From: Hoogen (hoogen82@gmail.com)
Date: Sat Mar 08 2008 - 17:44:49 ARST
Do a show ip rpf for the source from your R4 router. RPF check would have
taken one particular link. The RPF check doesn't follow the highest IP rule
or something for multipath command changes completely the way RPF check
works. The multipath command must be enabled consistenly on the route for
the traffic to route properly.
-Hoogen
On 3/7/08, Reza Toghraee <reza@toghraee.com> wrote:
>
> 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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