MBR (Multicast Border Router)

Intro

Enabling PIM MBR on an interface (where we don’t have an upstream PIM neighbor) will allow multicast traffic from remote sources that are outside of our PIM domain to be treated as locally connected sources.

We typically see this scenario when we are receiving multicast feeds from a remote Exchange and a PIM neighbourship is not established on our upstream links. In the current PIM implementation (EOS 4.14.0F and later) EOS will drop multicast traffic that is not considered to be locally connected by default and we need to configure MBR to allow this multicast data.

In the interfaces receiving unsolicited multicast, we need to disable PIM sparse mode and configure the interface with the “ip pim border-router” command to enable MBR. Sources learned through the MBR interfaces will be treated as local (directly connected to the switch) and we will need an RP in our PIM domain to complete the registration of these sources. The border-bit will be set in all PIM register messages that are sent for them.

mbr-pic1

Configuration

Set the L3 interface towards the remote source as MBR. Also remove “ip pim sparse-mode” command, since both features are mutually exclusive.

interface Vlan100
ip address 192.168.200.1/24
no ip pim sparse-mode
ip pim border-router
switch(config)#show ip pim interface
Address    Interface   Mode    Neighbor   Hello DR   DR Address  PktsQed  PktsDropped  Count      Intvl Pri
192.168.200.1  Vlan100     mbr     0          30    1    192.168.200.1   0        0

RPF check

RPF check is ignored as long as the route to the source does not point to any other PIM enabled interface.

mbr-pic3

If no route exists for remote source – works

If route via E1 – works

If route via E3 – works

if route via E2 – RPF check results in traffic being dropped

Limitations

Note MBR and pim sparse mode are mutually exclusive i.e. they must never be configured together on the same interface. The command “show ip pim config-sanity interface” will warn you about this configuration.