You have configured MSDP peering between two autonomous systems that pass traffic between two sites, but the peering has failed to come up. Which task do you perform to begin troubleshooting the problem?

Multicast Source Discovery Protocol (MSDP) peering relies on a TCP connection between the peers, which uses TCP port 639. The most fundamental requirement for this TCP session to be established is IP reachability between the source interfaces configured for the MSDP peering on both routers. If the peers cannot reach each other's source IP address, the TCP three-way handshake will fail, and the peering session will not come up. Therefore, verifying this bidirectional reachability is the primary and initial step in troubleshooting a failed MSDP peering.

A. Multicast routing must be enabled globally for MSDP to function; disabling it would be counterproductive.
B. MSDP is independent of the PIM mode (Sparse, Dense, or Sparse-Dense) used in the multicast domain. PIM-DM is not a specific requirement.
D. While asymmetric routing can sometimes cause issues, verifying basic IP connectivity is a more fundamental and initial troubleshooting step than investigating the routing path.

1. Cisco Systems, "Configuring MSDP," IP Multicast: PIM Configuration Guide, Cisco IOS XE Gibraltar 16.12.x. Section: "Prerequisites for MSDP". "You must have IP multicast configured in your network... You must have IP reachability between MSDP peers."

2. Cisco Systems, "Troubleshooting MSDP," IP Multicast Configuration Guide, Cisco IOS Release 15M&T. Section: "MSDP Peer Is Not Coming Up". "The first thing to check is whether there is IP reachability between the MSDP peers."