Add Book to My BookshelfPurchase This Book Online

Chapter 6 - Protocol Independent Multicast - Dense Mode

Cisco Multicast Routing & Switching
William R. Parkhurst
  Copyright © 1999 The McGraw-Hill Companies

PIM-DM Interface Pruning
When the oilist for a particular interface becomes null, there are no downstream PIM-DM routers or multicast receivers attached to the network. The interface does not need to transmit multicast traffic and can, therefore, be pruned from the source-based delivery tree. In Figure 6-13, router A initially receives multicast traffic from the source and floods the traffic onto all interfaces in the oilist. Router B is a PIM-DM-enabled router, but has no attached downstream PIM-DM routers or mulitcast receivers. Router B will send a prune message to its upstream router for this particular multicast source. When router A receives the prune from router B, router A’s oilist for the serial link will become null, halting the forwarding of multicast traffic to router B.
Figure 6-13: The pruning of a PIM-DM interface
The packet format used for Prune, Join, or Graft messages is illustrated in Figure 6-14.
Figure 6-14: PIM Join/Prune Packet Format
The Upstream Neighbor Address is where the Join/Prune packet is sent. For the network in Figure 6-13, router B sends the message to router A so the upstream neighbor address equals the IP address of router A’s serial interface. The holdtime indicates the lifetime of the prune. PIM-DM is a cyclic protocol. Initially all packets are forwarded onto interfaces in the oilist. When a prune is received, traffic from the source/group indicated in the prune message no longer forwards onto the interface. The prune remains in effect until the holdtime for the prune expires. When the prune timer expires, the interface is added back to the oilist for the source group. Multicast traffic is again forwarded onto the interface. Join or graft messages can be used to add a pruned interface to the oilist before the prune holdtime expires. The mask length (mask len) and address length (adr len) fields indicate the length in bytes of the mask and the address for the group or groups to be pruned from or grafted onto the source-based delivery tree. Either the prune list or the join list may be empty, but a join/prune packet should never be sent when both the join and prune lists are empty. The format for the group list is shown in Figure 6-15. The number of groups in the group list is given by the Num. of Groups parameter in Figure 6-14. Each group is identified by the address and mask of the group to be pruned or joined. Following the address and mask pair is the number of join and prune sources for the group. Join sources are all listed first, followed by the prune sources represented by the encoded format of Figure 6-16.
Figure 6-15: Group List format
Figure 6-16: Encoded Source Address format
The S bit in the encoded source address format indicates whether or not this is a sparse mode group and should be set to 0 for dense mode groups. The W bit is the wildcard bit and indicates whether the entry applies to a specific source/group (S,G), W = 0 or if the entry applies to all sources of the group (*,G), W = 1. The R bit applies to PIM Sparse Mode (PIM-SM). The Len field is the length of the source mask in bits and the source address is the IP address of the source to be joined or pruned.

 


 
Books24x7.com, Inc © 2000 –  Feedback