HSRP(Hot Standby Router Protocol) is a Cisco Proprietary First-Hop Redundancy Protocol (FHRP) which is used to maintain a fault tolerant default gateway (redundant gateways).
There are three FHRP protocols available in today’s networking.
1. HSRP (Hot Standby Router Protocol):- Cisco Proprietary.
2. VRRP (Virtual Router Redundancy Protocol):- Open Standard.
3. GLBP (Gateway Load Balancing Protocol):-Cisco Proprietary, which also supports load balancing.
In this blog, we will focus only on HSRP in great detail. Firstly, we need to understand the need of a gateway redundancy protocol (for example HSRP). Let’s say we have a LAN of 5 Computers connected to a switch(SW1), and the switch is connected to a router(R1) as shown in the picture below.
If the router (R1) fails or the link between R1 and SW1 fails, the Hosts in the LAN will not be able to reach the devices located beyond this LAN. To avoid such a situation, we should have at least two routers used for default Gateway(as sown in the picture below).
Note:- HSRP can be configured on Cisco Routers as well as Multi layer switches.
HSRP is not a routing protocol and it has no impact on routing protocol.
Now, we completely understood the need of gateway redundancy protocol like HSRP.
There are 2 versions of HSRP (Version 1 and version 2).They are pretty much similar with little difference. However, version 1 and version 2 are not compatible with each other.
HSRP v1 uses the Multicast address 224.0.0.2 with UDP port 1985 for sending the Hello Messages. It uses virtual MAC address 00:00:0c:07:ac:XX.
HSRP v2 uses the Multicast address 224.0.0.102 with UDP port 1985 for sending the Hello Messages for IPv4. It uses virtual MAC address 00:00:0c:9f:fX:XX for IPv4.
HSRP v2 uses the Multicast address ff02::66 with UDP port 2029 for sending the Hello Messages for IPv6. It uses virtual MAC address 00:05:73:a0:0X:XX for IPv6.
Note:- X represents HSRP group number in Hexadecimal.