3Com 4007, Руководство по внедрению, Страница: 437 / 684

How DVMRP Supports IP Multicast 437
Inside the prune message is a prune lifetime , or prune timer , which is a
period of time for which the prune message is valid. When the prune
lifetime expires, the interface is added back into the multicast delivery
tree — that is, until it generates another prune message.
Even though routers must use memory to store membership and prune
information, this approach regains the bandwidth that would have been
wasted on branches that do not lead to group members.
Grafting
If a router that has previously sent a prune message discovers a new
group member (from IGMP Reports) on one of its connections, it sends a
graft message to the previous hop router. When an upstream router
receives this message, it cancels the prune message it previously received.
Hop by hop, graft messages cascade back toward the source until they
reach the nearest live branch point on the IP multicast spanning tree.
DVMRP Interface
Characteristics
All DVMRP interfaces and DVMRP tunnels have two characteristics: a
metric that specifies the
cost
for the interface and a time-to-live (TTL)
threshold.
■
Metric Value
— The DVMRP metric is a numeric value or cost for that
path. The higher the assigned cost, the less likely it is that the
multicast packets will be routed over that interface (provided that
other path options exist).
■
TTL Threshold — Each IP multicast packet uses the TTL field of the IP
header to provide a scope-limiting parameter for routers to work with.
The initial value that the source sets in the TTL field controls the
number of router hops that the IP multicast packet can make through
the network. Each time that a router forwards a packet, it decrements
the packet TTL by one. As long as the multicast packet TTL is greater
than the TTL threshold of the multicast router interface, the router
forwards the packet. If not, the router filters (drops) the packet.
In all cases where the multicast router drops multicast packets, the router
does not provide an error notification to the source because IP multicast
is a connection-less technology.