HSRP optimization options

Three options are available to optimize the operation of HSRP in the campus network:

HSRP Standby Preempt
In the event of an active router failure, a standby router will assume the role of active router. When the original active router comes back on-line, this new active router will retain the role of active router even when the former active router has a higher priority. The Preempt option allows a router to regain its role of active router even if there is an existing active router on the segment.

HSRP Hello Message Timer Adjustment
An HSRP-enabled router sends hello messages to indicate that the router is running and is capable of becoming either the active or standby router. The hello message contains the priority of the router as well as hellotime and holdtime parameter values. The hellotime parameter value indicates the interval between the hello messages that the router sends. The holdtime parameter value indicates the amount of time that the current hello message is considered valid. The standby timer includes an "msec" parameter for faster failovers.

HSRP Interface Tracking
In some situations, it is not the status of the HSRP interface itself that should cause an HSRP failover but the failure of another route used by the active HSRP router. For example if the active router has a WAN link back to corporate, and that WAN link goes down, it would be desirable for the LAN interface of that router to give up its role as active HSRP router so it could be assumed by a standby router with a functioning link back to corporate. In this case, HSRP would be configured to track the status of the WAN interface and give up its roll as active router if its WAN interface went down.

HSRP Standby Preempt
The standby router automatically assumes the active router role when the active router fails or is removed from service. This new active router remains the forwarding router even when the former active router with the higher priority regains service in the network.

The former active router can be configured to resume the forwarding router role from a router with a lower priority. To enable a router to resume the forwarding router role, enter the following command in interface configuration mode:

Switch(config-if)#standby [group-number] preempt [{ delay } [ minimum delay ] [sync delay]]

When the standby preempt command is issued, the interface changes to the appropriate state.

To remove the interface from preemptive status, enter the no standby group preempt command.

Example: Displaying HSRP Preempt
The following example states that interface VLAN10 is configured to resume its role as the active router in HSRP group 47, assuming that interface VLAN10 on this router has the highest priority in that standby group.

Switch#show running-config
Building configuration...
Current configuration:
!
(text deleted)
interface Vlan10
ip address 172.16.10.82 255.255.255.0
no ip redirects
standby 47 priority 150
standby 47 preempt
standby 47 ip 172.16.10.110

Hello Message Timers
An HSRP-enabled router sends hello messages to indicate that the router is running and is capable of becoming either the active or standby router.

The hello message contains the priority of the router and also hellotime and holdtime parameter values. The hellotime parameter value indicates the interval between the hello messages that the router sends. The holdtime parameter value indicates the amount of time that the current hello message is considered valid.

If an active router sends a hello message, receiving routers consider that hello message to be valid for one holdtime. The holdtime value should be at least three times the value of the hellotime. The holdtime value must be greater than the value of the hellotime.

Both the hellotime and the holdtime parameters are configurable. To configure the time between hello messages and the time before other group routers declare the active or standby router to be nonfunctioning, enter this command in interface configuration mode:

Switch(config-if)#standby group-number timers hellotime holdtime

HSRP Interface Tracking
In some situations, the status of an interface directly affects which router needs to become the active router. This is particularly true when each of the routers in an HSRP group has a different path to resources within the campus network.

In this example , router A and router B reside in a branch office. These two routers each support a T1 link to the headquarters. Router A has the higher priority and is the active forwarding router for standby group 47. Router B is the standby router for that group. Router A and B are exchanging hello messages through their E0 interfaces.

The T1 link between the active forwarding router for the standby group and the headquarters experiences a failure. Without HSRP enabled, router A would detect the failed link and send an ICMP redirect to router B. However, when HSRP is enabled, ICMP redirects are disabled. Therefore, neither router A nor the virtual router sends an ICMP redirect. In addition, although the S1 interface on router A is no longer functional, router A still communicates hello messages out interface E0, indicating that router A is still the active router. Packets sent to the virtual router for forwarding to the headquarters cannot be routed.

Interface tracking enables the priority of a standby group router to be automatically adjusted, based on availability of the interfaces of that router. When a tracked interface becomes unavailable, the HSRP priority of the router is decreased. The HSRP tracking feature reduces the likelihood that a router with an unavailable key interface will remain the active router.

In this example , the E0 interface on router A tracks the S1 interface. If the link between the S1 interface and headquarters fails, the router automatically decrements the priority on that interface and stops transmitting hello messages out interface E0. Router B assumes the active router role when no hello messages are detected for the specific holdtime period.

Configuring HSRP Tracking
To configure HSRP tracking, enter this command in interface configuration mode:

To disable interface tracking, enter the no standby group track command.  – 

The command to configure HSRP tracking on a multilayer switch is the same as on the external router, except that the interface type can be identified as a switch virtual interface ( vlan followed by the vlan number assigned to that interface) or by a physical interface.

The internal routing device uses the same command as the external routing device to disable interface tracking.

Multiple tracking statement may be applied to an interface. An example of where this may be useful would be where the currently active HSRP interface would only relinquish its status upon the failure of two (or more) tracked interfaces.

Lab Activity

Lab Exercise 1: Hot Standby Router Protocol

Configure Hot Standby Router Protocol (HSRP) on a pair of routers to provide redundant fault tolerant router services to a network.

Lab Activity

Lab Exercise 2 Multigroup Hot Standby Router Protocol

Configure Multigroup Hot Standby Router Protocol (MHSRP) on a pair of routers to provide redundant router services to a network.