PIM ASSERT

Answered Question
Mar 30th, 2009
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WAN WAN

| |

R1 R2

| |

--LAN-


When Pim asserts were sent over the LAN between R1 and R2. What metrics will be included? Only the WAN metric to the source or metric from the LAN interface to the source?


Thanks,

Correct Answer by Harold Ritter about 8 years 3 months ago

Yuhui,


Section 3.6 of RFC4601 gives explains the exact scenarios in which the assert mechanism is needed in a sparse mode context.


http://www.ietf.org/rfc/rfc4601.txt


Regards

Correct Answer by Giuseppe Larosa about 8 years 3 months ago

Hello Yuhui,


>> 1. pim assert only works with PIM DM.


The title of the RFC is:


Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol

Specification


I would suggest you a quick read to it


Hope to help

Giuseppe



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yuhuiyao Mon, 03/30/2009 - 11:58
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WAN WAN

| |

R1 R2

| |

--LAN-


When Pim asserts were sent over the LAN between R1 and R2. What metrics will be included? Only the WAN metric to the source or metric from the LAN interface to the source?


Thanks,



yuhuiyao Mon, 03/30/2009 - 12:00
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Seemed I can not put a good diagram here.


Here is the connection:


WAN---R1---LAN


WAN---R2---LAN


R1 and R2 are connected via LAN.

Giuseppe Larosa Mon, 03/30/2009 - 13:09
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Hello Yuhui,


>> When Pim asserts were sent over the LAN between R1 and R2. What metrics will be included? Only the WAN metric to the source or metric from the LAN interface to the source?


the unicast metric to reach the source and the AD, the unicast metric of the LAN segment should be not important.


The target of the pim assert is to understand who is the best router to take the role to forward multicast traffic into the lan segment


see


rfc2362


sec. 3.5.1


Do unicast routing table lookup on source address from data

packet, and send assert on interface "I" for source address in

data packet; include metric preference of routing protocol and

metric from routing table lookup.



http://www.ietf.org/rfc/rfc2362.txt?number=2362


Hope to help

Giuseppe


yuhuiyao Mon, 03/30/2009 - 13:16
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Thanks. If unicast metric of the LAN is not considered, a downstream router may have RPF neighbor as router A while the router B is actually passing mcast traffic because it wins the pim assert over the wan but did not win over the lan from the downstream router's perspective.

Giuseppe Larosa Mon, 03/30/2009 - 13:54
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Hello Yuhui,

your note is good.


I think the PIM assert is sent on the shared link to elect a forwarder.


Downstreams routers have faculty to direct their PIM join messages to their preferred upstream neighbor.


So we could say the assert is useful on final client vlans but PIM joins messages from downstream routers shape the tree.


see


1 Downstream routers will select the upstream router with the

smallest metric preference and metric as their RPF neighbor. If

two metrics are the same, the highest network layer address is

chosen to break the tie. This is important so that downstream

routers send subsequent Joins/Prunes (in SM) to the correct

neighbor. An Assert-timer is initiated when changing the RPF

neighbor to the Assert winner. When the timer expires, the

router resets its RPF neighbor according to its unicast routing

tables to capture network dynamics and router failures.


sec 3.5.2 Receiving Asserts


http://www.ietf.org/rfc/rfc2362.txt?number=2362


Hope to help

Giuseppe



yuhuiyao Mon, 03/30/2009 - 14:23
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1. pim assert only works with PIM DM.

2. I think downstream routers do not care if the mcast traffic is coming from the rpf neighbor or not, as long as the mcast is from the right RPF interface.

3. there "might" be some issues with PIM prune message. If PIM prune message is mcasted to everyone over the LAN and everyone over the LAN will process it, ecerything should be fine. (I guess it is how it works with PIM prune).


However, if PIM prune works like PIM join, though mcast but dedicated to a specific RPF neighbor, problem happens:


PIM prune might be sent to the RPF neighbor (LAN RPF winner but WAN assert loser) who is not actual passing mcast traffic. The result will be PIM prune will not prune. (I guess this is NOT how PIM prune works).





Correct Answer
Giuseppe Larosa Mon, 03/30/2009 - 15:00
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Hello Yuhui,


>> 1. pim assert only works with PIM DM.


The title of the RFC is:


Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol

Specification


I would suggest you a quick read to it


Hope to help

Giuseppe



yuhuiyao Mon, 03/30/2009 - 15:03
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Hi,


I think PIM SM does not need PIM assert. Correct me if I am wrong.


Thanks,


Yuhui

Harold Ritter Mon, 03/30/2009 - 18:03
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Yuhui,


The PIM assert mechanism is used in sparse mode as well.


Regards

yuhuiyao Tue, 03/31/2009 - 05:33
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Hi,


Can you give me an example where PIM assert is used at sparse mode?


Thanks,

yuhuiyao Tue, 03/31/2009 - 14:40
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Thanks for you help. Just one more question: who will be "Encoded-Unicast-Upstream Neighbor Address" in pim join? Will that be RPF neighbor's address?

Harold Ritter Wed, 04/01/2009 - 06:16
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Yuhui,


Yes, it is indeed the RPF neighbor address.


Regards

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