Sunday, April 19, 2009

default gateway timing

I came across a default gateway lab scenario that initially had me stumped. The requirement could be achieved via standard HSRP or VRRP configuration.

So the basic solution making R2 the active default gateway is as follows:

interface FastEthernet0/0
vrrp 1 ip
vrrp 1 preempt

interface GigabitEthernet0/1
vrrp 1 ip
vrrp 1 preempt
vrrp 1 priority 105

The sting in the requirement (and in my experience with CCIE there usually is one) is to ensure that if R2 has relinquished the active default gateway role, is to not allow resumption of the role, until gi0/1 has been up for a minimum of 5 minutes.

My first thought was to adjust the VRRP timers. INCORRECT since the max timer is 255 seconds - not eneough.

R2(config-if)#vrrp 1 timers advertise ?
<1-255> Advertisement interval in seconds

My second thought was introduce a delay on the interface tracking. INCORRECT since the max timer is 180 seconds - not enough

R2(config)#track 1 interface Fastethernet0/0 line-prot
R2(config-track)#delay up ?
<0-180> Seconds to delay

I thought about combining the two features together but this felt a little too convoluted.

After some further investigation i discovered there is a delay parameter on the preempt. This features has a parameter allowing up to 3600 seconds or 1 hour delay.

R2(config-if)#vrrp 1 preempt delay min ?
<0-3600> Seconds to delay

Hence i added the following line to my lab solution

R2(config-if)#vrrp 1 preempt delay min 300

As ever, it was easy, once i knew how:-)

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