Subject: Re: How to disable DR election on an interface ?

From: Bill Fenner <[email-masked]>
Date: Mon, 4 Jun 2001 12:14:28 -0700

If you don't send HELLO messages, then your upstream neighbor will
discard Join/Prune messages from you.  I guess this isn't in the spec,
but it should be.  (General rule: don't send messages to and don't
accept messages from people that you don't have a HELLO relationship
with).

If all routers on the link support DR priority, then advertising a
DR priority of zero will cause you to only become the DR if all
routers on the link are advertising a DR priority of zero.  However,
if not all routers support DR priority then the only way you can
affect the DR election is by configuring the system with a low IP
address.

  Bill

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From: Dino Farinacci <[email-masked]>
Date: Mon, 4 Jun 2001 13:50:46 -0700

>> accept messages from people that you don't have a HELLO relationship

    I can understand not sending Join/Prune mesasges to a router which 
    you haven't received Hello messages from because they may not be
    a PIM router (unicast/multicast topology incongruence). But why not 
    accept Join/Prune messages from a router that hasn't sent a Hello. 
    Seems overly restrictive.

Dino

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From: Bill Fenner <[email-masked]>
Date: Mon, 4 Jun 2001 14:45:50 -0700

>    why not 

Because there may be options in the Hello message that need to be
consistent (e.g. join/prune override time).  e.g. if you're the DR and
you hear a Join/Prune from a router with a higher IP address, do you
decide that he's the DR?  Why or why not?

What I'd really like is for the Hello messages to have neighbor lists
and ignore a neighbor until you get a Hello message with you in his
neighbor list, but that got shot down a long time ago.  It's much nicer
for debugging purposes for the peering to go down on both ends than to
have a mysteriously one-way peering.

  Bill

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From: Dino Farinacci <[email-masked]>
Date: Mon, 4 Jun 2001 14:55:11 -0700

>> decide that he's the DR?  Why or why not?

    According to the current specs, you are DR because you haven't heard a
    Hello from the joiner. So you should decide nothing.

    Since there are implementations now that don't require this "Hello-before
    -accept" behavior, why make it required when there is currently no such 
    consistency option.

Dino

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From: Bill Fenner <[email-masked]>
Date: Mon, 4 Jun 2001 15:02:40 -0700

>    -accept" behavior, why make it required when there is currently no such 

Because it's backwards compatible (older implementations are required to
send Hellos) and improves consistency.  However, it does increase
convergence time in the restart case, so it's not a completely obvious
tradeoff; it's something we need to think about more.

  Bill