HSRP (Hot Standby Routing Protocol) is a protocol that makes High-Availability available and makes almost immediate cutover to a secondary router if the primary router fails. If is defined in RFC 281. On MLS, HSRP can be configured on routed ports, SVIs and Etherchannels with IP addresses. However, to run HSRP on a L3 Switch, it must have Enhanced Mulilayer Software IMage (EMI).
HA is available because HSRP creates a virtual router complete with a virtual IP address and MAC address for HSRP router group. In that way, the host will communicate to the virtual router, without knowing the actually physical router behind it. If the primary (hot) router fails, then the virtual router will switch the traffic to a secondary (standby) router almost instantly. HSRP has also an interface tracking feature, where it can monitor interfaces on the same router group and will be able to reduce to the priority of a router if its interface is down.
HSRP has 7 states:
1. Disabled: The interface is not running HSRP at all
2. Initial (Init): Happens when a router with HSRP comes up
3. Learn: The state where a router waits to other active router's in the group and waits for its Virtual IP Address.
4. Listen: Router knows its Virtual IP address, but still listens for hello packets from other routers.
5. Speak: The router sends hello messages and participate in the election for a primary or standby router.
6. Standby: The router sends hello messages because it is a candidate to become an active router.
7. Active: The router is active and forwards packets to the Virtual IP address.
To configure a router for HSRP:
(config)#interface fa
(config-if)#standby ip // standby command is used to do HSRP configuration, group-number is used to define the router group
To show HSRP configuration type the command:
RX#show standby
The default MAC address of the virtual router is 0000-0C07-ACXX where XX is the number of the router group in hexadecimal. So, if the router group is 26 then the hexadecimal number is 16+10 which is 1a so the default MAC address will be 0000-0C07-AC1A. To change the MAC address of the virtual router put the command:
(config-if)#standby mac-address // mac-address should be noted as XXXX.XXXX.XXXX
To change hello intervals on HSRP type the command:
(config-if)#standby timers
Router with higher priority will be the default servicing router. To change priority of a router:
(config-if)#standby priority
However, this command will not change the service directly to a more higher priority number, but will be applied if the current servicing router is restarted. To be able to switch service to the higher priority number directly, the command preempt should be used. There will be also a message on the router indicating the state change.
(config-if)#standby preempt
-- 23 November 2010 --
HA is available because HSRP creates a virtual router complete with a virtual IP address and MAC address for HSRP router group. In that way, the host will communicate to the virtual router, without knowing the actually physical router behind it. If the primary (hot) router fails, then the virtual router will switch the traffic to a secondary (standby) router almost instantly. HSRP has also an interface tracking feature, where it can monitor interfaces on the same router group and will be able to reduce to the priority of a router if its interface is down.
HSRP has 7 states:
1. Disabled: The interface is not running HSRP at all
2. Initial (Init): Happens when a router with HSRP comes up
3. Learn: The state where a router waits to other active router's in the group and waits for its Virtual IP Address.
4. Listen: Router knows its Virtual IP address, but still listens for hello packets from other routers.
5. Speak: The router sends hello messages and participate in the election for a primary or standby router.
6. Standby: The router sends hello messages because it is a candidate to become an active router.
7. Active: The router is active and forwards packets to the Virtual IP address.
To configure a router for HSRP:
(config)#interface fa
(config-if)#standby
To show HSRP configuration type the command:
RX#show standby
The default MAC address of the virtual router is 0000-0C07-ACXX where XX is the number of the router group in hexadecimal. So, if the router group is 26 then the hexadecimal number is 16+10 which is 1a so the default MAC address will be 0000-0C07-AC1A. To change the MAC address of the virtual router put the command:
(config-if)#standby
To change hello intervals on HSRP type the command:
(config-if)#standby
Router with higher priority will be the default servicing router. To change priority of a router:
(config-if)#standby
However, this command will not change the service directly to a more higher priority number, but will be applied if the current servicing router is restarted. To be able to switch service to the higher priority number directly, the command preempt should be used. There will be also a message on the router indicating the state change.
(config-if)#standby
-- 23 November 2010 --
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