First Hop Routing Protocol (FHRP) Info

Posted by: Pete Welcher on Dec 16, 2010 Print 

Pete's FaceIt 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:

HSRP

Hello/advertisement: Sent to the general all-router multicast IP 224.0.0.2, UDP port 1985

Virtual MAC (VMAC) used: 0000.0c07.acXX, XX = HSRP group number in hex

http://en.wikipedia.org/wiki/Hot_Standby_Router_Protocol

I was amused to see that the wikipedia article referenced an old article of mine:

http://www.netcraftsmen.net/resources/archived-articles/441-hot-standby-routing-protocol.html

VRRP

Hello/advertisement: Sent to dedicated multicast IP 224.0.0.18, IP sub-protocol 112

VMAC: 00-00-5E-00-01-XX, XX = the Virtual Router IDentifier (VRID),

http://www.cisco.com/en/US/docs/ios/ipapp/configuration/guide/ipapp_vrrp.html#wp1054602

http://en.wikipedia.org/wiki/Virtual_Router_Redundancy_Protocol

GLBP

Hello/advertisement: IP multicast address 224.0.0.102, UDP 3222

VMAC: 0007.b4xx.xxxx

http://en.wikipedia.org/wiki/Gateway_Load_Balancing_Protocol

Overall

All of the above turned out to be at the following URL, with a bit more good summary info about how the protocols work:

http://routerjockey.com/2010/10/21/hsrp-vrrpd-and-glbp-compared/

 

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0
Manual Filtering?
written by Jason Edelman, January 19, 2011
Has this changed? From what I had read, OTV did not require manual filtering?

Thanks,
Jason
Pete Welcher
Response to Jason
written by Pete Welcher, January 20, 2011
The first release of OTV doesn't do some things. I will shortly be posting a presentation I did yesterday on OTV for CMUG (1/19/11), containing a list of the things I know OTV lacks / might have in the future. The two main ones are: manual filtering for the FHRP, and adjacency server functionality for operation over a unicast WAN. These are supposed to be in the forthcoming next NX-OS code release.

I intend to post a follow-up blog to this one, because in presenting, it occurred to me that one probably wants to not only block the FHRP itself between sites, but also block any ARP directed towards the Virtual IP used by the FHRP. I intend to revisit a Networkers presentation from last Summer and review the ACL I noted there, and will post a blog supplementing the above material to make sure readers get the full story, if that is necessary.

Thanks!
Pete Welcher
Follow-up
written by Pete Welcher, January 20, 2011
It turns out the Cisco Networkers example is pretty good for HSRP, and does block ARP to the VIP cross-site, although not in the most obvious way (to me). I'd have used an ARP ACL, however using a MAC ACL also works.

If you don't block ARP reply from the VIP at the remote site, I would expect the local VIP to probably win the race to answer an ARP for the MAC for the VIP. However, one might want to make sure it does. (Good question someone asked yesterday during my presentation!)
Pete Welcher
Sample config for HSRP / ARP filtering
written by Pete Welcher, January 20, 2011

See the new posting at http://www.netcraftsmen.net/re...n-otv.html.
0
HSRPv2
written by Asbjorn Hojmark, January 24, 2011
HSRPv2 should be very relevant to people as well (think: IPv6):

Hello/advertisement: Sent to the dedicated HSRPv2 multicast IP 224.0.0.102, 2029/UDP
VMAC: 0000.0C9F.Fxxx, where xxx = HSRP group number in hex (max 4095).

-A
0
Reply: HSRPv2 -- Good point
written by Peter Welcher, January 24, 2011
Good point! And thanks for the info. I'm a bit surprised I didn't think of discussing this for IPv6.

I've been telling folks about the impending dearth of IPv4 addresses (and I hear they will run out really soon now). I also deployed IPv6 for a 13,000-person campus (government agency), in part because it was easy to do along with the rest of the deployment, in part to get the security and server teams moving on IPv6. Not in a nasty way, just removing the chicken and egg factor of "the network doesn't support it". (The short-term answer to that seems to be a common one: turn IPv6 off on servers and laptops.)

I'm puzzled in that I'd expected companies to be rolling out IPv6 DMZ / Internet servers (IPv6 edge) at least, to allow business with IPv6-only customers. And I see the tipping point as perhaps being when there are more and more IPv6-only servers. The U.S. seems to be lagging on IPv6 (lack of time, not an apparent crisis), and I find myself wondering when the stampeded to deploy it will begin. (Sort of feels like the decade where every year was going to be the Year of Unix :-).)

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