I was working with VRRP on a small setup. I came across command switch-back-delay. According to us we can set MIN and MAX limit for switch-back-delay by changing the code in our program, but I wanted to know whats the MIN and MAX limit for VRRP switch-back-delay and why? I saw in RFC there I could not find the answer.

Thanks in advance.

  • @Singh_007 Are we talking about any specific platform? – Ryan Foley Nov 22 '14 at 2:31
  • No. Not to any specific platform. In general according standards there will be some limiting value right ? – Singh_007 Nov 25 '14 at 8:49

I’ve never heard the term switch-back-delay, nor can I find anything referencing it on the internet or in RFC 5798.

The you’re really looking for is Preemption Hold Time. This is the time a higher priority VRRP speaker will wait until preemption of the mastership role happens. There isn't a hold timer reference in the RFC, so that’s a vendor specific implementation.

VRRP only operates in 3 different modes.

                  +--------->|               |<-------------+
                  |          |  Initialize   |              |
                  |   +------|               |----------+   |
                  |   |      +---------------+          |   |
                  |   |                                 |   |
                  |   V                                 V   |
          +---------------+                       +---------------+
          |               |---------------------->|               |
          |    Master     |                       |    Backup     |
          |               |<----------------------|               |
          +---------------+                       +---------------+

RFC 5798 - 6.3. State Transition Diagram

When a router with a higher priority becomes active and has the preemption flag set, it actively tries to usurp that state. But in some instances, that router might have something wrong with it. Maybe it’s flapping for some reason. That problem is even more aggravated when you have sub-second operation; a couple missed packets result in a network-wide flooding of gratuitous ARPs.

To protect from this, some vendors offer the ability to back-off that preemption time a bit. In networks where you're utilizing aggressive timing, you’ll still have the power to react to offline nodes at sub-second intervals. In addition to that, you can also make sure devices coming on/off at a rapid pace don’t affect the overall state of VRRP within your boundary.

I wanted to know whats the MIN and MAX limit for VRRP switch-back-delay and why?

This will be totally vendor dependent. As an example, Juniper devices have a range between 0 to 3600 seconds. 0 seconds being the default (instant preemption). 3600 seconds, or 1 hour, being the extreme.

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  • Even I was searching for the term switch-back-delay on internet but I could not find any, as you mentioned in RFC 5798 also there was nothing related to switch-back-delay. Thank you for good explanation. – Singh_007 Nov 26 '14 at 9:25

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