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CCNP - BCMSN ( 642-812 )
The HSRP MAC Address
To pass the BCMSN exam and earn your CCNP,
you've got to know HSRP inside and out! Part of that is knowing how
the MAC address of the virtual router is derived, and another part
is knowing how to change this address. We'll look at both features
in this tutorial.
We've got two routers on a segment running HSRP, so first we need to
find out what the MAC address of the HSRP virtual router is. The
show command for HSRP is show standby, and it's the first command
you should run while configuring and troubleshooting HSRP. Let's run
it on both routers and compare results.
R2#show standby
Ethernet0 - Group 5
Local state is Standby, priority 100
Hellotime 3 sec, holdtime 10 sec
Next hello sent in 0.776
Virtual IP address is 172.12.23.10 configured
Active router is 172.12.23.3, priority 100 expires in 9.568
Standby router is local
1 state changes, last state change 00:00:22
And on the 2nd router...
R3#show standby
Ethernet0 - Group 5
Local state is Active, priority 100
Hellotime 3 sec, holdtime 10 sec
Next hello sent in 2.592
Virtual IP address is 172.12.23.10 configured
Active router is local
Standby router is 172.12.23.2 expires in 8.020
Virtual mac address is 0000.0c07.ac05
2 state changes, last state change 00:02:08
R3 is in Active state, while R2 is in Standby. The hosts are using
the 172.12.123.10 address as their gateway, but R3 is actually
handling the workload. R2 will take over if R3 becomes unavailable.
An IP address was statically assigned to the virtual router, but not
a MAC address. However, there is a MAC address under the show
standby output on R3, the active router. How did the HSRP process
arrive at a MAC of 00-00-0c-07-ac-05?
Well, most of the work is already done before the configuration is
even begun. The MAC address 00-00-0c-07-ac-xx is reserved for HSRP,
and xx is the group number in hexadecimal. That's a good skill to
have for the exam, so make sure you're comfortable with hex
conversions. The group number is 5, which is expressed as 05 with a
two-bit hex character. If the group number had been 17, we'd see 11
at the end of the MAC address - one unit of 16, one unit of 1.
On rare occasions, you may have to change the MAC address assigned
to the virtual router. This is done with the standby mac-address
command. Just make sure you're not duplicating a MAC address that's
already on your network!
R2(config-if)#standby 5 mac-address
0000.1111.2222
1d12h: %STANDBY-6-STATECHANGE: Ethernet0 Group 5 state Active ->
Learn
R2#show standby
Ethernet0 - Group 5
Local state is Active, priority 150, may preempt
Hellotime 4 sec, holdtime 12 sec
Next hello sent in 3.476
Virtual IP address is 172.12.23.10 configured
Active router is local
Standby router is 172.12.23.3 expires in 10.204
Virtual mac address is 0000.1111.2222 configured
4 state changes, last state change 00:00:00
1d12h: %STANDBY-6-STATECHANGE: Ethernet0 Group 5 state Listen ->
Active
The MAC address will take a few seconds to change, and the HSRP
routers will go into Learn state for that time period.
A real-world HSRP troubleshooting note: If you see constant state
changes with your HSRP configuration, do what you should always do
when troubleshooting - check the physical layer first. Best of luck
on your BCMSN exam!