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Hot Standby Routing Protocol Interface Tracking:
A CCNP SWITCH Exam Training Tutorial
By Chris Bryant, CCIE #12933
Now includes an exclusive Video Practice Exam On HSRP!
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#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
We'll continue this CCNP SWITCH tutorial after this brief and important message!
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.
Whew!
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