H3CSE: detailed explanation of multicast forwarding mechanism and h3cse multicast forwarding

H3CSE: detailed explanation of multicast forwarding mechanism and h3cse multicast forwarding

Different from unicast, the multicast data forwarding path is based on the tree structure.

Different from unicast, multicast forwarding needs to check whether the inbound interface of the packet is optimal.

All multicast forwarding trees are source ------ >>> receiver gets one-way tree

Concept of multicast distribution tree:

1. the multicast distribution tree refers to the forwarding path of multicast data in the network, which is established by the multicast routing protocol.

2. Based on different root nodes, the multicast distribution tree can be divided into the Shortest Path Tree and the sharing tree.

SPT (Shortest Path Tree, Shortest Path Tree): the root of the SPT is the specified vro connected to the multicast source.

RPT (Rendezvous Point Tree, shared Tree): RP (Rendezvous Point, aggregation Point) is the root of the Tree)

A spt tree must be created for each source to each multicast group. The path from any source to any multicast receiver is the shortest.

Any source has the same path to the same multicast group.

RPF introduction:

RPF (Reverse Path Forwarding, Reverse Path Forwarding)

1. Ensure that multicast data is transmitted along the correct path

2. Avoid generating multicast path loops

The RPF check process is as follows:

1. If the data packet arrives at the optimal path to the Multicast Source, RPF check is successful and the data packet is forwarded.

2. if RPF check fails, discard the data packet

The RPF check is based on the unicast route table.

In short, the multicast routing device receives multicast data, checks the Source IP in the IP header of the multicast data, matches the unicast route table (display ip routing-table ), if the outbound interface in the matched route information is the same as the interface for receiving multicast data, the RPF check is successful and the multicast data can be forwarded. Otherwise, the data is discarded.

Because multicast forwarding relies on the RPF mechanism, the routing device in the multicast environment must run the unicast routing protocol, and each device must learn the IP network segment of the source.

Note: For multicast data from a source to a multicast group, the local route can only receive data from a specific upstream neighbor through a specific local port.