Answers: HSRP 2
This latest #HSRP lab requires you to think about the variety of optional settings and choose which non-default settings need to be configured. For instance, how do you control which router becomes primary, and whether that router preempts the other router if it fails and later recovers? This latest lab works through those options. As always, check the original lab post first, and then check your answers here in the matching answers post.
Figure 1: HSRP Design, One LAN Switch
Figure 2: HSRP Design, Two LAN Switches
Answers
Example 5: R1 Config
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interface GigabitEthernet0/1 standby 20 ip 172.16.100.126 standby version 2 standby 20 name HSRPLab standby 20 priority 110 |
Example 6: R2 Config
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interface GigabitEthernet0/1 standby 20 ip 172.16.100.126 standby version 2 standby 20 name HSRPLab standby 20 priority 90 |
Commentary
Hot Standby Router Protocol (HSRP) is a Cisco proprietary first hop redundancy protocol (FHRP) that enables hosts to have redundant gateways. This is important because many LANs are configured with a single gateway that is used by all devices. If this single gateway were to fail, then all of the devices are left without access to external networks (not on the local LAN).
With HSRP, multiple routers reference a single IP address to be used as the default gateway. One router actively provides the data plane function of receiving packets from hosts, while the other waits to take over if the first router fails.
First, look at the two IP addresses in subnet 172.16.100.120/29 that we know about from the router configuration: 172.16.100.122 and .123. Then look at the figure, and see that that two PCs have been assigned 172.16.100.124 and 172.16.100.125. That leaves two usable addresses: the .121 and .126 address. (The 172.16.100.120/29 subnet has subnet ID 172.16.100.120, subnet broadcast address 172.16.100.127, with the addresses between those numbers available for use.) So .126 is available, and is the highest IP address in the subnet.
So the standby 20 ip 172.16.100.126 command sits in both routers’ configurations, under interface G0/1.
For the requirements to name the HSRP group and to set the HSRP version, two obvious commands on each router fill those requirements: the standby 20 name HSRPLab command and the standby version 2 command.
The default priority (which determines which router will become active) is 100. Per the directions, R1 has its HSRP priority set to 110 using the standby 20 priority 110 command, with R2’s value set to 90 with the standby 20 priority 90 command.
Finally, the lab asks (with subtle wording) to make sure R1 does not preempt R2 if it fails and then recovers. HSRP uses a default of no standby 20 preempt, that is, by default HSRP does not preempt. So the R1 configuration did not need the addition of a no standby 20 preempt command.
Wendell
I configured two real C3825 routers in the above configuration. I also triple checked the show standby command that Preemption was disabled on both routers. I then did a shut on the R1 int gi0/0 (that’s how my router has it named) and R2 took over. I then did a no shut on R1 int gi0/0 and R1 still took over and R2 went from active to standby. What did I do wrong?
Hi Jonathan,
Thanks for the note.
I can’t tell what went wrong from what you wrote. It’s a pretty easy thing to test, so I’m sure that’s why neither of us has a good guess as to what’s happening. I’d suggest (just things to try) if you want to try more.
term mon on both routers when you test, to make sure you see all log messages.
Make sure the router interfaces aren’t failing and recovering as part of the test. That would drive the HSRP process to elect a new active router. (That is – when you test, at no point does R2’s interface go down – right?
I hate to go here on something so simple, but it’s possible that it’s a bug.
I’ll mark it as something to test myself in lab, just for grins. Easily done. I’ll be in lab a lot this week. I’ll post back.
Wendell
Might want to make a note that PT doesn’t support ‘standby name’.
Hi Wendell,
how you confirm or verify if the passive router will become active once active router failed? Also, will it work if the host/client is statistically configured with the GW? I am bit confused.
Hi Riol,
Indeed, the passive router takes over for the active when the active fails. Both function, one wins the election and is active, while the other monitors the active router ready to take over. When the active router fails, the passive router takes over.
All that happens independently from the hosts. The hosts are unaware. So, whether statically configured with IP settings or learned with DHCP, as long as the default gateway of the host is set to the address managed by HSRP, it all works.
FYI, by design, these labs let you practice things you’ve read about already, so if you want some more background, check out CCNA 200-301 Official Cert Guide, Vol 2, Chapter 12, for more on HSRP.