Port 496 carries PIM-RP-DISC (Protocol Independent Multicast - Rendezvous Point Discovery), better known as Auto-RP, Cisco's proprietary protocol for announcing rendezvous points in multicast networks.
What Port 496 Does
Auto-RP uses UDP port 496 to solve one of the hardest problems in multicast networking: how do you tell thousands of routers where the rendezvous point is without configuring each one manually?1
In multicast networks running PIM Sparse Mode, a rendezvous point (RP) is the meeting place where sources and receivers find each other. Before Auto-RP, network engineers had to manually configure every router with the RP address. One mistake, one forgotten router, and multicast traffic would fail.2
Auto-RP automates this. Candidate rendezvous points announce themselves to mapping agents using multicast group 224.0.1.39. Mapping agents then broadcast the elected RP information to all PIM routers using multicast group 224.0.1.40. All of this discovery traffic flows over port 496.3
The Protocol
Auto-RP uses two packet types:4
- RP-announce packets: Sent by candidate RPs to mapping agents at 224.0.1.39
- RP-discovery packets: Sent by mapping agents to all PIM routers at 224.0.1.40, announcing which RPs were elected for which multicast groups
The protocol supports IPv4 only. IPv6 multicast networks use Bootstrap Router (BSR) instead, which was introduced later in PIM-SM version 2 as an open-source alternative to Cisco's proprietary mechanism.5
The History
Protocol Independent Multicast - Sparse Mode (PIM-SM) was developed in the mid-1990s. An Internet Draft on PIM architecture was published in January 1995, and the protocol specification was released as a Proposed Experimental RFC in September 1996, authored by D. Estrin, D. Farinacci, and others.6
Dino Farinacci, who worked at Cisco Systems, was one of the primary authors of the PIM-SM specification. He spent 15 years designing and implementing IP multicast routing technology, notably PIM and DVMRP. The implementation he wrote from scratch at Procket Networks is now owned by Cisco and has been extensively tested in production deployments.7
Auto-RP was Cisco's solution to the static RP problem, created before the standardized Bootstrap Router mechanism. Even though it's proprietary, IANA assigned it port 496, recognizing that what works in production networks matters as much as what's in the RFCs.8
Why Multicast Needs Rendezvous Points
In PIM Sparse Mode, multicast traffic doesn't flood the entire network. Instead, routers only forward multicast packets when receivers explicitly request them. The rendezvous point is where this connection happens—where sources register their multicast streams and receivers send join requests.9
Without automated RP discovery, scaling multicast across large networks would be a configuration nightmare. Auto-RP made enterprise multicast practical.
Proprietary but Assigned
Port 496 is unusual. Auto-RP is Cisco proprietary, yet it has an official IANA port assignment. This reflects a reality of Internet infrastructure: sometimes the protocol that's deployed everywhere matters more than whether it's an open standard.
BSR eventually provided a standardized alternative, but Auto-RP came first and is still widely deployed in Cisco-based networks.10
Related Ports
- Port 520: RIP (Routing Information Protocol) — another routing protocol
- Port 521: RIPng (RIP next generation) — RIP for IPv6
Security Considerations
Auto-RP itself has no authentication mechanism. Malicious actors on the network could announce themselves as candidate RPs or mapping agents, potentially disrupting multicast traffic or redirecting it through compromised routers.
Modern deployments should:
- Filter Auto-RP messages at network boundaries
- Use access control lists to restrict which devices can send RP-announce or RP-discovery messages
- Consider migrating to BSR, which has better security features
- Monitor for unexpected RP announcements
Frequently Asked Questions
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