Subject: [magma] Question about the relation between the igmpv3 group information and multicast routing table

From: [email-masked]
Date: Tue, 20 Jun 2006 19:07:23 +0800

I have a question about the relation between the igmp group information
and multicast routing table. The scenario is very simple, PC1 and PC2 are
connected with cisco 3725 through fa0/1 and fa0/0 respectively,like this:
        PC1------fa0/1------fa0/0------PC2
    PC1 simulates igmp host, sends out igmpv3 report, PC2 generates
multicast traffic, PC1 receives traffic and verifies.
    Cisco 3725 runs pim and igmpv3, with "ip pim ssm default" configured,
the IOS version is: "Cisco IOS Software, 3700 Software (C3725-JSX-M),
Version 12.3(8)T, RELEASE SOFTWARE (fc2)"

    Sometime, the igmp group at fa0/1 is:
Router#sh ip igmp groups fastEthernet 0/1 detail
Flags: L - Local, U - User, SG - Static Group, VG - Virtual Group,
SS - Static Source, VS - Virtual Source

Interface: FastEthernet0/1
Group: 225.1.1.1
Flags:
Uptime: 00:00:44
Group mode: INCLUDE
Last reporter: 120.0.0.1
Group source list: (C - Cisco Src Report, U - URD, R - Remote, S - Static,
V - Virtual, Ac - Accounted towards access control limit,
M - SSM Mapping)
Source Address Uptime v3 Exp CSR Exp Fwd Flags
10.1.1.1 00:00:44 00:04:28 stopped Yes R
10.1.1.2 00:00:44 00:04:28 stopped Yes R

    Here, we can find that at fa0/1, in terms of group 225.1.1.1, the group
mode is INCLUDE and the source addresses are 10.1.1.1 and 10.1.1.2，so this
port can only receive traffic sending to 225.1.1.1 from 10.1.1.1 or
10.1.1.2, right?

    Ok, let's check the multicast routing table now:
Router#sh ip mroute
IP Multicast Routing Table
Flags: D - Dense, S - Sparse, B - Bidir Group, s - SSM Group, C -
Connected,
L - Local, P - Pruned, R - RP-bit set, F - Register flag,
T - SPT-bit set, J - Join SPT, M - MSDP created entry,
X - Proxy Join Timer Running, A - Candidate for MSDP Advertisement,
U - URD, I - Received Source Specific Host Report, Z - Multicast Tunnel
Y - Joined MDT-data group, y - Sending to MDT-data group
Outgoing interface flags: H - Hardware switched, A - Assert winner
Timers: Uptime/Expires
Interface state: Interface, Next-Hop or VCD, State/Mode

(*, 225.1.1.1), 00:00:54/stopped, RP 0.0.0.0, flags: DC
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list:
FastEthernet0/1, Forward/Sparse-Dense, 00:00:19/00:00:00

(10.1.1.1, 225.1.1.1), 00:00:46/00:02:51, flags: T
Incoming interface: FastEthernet0/0, RPF nbr 0.0.0.0
Outgoing interface list:
FastEthernet0/1, Forward/Sparse-Dense, 00:00:19/00:00:00

(10.1.1.2, 225.1.1.1), 00:00:47/00:02:50, flags: T
Incoming interface: FastEthernet0/0, RPF nbr 0.0.0.0
Outgoing interface list:
FastEthernet0/1, Forward/Sparse-Dense, 00:00:20/00:00:00

(10.1.1.3, 225.1.1.1), 00:00:47/00:02:50, flags: T
Incoming interface: FastEthernet0/0, RPF nbr 0.0.0.0
Outgoing interface list:
FastEthernet0/1, Forward/Sparse-Dense, 00:00:20/00:00:00

(10.1.1.4, 225.1.1.1), 00:00:48/00:02:59, flags: T
Incoming interface: FastEthernet0/0, RPF nbr 0.0.0.0
Outgoing interface list:
FastEthernet0/1, Forward/Sparse-Dense, 00:00:21/00:00:00

(*, 224.0.1.40), 07:19:38/00:04:04, RP 0.0.0.0, flags: DCL
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list:
FastEthernet0/0, Forward/Sparse-Dense, 07:19:38/00:00:00

    We can find that (10.1.1.1, 225.1.1.3) and (10.1.1.4, 225.1.1.1) are
included, with incoming interface fa0/0 anb outgoing interface fa0/1.
    But, fa0/1 only accepets the traffic from 10.1.1.1 or 10.1.1.2, how to
explain the variance between igmp group and mroute?
    PS: the traffic captured at fa0/1 contains the traffic with source
address 10.1.1.3 and 10.1.1.4, as is consistent with mroute.

    Any suggestion is welcome.

    Best Regards

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