3.6. Multi-Access Transit LANs

Multi-access LANs (e.g., Ethernet) used for transit introduce complexity when multiple routers join the same group or source.

Challenges:

1. Multiple Upstream Routers: Two or more routers may issue (*,G) or (S,G) Joins to different upstream routers on the same LAN
2. Duplicate Traffic: Multiple paths established, causing duplicate packets on the LAN
3. Mixed Trees: Traffic from same source may arrive via both RPT and SPT

Assert Mechanism:

• PIM Assert Messages: Routers detect duplicate packets and elect a single forwarder
• Election Criteria: Favors router with (S,G) state; or best metric to RP (for shared tree) or source (for SPT)
• Downstream Updates: Downstream routers receive Asserts and send subsequent Joins to the Assert winner

note

For detailed Assert message format and election procedures, see Section 3.6 and Section 4.9 of RFC 7761.