dhclients) will often ping the ip first and barf if it answers. Now mac addresses exist on the same ethernet segment (vlan). So the two ways around this are a) multicast mac addresses are easy because ethernet is a broadcast medium to begin with & every hosts sees every packet. All you have to do is put the nic in promiscuous mode and grab the packets with that "special" mac address on them. b) take arp out of the equation altogether and work at ip level -- that means routing. Once you route the same packet to multiple subnets, within each subnet your "special" ip maps to only one mac address & arp is happy. ... I was in no way trying to say that ANYCAST and CLUSTERIP can be used for the same problems. They usually cannot. I think faster than I type so I often enough end up typing a sentence that starts with apples and ends with oranges myself. I know you meant that anycast won't work for an active-active failover cluster, not that clusterip will work over routed subnets. It just didn't sound quite that way. Here's a free PhD title if anyone needs one: a multiply-resilient geografically distributed cloud by anycast'ing to a clusters of clusterip's. -- Dimitri Maziuk Programmer/sysadmin BioMagResBank, UW-Madison -- http://www.bmrb.wisc.edu
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