Four routers share a multi-access network. Router A is configured with a PIM DR priority of 10, Router B is configured with a PIM DR priority of 100, Router C and Router D are Alcatel-Lucent 7750 SRs set at the default value. Router D has the highest IP address. Which router is selected as the PIM DR on this multiaccess network?

The Protocol Independent Multicast (PIM) Designated Router (DR) on a multi-access network is elected based on the highest DR priority. If multiple routers share the same highest priority, the router with the highest IP address is chosen as a tie-breaker. In this scenario, Router B has a configured priority of 100, which is the highest among all routers (Router A: 10; Routers C & D: default of 1). Therefore, Router B is elected as the PIM DR, and the IP address tie-breaker rule is not needed.

A. Router A: Its priority of 10 is lower than Router B's priority of 100.
C. Router C: Its default priority of 1 is lower than Router B's priority of 100.
D. Router D: The highest IP address is only used as a tie-breaker when priorities are equal. Its default priority of 1 is lower than Router B's priority of 100.

1. Nokia 7750 SR OS MPLS Guide, Release 21.10.R1: In the "PIM" chapter, section "Designated Router", it states: "The DR is the router with the highest DR priority. If more than one router has the same DR priority, the router with the highest IP address is selected. The default priority is 1." (Document ID: 3HE 17179 AAAA TQZZA, Page 288). This confirms the election process and the default priority value for the 7750 SR.

2. RFC 7761 - Protocol Independent Multicast - Sparse Mode (PIM-SM): Section 4.3.1, "DR Election", specifies the standard election mechanism: "The PIM router with the highest DR priority on the LAN is elected the DR. If there is a tie in the DR priority, or if all routers on the LAN have the default DR priority, the PIM router with the highest IP address is elected the DR." This corroborates the priority-first, IP-address-as-tie-breaker rule.