Redundancy: HSRP design consideration

Suppose the pair only takes care of one group per vlan (Vlan 10 for example), one is the active router the other one is the standby router (router, L3switch and MLS will be used interchangeably because Layer3 switch or multilayer switch in essence is also a router)

The active router has these:

sh run | b standby
standby 1 ip
standby 1 timers msec 500 msec 1500
standby 1 preempt
standby 1 track GigabitEthernet1/0/1 60
standby 1 track GigabitEthernet1/0/2 60
standby 1 track GigabitEthernet1/0/3 60
standby 1 track GigabitEthernet1/0/4 60
standby 1 track GigabitEthernet1/0/7 60
standby 1 track GigabitEthernet1/0/8 60
standby 1 track Port-channel1 60

The standby router also has the same standby configuration syntax, the question came to me when I was looking at this was, “Does the standby router really need to decrement itself when a link has failed? The purpose of decrementing priority value when a link has failed is to inform (or let Standby router knows when to take over) standby router to know when to take over. So does standby router really need to decrement its priority value? For what? For letting the third standby router to take over? LOL!”

HSRP is designed to work as a pair, unlike GLBP which works as a group of routers, one Active virtual gateway is the manager of the group while other active virtual forwarders as workers of the group and GLBP can have one AVG and many AVF.

So there’s no third standby router for HSRP, the third HSRP router will have unknown status instead of Active or Standby status. Yeah HSRP advocates monogamy, mistress has unknown status (third hsrp router with unknown status) when the wifey (standby router) is around LOL! :p

Standby router is a dummy router, so what it needs are HSRP timers to match Active’s and preemption, priority is a choice, the default priority is 100, as long as the active router has a higher priority than 100 it will be the boss (^_^ )b I am not saying HSRP timer mismatch is mandatory, timer mismatch can still work, but may have unexpected results.

Also, a question came to me about the need to track etherchannel link, this etherchannel link is the link that connects between these HSRP pair, why this question arose? Because when the active router fails and there’s no hello packets received for a period of (3x hello) standby router will assume the role as active.

Also another consideration is the decrement value, the number of groups needed, there’s many design considerations for HSRP. Discuss it within the comment box if you are interested.

This entry was posted in High Availability and tagged . Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s