Introduction
This document describes how thestandby preempt andstandby track commands work together and how to use them.
Prerequisites
Requirements
There are no specific requirements for this document.
Components Used
The information in this document is based on these software versions:
Routers running Cisco IOS®Software Release 12.2(10b)
The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, ensure that you understand the potential impact of any command.
Conventions
Refer toCisco Technical Tips Conventions for more information on document conventions.
Background Information
Thestandby preemptcommand enables the Hot Standby Router Protocol (HSRP) router with the highest priority to immediately become the active router. Priority is determined first by the configured priority value, and then by the IP address. In each case, a higher value is of greater priority. When a higher priority router preempts a lower priority router, the router sends a Coup message. When a lower priority active router receives a Coup message or a Hello message from an active, higher priority router, the router changes to the Speak state and sends a resign message.
Thestandby trackcommand allows you to specify another interface on the router for the HSRP process to monitor in order to alter the HSRP priority for a given group. If the line protocol of the specified interface goes down, the HSRP priority is reduced. This means that another HSRP router with higher priority can become the active router if that router has thestandby preempt enabled.
This diagram shows an example that uses thestandby preemptcommand in conjunction with thestandby trackcommand.
Standby Preempt and Standby Track Configuration
Configuration Examples
In the network diagram, HSRP is configured in this manner:
R1 is the active router and tracks the R1 Serial 0 interface state. When R1 is the active router all the traffic from the hosts (Host 1, 2, 3) to the servers is routed through R1.
R2 is the standby router and tracks the R2 Serial 1 interface state.
If the R1 Serial 0 interface goes down, the R1 HSRP priority is decreased by 10. At this point the R2 HSRP priority is higher than R1, and R2 takes over as the active router.
When R2 becomes the active router all the traffic from the hosts to the servers is routed through R2.
Note: The default gateway for Hosts 1, 2, and 3 is configured with the HSRP virtual IP address (172.16.6.100, in this case). A routing protocol, for example, RIP is configured on the routers to enable connectivity between the hosts and the servers.
This is the configuration for each router:
| Router 1 ( Cisco 2503) |
|---|
interface Ethernet0 ip address 172.16.6.5 255.255.255.0!--- Assigns an IP address to the interface. no ip redirects standby 1 ip 172.16.6.100!--- Assigns a standby group and standby IP address standby 1 priority 105!--- Assign a priority (105 in this case) to the router interface (e0) |
| Router 2 ( Cisco 2503) |
|---|
interface Ethernet0 ip address 172.16.6.6 255.255.255.0!--- Assigns an IP address to the interface. no ip redirects standby 1 ip!--- Indicates the hot standby group. Here the IP address of the virtual |
Note: R2 does not have a standby IP address configured. This is intentional in order to demonstrate that this is a valid configuration. When R1 and R2 exchange HSRP hellos, R2 learns the standby IP address from R1. To configure R2 with a standby IP address (same standby address configured on R1) is also a valid configuration.
R1#show standbyEthernet0 - Group 1Local state is Active, priority 105, may preemptHellotime 3 sec, holdtime 10 secNext hello sent in 1.458Virtual IP address is 172.16.6.100 configuredActive router is localStandby router is 172.16.6.6 expires in 8.428Virtual mac address is 0000.0c07.ac012 state changes, last state change 02:09:49IP redundancy name is "hsrp-Et0-1" (default)Priority tracking 1 interface, 1 up:Interface Decrement StateSerial0 10 Up R2#show standbyEthernet0 - Group 1Local state is Standby, priority 100, may preemptHellotime 3 sec, holdtime 10 secNext hello sent in 1.814Virtual IP address is 172.16.6.100Active router is 172.16.6.5, priority 105 expires in 9.896Standby router is local3 state changes, last state change 00:10:21IP redundancy name is "hsrp-Et0-1" (default)Priority tracking 1 interface, 1 up:Interface Decrement StateSerial1 10 Up
Although R2 does not have an HSRP priority configured, the output shows the R2 priority as the HSRP default value, 100. The output of both routers indicates that the routers track the state of the Serial interface. The state of R1 is active and the state of R2 is standby. Lastly, both routers are configured with the standby preempt command.
What happens if the serial 0 interface of R1 goes down? The output of the show standby command looks like this:
R1#show standbyEthernet0 - Group 1Local state is Standby, priority 95 (confgd 105), may preemptHellotime 3 sec, holdtime 10 secNext hello sent in 2.670Virtual IP address is 172.16.6.100 configuredActive router is 172.16.6.6, priority 100 expires in 8.596Standby router is local4 state changes, last state change 00:01:45IP redundancy name is "hsrp-Et0-1" (default)Priority tracking 1 interface, 0 up:Interface Decrement StateSerial0 10 Down R2#show standbyEthernet0 - Group 1Local state is Active, priority 100, may preemptHellotime 3 sec, holdtime 10 secNext hello sent in 0.810Virtual IP address is 172.16.6.100Active router is localStandby router is 172.16.6.5 expires in 9.028Virtual mac address is 0000.0c07.ac014 state changes, last state change 00:01:38IP redundancy name is "hsrp-Et0-1" (default)Priority tracking 1 interface, 1 up:Interface Decrement StateSerial1 10 Up
Notice in the output that the HSRP priority of R1 is reduced by 10 to 95. This change makes the R2 priority of 100 higher. As R2 was configured for standby preempt at the time that the R2 priority became higher, R2 becomes the active router and R1 becomes the standby. Now, if the R1 serial 0 interface comes back up, the R1 priority is 105, again. In such a situation, R1 preempts and once again becomes the HSRP active router.
Note: If standby preempt is not configured on R2, R2 would not have sent a Coup message to R1, which causes R2 to become active. Instead R1 would have remained the active router.
For example, consider this configuration:
standby priority 120standby track serial 0standby track serial 1
An HSRP priority of 120 is configured with the standby priority command and HSRP is configured to track the state of two interfaces, Serial0 and Serial1. Because no decrement value is specified in the standby track command, the HSRP priority is decremented by the default value of 10 when the tracked interface goes down. Initially, both the interfaces are up and the HSRP priority of the interface is 120, as in the show standby command output:
R1#show standbyEthernet0 - Group 1 Local state is Active, priority 120, may preempt Hellotime 3 sec, holdtime 10 sec Next hello sent in 1.034 Virtual IP address is 10.0.0.5 configured Active router is local Standby router is unknown Virtual mac address is 0000.0c07.ac01 2 state changes, last state change 00:00:04 IP redundancy name is "hsrp-Et0-1" (default) Priority tracking 2 interfaces, 2 up: Interface Decrement State Serial0 10 Up Serial1 10 Up
Now, interface Serial 0 is brought down.
R1#1w0d: %LINK-3-UPDOWN: Interface Serial0, changed state to down1w0d: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0, changed state to downR1#
This reduces the HSRP priority by 10 from 120 to a value of 110. You can use the show standby command to verify this:
R1#show standbyEthernet0 - Group 1 Local state is Active, priority 110 (confgd 120), may preempt Hellotime 3 sec, holdtime 10 sec Next hello sent in 2.544 Virtual IP address is 10.0.0.5 configured Active router is local Standby router is unknown Virtual mac address is 0000.0c07.ac01 2 state changes, last state change 00:00:48 IP redundancy name is "hsrp-Et0-1" (default) Priority tracking 2 interfaces, 1 up: Interface Decrement State Serial0 10 Down (line protocol down) Serial1 10 Up
Next, the second tracked interface—Serial 1—is brought down:
R1#1w0d: %LINK-3-UPDOWN: Interface Serial1, changed state to down1w0d: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state to downR1#
This again reduces the HSRP priority by 10 from 110 to a value of 100. You can use the show standby command to verify this:
R1#show standbyEthernet0 - Group 1 Local state is Active, priority 100 (confgd 120), may preempt Hellotime 3 sec, holdtime 10 sec Next hello sent in 1.846 Virtual IP address is 10.0.0.5 configured Active router is local Standby router is unknown Virtual mac address is 0000.0c07.ac01 2 state changes, last state change 00:01:06 IP redundancy name is "hsrp-Et0-1" (default) Priority tracking 2 interfaces, 0 up: Interface Decrement State Serial0 10 Down (line protocol down) Serial1 10 Down (line protocol down)
When any one of the tracked serial interfaces (either serial 0 or serial 1) go down, the resultant priority is 110. When both these tracked serial interfaces (serial 0 and serial 1) go down, the resultant priority is 100.
Related Information
- HSRP Support Page
