The BEBs act as the boundary between the multicast domain (currently only IGMP dynamic or static) and the SPBM domain. Multicast senders (sources) and receivers connect directly or indirectly (using Layer 2 switches) to the BEBs. You can enable IP Multicast over Fabric Connect services at the Layer 2 VSN level or the Layer 3 VSN level (including the GRT).
The following figure shows how multicast senders and receivers connect to the SPBM cloud using BEBs.
The sender transmits multicast traffic with group IP address 233.252.0.1.
After the BEB receives the IP multicast stream from the sender, the BEB allocates data I-SID 16000001 for the S,G multicast stream. The BEB sends an LSP with the TLV 185 (for Layer 2 VSN multicast and Layer 3 VSN multicast) or TLV 186 (for IP Shortcuts multicast) with the transmit bit set. The BEB also sends an IS-IS service identifier and unicast address sub-TLV (where the unicast address has the multicast bit set and the I-SID is the Data I-SID).
The receiver sends a join request to Group 233.252.0.1.
The BEB (acting as the IGMP Querier) queries the IS-IS database to find all senders for group 233.252.0.1. If the group exists, the BEB sends an LSP with the IS-IS service identifier and unicast address sub-TLV (where the unicast address has the multicast bit set and the nickname is the stream transmitter BEB and the I-SID is the data I-SID).
The multicast tree is calculated for the data I-SID and the data starts flowing from the sender.
IP Multicast over Fabric Connect constrains all multicast streams within the level in which they originate, which is called the scope level. In other words, if a sender transmits a multicast stream to a BEB on a C-VLAN (a VLAN that is mapped to an I-SID, for instance, a Layer 2 VSN) with IP Multicast over Fabric Connect enabled, only receivers that are part of the same Layer 2 VSN can receive that stream. Similarly, if a sender transmits a multicast stream to a BEB on a VLAN that is part of the GRT or a Layer 3 VSN with IP Multicast over Fabric Connect enabled, only receivers that are part of the same Layer 3 instance (GRT or L3 VSN) can receive that stream.
Note
In the context of IP Multicast over Fabric Connect, scope is either the Global Routing Table or the I-SID value of the Layer 2 or Layer 3 VSN associated with the local VLAN on which the IP multicast data was received.
After the BEB receives the IP multicast stream from the sender, a BEB allocates a data Service Identifier (I-SID) in the range of 16,000,000 to 16,512,000 for the stream. The stream is identified by the S,G, V tuple, which is the source IP address, the group IP address, and the local VLAN the multicast stream is received on.
The BEB propagates this information through the SPBM cloud by using IS-IS TLV updates in LSPs, which results in the creation of a multicast tree for that stream. All BEBs now know what data I-SID to use for that stream and its scope. The data I-SID is a child of the scope or VSN I-SID. If no receiver requests the IP multicast stream, the ingress BEB does not forward the multicast stream.
After a BEB receives an IGMP join message from a receiver, a BEB queries the IS-IS database to check if a sender exists for the requested stream within the scope of the receiver. If the requested stream does not exist, the IGMP information is kept, but no further action is taken. If the requested stream exists, the BEB sends an IS-IS TLV update to its neighbors to inform them of the presence of a receiver, and this information is propagated through the SPBM cloud.
IS-IS acts dynamically using the TLV information it receives from BEBs that connect to the sender and the receivers to create a multicast tree between them. IS-IS creates very efficient multicast trees for the data I-SID allocated at the sender edge of the SPBM cloud to transport data between the sender and the receivers. The data I-SID uses Tx/Rx bits to signify whether the BEB uses the I-SID to transmit, receive, or both transmit and receive data on that I-SID. After IS-IS creates the multicast tree, the sender transports data to the receiver across the SPBM cloud using the data I-SID.
The trigger to send IS-IS updates to announce a multicast stream into the SPBM cloud is the multicast traffic arriving at the BEB. Because the BEB only interacts with IGMP and not PIM, all multicast traffic must be drawn towards the BEB for the stream to be announced, which SPBM accomplishes by making the BEB an IGMP Querier. In a VLAN, the IGMP Querier sends out periodic IGMP queries.
Note
The BEB must be the only IGMP Querier in the VLAN. If the BEB receives an IGMP query from any other device, it causes unexpected behavior, including traffic loss.