Source Registration

The fundamental concept of shared trees, mentioned several times already, is that the multicast tree is rooted at a core or rendezvous point rather than at the source The question arises, then, of how the source delivers multicast packets to the RP for delivery over the branches of the tree Recall that CBT resolves the question by using bidirectional trees— packets can flow both down a branch from the core and up the branch toward the core The source's directly connected router joins the shared tree to the core and then sends its traffic up the branch to the core The problem with bidirectional trees is that it is very hard to ensure a loop-free topology, because RPF checks cannot be performed when there is no distinct "upstream" and "downstream "

Unlike CBT, PIM-SM uses RPF checks Therefore, its trees must be unidirectional—that is, traffic can flow only down tree branches from the RP The unidirectional traffic ensures a clearly defined incoming or RPF interface If traffic flows only from the RP outward, however, how does a source deliver its multicast traffic to the RP?

When a PIM-SM router first receives a multicast packet from a directly connected source, it looks in its group-to-RP mappings to find the correct RP for the destination group, as demonstrated in the output in Example 5-17 This step is the same as when a member signals a group join with an IGMP message

Example 5-17 The Group-to-RP Mapping of Router Aluminum in Figure 5-55 Compare This to Example 5-14, Iron Has a Static RP Napping, Whereas Aluminum Has Learned the RP Address Dynamically

Aluminum#show ip pim rp mapping

PIM Group-to-RP Mappings

Group(s) 224 0 0 0/4, uptime: 00:02:39, expires: 00:02:17 RP 172 16 224 1 (?), PIMv2 v1 Info source: 172 16 2 245 (?) Aluminum#

After the group's RP is determined, the router encapsulates the multicast packet in a PIM Register message and sends the message to the RP Instead of multicasting, the Register message is unicast to the RP address, as illustrated by Figure 5-57

When the RP receives the Register message, the multicast packet is decapsulated If the multicast routing table already has an entry for the group, copies of the multicast packet are forwarded out all interfaces on the outgoing interface list, as illustrated by Figure 5-58

Figure 5-57 The First Multicast Packet Is Encapsulated in a PIM Register Message and Is Unicast to the RP

Source, Group 236 82 134 23

Shared tree

Aluminum .

S1 309 172 16 2 233

S1 903

172 16 2 234 Platinum

Register Destination: 172 16 224 1

Multicast packet

16 2 245 S1508

172 16 2 237

172 16 2 246

16 2 245 S1508

172 16 2 237

Tin si 807 iron

172 16 2 242

Brass (RP 172 16 2241)

S1 805 172 16 2 238

S1 708 172 16 2 241

Member, Group 236 82 134 23

Figure 5-58 The Multicast Packet Is Removed from the Register Message and Is Forwarded Out All Interfaces on the Group's Outgoing Interface List

Source, Group 236 82 134 23

Aluminum

Platinum

Multicast packet

Brass (RP 172 16 2241)

Shared tree

Iron

Member, Group 236 82 134 23

If there is a significant amount of multicast traffic to be sent to the RP, it is inefficient to continue encapsulating the packets in Register messages to get them to the RP Therefore, the RP creates an (S, G) entry in its multicast table and initiates an SPT to the source DR by multicasting a Join/Prune message, as illustrated by Figure 5-59 In this message, the source address is included, WC-bit = 0, and RPT-bit = 0 to indicate that the path is a source-based SPT rather than a shared RPT

Figure 5-59 The RP Creates a Source-Based, Shortest Path Tree to the Source's DR

Source 172 16 1 1, Group 236 82 134 23

Aluminum

Join

Shared tree

Platinum

Join

Iron

Brass (RP 172 16 2241)

Iron

Member, Group 236 82 134 23

Member, Group 236 82 134 23

Once the SPT is established and the RP is receiving the group traffic over that tree, it sends a Register Stop message to the source's DR to tell the router to stop sending the multicast packets in Register messages, as illustrated by Figure 5-60

If there are no group members when the source begins sending multicast traffic to the RP, the RP does not build an SPT Instead, it just sends a Register Stop to the source's DR, telling it to stop sending the encapsulated multicast packets in Register messages The RP has a (*, G) entry for the group, and when a member joins, the RP can then initiate the SPT

A mechanism known as Register Suppression helps protect against the DR continuing to send packets to a failed RP When a DR receives a Register Stop, it starts a 60-second Register-Suppression timer When the timer expires, the router again sends its multicast packets to the RP in Register messages However, 5 seconds before this occurs, the DR sends a Register message with a flag set, called the Null-Register bit, and with no encapsulated packets If this message triggers a Register Stop from the RP, the Register-Suppression timer is reset

Figure 5-60 The RP Sends a Register Stop Message to Stop the Register Messages The Source 's Multicast Packets Are Now Sent to the RP Over the SPT

Source 172 16 1 1,

Group 236 82 134 23

Figure 5-60 The RP Sends a Register Stop Message to Stop the Register Messages The Source 's Multicast Packets Are Now Sent to the RP Over the SPT

Source 172 16 1 1,

Group 236 82 134 23

Member, Group 236 82 134 23

The debug messages in Example 5-18 show the sequence of events that occurs when router Aluminum begins sending multicast traffic to group 236 82 134 23 In this particular case, no members have yet joined the group As a result, the RP (Brass) immediately sends a Register Stop message to Aluminum in response to the Register

Example 5-18 This RP Has No Members for Group 236 82 134 23 As a Result, It Immediately Replies to the

Register Message from Aluminum (172 16 2 233) with a Register Stop Message Notice That Both Messages Are Unicast Rather Than Multicast

Example 5-18 This RP Has No Members for Group 236 82 134 23 As a Result, It Immediately Replies to the

Register Message from Aluminum (172 16 2 233) with a Register Stop Message Notice That Both Messages Are Unicast Rather Than Multicast

Brass#debug ip pim 236.82.134.23

PIM debugging is on

Brass#

PIM: Received Register on Seriall

509 from 172 16 2 233 for 172 16 1

1, group

236 82 134 23

PIM: Send Register-Stop to 172 16

2 233 for 172 16 1 1, group 236 82

134 23

Example 5-19 shows the route entry for the group Notice that there are both (*, G) and (S, G) entries for the group The (*, G) entry shows a null incoming interface and an RPF neighbor of 0 0 0 0, indicating that this router is the root of the shared tree The (S, G) entry shows that router Platinum (172 16 2 246), the upstream neighbor toward the source, is the RPF neighbor There are no interfaces on the outgoing interface list, so the entry is pruned

Example 5-19 The Routing Entry for Group 236.82.134.23 at the RR No Members Have Joined the Group

Brass#show ip mroute 236.82.134.23

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 v

(*, 236.82.134.23), 00:07:38/00:02:59, RP 172.16.224.1, flags: S Incoming interface: Null, RPF nbr 0.0.0.0 Outgoing interface list:

Serial1.509, Forward/Sparse, 00:03:06/00:02:50

(172.16.1.1, 236.82.134.23), 00:07:38/00:01:21, flags: P Incoming interface: Serial1.509, RPF nbr 172.16.2.246 Outgoing interface list: Null

Brass#

Example 5-20 shows the route entries for the group at Aluminum, the source's DR. He*e, the (*, G) entry also exists, with the Ethernet interface connecting to the source in the outgoing interface list. The incoming interface list is null. The (S, G) entry shows the same Ethernet interface on the incoming interface list. The entries have two flags in common: One flag indicates that the source is directly connected; the other (F) indicates that the router must send a Register message for the group traffic.

Example 5-20 The Corresponding Route Entry at the Source \s DR Shows a Pruned SPT Entry

Aluminum#show ip mroute 236.82.134.23

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

(*, 236.82.134.23), 00:15:30/00:02:59, RP 172.16.224.1, flags: SJCF Incoming interface: Null, RPF nbr 0.0.0.0 Outgoing interface list:

Ethernet0/0, Forward/Sparse, 00:15:23/00:02:28

(172.16.1.1/32, 236.82.134.23), 00:00:29/00:02:30, flags: PCFT Incoming interface: Ethernet0/0, RPF nbr 0.0.0.0 Outgoing interface list: Null

Aluminum#

The T flag on the (S, G) entry indicates that the entry represents an SPT, and the P entry indicates that there are no interfaces on the outgoing interface list. If there were an RPF neighbor, the router would send a Prune message to it for the group.

The final flag of interest is the J flag on the (*, G) entry. This flag indicates that the router switches to the SPT when a packet is received on the shared tree. Just how PIM-SM routers switch from shared trees to SPTs is the subject of the following section.

The debug messages in Example 5-21 show the sequence of events that occurs when the host attached to router Iron joins the group. The Join/Prune message, which was generated by Iron and multicast hop by hop to the RP, is received from Tin. The interface to Tin is added to the (*, G) entry; the interface is also added to the (S, G) entry, because the SPT to Aluminum will be used. Next, an SPT Join message is sent to Aluminum.

Example 5-21 These debug Messages Show the Member Attached to Router Iron Joining Group 236.82.134.23

Brass#debug ip pim 236.82.134.23

PIM debugging is on Brass#

PIM: Received v2 Join/Prune on Serial*!.508 from 172.16.2.238, to us

PIM: Join-list: (*, 236.82.134.23) RP 172.16.224.1, RPT-bit set, WC-bit set, S-bit set

PIM: Add Seriall.508/172.16.2.241 to (*, 236.82.134.23), Forward state

PIM: Add Seriall.508/172.16.2.241 to (172.16.1.1/32, 236.82.134.23)

PIM: Building Join/Prune message for 236.82.134.23

PIM: Send periodic Join/Prune to 172.16.2.246 (Seriall.509)

Example 5-22 shows the resulting route entries at the RP, and Example 5-23 shows the resulting route entries at the source's DR.

Example 5-22 When a Group Member Joins, Its Interface Is Added to the (*, G) Entry. It Also Is Added to the (S, G) Entry Because of the SPT to Aluminum

Brass#show ip mroute 236.82.134.23

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

(*, 236.82.134.23), 00:29:58/00:03:05, RP 172.16.224.1, flags: S Incoming interface: Null, RPF nbr 0.0.0.0 Outgoing interface list:

Seriall.509, Forward/Sparse, 00:29:58/00:02:52 Seriall.508, Forward/Sparse, 00:24:36/00:03:05

(172.16.1.1, 236.82.134.23), 00:24:54/00:02:59, flags: T Incoming interface: Seriall.503, RPF nbr 172.16.2.246 Outgoing interface list:

Seriall.508, Forward/Sparse, 00:24:36/00:02:35

Brass#

Example 5-23 The Interface Toward the RP Has Been Added to the Outgoing Interface List of Aluminum's (S, G) Entry, and the Entry Is No Longer in Prune State

Aluminum#show ip mroute 236.82.134.23

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

(*, 236.82.134.23), 00:00:47/00:02:59, RP 172.16.224.1, flags: SJCF Incoming interface: Serial0/1.309, RPF nbr 172.16.2.245 Outgoing interface list:

(172.16.1.1/32, 236.82.134.23), 00:00:47/00:02:59, flags: CFT Incoming interface: Ethernet©/©, RPF nbr 0.0.0.0 Outgoing interface list:

Serial0/1.309, Forward/Sparse, 00:00:34/00:02:58

Aluminum#

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