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Re: Where I should place "ip pim spt-threshold " ? posted 10/15/2005
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Thanks Andrew as you clear my question on that!


2005/10/16, Andrew Lissitz (alissitz) <alissitz@xxxxxxxxx>:
>
> 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@xxxxxxxxx]
> 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@xxxxxxxxx>:
> > 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@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx] 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@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx] 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@xxxxxxxxx>:
> > > 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@xxxxxxxxx>
> > > To: Cisco certification <ccielab@xxxxxxxxxxxxxx>
> > > 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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