Case Study Configuring Sparse Dense Mode

A slight "cheat" was used in the examples of the preceding case study. Examining Figure 6-5, notice that the C-RPs are directly connected to the mapping agent, and the mapping agent is directly connected to Bowler. In Figure 6-6, Homburg is now configured as the Auto-RP mapping agent. This topology gives rise to an interesting dilemma: Homburg advertises the RPs to all routers in RP-Discovery messages, using the reserve address 224.0.1.40. All PIM-SM routers listen for this address. In a sparse-mode environment, however, multicast packets must initially be forwarded on shared trees. That means the routers listening for 224.0.1.40 must notify their RP that they want to join that group, in order to receive the RP-Discovery messages. But how do the routers know where the RP is if they have not yet received the RP-Discovery messages?

Figure 6-6 Homburg Is Now the Mapping Agent

10.1.1.88/24 Source, Group 228.13.20.216

Homburg Mapping Agent

Fedora C-RP

S1.503 10224J:2 10.2.1.2/24 j """"

Stetson C-RP

10.224.1.1

S1.503 10224J:2 10.2.1.2/24 j """"

Stetson C-RP

10.224.1.1

Bowler

10.1.2.113/24 Member, Group 228.13.20.216

10.1.2.113/24 Member, Group 228.13.20.216

Bowler

The same Catch-22 would apply to the C-RPs if they were not directly connected to the mapping agent. The mapping agent must receive RP-Announce messages from the C-RPs in order to select an RP, and to do this, it must join group 224.0.1.39. It cannot join this group, however, if it does not know where the RPs are, and it cannot know where the RPs are unless it receives RP-Announce messages.

PIM sparse-dense mode was created to overcome this problem. When an interface is configured in this mode, it uses sparse mode if an RP is known for the group. If no RP is known, it uses dense mode. In the case of 224.0.1.39 and 224.0.1.40, the groups are assumed to be in dense mode. Example 6-36 shows the sparse-dense mode configuration for Homburg.

Example 6-36 PIM Sparse-Dense Mode Configuration for Router Homburg hostname Homburg i ip multicast-routing i interface Loopback© ip address 10.224.1.4 255.255.255.0 ip pim sparse-mode %

interface Ethernet©/© ip address 10.1.1.1 255.255.255.0 ip pim sparse-dense-mode no ip mroute-cache i interface Serial0/1 no ip address encapsulation frame-relay no ip mroute-cache i interface Serial©/1.305 point-to-point description PVC to R5 ip address 10.2.1.1 255.255.255.0 ip piift sparse-dense-mode no ip mroute-cache frame-relay interface-dlci 305

i interface Serial©/1.309 point-to-point description PVC to R9 ip address 10.2.2.1 255.255.255.0 ip pim sparse-dense-mode no ip mroute-cache frame-relay interface-dlci 309

router ospf 1 network 10.0.0.0 0.255.255.255 area 0

i ip pim send-rp-discovery LoopbackO scope 5

The command ip pim sparse-dense-mode is used on all the physical interfaces, and it is configured similarly on all physical interfaces of all routers in the topology of Figure 6-6. The loopback interface is only in sparse mode, because it is needed only as the mapping agent address and never must make any sparse/dense determinations. Interface E0/0 could also be put into sparse mode, because it does not face any downstream routers and would not have to make sparse/dense decisions. However, it is good practice to place all interfaces in sparse-dense mode for consistency. In fact, it is commonly advised to use this mode in all modern PIM domains as long as all routers support the mode.

Example 6-37 shows the multicast routing table on Homburg after the reconfiguration. Notice that the entries for (*, 224.0.1.39) and (*, 224.0.1.40) have D flags, indicating that they are operating in dense mode. All other (*, G) entries are flagged as sparse.

Example 6-37 The Flags Associated with (*,224.0.1.39) and (*,224.0.1.40) in Homburg's mroute Table Show That Those Groups Are Operating in Dense Mode

Homburg#show ip mroute 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, State/Mode (*, 228.13.20.216), 00:20:42/00:02:59, RP 10.224.1.2, flags: SJCF Incoming interface: Serial0/1.305, RPF nbr 10.2.1.2 Outgoing interface list:

Ethernet©/©, Forward/Sparse-Dense, 00:20:42/00:02:43 (10.1.1.88/32, 228.13.20.216), 00:20:42/00:02:59, flags: CFT Incoming interface: Ethernet0/0, RPF nbr 0.0.0.0 Outgoing interface list:

Serial0/1.305, Forward/Sparse-Dense, 00:20:04/00:02:47 (*, 224.2.127.254), 00:20:34/00:02:59, RP 10.224.1.2, flags: SJCF Incoming interface: Serial0/1.305, RPF nbr 10.2.1.2 Outgoing interface list:

Ethernet0/0, Forward/Sparse-Dense, 00:20:34/00:02:42 (10.1.1.88/32, 224.2.127.254), 00:20:34/00:02:56, flags: CFT Incoming interface: Ethernet0/0, RPF nbr 0.0.0.0 Outgoing interface list:

Serial0/1.305, Forward/Sparse-Dense, 00:20:06/00:02:44 (*, 224.0.1.39), 00:20:32/00:00:00, RP 0.0.0.0, flags: DJCL Incoming interface: Null, RPF nbr 0.0.0.0 Outgoing interface list:

Ethernet0/0, Forward/Sparse-Dense, 00:20:32/00:00:00 Serial0/1.305, Forward/Sparse-Dense, 00:20:32/00:00:00 Serial0/1.309, Forward/Sparse-Dense, 00:20:32/00:00:00 (10.224.1.1/32, 224.0.1.39), 00:20:32/00:02:27, flags: CLT Incoming interface: Serial0/1.309, RPF nbr 10.2.2.2 Outgoing interface list:

Ethernet0/0, Forward/Sparse-Dense, 00:20:32/00:00:00 Serial0/1.305, Forward/Sparse-Dense, 00:20:32/00:00:00

(10.224.1.2/32, 224.0.1.39), 00:19:54/00:02:05, flags: CLT Incoming interface: Serial0/1.305, RPF nbr 10.2.1.2 Outgoing interface list:

Ethernet©/©, Forward/Sparse-Dense, 00:19:54/00:00:00 Serial0/1.309, Forward/Sparse-Dense, 00:19:54/00:02:08

(*, 224.0.1.40), 00:20:13/00:00:00, RP 0.0.0.0, flags: DJCL Incoming interface: Null, RPF nbr 0.0.0.0 Outgoing interface list:

Ethernet0/0, Forward/Sparse-Dense, 00:20:14/00:00:00

Example 6-37 The Flags Associated with (*,224.0.1.39) and (*, 224.0.1.40) in Homburg's mroute Table Show That Those Groups Are Operating in Dense Mode (Continued)

Serial0/1.305, Forward/Sparse-Dense, 00:20:14/00:00:00 Serial0/1.309, Forward/Sparse-Dense, 00:20:14/00:00:00

(10.224.1.4/32, 224.0.1.40), 00:20:06/00:02:48, flags: CLT Incoming interface: Loopback0, RPF nbr 0.0.0.0 Outgoing interface list: %

Ethernet©/©, Forward/Sparse-Dense, 00:20:06/00:00:00 Serial0/1.305, Forward/Sparse-Dense, 00:20:06/00:00:00 Serial0/1.309, Forward/Sparse-Dense, 00:20:06/00:00:00

Homburg#

Besides the two Auto-RP groups, sometimes you might want to have some groups operating in sparse mode and others operating in dense mode. By using the ip pim send-rp-announce group-list command at the C-RPs, as demonstrated in the preceding case study, you can regulate what groups are mapped to the RP, and hence operate in sparse mode. Any groups not mapped to an RP will operate in dense mode.

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