HSRP Preemption delay and downtime

[AK59]

Hello everybody,

I have a little question regarding downtime and HSRP. 

I plan to replace old routers by new ones. Configuration are the same except the IP addresses.  

My plan is to add the new routers to the actual HSRP group and then modify the priority number so that the new routers preempt the old ones and became primary. 

My question is : Will there be a downtime or will it be 'transparent' ? 

Thanks in advance, 

[balaji.bandi]

I do not see any down time, as long as you follow right proceedure.

I leave Active one to process the traffic, and replace the standby one with new router, when  the active go down, standby pickup the active role, here you see sub millseonds convergance (user should not notice any interuption) - until you have very legacy application, which required some TCP session open all time, may have some break.

Notes i have noed learning things below :

The basic behavior in HSRP is that once a router has become the active HSRP device it will continue as active even if some other device obtains a priority that is higher than the currently active device. Use of preemtion allows a device whose priority has become higher to take over the role as the active router in HSRP.

[Georg Pauwen]

Hello,

if you want to completely eliminate any down time (during business hours), you could also configure an EEM script and run it e.g. at midnight, so the failover will occur then:

event manager applet HSRP_PREEMPT
event timer cron cron-entry "0 0 * * *"
action 1.0 cli command "enable"
action 2.0 cli command "conf t"
action 3.0 cli command "interface GigabitEthernet0/0"
action 4.0 cli command "standby 1 preempt"
action 5.0 cli command "end"

[ThrowingPackets86]

As said here before, and the way I have done this is to stagger the replacements. So that your Active router is not disturbed. Should be pretty easy process.

[Joseph W. Doherty]

I've, in the past, changed the active/hot HSRP router (with preempt) on multiple occassions, and don't recall anyone noticing a network interruption. So, it should be, more or less, transparent. (NB: In the past, I've also suggested using HSRP, even when there's only one router, so that you can replace it w/o interrupting the gateway.)

That said, it's always best to do any kind of changes during a maintenance period, or a least during minimal business usage, to minimize the impact of any unexpected result.

[mlund]

Hi

As noted by the others, there will be no downtime. If you have preempt, and then configure a higher priority, the router new router will take over the active role. When it does so it will send at a gratitious arp so that switches can update there cam tables.

Before you disconnect the old routers, be sure that the routing is correct for the return traffic. Otherwise there will be traffic drops during routing recalculation.

/Mikael