Flooding/Multicast Trees

TRILL networks require a minimum of one distribution tree for the purpose of flooding broadcast, unknown unicast, and multicast traffic so that each egress RBridge receives one copy of the packet. RBridge trees are bi-directional. All RBridges in the TRILL network must know:

How many trees to compute.
Which trees to compute.
What the nickname (16-bit number) is for each tree.
Which trees each ingress bridge might choose.

At a minimum, each RBridge will request one tree be computed with itself as the root (default configuration). Since all RBridges in the network must use the same set of distribution trees to forward flood and multicast traffic and there are a limited number of trees that can be reasonably supported (due to computational load on the CPU), all of the RBridges compute the number of trees supported by the RBridge advertising the minimum number of supported trees and which trees computed are determined by the distribution tree priority.

Distribution tree nicknames may be configured or auto-generated. Configured distribution tree nicknames always have priority over auto-generated nicknames. So as to minimize nickname collisions, RBridges attempt to reuse auto generated nicknames across system restarts. If multiple distribution trees are maintained within the TRILL network, by default the ingress RBridge forwards flood traffic to the tree whose root is least cost from itself.

The figure below shows how trees are determined and used. In the network example, all of the RBridges advertise the number of trees to compute is four. They also advertise the number of trees to use as two. Each RBridge also advertises that it wants all of the other RBridges to compute a tree with itself as the root. Thus, the total number of potential trees is six. The tree priorities are set in the following order from highest to lowest priority: B, D, A, C, E, and then F. Thus, all RBridges compute four trees with an RBridge root of B, D, A, and C. But all RBridges will forward using only the tree rooted at RBridge B or D. If RBridge E needed to flood a packet, it would choose the distribution tree rooted at RBridge B‘s nickname as the egress RBridge nickname. Similarly, RBridge A would also flood packets using the tree rooted at RBridge B. RBridge C on the other hand would choose the tree rooted at RBridge D.

Multiple TRILL Distribution Trees

Trees are bidirectional and transit nodes must only forward to next-hop RBridges that are represented in the tree. except for the RBridge from which the packet was received. Reverse Path Forwarding is recommended as a loop avoidance mechanism and to provide network security from spoofing attacks.

Published March 2020prev | next

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