Layer 2 Multicast Over MCT
SLX-OS devices support Layer 2 multicast
control packets over multichassis trunks (MCT). Multicast state information is
synchronized between MCT peers using MP-BGP EVPN transport. Multicast protocol packets
are sent on the peer link only when required.
Internet Group Management Protocol (IGMP)
protocol packets are of three types.
- IGMP Query: General query
and Group-specific query
- IGMP Report: Version 1,
Version 2, and Version 3 Membership reports. In a report, the multicast address
field contains the specific multicast address to which the sender is
listening.
- IGMP Leave: Version 2
group leave
Protocol Independent Multicast (PIM) protocol packets are of 4 types.
- PIM hello
- PIM join/prune
- BSR and candidate-RP advertisements
- RP registration & null-registration
IGMP Query Packet Processing
Each EVI is associated with a
multicast group ID (MGID) that is BUM-suppressed (Broadcast, unknown-unicast and
multicast). However, query packets need to be transmitted on an interchassis link
(ICL) to address the following scenarios.
- The querier connected to only one
of the MCT peer switches is the elected querier.
- Only one of the peer switches is
configured as a querier.
- The switch ages out IGMP routes if memberships are
not confirmed during the timeout interval. Although query packets are
received on the MCT peer link, the mrouter port is not learned or considered
on that peer link.
IGMP Membership Reports
- Traditionally, each peer switch learns about Layer
2 multicast memberships by snooping the IGMP membership reports. The membership
reports are then flooded on multicast router (mrouter) ports.
- For MCT, because an mrouter port is not
learned on the peer link, membership reports are not flooded between the peer
switches. Peer switches exchange learned routes by using EVPN NLRI messages
between Border Gateway Protocol (BGP) peers running on the MCT cluster control
VLAN.
- MCT module (L2RIB) which handles the exchange of
information across MCT cluster, communicates Multicast routes to Multicast
module.
- If a general query or group-specific query is
received from any port other than a peer link, each peer switch generates a
proxy report for the IGMP routes learned across MCT.
Duplicate IGMP Query Packets on
CCEP
If a query is configured on both MCT peers of a member VLAN, duplicate query packets
reach clients that are connected to the MCT domain by means of a Cluster Client Edge
Port (CCEP) or Cluster Edge Port (CEP).
Same BGP EVPN IGMP Join Sync Route is used to exchange IGMP Querier configuration on
the member VLANs. Only one MCT Peer per VLAN is be elected as IGMP Querier based on
higher MCT Peer IP address.
IGMP Leave
When fast-leave is not configured and an MCT
peer receives a leave membership report from one of its clients for group G, the
switch or router informs other MCT peers about the group-specific query and latency
by using Leave Sync Route. The peer switch, which runs the querier, sends
group-specific queries and group queries to the local VLAN ports.
Mrouter Synchronization
Mrouter synchronization helps in
achieving optimal path selection for unknown multicast traffic and optimal MP-BGP
message exchange between MCT peers. Mrouter port information is synchronized to MCT
Peer using the same BGP EVPN IGMP Join Sync route. For Mrouter detection on CCEP
client port, the traffic is forwarded based on Local Bias forwarding behavior. For
all CEP Mrouter ports learnt, only one Sync is used, first add and last delete,
unlike CCEP Mrouter ports which is per Client ESI.
Device Support
Extreme
Networks supports
Layer 2 multicast over MCT on the following devices:
- SLX 9540
- SLX 9640
- SLX 9150
- SLX 9250
