From: The Great Ryan (pv.ryan@gmail.com)
Date: Sat Oct 15 2005 - 12:41:47 GMT-3
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
> >
> > ______________________________________________________________________
> > _ Subscription information may be found at:
> > http://www.groupstudy.com/list/CCIELab.html
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html
This archive was generated by hypermail 2.1.4 : Sun Nov 06 2005 - 22:00:51 GMT-3