PIM Forwarder Election

In Figure 5-52, both Mercury and Copper have a route to source 172 16 1 1 They also have downstream interfaces to a member of group 239 70 49 238 that are connected to a common multiaccess network Both Meicury and Copper are receiving copies of the same multicast packets from the source, but it would obviously be inefficient for both routers to forward the packets onto the same network

Figure 5-52 Both Copper and Mercury Are Receiving Copies of the Multicast Packets Sent by Source 172 161 1, but Only One Router Should Forward the Packets onto Subnet 172 16 3 0/24

Copper

Mercury

Member of 239 70 49 238

Member of 239 70 49 238

To prevent such a situation, PIM routers select a single forwarder on the shared network Recall that DVMRP has a similar function, the designated forwarder DVMRP-designated forwarders are selected as part of the route exchange across the multiaccess network Because PIM does not have its own routing protocol, however, it instead uses Assert messages to select the forwarder

When a router receives a multicast packet on an outgoing multiaccess interface, it sends an Assert message on the network The Assert message includes the source and group address, the metric of the unicast route to the source, and the metric preference (in Cisco terms, the administrative distance) of the unicast protocol used to discover the route The routers producing the duplicate packets compare the messages and determine the forwarder based on the following criteria

• The router advertising the lowest metric preference (administrative distance) is the forwarder The routers would advertise only different metric preferences if their routes to the source have been discovered via different unicast routing protocols

• If the metric preferences are equal, the router advertising the lowest metric is the forwarder In other words, if the routers are running the same unicast routing protocol, the router metrically closest to the source becomes the forwarder

• If both the metric preferences and the metrics are equal, the forwarder is the router with the highest IP address on the network

The forwarder continues forwarding group traffic onto the multiaccess network The other routers stop forwarding that group's traffic and remove the multiaccess interface from their outgoing interface list

When the multicast source in Figure 5-52 first begins sending packets to group 239 70 49 238, for example, both Copper and Mercury receive copies of the packets, and both routers forward the packets onto subnet 172 16 3 0/24, as illustrated in Part A of Figure 5-53 When Copper receives a packet froift Mercury for (172 16 1 1,239 70 49 238) on its Ethernet interface, it sees that the interface is on the outgoing interface list for that (S, G) pair As a result, it sends an Assert message on the subnet When Mercury receives a multicast packet from Copper on the same interface, it takes the same action, as illustrated in Part B of Figure 5-53

Figure 5-53 When Copper and Mercury Detect Packets for (172 16 1 1, 239 70 49 238) on Their Downstream Multiaccess Interfaces, They Originate Assert Messages to Determine the Forwarder for the Group

Packets for group 239 70 49 238

Source 172 16 1 1

Copper t ^ I gf Mercury

il Silver

Copper t ^ I gf Mercury

Assert ^

^ Assert

Member of 239 70 49 238

Member of 239 70 49 238

Example 5-13 shows Silver's unicast routing table and its multicast forwarding table The unicast table indicates equal-cost OSPF paths to the source 172 16 1 1 via either Copper (172 16 3 2) or Mercury (172 16 3 1) Because routes are OSPF, they have an equal administrative distance of 110 And because both routes have an OSPF cost of 74, the forwarder is the router with the highest IP address

Example 5-13 Silver's Unicast Routing Table Shows Two Next-Hop Routers to the Subnet of Source 172 16 1 1 The Multicast Routing Table Shows That the Next-Hop Router with the Highest IP Address Has Been Chosen as the Forwarder

Silver#show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, 0 - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default U - per-user static route, o - ODR T - traffic engineered route Gateway of last resort is not set

172 16 0 0/16 is variably subnetted, 8 subnets, 2 masks 0 172 16 2 252/30 [110/138] via 172 16 3 1, 00:02:16, Ethernetl

[110/138] via 172 16 3 2, 00:02:16, Ethernetl 0 172 16 2 248/30 [110/74] via 172 16 3 1, 00:02:16, Ethernetl

0 172 16 2 244/30 [110/74] via 172 16 3 2, 00:02:16, Ethernetl

[110/74] via 172 16 3 1, 00:02:16, Ethernetl 0 172 16 2 240/30 [110/138] via 172 16 3 1, 00:02:16, Ethernetl

0 172 16 2 236/30 [110/74] via 172 16 3 1, 00:02:16, Ethernetl

C 172 16 5 0/24 is directly connected, Ethernet©

0 172 16 1 0/24 [110/84] via 172 16 3 1, 00:02:16, Ethernetl

[110/84] via 172 16 3 2, 00:02:16, Ethernetl C 172 16 3 0/24 is directly connected, Ethernetl

Silver#

Silver#show ip mroute 172 16 1 1 239 70.49 238

IP Multicast Routing Table

Flags: D - Dense, S - Sparse, C - Connected, L - Local, P - Pruned

R - RP-bit set, F - Register flag, T - SPT-bit set, J - Join SPT Timers: Uptime/Expires

Interface state: Interface, Next-Hop or VCD, State/Mode

(172 16 1 1, 239 70 49 238), 00:02:02/00:02:59, flags: CT Incoming interface: Ethernetl, RPF nbr 172 16 3 2 Outgoing interface list:

Ethernet©, Forward/Dense, 00:01:50/00:00:00

Silver#

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