From: Andrew Lissitz \(alissitz\) (alissitz@cisco.com)
Date: Sat Oct 15 2005 - 12:24:07 GMT-3
Your example is a little difficult to work, since this would not be a
very good design (as drawn). It is common to make the RP outside the
forwarding plane, but in your example it appears these are back to back.
Lets consider this:
R1 (source) -------- R2 (RP)
\ |
\ |
\ R3 (standard multicast configs)
\ |
\ |
R4 (receiver)
|
R1 - (source) sends the group traffic to the RP when the source begins.
RP now records the source, and forwards the traffic towards all the
receivers that have requested this group. The RP has a OIL that
specifies these interfaces that have requested this group. If no
receivers have requested this group then the traffic is dropped since
the RP would not have a OIL built for this group. The RP only knows
where to send this traffic when requests for it have come in.
The RP sends traffic towards R3. R3 saw the request from R4 and
forwards traffic to R4. When the first packets get to R4, R4 will then
begin to build the SPT back to the source. Before the first packets
come, R4 does not know the source and can not build a SPT. After it
gets the packets it now performs a RPF and send join messages towards
the course. R4 does not want to use the default shared tree because
going to straight to the source is faster.
If you want traffic to stay on the default / shared tree, then you need
to tell the routers who would otherwise build a SPT, to stay on it via
the spt-threshold infinity command. In this case R4 will want to build
a SPT, since this router has a receivers registered.
As Carl said, when you want traffic to stay on the default tree,
companies will typically make this command common for all routers. It
is needed on routers that will build SPTs; ones with interested
receivers.
I have a good presentation, about 2MB... If anyone is interested;
Unicast me and I will send it tonight. Networkers 2005 also has some
good presentations ... I do not have the links for these...
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Ashok Ananda (aananda)
Sent: Saturday, October 15, 2005 10:38 AM
To: The Great Ryan; Carl Willias
Cc: Cisco certification
Subject: RE: Where I should place "ip pim spt-threshold " ?
My understanding is that it should be on the router which connects to
the member device. That is because the last hop router stops switching
over to SPT when (S,G) is received by the member device.
Thanks & Regards,
Ashok M A
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
The Great Ryan
Sent: Saturday, October 15, 2005 7:59 PM
To: Carl Willias
Cc: Cisco certification
Subject: Re: Where I should place "ip pim spt-threshold " ?
Hi,
R1(ip igmp join-group)--R2(multicast source)--R3(RP)
When R2 act as multicast source, it will contact RP(R3) and then forward
to R1 i.e. R2 -> R3 -> R1 Then it will switch to SPT i.e R2 -> R1
Thus, I want to know where "ip pim spt-threshold infinity" should be
needed?
in R2 ? in R1?
Ryan
2005/10/15, Carl Willias <mandingo2073@yahoo.com>:
> I assume you mean the infinity option. the answer is that it depends.
> Your topology does not really have an spt :-). But in reality you
> would put the command on every router up to the RP. Think of it like
> this, multicast will stay on the shared tree until a router that finds
> it has a shorter path to the multicast source, this box needs to be
> told to ignore your routing table that says if has a better way to
> get the the source and keep the tree back to the RP. That router has
> to have the ip pim spt-threshold command. In essence the choice to
> stay or leave the shared tree is a router by router choice. If you
> know the point that those tree diverge you can get away with putting
> it on one box. But it is sound practice in the real world to put it
> along the whole shared tree
>
> CW
>
> ----- Original Message ----
> From: The Great Ryan <pv.ryan@gmail.com>
> To: Cisco certification <ccielab@groupstudy.com>
> Sent: Saturday, October 15, 2005 6:21:04 AM
> Subject: Where I should place "ip pim spt-threshold " ?
>
>
> Hi Group
>
> R1--R2--R3
>
> R1 is RP
> R3 is running "ip igmp join-group" on its loopback interface
>
> Where I should place "ip pim spt-threshold " such that it will never
> switch to shortest-path tree?
>
> Place this command only on R2 or all of them ?
>
> Thanks!
> Ryan
>
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