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CCIE Must Know:PIM Technology Overview

CCIE Must Know:PIM Technology Overview

PIM (Protocol Independent Multicast) is called protocol independent multicast. The protocol-independent here refers to the unicast routing protocol, that is, PIM does not need to maintain special unicast routing information. As a multicast routing solution, it directly uses the routing information of the unicast routing table to perform RPF (Reverse Path Forwarding, reverse path forwarding) check on the multicast packet, and creates a multicast routing entry after the check is passed, thereby forwarding the multicast Message.

PIM (Protocol Independent Multicast) means that it does not depend on a specific IP routing protocol. It just uses the unicast routing table to implement the RPF check function. Therefore, it can work on any IP routing protocol that can generate unicast routing tables, including static routing. It is worth emphasizing that protocol irrelevance does not mean that unicast routing protocols are not required; on the contrary, the multicast protocol must be supported by unicast routing protocols because it does not have unicast routing capabilities. The PIM protocol message is based on the UDP protocol and its port number is 103. PIM also has a special multicast address of, which means all PIM routers.

Currently, the PIM protocols actually supported by the device include: PIM-DM (PIM-Dense Mode, Protocol Independent Multicast-Dense Mode) and PIM-SM (PIM-Sparse Mode, Protocol Independent Multicast-Sparse Mode).

Basic working principle of PIM-DM:

PIM-DM uses the “Push (Push) mode” to forward multicast messages, and is generally applied to networks with relatively small and dense multicast group members. The key working mechanisms of PIM-DM include neighbor discovery, diffusion, pruning, grafting, and assertion.

1. Neighbor discovery: The router periodically sends PIM Hello messages (hereinafter referred to as Hello messages) to all PIM routers ( in a multicast manner to discover PIM neighbors and maintain PIM neighbors between routers Relationships to build and maintain SPT.

2. Diffusion: When an active multicast source appears in the PIM-DM network, the multicast packets sent by the multicast source will spread throughout the network. When the PIM router receives the multicast packet and passes the RPF check according to the unicast routing table, it creates an (S, G) entry on the router and forwards the packet to all downstream nodes in the network. After proliferation, each router in the PIM-DM domain will create (S, G) entries.

3. Pruning: After the PIM router receives the multicast packet, the RPF check passes, but there is no multicast packet requirement on the downstream network segment. At this time, the PIM router sends a pruning message to the upstream to notify the upstream router to prohibit the forwarding of the corresponding downstream interface and delete it from the downstream interface list of the (S, G) entry. The pruning operation is initiated by the leaf router, hop-by-hop upwards, and finally there are only branches connected to group members on the multicast forwarding path.

4. Grafting: PIM-DM uses the grafting mechanism to quickly get multicast packets on the network segment where new group members join. The leaf router learned through IGMP that on the user network segment connected to it, a new group member has joined the multicast group G. Then the leaf router will send a Graft message to the upstream, requesting the upstream router to resume the forwarding of the corresponding outgoing interface, and add it to the downstream interface list of the (S, G) entry. The grafting process starts from the leaf router and ends with the router where multicast packets arrive.

5. Assertion: If there are multiple multicast routers in a network segment, the same multicast message may be repeatedly sent to the network segment. In order to avoid this situation, it is necessary to select the only multicast data forwarder through the Assert mechanism.

Basic working principle of PIM-SM:

PIM-DM uses SPT constructed in a “diffusion-prune” way to transmit multicast data. Although the path of SPT is the shortest, the efficiency of its establishment process is low, which is not suitable for large and medium-sized networks. PIM-SM uses “Pull” mode to forward multicast packets, which is generally applied to networks with relatively large and relatively sparse multicast group members.

The key mechanisms of PIM-SM include neighbor discovery, DR election, RP discovery, RPT construction, multicast source registration, SPT switching, and assertion; at the same time, a single PIM-SM domain can be refined by configuring the BSR (Bootstrap Router) management domain化管理。 Management.

1. Neighbor discovery: similar to PIM-DM mechanism;

2. DR election: With the help of Hello messages, DR can also be elected for a shared network (such as Ethernet). The DR will act as the only forwarder of multicast data in the shared network. Either the network connected to the multicast source or the network connected to the receiver needs to elect the DR. The DR on the receiver side is responsible for sending join messages to the RP; the DR on the multicast source side is responsible for sending registration messages to the RP.

3. RP discovery: The convergence point RP is an important PIM router in the network, which is used to process the source DR registration information and group member join request. All PIM routers in the network must know the RP address, similar to a supply and demand information Convergence center. An RP can serve multiple multicast groups at the same time, but a multicast group can only correspond to one RP.

4. RPT construction: RPT is a multicast distribution tree with RP as the root and PIM router with group membership as leaves.

5. Multicast source registration: In a PIM-SM network, any newly emerging multicast source must first “register” at the RP before it can transmit multicast messages to group members.

6. SPT switching: When the DR on the receiver side finds that the multicast data rate sent from the RP to the multicast group G exceeds a certain threshold, it will initiate the switching from RPT to SPT.

7. Assertions: similar to PIM-DM.

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