Hot Standby Router Protocol

HSRP (Cisco-proprietary) lets two or more routers share one virtual IP that clients use as their default gateway. One router is active and answers for the virtual IP; a standby router takes over when the active one stops sending hellos.

flowchart LR
  pcs[PCs] --> vip((10.0.0.1))
  vip ==> r1["R1 · active<br/>priority 110"]
  vip -.-> r2["R2 · standby<br/>priority 100"]
  r1 <-.->|hellos| r2

Configuration

Configure the same group number and virtual IP on every router; only the priority differs.

R1

R1# configure terminal
R1(config)# interface GigabitEthernet0/0
R1(config-if)# ip address 10.0.0.2 255.255.255.0
R1(config-if)# standby version 2
R1(config-if)# standby 1 ip 10.0.0.1
R1(config-if)# standby 1 priority 110
R1(config-if)# standby 1 preempt
R1(config-if)# standby 1 track GigabitEthernet0/1 20
R1(config-if)# end
R1# write memory

Command	Effect
standby <group> ip <vip>	Joins the group and sets the virtual IP.
standby <group> priority <0-255>	Highest priority becomes active; ties go to the highest interface IP. Default 100.
standby <group> preempt	Lets a higher-priority router take the active role back after it recovers.
standby <group> track <if> <n>	Lowers the priority by n when the tracked (uplink) interface goes down.
standby <group> timers <hello> <hold>	Hello and hold time in seconds. Default 3 and 10.
standby version 2	Groups 0–4095, IPv6 support and millisecond timers. Must match on all routers.

Tip

Without preempt, the router that became active stays active even after a router with a higher priority comes back. Tracking only helps together with preempt on the standby router.

States

stateDiagram-v2
  [*] --> Listen: interface up
  Listen --> Speak: no active router heard
  Speak --> Active: highest priority
  Speak --> Standby: second highest
  Standby --> Active: hold time expired
  Active --> Speak: preempted

Before Listen a router passes Initial and Learn (waiting for the virtual IP). Routers that are neither active nor standby stay in Listen.

Verification

Command	Shows
show standby brief	Group, priority, state, active and standby router per interface.
show standby	Details including timers, tracked objects and the virtual MAC.
debug standby events	State changes as they happen.

The virtual MAC is 0000.0c07.acXX for version 1 and 0000.0c9f.fXXX for version 2, where X is the group number in hex. Clients keep this MAC in their ARP cache, so a failover needs no ARP update.