Enterasys C5G124-24 Configuration Manual page 329

1. Decides if the upstream neighbor is capable of receiving prunes.
• If it is not, then the sending device proceeds no further.
• If it is, then the sending device proceeds as follows.
2. Stops any pending grafts awaiting acknowledgments.
3. Determines the prune lifetime.
This value should be the minimum of the default prune lifetime (randomized to prevent
synchronization) and the remaining prune lifetimes of the downstream neighbors.
4. Forms and transmits the packet to the upstream neighbor for the source.
To ensure the prune is accepted, the DVMRP-enabled device sets a negative cache prune entry for
three seconds. If the traffic has not stopped after three seconds, the device sends another prune
and doubles the cache entry. This method is called exponential back-off. The more prunes that are
dropped, the longer the back-off becomes.
After the prune lifetime expires (two hours), the prune transmission process is repeated.
When receiving a prune, the upstream device:
1. Decides if the sending neighbor is known.
• If the neighbor is unknown, it discards the received prune.
• If the neighbor is known, the receiving device proceeds as follows.
2. Ensures the prune message contains at least the correct amount of data.
3. Copies the source address, group address, and prune time-out value, and, if it is available in
the packet, the netmask value to determine the route to which the prune applies.
4. Determines if there is active source information for the source network, multicast group (S,G)
pair.
• If there is not, then the device ignores the prune.
• If there is, then the device proceeds as follows.
5. Verifies that the prune was received from a dependent neighbor for the source network.
• If it was not, then the device discards the prune.
• If it was, then the device proceeds as follows.
6. Determines if a prune is currently active from the same dependent neighbor for this S,G pair.
• If not active, creates a state for the new prune and sets a timer for the prune lifetime
• If active, resets the timer to the new time-out value.
7. Determines if all dependent downstream devices on the interface from which the prune was
received have now sent prunes.
• If they have not, removes the interface from all forwarding cache entries for this group
instantiated using the route to which the prune applies.
• If they have, determines if there are group members active on the interface and if this
device is the designated forwarder for the network.
Graft Messages
Leaf devices send graft messages when the following occur:
• A new local member joins a group that has been pruned upstream and this device is the
designated forwarder for the source.
Using Multicast in Your Network
Fixed Switch Configuration Guide 19-9