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Overview of Multicast Forwarding with IGMP or MLD Snooping in an EVPN-MPLS Environment
Starting with Junos OS Release 18.2R1, MX Series routers with MPCs, vMX routers, and EX9200 switches support IPv4 multicast traffic forwarding with Internet Group Management Protocol (IGMP) snooping in an Ethernet VPN (EVPN) over MPLS environment. Starting with Junos OS Release 18.4R1, MX Series routers with MPCs, vMX routers, and EX9200 switches also support IPv6 multicast traffic forwarding with Multicast Listener Discovery (MLD) snooping in an Ethernet VPN (EVPN) over MPLS environment.
Multicast source and receiver hosts in the EVPN instance (EVI) can each be single-homed to one provider edge (PE) device or multihomed in all-active mode to multiple provider edge (PE) devices. Sources and receivers can also be attached to PE devices in the EVI at the same site or at different sites. For receivers that are multihomed to multiple PE devices, IGMP or MLD state information is synchronized among the peer PE devices. IGMP and MLD snooping can be enabled for multiple EVIs, and either for specific bridge domains or VLANs in an EVI or all bridge domains or VLANs within an EVPN virtual switch instance. Multicast traffic can be forwarded within a bridge domain or VLAN, and can be routed across bridge domains or VLANs at Layer 3 using IRB interfaces.
This feature provides multicast support in an EVPN-MPLS network using IGMPv2, MLDv1, and MLDv2.
Benefits of Multicast Forwarding with IGMP or MLD Snooping in an EVPN-MPLS Environment
In an environment with significant multicast traffic, using IGMP or MLD snooping constrains multicast traffic in a broadcast domain or VLAN to interested receivers and multicast devices, which conserves network bandwidth.
Synchronizing IGMP or MLD state among all EVPN PE devices for multihomed receivers ensures that all subscribed listeners receive multicast traffic, even in cases such as the following:
IGMP or MLD membership reports for a multicast group might arrive on a PE device that is not the Ethernet segment’s designated forwarder (DF).
An IGMP or MLD message to leave a multicast group arrives at a different PE device than the PE device where the corresponding join message for the group was received.
Multicast Forwarding with IGMP or MLD Snooping in Single-homed or Multihomed Ethernet Segments
Hosts in the network send IGMP or MLD reports expressing interest in particular multicast groups from IPv4 multicast sources (using IGMP) or IPv6 multicast sources (usng MLD). PE devices with IGMP or MLD snooping enabled listen to IGMP or MLD packets and use that snooped information on the access side to establish multicast routes that only forward traffic for a multicast group to interested receivers.
For redundancy, an EVPN-MPLS environment can have multicast sources and receivers multihomed to a set of peer PE devices that are in all-active mode. When all PE devices in the EVI have snooping enabled, the IGMP or MLD state is synchronized among the multihomed peer PE devices so multicast traffic can reach all listeners.
IGMP or MLD Join and Leave Route Synchronization Among Multihoming Peer PE Devices
In an EVI with receivers that are multihomed to multiple PE devices, corresponding IGMP or MLD join and leave messages for multicast group management might not be sent to the same PE device, so all the PE devices must synchronize and share IGMP and MLD state. PE devices with snooping enabled in this environment exchange BGP EVPN Type 7 (Join Sync Route) and Type 8 (Leave Synch Route) network layer reachability information (NLRI) to synchronize IGMP or MLD membership reports received on multihomed interfaces. IGMP and MLD snooping in this environment is supported only with multihoming PEs in all-active mode.
The advertised EVPN Type 7 and Type 8 routes also carry EVI route target extended community attributes associated with multihomed EVIs, so multiple EVPN routing instances can be supported simultaneously. These routes are only imported by PE devices that share the same Ethernet segment ID.
EVPN Type 7 and Type 8 route advertisement is not needed for hosts that are single-homed. However, to save bandwidth on the access side by only forwarding multicast traffic to interested receivers, IGMP or MLD snooping should also be configured in this case.
Figure 1 shows a multicast source behind customer edge device CE3, and a multicast receiver Host 1 behind CE2 that is multihomed to PE devices PE1 and PE2.
In Figure 1, PE2 receives a membership request from Host 1, and advertises the EVPN Type 7 Join Sync Route to the EVPN core. PE1 imports the Type 7 route to synchronize the IGMP or MLD state among the PE devices to which the receiver is multihomed.
Processing leave messages and membership route withdrawals in a multihomed environment is more complicated when the leave message is not received by the same PE device that processed the join message, but is facilitated using BGP EVPN Type 8 routes as follows:
A PE device that receives an IGMP or MLD leave message for a group advertises a Type 8 route. Other PE devices import the Type 8 route.
The PE device that advertised the Type 8 route originates a membership query for any remaining group members, and starts a leave timer. The other PE devices that imported the Type 8 route likewise start a leave timer.
If no join membership reports are received by the time the timer expires, the PE device that advertised the Type 7 route withdraws the Type 7 route. The PE device that originated the Type 8 route withdraws the Type 8 route.
IGMP or MLD Versions and Supported Group Membership Report Modes
By default, the EVPN-MPLS network can process (*,G) (any-source multicast [ASM]) membership reports with IGMPv2, MLDv1, and MLDv2. If the network has hosts sending both MLDv1 and MLDv2 ASM reports for a given group, PE devices will process MLDv1 and MLDv2 reports for the group as MLDv1 membership reports.
You can alternatively configure PEs to process only (S,G) (source-specific multicast [SSM]) membership reports as an MLD snooping configuration option with MLDv2. You can enable SSM-only processing for one or more bridge domains in the EVI. When enabling this option with MLD snooping for a virtual switch, the behavior applies to all bridge domains in the virtual switch instance. When this option is enabled, ASM reports are not processed and are dropped.
Table 1 summarizes the membership report modes supported with each IGMP or MLD version:
Table 1: Group Membership Report Modes Supported for Each IGMP and MLD Version
IGMP or MLD Version
ASM (*,G) Only
SSM (S,G) Only
ASM (*,G) + SSM (S,G)
Yes (explicitly configured)
Multicast Traffic Forwarding with Single-homed or Multihomed Receivers
In an EVPN-MPLS network where hosts might be multihomed to more than one PE device, when a bridge domain (or VLAN) is configured on a PE device, the PE device signals a BGP EVPN Type 3 (Inclusive Multicast Ethernet Tag [IMET]) route to the other PE devices in the instance to build a core multicast replication tree for each configured bridge domain.
With IGMP or MLD snooping enabled:
A PE device receiving multicast traffic to be forwarded is referred to as the ingress PE device. To ensure multicast traffic reaches all remote PE devices in the EVI, the ingress PE device uses the IMET routing information with ingress replication in the EVPN core, replicating and flooding the packets on the EVPN tunnels to all of the other PE devices (or external edge routers) in the EVI that might need to forward the traffic. IGMP or MLD snooping is not performed in the EVPN core.
If a multihoming PE device receives multicast traffic from the EVPN core and it is the DF for an interested receiver for the multicast group, the PE device forwards the traffic.
If a multihoming PE device receives multicast traffic from the EVPN core and it is not the DF for any interested receivers, the PE device does not forward the traffic.
On the access side, upon receiving multicast data from the EVPN core, PE devices selectively forward the multicast traffic only to interested receivers. Single-homing PE devices use learned IGMP or MLD snooping information, and multihoming PE devices use both IGMP or MLD snooping information and EVPN Type 7 routes.
For example, in Figure 1, PE3 is the ingress PE device for Multicast Source A and forwards the multicast traffic to the EVPN core. Host 1 is a multicast receiver that is multihomed to PE1 and PE2 by way of customer edge device CE2. PE1 and PE2 have synchronized IGMP or MLD state information about receiver Host 1, and both PE devices receive the multicast traffic from the EVPN core. Because PE1 is currently the elected DF for Host 1, PE1 forwards the data to Host 1, but based on the IGMP snooping information and imported EVPN Type 7 routing information, PE1 does not forward the data to CE1. PE2 is not the DF, so PE2 does not forward the traffic toward Host 1.
IGMP or MLD Snooping with Multicast Forwarding Between Bridge Domains or VLANs Using IRB Interfaces
For multicast forwarding between bridge domains or VLANs in this environment, PE devices use Protocol Independent Multicast (PIM) in distributed designated router (DDR) mode on IRB interfaces.
The IRB interfaces on PE devices route multicast traffic between bridge domains or VLANs as follows:
Upon receiving multicast traffic on an IRB interface from a multicast source, the PE device routes the traffic to any IRBs that have PIM enabled and are configured for bridge domains or VLANs with interested local receivers for the multicast group.
With PIM DDR configured, PE devices with IRB interfaces route multicast traffic to local receivers whether or not the IRB is the elected PIM designated router (DR).
To prevent multicast traffic duplication, IRB-routed multicast traffic is not forwarded back to the EVPN core.
For example, Figure 2 shows an EVPN instance has three PE devices, PE1, PE2, and PE3, with IRB interfaces configured on each PE device for two bridge domains or VLANs. The multicast source on bridge domain 1 (VLAN 1) sends multicast traffic to PE1, and PE1 uses inclusive multicast forwarding with ingress replication to forward the traffic to the EVPN core to reach other PE devices with interested receivers. PE1 also routes the traffic to interested receivers on bridge domain 2 (VLAN 2) locally through the IRB interfaces, even though PE3 is the PIM DR for bridge domain 2 (VLAN 2). The IRB interfaces on all PE devices that route traffic to receivers on bridge domain 2 (VLAN 2) do not forward the routed traffic back out into the EVPN core. PE2 receives the multicast traffic from the EVPN core and forwards or locally routes the traffic on each bridge domain (VLAN) that has interested receivers. PE3 routes the traffic locally to bridge domain 2 only, because there are no interested receivers on bridge domain 1 (VLAN 1).
Requirements and Limitations of Multicast with IGMP or MLD Snooping in an EVPN-MPLS Environment
This environment requires the following:
IGMP or MLD snooping is supported with single-homing and multihoming PEs in all-active mode only.
Only IGMPv2 is supported with IGMP snooping.
MLDv1 and MLDv2 are supported with MLD snooping under the following constraints:
By default, when MLD snooping is enabled, only MLDv1 and MLDv2 (*,G) ASM reports are accepted and processed.
Alternatively, you can configure the device to accept and process MLDv2 (S,G) SSM reports only. (Processing both MLDv2 (*,G) and MLDv2 (S,G) reports are not supported at the same time.)
All bridge domains or VLANs with multicast sources and receivers are configured on all PE devices in the EVI.
IRB interfaces for routing multicast traffic between bridge domains or VLANs must be configured on all PE devices in the EVI for multicast traffic to be routed locally on each PE device.
If the bridge domain (VLAN) and an IRB interface are configured on all PE devices, but an IRB interface is down on a PE device, traffic routing between bridge domains (VLANs) for receivers served by that IRB will be impacted.
For Layer 3 routing of multicast traffic between bridge domains or VLANs, PIM DDR mode must be configured on all participating IRB interfaces.
All multihomed peer PE devices must support BGP EVPN Type 7 Join Sync Routes and Type 8 Leave Sync Routes.
PE devices are configured in enhanced IP Network Services mode on MX Series routers. (See Network Services Mode Overview.)
This environment has the following limitations:
Using PIM passive mode on IRB interfaces in the EVPN instance is not supported.
IGMP or MLD snooping in this environment with an external multicast router instead of IRB interfaces is not supported.
Receiving or forwarding multicast traffic from or to devices outside of this environment using a PIM gateway is not supported.
Selective multicast forwarding (advertising EVPN Type 6 Selective Multicast Ethernet Tag (SMET) routes for forwarding only to interested receivers) in the EVPN core is not supported.
IGMP snooping is not supported with point-to-multipoint (P2MP) multipoint LDP/RSVP replication; only ingress replication is supported.