It can be useful to have some basic information about how the various First Hop Routing Protocols (FHRP’s) work. When I read that the first release of Cisco OTV (Overlay Transport Virtualization) requires manual FHRP filtering, I started wondering how I would do it. The Cisco documents talk about MAC address-based filters. Ok, to do that, you need some basic info. Hence this quick note with what I found.
Murphy’s Law applied: I looked for info for a while, found most of it, and then found a web page listing all the information (URL is at the end of this article). And I probably should have started with Wikipedia (it’s been pretty useful as a technical reference lately!) My hope is that by repeating the info in one place it’ll be helpful.
OTV tip: The Cisco documents also mention that your FHRP gateway should not be on the OTV devices, i.e. the SVI (interface VLAN) must not be in a VLAN transported by OTV. One guess is that this is because manual or automatic FHRP hello/advertisement filtering won’t work on such an interface. The workaround if your Aggregation layer is the datacenter Layer 3 switch AND the OTV edge device is to do OTV in a separate VDC.
The promised info:
Hello/advertisement: Sent to the general all-router multicast IP 126.96.36.199, UDP port 1985
Virtual MAC (VMAC) used: 0000.0c07.acXX, XX = HSRP group number in hex
Hello/advertisement: Sent to dedicated multicast IP 188.8.131.52, IP sub-protocol 112
VMAC: 00-00-5E-00-01-XX, XX = the Virtual Router IDentifier (VRID),
Hello/advertisement: IP multicast address 184.108.40.206, UDP 3222
All of the above turned out to be at the following URL, with a bit more good summary info about how the protocols work: