Hot Standby Routing Protocol Interface Tracking:

A Free Cisco CCNP / BCMSN Exam Training Tutorial

By Chris Bryant, CCIE #12933

Using HSRP interface tracking can be a little tricky at first, but it's a feature that can really come in handy. Basically, this feature enables the HSRP process to monitor an additional interface; the status of this interface will dynamically change the HSRP priority for a specified group.  When that interface's line protocol shows as "down", the HSRP priority of the router is reduced.  This can lead to another HSRP router on the network becoming the active router - but that other router must be configured with the preempt option.

In the following network, R2 is the primary due to its priority of 105.  R3 has the default priority of 100.  R2 will therefore be handling all the traffic sent to the virtual router's IP address of 172.12.23.10. 

If R2's Serial0 interface fails, the hosts will be unable to reach the server farm.  HSRP can be configured to drop R2's priority if the line protocol of R2's Serial0 interface goes down, making R3 the primary router.  (The default decrement in the priority when the tracked interface goes down is 10.)

R2(config)#interface ethernet0
R2(config-if)#standby 1 priority 105 preempt
R2(config-if)#standby 1 ip 172.12.23.10
R2(config-if)#standby 1 track serial0

R3(config)#interface ethernet0
R3(config-if)#standby 1 priority 100 preempt
R3(config-if)#standby 1 ip 172.12.23.10

R2#show standby
Ethernet0 - Group 1
  Local state is Active, priority 105, may preempt
  Hellotime 3 sec, holdtime 10 sec
  Next hello sent in 1.424
  Virtual IP address is 172.12.23.10 configured
  Active router is local
  Standby router is 172.12.23.3 expires in 9.600
  Virtual mac address is 0000.0c07.ac01
  2 state changes, last state change 00:01:38
  Priority tracking 1 interface, 1 up:
    Interface                  Decrement  State
    Serial0                       10       Up

R3#show standby
Ethernet0 - Group 1
  Local state is Standby, priority 100, may preempt
  Hellotime 3 sec, holdtime 10 sec
  Next hello sent in 0.624
  Virtual IP address is 172.12.23.10 configured
  Active router is 172.12.23.2, priority 105 expires in 9.452
  Standby router is local
  1 state changes, last state change 00:01:33

The show standby output on R2 shows the tracked interface, the default decrement of 10, and that the line protocol of the tracked interface is currently up.  We'll test the configuration by shutting the interface down manually.

R2(config-if)#int s0
R2(config-if)#shutdown

1d14h: %STANDBY-6-STATECHANGE: Ethernet0 Group 1 state Active -> Speak

1d14h: %LINK-5-CHANGED: Interface Serial0, changed state to administratively down

1d14h: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0, changed state to down

R2#show standby
Ethernet0 - Group 1
  Local state is Standby, priority 95 (confgd 105), may preempt
  Hellotime 3 sec, holdtime 10 sec
  Next hello sent in 0.446
  Virtual IP address is 172.12.23.10 configured
  Active router is 172.12.23.3, priority 100 expires in 9.148
  Standby router is local
  4 state changes, last state change 00:00:02
  Priority tracking 1 interface, 0 up:
    Interface                  Decrement  State
    Serial0                       10      Down  (administratively down)

Not only does the HSRP tracking work to perfection - R2 is now the standby and R3 the primary - but the show standby command even shows us that the line protocol is administratively down, rather than just "down".  Running show standby on R3 verifies that R3 now sees itself as the Active router.

R3#show standby
Ethernet0 - Group 1
  Local state is Active, priority 100, may preempt
  Hellotime 3 sec, holdtime 10 sec
  Next hello sent in 0.706
  Virtual IP address is 172.12.23.10 configured
  Active router is local
  Standby router is 172.12.23.2 expires in 8.816
  Virtual mac address is 0000.0c07.ac01
  2 state changes, last state change 00:02:34

We'll now reopen the Serial0 interface on R2.  Since we also put the preempt option on that router's HSRP configuration, R2 should take over as the Active router.

R2(config)#int s0
R2(config-if)#no shut

1d14h: %STANDBY-6-STATECHANGE: Ethernet0 Group 1 state Standby -> Active

1d14h: %LINK-3-UPDOWN: Interface Serial0, changed state to up
1d14h: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0, changed state to up

R2#show standby
Ethernet0 - Group 1
  Local state is Active, priority 105, may preempt
  Hellotime 3 sec, holdtime 10 sec
  Next hello sent in 0.852
  Virtual IP address is 172.12.23.10 configured
  Active router is local
  Standby router is 172.12.23.3 expires in 9.276
  Virtual mac address is 0000.0c07.ac01
  5 state changes, last state change 00:00:16
  Priority tracking 1 interface, 1 up:
    Interface                  Decrement  State
    Serial0                      10        Up

Just that quickly, R2 is again the Active router.   If you're running HSRP interface tracking, it's a very good idea to configure the preempt option on all routers in the HSRP group.

The #1 problem with an HSRP Interface Tracking configuration that is not working properly is a priority / decrement value issue.   As I mentioned earlier, the default decrement is 10, and that's fine with the example we just worked through.  If R2 had a priority of 120, the decrement of 10 would not be enough to make R3 the Active router. 

You can change the default decrement at the end of the standby interface command.  The following configuration would result in a priority value decrement of 25 when the tracked interface goes down.

R1(config)#int ethernet0
R1(config-if)#standby 5 track s0/0?
  <1-255>  Decrement value
  <cr>

R1(config-if)#standby 5 track s0/0 25

That does not change the decrement value for all interfaces - just the one we're tracking with that particular statement, serial0.  If we configure a second interface for tracking and do not supply a decrement value, that interface will have a decrement value of 10.

FastEthernet0/0 - Group 5
  State is Init (virtual IP in wrong subnet)
  Virtual IP address is 172.12.34.10 (wrong subnet for this interface)
  Active virtual MAC address is unknown
    Local virtual MAC address is 0000.0c07.ac05 (v1 default)
  Hello time 3 sec, hold time 10 sec
  Preemption disabled
  Active router is unknown
  Standby router is unknown
  Priority 65 (default 100)
    Track interface Serial0/0 state Down decrement 25
    Track interface Serial0/1 state Down decrement 10

Note that this interface's priority is now 65 in Group 5!  At the bottom of that output, we see that it's using the default of 100, then has 25 decremented from that because serial0/0 is down, and then another 10 decremented because serial0/1 is down.

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To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com