HSRP est un protocole de redondance de niveau 3, Active/Standby Failover, propriétaire cisco, créé en 1994.

Le principe est d’avoir une IP/mac virtuelle partagée entre plusieurs routeurs. Seul un routeur est actif (Active) à la fois, celui qui a la priorité la plus élevée. Les autres sont en état « Standby ».
Le principe est d’envoyer pédiodiquement des messages « Hello » à une adresse multicast: 224.0.0.2 pour HRSPv1 et 224.0.0.102 pour HSRPv2.

Configuration sur un c2621XM Cisco IOS 12.4(15)T14:

philb@core2:~$ ssh router
Password:
Authorized
router>enable
Password:
router#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
router(config)#interface FastEthernet0/1.2
router(config-subif)#standby version 2
router(config-subif)#standby 1 ip 192.168.100.33
router(config-subif)#standby 1 priority 110
router(config-subif)#standby 1 preempt
router(config-subif)#standby 1 authentication md5 key-string 0 motdepasse
router(config-subif)#standby 1 name hsrpPhilbLocal
router(config-subif)#^Z
router#

Configuration sur un c1803 Cisco IOS 15.1(2)T1

philb@core2:~$ ssh c1803
Password: 

c1803>enable
Password:
c1803#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
c1803(config)#interface Vlan2
c1803(config-if)#standby version 2
c1803(config-if)#standby 1 ip 192.168.100.33
c1803(config-if)#standby 1 preempt
c1803(config-if)#standby 1 authentication md5 key-string 0 motdepasse
c1803(config-if)#standby 1 name hsrpPhilbLocal
c1803(config-if)#^Z
c1803#

Vérification en analysant les trames réseaux:

root@core2:~# tcpdump -ni eth0
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
22:00:18.713324 IP 192.168.100.62.1985 > 224.0.0.102.1985: HSRPv1
22:00:19.247917 IP 192.168.100.61.1985 > 224.0.0.102.1985: HSRPv1
22:00:21.713573 IP 192.168.100.62.1985 > 224.0.0.102.1985: HSRPv1
22:00:21.968158 IP 192.168.100.61.1985 > 224.0.0.102.1985: HSRPv1

Vérification par ping:

root@core2:~# ping 192.168.100.33
PING 192.168.100.33 (192.168.100.33) 56(84) bytes of data.
64 bytes from 192.168.100.33: icmp_req=1 ttl=255 time=3.45 ms
64 bytes from 192.168.100.33: icmp_req=2 ttl=255 time=2.04 ms
64 bytes from 192.168.100.33: icmp_req=3 ttl=255 time=2.01 ms
^C
--- 192.168.100.33 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2002ms
rtt min/avg/max/mdev = 2.014/2.503/3.450/0.669 ms
root@core2:~#

Vérification de la table arp

root@core2:~# arp -an
? (192.168.100.61) at 00:21:a0:69:fa:80 [ether] on eth0
? (192.168.100.62) at 00:0f:23:a1:57:41 [ether] on eth0
? (192.168.100.33) at 00:00:0c:9f:f0:01 [ether] on eth0

Vérification de l’état HSRP sur les routeurs:

c1803#show standby
Vlan2 - Group 1 (version 2)
  State is Standby
    1 state change, last state change 00:09:01
  Virtual IP address is 192.168.100.33
  Active virtual MAC address is 0000.0c9f.f001
    Local virtual MAC address is 0000.0c9f.f001 (v2 default)
  Hello time 3 sec, hold time 10 sec
    Next hello sent in 0.048 secs
  Authentication MD5, key-string
  Preemption enabled
  Active router is 192.168.100.62, priority 110 (expires in 10.704 sec)
    MAC address is 000f.23a1.5741
  Standby router is local
  Priority 100 (default 100)
  Group name is "hsrpPhilbLocal" (cfgd)
c1803#

router#sh standby
FastEthernet0/1.2 - Group 1 (version 2)
  State is Active
    1 state change, last state change 00:14:36
  Virtual IP address is 192.168.100.33
  Active virtual MAC address is 0000.0c9f.f001
    Local virtual MAC address is 0000.0c9f.f001 (v2 default)
  Hello time 3 sec, hold time 10 sec
    Next hello sent in 2.176 secs
  Authentication MD5, key-string "motdepasse"
  Preemption enabled
  Active router is local
  Standby router is 192.168.100.61, priority 100 (expires in 9.420 sec)
  Priority 110 (configured 110)
  Group name is "hsrpPhilbLocal" (cfgd)
router#