MSDP Multicast Source Discovery Protocol

The Multicast Source Discovery Protocol (MSDP) is a protocol used in multicast routing to interconnect multiple Protocol Independent Multicast (PIM) domains. The primary purpose of MSDP is to enable the sharing of multicast source information across multiple PIM domains, which is essential for interdomain multicast routing.

Multicast is a communication method that allows one-to-many or many-to-many communication over a network. It is used to transmit data to a group of hosts simultaneously, which is more efficient than transmitting the same data to each host individually. Multicast routing is the process of routing multicast traffic through the network. The PIM protocol is the most widely used multicast routing protocol, and it is used to manage the multicast distribution tree in each domain.

PIM has two modes of operation: sparse mode and dense mode. In sparse mode, multicast traffic is only forwarded to networks that have receivers for that traffic. In dense mode, multicast traffic is forwarded to all networks within a domain, regardless of whether there are receivers. In a large network, it is common to have multiple PIM domains, each with its own multicast distribution tree. However, there may be multicast sources in one domain that have receivers in another domain. MSDP is used to discover and share multicast source information across these domains.

MSDP uses a client-server model, where the client is a PIM domain, and the server is an MSDP domain. Each MSDP domain has one or more MSDP routers, which are responsible for communicating with other MSDP routers in other domains. MSDP routers use the TCP protocol to communicate with each other and exchange multicast source information.

When a multicast source sends data, the PIM router in the source domain sends a multicast Join message towards the RP (Rendezvous Point) for that group. The RP is a designated router that is responsible for forwarding multicast traffic to receivers in that group. The RP for a group can be located in the same domain as the multicast source or in a different domain. If the RP for the group is located in a different domain, the PIM router in the source domain sends the Join message to the RP's IP address.

When the RP receives the Join message, it sends a Register message to the MSDP router in its domain. The Register message contains the source IP address, group IP address, and the RP's IP address. The MSDP router then sends this information to other MSDP routers in other domains. MSDP routers keep track of the source IP address and the RP's IP address for each group.

When a PIM router in a domain receives a multicast data packet, it first checks if the source IP address and group IP address are in its multicast routing table. If the source IP address and group IP address are not in its table, it sends an MSDP SA (Source Active) message to its MSDP router. The SA message contains the source IP address and the group IP address. The MSDP router then searches its MSDP database for the RP's IP address for that group.

If the RP's IP address is in the database, the MSDP router sends a multicast Join message to the RP's IP address. If the RP's IP address is not in the database, the MSDP router sends an MSDP SA request message to all other MSDP routers in other domains. The MSDP routers respond with the RP's IP address for that group. The MSDP router then sends a Join message to the RP's IP address.

MSDP has two main functions: SA message exchange and SA filtering. The SA message exchange function enables MSDP routers to exchange SA messages that contain source IP addresses and group IP addresses. The SA filtering function enables MSDP routers to filter out duplicate SA messages and prevent loops in the multicast distribution tree.

MSDP routers use a timer mechanism to expire SA messages and prevent stale information from being stored in the database. When an SA message expires, the MSDP router removes the source IP address and group IP address from its database.

MSDP also supports authentication to ensure that only authorized MSDP routers can exchange SA messages. MSDP routers can use MD5 or SHA-1 authentication to secure their communication.

One of the benefits of using MSDP is that it enables interdomain multicast routing, which is important in large networks with multiple PIM domains. MSDP allows multicast traffic to be forwarded across different domains, which reduces the amount of traffic and reduces the complexity of the network.

MSDP is also efficient in terms of bandwidth usage. MSDP routers only exchange SA messages when new sources are detected or when the RP for a group changes. This means that MSDP does not generate a lot of overhead traffic, which is important in networks with limited bandwidth.

MSDP is widely used in enterprise networks and service provider networks. It is supported by many networking vendors, including Cisco, Juniper, and Huawei.

In summary, MSDP is a protocol used in multicast routing to enable the sharing of multicast source information across multiple PIM domains. MSDP routers exchange SA messages to discover and share multicast source information, and they use SA filtering to prevent loops in the multicast distribution tree. MSDP supports authentication to secure communication between MSDP routers and is efficient in terms of bandwidth usage. MSDP is important for interdomain multicast routing and is widely used in enterprise and service provider networks.