In Multicast Source Discovery Protocol (MSDP), each peer's state plays a critical role in the establishment and exchange of multicast source information between routers. It is common to observe MSDP peers in either the Inactive or Listen state, especially before a routing path or active connection is established between the devices. These states are part of MSDP's designed behavior for managing multicast source information across network boundaries and do not necessarily indicate an error.

When examining MSDP peer connections, you may observe that some peers are in the Listen or Inactive states. Example output might look like:

> show msdp

Peer address       Local address       State     Last up/down   Peer-Group     SA Count
X.X.X.Y              X.X.X.X          Listen      00:07:05       test           0/0
X.X.X.Z              X.X.X.X          Inactive    00:00:04       test1          0/0

Listen: The router is in a passive state, waiting to accept an incoming TCP connection from the remote peer. No MSDP session is fully established yet, and the router will not exchange Source-Active (SA) messages until the connection is active.

Inactive: The MSDP session is not established. This may occur if the peer relationship has never been successfully set up or if the connection has gone down recently. Common causes include connectivity issues or configuration mismatches.

It is normal for MSDP peers to be in the Listen or Inactive state until a routing path is established or an active TCP connection is initiated. This behavior is expected and does not inherently indicate a network problem unless the states persist unexpectedly.

If the Listen or Inactive states are expected, they do not require immediate remediation. However, for successful MSDP session establishment and SA message exchange, ensure the following:

Routing Verification: Verify that there is a valid IP route to the MSDP peer. Without a valid route, the session cannot be initiated or completed.

Connectivity: Check for connectivity between peers using tools like ping or traceroute. Confirm that no firewalls or access lists are blocking TCP port 639 (default for MSDP).

Peer Configuration: Double-check MSDP peer configuration, ensuring the IP addresses and ports are correctly defined on both ends.

Session Monitoring: Use commands like show msdp periodically to observe if the state transitions to Established, indicating a successful session.

To troubleshoot MSDP problems, please refer to KB: https://supportportal.juniper.net/s/article/M-MX-PTX-T-Data-Collection-Checklist-Logs-data-to-collect-for-troubleshooting