From: Chris Lewis (chrlewiscsco@gmail.com)
Date: Mon May 08 2006 - 11:39:58 ART
A good sumary Pierre,
For question 1, there are some differences between IGMP version 1 and
version 2 relevant to your statements. In version 1 the querier and DR are
the same device, in version 2 the DR is the router with the highest IP
address, whereas the querier is the one with the lowest address.
For question 2, I am aware that PIM v2 uses the assert message to determine
which of multiple possible forwarders of a multicast stream will be the one
to forward packets on to a multi-access segment (to avoid duplication of a
steam on to that segment). The lowest IGP cost to the source wins the assert
mechanism, highest IP address is the tie breaker.
I am not aware of a difference between Dense mode and sparse mode operation
for the assert process. Could you give a link of some kind that suggests
this? Possibly a reference to the Designated Forwarder, which is a PIM BiDir
enhancement is mixed in here? RFC 2362 defines assert for sparse mode, so I
think you should re-visit that. In sparse mode the DR is responsible for
sending triggered Join/Prune and Register messages toward the RP, it is not
involved in the process of selecting routers to be used to forward multicast
packets on to the multi-access segment.
For 3, again the DR is not necessarily the forwarder for a multi-access
segment.
For more information on the BiDir enhancement for a DF, refer to
http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration
_guide_chapter09186a00800ca796.html
Specifically the following (noting that it is only relevant to BiDir, not
sparse mode in general):
DF Election
In bidir-PIM, the packet forwarding rules have been improved over PIM-SM,
allowing traffic to be passed up the shared tree toward the RP. To avoid
multicast packet looping, bidir-PIM introduces a new mechanism called
designated forwarder (DF) election, which establishes a loop-free SPT rooted
at the RP.
On every network segment and point-to-point link, all PIM routers
participate in a procedure called DF election. The procedure selects one
router as the DF for every RP of bidirectional groups. This router is
responsible for forwarding multicast packets received on that network
upstream to the RP.
The DF election is based on unicast routing metrics and uses the same
tie-break rules employed by PIM assert processes. The router with the most
preferred unicast routing metric to the RP becomes the DF. Use of this
method ensures that only one copy of every packet will be sent to the RP,
even if there are parallel equal cost paths to the RP.
A DF is selected for every RP of bidirectional groups. As a result, multiple
routers may be elected as DF on any network segment, one for each RP. In
addition, any particular router may be elected as DF on more than one
interface.
Chris
On 5/7/06, Pierre-Alex <paguanel@hotmail.com> wrote:
>
> Can someone please review the following statements I am making?
>
> Some of them are not clearly stated in the RFCs or the Cisco Doc but
> appear to
> be verified while labing.
>
> Much thanks,
>
> Pierre
>
> ======
>
> 1. The router with the lowest ip address is the IGMP querrier.
>
> 2. In PIM Dense mode to determine who will be the forwarder on a shared
> LAN,
> pim asserts are used.
>
> In PIM Spare mode to determine who will be the forwarder on a shared
> LAN,
> a DR election is run :
>
> The designated router is the router with the highest IP address or the
> router
> configured with the highest "ip pim dr-priority" value
>
> NB: in PIM Dense mode there is still an election but it serves no purpose
> (except in IGMP v1 m to choose the IGMP Querier)
>
> There is no need for an assert mechanism in SPARSE mode since there is
> already
> a DR election.
>
> (not exactly what the RFC is saying:
> http://www.zvon.org/tmRFC/RFC2362/Output/chapter2.html !!!!
>
>
> 3. In sparse mode, the designated router (DR) sends SOURCE REGISTRATION
> messages to the rendezvous point (RP).
>
> The DR is also responsible for sending joins towards the RP.
>
> The DR is the fowarder on a shared LAN.
>
>
>
> ========
>
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