Subject: [pim] How do BSR messages stop flowing? Cannot find answer in "draft-ietf-pim-sm-bsr-06.txt" ...

From: "John Spence" <[email-masked]>
Date: Wed, 30 Nov 2005 15:17:35 -0800

As I read draft-ietf-pim-sm-bsr-06.txt, I see how C-BSR's send
PIM-SM messages "Bootstrap" (message type 4) during the election
process.  I am particularly interested in the passage in 3.4:
 
"When a Bootstrap message is forwarded, it is forwarded out of
every multicast-capable interface which has PIM neighbors
(including the one over which the message was received).".
 
I understand that PIM-SM BSR messages are sent form the
link-local address of the sending interface, and are sent to the
All-PIM-Capable-Routers" multicast address FF02::D.

How do the messages stop flowing - what keeps two or more routers
which are PIM neighbors from re-originating and flooding
back-and-forth PIM Bootstrap messages indefinitely?  There must
be a mechanism where a PIM router recognizes a Bootstrap message
it has seen before, but I cannot find the reference.

Please point me to place in the Spec where that is addressed.
Thanks very much.

John Spence
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From: John Zwiebel <[email-masked]>
Date: Wed, 30 Nov 2005 15:57:29 -0800

Think RPF.  In 3.1.3

if (BSM.dst_ip_address == ALL-PIM-ROUTERS) {
   if (BSM.src_ip_address != RPF_neighbor(BSM.BSR_ip_address)) {
     drop the Bootstrap message silently
   }

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From: "John Spence" <[email-masked]>
Date: Wed, 30 Nov 2005 16:50:35 -0800

Ah - very good.  I see that the BSR global-scope address (or at
least non link-local scope) is included as a field in the
Bootstrap message (which if course it must be), and that the RPF
check would ensure I only forward "downstream", away from the
C-BSR.

Thanks for the pointer John.

Spence
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>-----Original Message-----
>From: John Zwiebel [mailto:[email-masked]] 
>Sent: Wednesday, November 30, 2005 3:57 PM
>To: John Spence
>Cc: John Zwiebel; [email-masked]
>Subject: Re: [pim] How do BSR messages stop flowing? Cannot 
>find answer in "draft-ietf-pim-sm-bsr-06.txt" ...
>
>Think RPF.  In 3.1.3
>
>> As I read draft-ietf-pim-sm-bsr-06.txt, I see how C-BSR's 
>send PIM-SM 
>> messages "Bootstrap" (message type 4) during the election 
>process.  I 
>> am particularly interested in the passage in 3.4:
>>
every 
>> multicast-capable interface which has PIM neighbors 
>(including the one 
link-local 
>> address of the sending interface, and are sent to the 
>> back-and-forth PIM Bootstrap messages indefinitely?  There
must be a 
>> mechanism where a PIM router recognizes a Bootstrap message it
has 
>> seen before, but I cannot find the reference.
>>