From: Brian McGahan (brian@cyscoexpert.com)
Date: Wed Jul 16 2003 - 12:27:29 GMT-3
Nate,
Auto-RP is a mystical and paradoxical technology that easily
boggles the mind. See if you can follow this reasoning:
The point of running auto-rp is to automate the assignment of
multiple rp's on a per-group basis without having lots of static configs
on your devices. Keep in mind that RP's are only used in the sparse PIM
domain.
Auto-RP announce and discovery use the multicast groups
224.0.1.39 and 224.0.1.40. In order for a device to learn about an RP,
it must first join 224.0.1.39 and 224.0.1.40. However, in order to join
224.0.1.39 and 224.0.1.40, it must know which RP to send these join
messages to. In other words, the device needs to know where the RP is
in order to discover where the RP is, and the device needs to discover
where the RP is in order to know where the RP is. As you can see, we
have a problem.
The easy fix for this problem is to run pim in 'sparse-dense'
mode. Sparse-dense means that for any groups which you have an RP, you
will be sparse, while any groups that you do not have an RP for, you
will be dense.
Assuming that your candidate RP's will be servicing all groups,
this simple means that devices in the PIM domain will be dense for
224.0.1.39 and 224.0.1.40, and sparse for everything else. Therefore
they don't have to send join messages for these two groups, and can
discover the RP without having to know who the RP is in the first place.
HTH,
Brian McGahan, CCIE #8593
Director of Design and Implementation
brian@cyscoexpert.com
CyscoExpert Corporation
Internetwork Consulting & Training
Toll Free: 866.CyscoXP
Fax: 847.674.2625
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
Of
> Nathan Chessin
> Sent: Tuesday, July 15, 2003 6:50 PM
> To: ccielab@groupstudy.com
> Subject: multicast sparse-dense mode
>
> Hi All,
>
> Doing some multicast reading, I came across this
>
http://www.cisco.com/en/US/partner/tech/tk828/tk363/technologies_white_p
ap
> er
> 09186a00800d6b63.shtml
> "A prerequisite of Auto-RP is that all interfaces must be configured
in
> sparse-dense mode using the ip pim sparse-dense-mode interface
> configuration
> command."
>
> I thought that sparse-mode needed an RP, but I wasn't aware that all
> interfaces had to be in sparse-dense mode with Auto-RP. Any ideas?
>
> Nate
>
>
>
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