First Hop Routing Protocol (FHRP) Info

Peter Welcher
Architect, Operations Technical Advisor

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, UDP port 1985

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


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

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


Hello/advertisement: IP multicast address, UDP 3222

VMAC: 0007.b4xx.xxxx


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

4 responses to “First Hop Routing Protocol (FHRP) Info

  1. 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.


  2. It turns out the Cisco Networkers example is pretty good for HSRP, but doesn’t block ARP to the VIP cross-site. The local VIP will probably win the race, but one might want to be sure. (Good question someone asked yesterday during my presentation!) I’ll leave that part as an exercise for the reader — the Nexus does support ARP ACL’s.

  3. See the new posting at [url][/url].

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Nick Kelly

Cybersecurity Engineer, Cisco

Nick has over 20 years of experience in Security Operations and Security Sales. He is an avid student of cybersecurity and regularly engages with the Infosec community at events like BSides, RVASec, Derbycon and more. The son of an FBI forensics director, Nick holds a B.S. in Criminal Justice and is one of Cisco’s Fire Jumper Elite members. When he’s not working, he writes cyberpunk and punches aliens on his Playstation.


Virgilio “BONG” dela Cruz Jr.

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Virgilio “Bong” has sixteen years of professional experience in IT industry from academe, technical and customer support, pre-sales, post sales, project management, training and enablement. He has worked in Cisco Technical Assistance Center (TAC) as a member of the WAN and LAN Switching team. Bong now works for Tech Data as the Field Solutions Architect with a focus on Cisco Security and holds a few Cisco certifications including Fire Jumper Elite.


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John is our CTO and the practice lead for a talented team of consultants focused on designing and delivering scalable and secure infrastructure solutions to customers across multiple industry verticals and technologies. Previously he has held several positions including Executive Director/Chief Architect for Global Network Services at JPMorgan Chase. In that capacity, he led a team managing network architecture and services.  Prior to his role at JPMorgan Chase, John was a Distinguished Engineer at Cisco working across a number of verticals including Higher Education, Finance, Retail, Government, and Health Care.

He is an expert in working with groups to identify business needs, and align technology strategies to enable business strategies, building in agility and scalability to allow for future changes. John is experienced in the architecture and design of highly available, secure, network infrastructure and data centers, and has worked on projects worldwide. He has worked in both the business and regulatory environments for the design and deployment of complex IT infrastructures.