Subject: Re: [pim] why PIM DR auto-switchover is more desired than VRRP master switchover in the case of uplink failure?

From: Xu Xiaohu <[email-masked]>
Date: Thu, 01 Apr 2010 17:39:47 +0800

Lenny,

Thanks for your response.

> This draft seems like a clever and reasonable solution to the problem you

> are trying to solve.  However, my point was that I do not think it is

> necessary to specify this solution in this WG, as there is nothing in the

> protocol that prevents you from doing it today.  This solution is simply

> an implementation feature that uses the existing tools in the protocol.

Yes, your observation is correct. The solution described in this draft
doesn't require any change to the PIM specification, that's why I propose it
as an informational draft.

> As an example, interface tracking is a useful and desired feature

> available in many VRRP implementations, yet I could find no mention of

> interface tracking in any of the VRRP WG docs.  The VRRP protocol

> specifies the tools available, and implementations are free to add

> features like interface tracking that build on the tools in the spec.  I

> don't think "desirability" makes it any more or less necessary to document

> an implementation feature that uses existing tools in the protocol.

There is some difference on the aspect of interface tracking usages between
VRRP and PIM. Since VRRP has the non-preemption option, the VRRP priority
adjustment according to the status of the tracked interfaces is much simple
and straightforward (IMHO, that's the major reason why there is no a
separate WG doc describing the interface tracking mechanism for VRRP).
However, PIM has no mechanism for non-preemption, so some extra designs for
DR priority adjustment according to the status of the tracked interfaces
must be considered so as to avoid PIM DR preemption in case that the tracked
interface on the old DR recovers from down to up.

Besides, as I said before, interface tracking feature is much more desirable
for PIM. Without the interface tracking feature, VRRP can still work well
with the help of a configured floating route. In contrast, the floating
route is not ideal for PIM since the transient interruption of multicast
forwarding due to multicast tree rebuilding will occur once the tracked
interface on the old DR recovers from down to up. 

Best wishes,

Xiaohu

> Thanks,

> Lenny