Technical Note: How to address duplicated multicast stream in a dual homed network using PIM sparse mode
Description
Solution
This article talks about use of a PIM Designated Router (PIM DR) to avoid duplicated multicast stream in a dual homed network.
Solution
In the following diagram, A is the receiver and B is the source sending multicast traffic to 239.1.1.1. The Rendezvous Point (RP) is R3.
Client (A) network is dual homed, R1 and R2 are configured with VRRP, R1 is the master router. OSPF is already configured with default cost, resulting in the following paths (asymroute is enabled as well):
The relevant parts of the configuration of R1 and R2 are:
R1
R2
Looking at a packet capture on the client A, it can be seen that the multicast packets for the same multicast stream are received twice:
1. from R1 MAC (52:54:00:16:bf:27)
2. from R2 MAC (52:54:00:8f:89:b4)
The issue is as follows:
R1 and R2 receive an IGMP group membership report for 239.1.1.1 from client A. Both R1 and R2 send a PIM join toward the RP, then 2 forwarding trees are created from the source toward the client A.
Both R1 and R2 have a S,G entry in the multicast routing table:
To address this issue, it is mandatory to enable PIM neighboring between R1 and R2 over their LAN interface (port2 and port3).
Once the PIM neighbor relationship is established, a PIM Designated Router (DR) will be elected. On a broadcast network, the PIM DR is used to send the PIM join toward the RP.
R1
R2
By default, the router with the highest IP wins the DR election, meaning that R2 will be the DR. Thus it will receive and forward the multicast stream (S,G is present in the multicast routing table):
R1 no longer receives the multicast stream (no S,G entry):
A packet capture on client A confirms that the multicast packets are received from R2 only:
The multicast stream is being received from R2, but it is possible to received it from R1. Simply configure R1 as the DR using a higher priority (by default priority = 1):
The multicast packets are now received from R1:
Client (A) network is dual homed, R1 and R2 are configured with VRRP, R1 is the master router. OSPF is already configured with default cost, resulting in the following paths (asymroute is enabled as well):
- A -> B : R1 -> R3 -> R4
- B -> A : R4 -> R2
The relevant parts of the configuration of R1 and R2 are:
R1
config router ospf
set router-id 1.1.1.1
config area
edit 0.0.0.0
next
end
config network
edit 1
set prefix 1.1.1.1 255.255.255.255
next
edit 2
set prefix 10.0.0.1 255.255.255.255
next
edit 3
set prefix 10.1.3.1 255.255.255.255
next
end
set passive-interface "port2"
config redistribute "connected"
end
config redistribute "static"
end
config redistribute "rip"
end
config redistribute "bgp"
end
config redistribute "isis"
end
end
config router multicast
set multicast-routing enable
config pim-sm-global
config rp-address
edit 1
set ip-address 3.3.3.3
next
end
end
config interface
edit "port2"
set pim-mode sparse-mode
set passive enable
config igmp
set access-group "MCAST_239.1.1.1"
end
next
edit "r1_r3_0"
set pim-mode sparse-mode
config igmp
set access-group "DENY_ALL"
end
next
end
end
R2
config router ospf
set router-id 2.2.2.2
config area
edit 0.0.0.0
next
end
config network
edit 1
set prefix 2.2.2.2 255.255.255.255
next
edit 2
set prefix 10.0.0.2 255.255.255.255
next
edit 3
set prefix 10.2.4.2 255.255.255.255
next
end
set passive-interface "port3"
config redistribute "connected"
end
config redistribute "static"
end
config redistribute "rip"
end
config redistribute "bgp"
end
config redistribute "isis"
end
end
config router multicast
set multicast-routing enable
config pim-sm-global
config rp-address
edit 1
set ip-address 3.3.3.3
next
end
end
config interface
edit "port3"
set pim-mode sparse-mode
set passive enable
config igmp
set access-group "MCAST_239.1.1.1"
end
next
edit "r2_r4_0"
set pim-mode sparse-mode
config igmp
set access-group "DENY_ALL"
end
next
end
end
Looking at a packet capture on the client A, it can be seen that the multicast packets for the same multicast stream are received twice:
1. from R1 MAC (52:54:00:16:bf:27)
2. from R2 MAC (52:54:00:8f:89:b4)
The issue is as follows:
R1 and R2 receive an IGMP group membership report for 239.1.1.1 from client A. Both R1 and R2 send a PIM join toward the RP, then 2 forwarding trees are created from the source toward the client A.
Both R1 and R2 have a S,G entry in the multicast routing table:
R1 # get router info multicast pim sparse-mode table 239.1.1.1
IP Multicast Routing Table
(*,*,RP) Entries: 0
(*,G) Entries: 1
(S,G) Entries: 6
(S,G,rpt) Entries: 6
FCR Entries: 0
(*, 239.1.1.1)
RP: 3.3.3.3
RPF nbr: 10.1.3.3
RPF idx: r1_r3_0
Upstream State: JOINED
Local:
port2
Joined:
Asserted:
port2: winner
FCR:
(10.4.0.10, 239.1.1.1)
RPF nbr: 10.1.3.3
RPF idx: r1_r3_0
SPT bit: 1
Upstream State: JOINED
Local:
Joined:
Asserted:
Outgoing:
port2
(10.4.0.10, 239.1.1.1, rpt)
RP: 3.3.3.3
RPF nbr: 10.1.3.3
RPF idx: r1_r3_0
Upstream State: NOT PRUNED
Local:
Pruned:
Outgoing:
R2 # get router info multicast pim sparse-mode table 239.1.1.1
IP Multicast Routing Table
(*,*,RP) Entries: 0
(*,G) Entries: 1
(S,G) Entries: 4
(S,G,rpt) Entries: 4
FCR Entries: 0
(*, 239.1.1.1)
RP: 3.3.3.3
RPF nbr: 10.2.4.4
RPF idx: r2_r4_0
Upstream State: JOINED
Local:
port3
Joined:
Asserted:
FCR:
(10.4.0.10, 239.1.1.1)
RPF nbr: 10.2.4.4
RPF idx: r2_r4_0
SPT bit: 1
Upstream State: JOINED
Local:
Joined:
Asserted:
Outgoing:
port3
(10.4.0.10, 239.1.1.1, rpt)
RP: 3.3.3.3
RPF nbr: 10.2.4.4
RPF idx: r2_r4_0
Upstream State: NOT PRUNED
Local:
Pruned:
Outgoing:
To address this issue, it is mandatory to enable PIM neighboring between R1 and R2 over their LAN interface (port2 and port3).
Once the PIM neighbor relationship is established, a PIM Designated Router (DR) will be elected. On a broadcast network, the PIM DR is used to send the PIM join toward the RP.
R1
config router multicast
config interface
edit "port2"
unset passive
next
end
end
R2
config router multicast
config interface
edit "port3"
unset passive
next
end
end
By default, the router with the highest IP wins the DR election, meaning that R2 will be the DR. Thus it will receive and forward the multicast stream (S,G is present in the multicast routing table):
R2 # get router info multicast pim sparse-mode neighbour
Neighbor Interface Uptime/Expires Ver DR
Address Priority/Mode
10.0.0.1 port3 00:11:05/00:01:40 v2 1 /
10.2.4.4 r2_r4_0 05:56:04/00:01:41 v2 1 / DR
R2 # get router info multicast pim sparse-mode table 239.1.1.1
IP Multicast Routing Table
(*,*,RP) Entries: 0
(*,G) Entries: 1
(S,G) Entries: 4
(S,G,rpt) Entries: 4
FCR Entries: 0
(*, 239.1.1.1)
RP: 3.3.3.3
RPF nbr: 10.2.4.4
RPF idx: r2_r4_0
Upstream State: JOINED
Local:
port3
Joined:
Asserted:
FCR:
(10.4.0.10, 239.1.1.1)
RPF nbr: 10.2.4.4
RPF idx: r2_r4_0
SPT bit: 1
Upstream State: JOINED
Local:
Joined:
Asserted:
Outgoing:
port3
(10.4.0.10, 239.1.1.1, rpt)
RP: 3.3.3.3
RPF nbr: 10.2.4.4
RPF idx: r2_r4_0
Upstream State: NOT PRUNED
Local:
Pruned:
Outgoing:
R1 no longer receives the multicast stream (no S,G entry):
R1 # get router info multicast pim sparse-mode neighbour
Neighbor Interface Uptime/Expires Ver DR
Address Priority/Mode
10.0.0.2 port2 00:12:52/00:01:29 v2 1 / DR
10.1.3.3 r1_r3_0 05:57:51/00:01:26 v2 1 / DR
R1 # get router info multicast pim sparse-mode table 239.1.1.1
IP Multicast Routing Table
(*,*,RP) Entries: 0
(*,G) Entries: 1
(S,G) Entries: 4
(S,G,rpt) Entries: 4
FCR Entries: 0
(*, 239.1.1.1)
RP: 0.0.0.0
RPF nbr: 0.0.0.0
RPF idx: None
Upstream State: NOT JOINED
Local:
port2
Joined:
Asserted:
FCR:
A packet capture on client A confirms that the multicast packets are received from R2 only:
conf router multicast
config interface
edit "port2"
set dr-priority 100
next
end
end
The multicast packets are now received from R1: