RE: Multicast RPF check Question

From: Reza Toghraee (reza@toghraee.com)
Date: Sat Mar 08 2008 - 04:26:13 ARST

Hi

Thanks for your reply, I got it.

Now for solving this problem I used 2 solutions

1) Static multicast static route

2) Using the IP MULTICAST MULTPATH command.

The first one is clear, but as I know the IP MULTICAST MULTIPATH command,
load balance the multicast traffic through 2 same metric links but HOW ?

In my case the problem is R4, normally it fails the RPF check, because the
link between R3-R4 doesn't have the IP MULTICAST DENSE-MODE command , but
the IP address of R3 is higher than R2, so the RPF check does not select
the R2 as RPF neighbor.

I tested with IP MULTICAST MULTIPATH command on R4, the problem solved and
R4 started forwarding the traffic, but I didn't understand why.

If the link between R4-R3 was multicast enabled, how the IP MULTICAST
MULTIPATH command works? R4 is downstream router, how it can load balance
between 2 INCOMING links?

 

 

Reza

 

 

 

From: Vinoth Kannan Ganapathy [mailto:ccieprep4vinoth@gmail.com]
Sent: Friday, March 07, 2008 1:21 AM
To: Reza Toghraee
Cc: ccielab@groupstudy.com
Subject: Re: Multicast RPF check Question

 

 

Hi,

 

I beleive, when are there are two routes to the source,

RPF will be elected based on the highest IP address of the neighbour.

Based on tht if your R3's IP address is the highest then it has been choose
as the RPF neighbour..

 

Thx and regds

VInoth G

 

On 3/6/08, Reza Toghraee <reza@toghraee.com> wrote:

Hello

I'm a little confused about how RPF happens where there are 2 routes through
the source.
Ospf is running in this diagram
R1 is attacxhed to multicast rource
PIM DENSE mode is in USE
AlL interfaces Except the R3 S0 have "ip pim dense-mode" command. (R3 S0
not in Multicast)
R4 has 2 routes to source, through R2 and R3 (via OSPF)
Assume that the metric of S0 on R3 and E0 on R2 are same.

R2 E0 : 172.16.21.2/24
R2 E0 : 172.16.31.3/24

                                                       ---------------
                                                       | R1 |
                                                       |att to SRC |
                                                       ---------------
                                                       S0 E0
                                                       / \
                                                    / \
                                           / \
                                      / \
                                   -------- -----------
                                               | R2 | |
R3 |
                                               | |
| |
                                               --------
------------
                                                  E0|
|S0
                                                       \
/
                                                        \
/
                                                         \
/
                                                          \
/
                                                           \
/
                                              ----------------
                                                               | R4
|
                                                               |
|

---------------

Now the question is about the RPF check in R4.
When R4 receives a multicast packet from R2 E0, it checks the unicast
routing, finds 2 routes, but it can only select 1 route (as I understood),
But what is the RPF neighbor for R4? It is R3 or R2? It depends on neighbor
IP , neighbor metric , neighbnor interface ?
I search in last posts, I found that Scott wrote to use Ip Multicast
multipath, and it fixes the problem but I donno what is this command.
Also I know with static multicast route we can solve it, I;m looking to know
when R4 selects R2 as RPF neighbor and when R3 ?

Thanks
Reza

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