View source for Hot Standby Router Protocol

{{short description|Network system for establishing a fault-tolerant default gateway}}
{{confuse|High-availability Seamless Redundancy}}

In computer networking, the '''Hot Standby Router Protocol''' ('''HSRP''') is a [[Cisco]] [[proprietary protocol|proprietary]] redundancy protocol for establishing a [[fault-tolerant]] [[default gateway]]. Version 1 of the protocol was described in {{IETF RFC|2281}} in 1998. Version 2 of the protocol includes improvements and supports IPv6 but there is no corresponding RFC published for this version.

The protocol establishes an association between gateways in order to achieve default gateway failover if the primary gateway becomes inaccessible.  HSRP gateways send [[multicast address|multicast]] ''hello'' messages to other gateways to notify them of their priorities (which gateway is preferred) and current status (''active'' or ''standby'').

==Operation==
The primary router with the highest configured priority will act as a ''virtual'' router with a pre-defined gateway [[Internet Protocol|IP address]] and will respond to the [[Address Resolution Protocol|ARP]] or [[Neighbor Discovery Protocol|ND]] request from machines connected to the [[LAN]] with a virtual [[MAC address]]. If the primary router should fail, the router with the next-highest priority would take over the gateway IP address and answer ARP requests with the same MAC address, thus achieving transparent default gateway failover.

{| class="wikitable"
! HSRP version || IP protocol !! Group address !! [[User Datagram Protocol|UDP]] port !! Virtual MAC address range
|-
| 1 || IPv4 || 224.0.0.2 (all routers)<ref name=ciscov2>{{citation |url=http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipapp_fhrp/configuration/15-mt/fhp-15-mt-book/fhp-hsrp-v2.html |title=First Hop Redundancy Protocols Configuration Guide, Cisco IOS Release 15M&T |publisher=[[Cisco Systems]] |accessdate=2016-05-28}}</ref> || 1985 || 00:00:0c:07:ac:XX
|-
| rowspan=2|2 || IPv4 || 224.0.0.102 (HSRP)<ref name=ciscov2/> || 1985 || 00:00:0c:9f:fX:XX
|-
| IPv6 || ff02::66 || 2029 || 00:05:73:a0:0X:XX
|}

In the virtual MAC address, Xs represent the group ID in [[hexadecimal|hex]].

HSRP is not a [[routing protocols|routing protocol]] as it does not advertise [[Internet Protocol|IP]] routes or affect the [[routing]] table in any way.{{cn|date=February 2025}}

HSRP has the ability to trigger a failover if one or more interfaces on the router go down.  This can be useful for dual branch routers each with a single link back to the [[Gateway (telecommunications)|gateway]].  If the link of the primary router goes down, the backup router will take over the primary functionality and thus retain connectivity to the gateway.

==Version 2==
Version 2 of the protocol introduces stability, scalability and diagnostic improvements. It is not compatible with version 1 HSRP.<ref name=ciscov2/> There is no RFC for version 2 of the protocol.
* Provides IPv6 support
* Increases the number of HSRP groups from 256 to 4096

==See also==
* [[Virtual Router Redundancy Protocol]] - standards-based alternative to HSRP
* [[Gateway Load Balancing Protocol]] - Cisco proprietary router redundancy solution providing load balancing
* [[Common Address Redundancy Protocol]] - open source, patent-free and unrestricted alternative to HSRP and VRRP
* [[First-hop redundancy protocols]] - List of default gateway redundancy protocols

==References==
{{reflist}}

==Further reading==
* {{Cite web |url=http://www.szukalak.ovh.org/ |title=Configuration of HSRP® Protocol on the sample network topology |last1=Gruszczyk |first1=Katarzyna |last2=Szukała |others=Supervised and led by Dawid Mentel. |first2=Krzysztof |archive-url=https://web.archive.org/web/20090606064844/http://www.szukalak.ovh.org/ |url-status=dead |archive-date=2009-06-06}}

==External links==
* [http://www.cisco.com/en/US/tech/tk648/tk362/technologies_tech_note09186a0080094a91.shtml Cisco: HSRP Features]

[[Category:Cisco protocols]]
[[Category:First-hop redundancy protocols]]