From: Andrew Lissitz \(alissitz\) (alissitz@cisco.com)
Date: Sat Oct 15 2005 - 13:14:48 GMT-3
Good question Ryan,
With this command on R4, R4 will not build a SPT, and will continue to
receive traffic from R3. R3 does not know much, other than it receives
traffic from the RP and forwards it to R4. It knows to forward to R4,
because it earlier received a PIM message from R4 asking for this
traffic. R4 was sending this message towards the RP, and R3 processed
it, cached the info, and forwarded it on to the RP.
There are no receivers on R3 so it does not care to build a SPT to the
source.
Typically when R4 builds a SPT and after it is done, R4 would send a
prune message to R3 telling it not to send anymore. Without this prune
message being sent, R3 is happy to continue forwarding.
A common practice for companies that desire this, is to place this
command on all routers, however only the routers with receivers build a
SPT.
PS ... If I do not answer any emails quickly .... I am not trying to be
rude ... Just out / or busy. I am going hiking with the family in the
Poconos today so no more answers from me till tonight ;-) ... Take care
Great Ryan and 'Great Group'
-----Original Message-----
From: The Great Ryan [mailto:pv.ryan@gmail.com]
Sent: Saturday, October 15, 2005 11:42 AM
To: Andrew Lissitz (alissitz)
Cc: Ashok Ananda (aananda); Carl Willias; Cisco certification; C&S
GroupStudy
Subject: Re: Where I should place "ip pim spt-threshold " ?
Your description is impressive !
Just want to further ask a question on your example.
Suppose "ip pim spt-threshold infinity" is added on R4 and add a
connection between R1 and R3, will R3 build the SPT back to the source?
Need to add "ip pim spt-threshold infinity" on R3 to ensure they are
using default share tree?
Thanks !
Ryan
2005/10/15, Andrew Lissitz (alissitz) <alissitz@cisco.com>:
> 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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