From: Brian McGahan (brian@cyscoexpert.com)
Date: Thu Jun 12 2003 - 18:25:05 GMT-3
David,
IP PIM NBMA mode is an optimization for PIM sparse multicast
over
non-broadcast media, it is never "required" for operation. Take the
following scenario:
R2--|
__/
( )
|--R1--( FR )
(__)
\
R3--|
R1 has a LAN segment that is sourcing a multicast feed X. R1 is
the
hub for the NBMA frame-relay network between R2 & R3. The segment
between
R1, R2, & R3 is the same IP subnet. A host on R2's LAN segment joins
multicast feed X. Since the frame-relay is a non-broadcast media, R1
must
send multicast packets as a layer 2 replicated unicast in order to
transport
it over the frame-relay. R1's config would look something like this:
R1#
Interface serial 0
Ip address 10.0.0.1 255.0.0.0
Encapsulation frame-relay
Frame-relay map ip 10.0.0.2 102 broadcast
Frame-relay map ip 10.0.0.3 103 broadcast
Ip pim sparse-mode
!
When a multicast packet arrives to be send out R1's S0
interface, R1
looks for L2 to L3 resolution. The 'broadcast' keywords on the
frame-relay
maps tell R1 that for any broadcast or multicast traffic sent out this
interface, replicate it as a layer 2 unicast for any VC's which support
this
feature. In this case, it's both VCs 102 and 103.
Multicast feed X now arrives on R1's serial interface. It is
replicated as a L2 unicast out VC 102 and 103. The problem, however, is
that hosts connected to R3 did not request this multicast feed. The
point
of PIM sparse mode is to minimize the flooding of multicast feeds. The
unnecessary flooding out VC 103 is counterintuitive to the goal of
sparse
mode PIM.
This is where 'ip pim nbma mode' comes in. If R1 is running PIM
in
NBMA mode, it tracks which endpoints of the NBMA network have joined
which
groups. When R1 goes to send feed X out serial 0, it realizes that only
R2
has joined feed X, and therefore only replicates the packet for VC 102.
For more information:
http://www.cisco.com/univercd/cc/td/doc/cisintwk/intsolns/mcst_sol/frm_r
lay.htm
HTH
Brian McGahan, CCIE #8593
Director of Design and Implementation
brian@cyscoexpert.com
CyscoExpert Corporation
Internetwork Consulting & Training
Toll Free: 866-CyscoXP
Outside US: 847.674.3392
Fax: 847.674.2625
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
Of
> Zhang, Ou (David)
> Sent: Thursday, June 12, 2003 12:05 AM
> To: 'ccielab@groupstudy.com'
> Subject: Multicast NBMA Mode
>
> All,
>
> PIM adjacency can be established between a hub and a spoke over frame
> relay
> network without configuring NBMA mode. Source is attached to the hub
and
> receiver is attached to the spoke. Configured either static RP or
auto
> RP.
> MRM testing is successful between source and receiver. In what
scenarios
> do
> I need to configure NBMA mode ?
>
> Thanks in advance.
>
> -David
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