VLAN based Tree Selection for Multi-destination Frames
TRILL uses distribution trees for multi-destination traffic. Multiple trees can be used by an ingress RBridge. Different ingress RBridges may choose different distribution trees for the same VLAN and/or multicast group traffic. Distribution trees are normally pruned based on VLAN. For any RBridge RBn, if RBn has downstream receivers of VLAN x in a distribution tree t, there will be an entry of (t, x, port list) in the multicast forwarding table on RBn. If there are n trees and m VLANs, the multicast forwarding table size on RBn is typically n*m entries. The value of m is up to 4096 and n is the total number of distribution trees in the campus. If finer granularity filtering such as L2/L3 multicast address is used, then the multicast forwarding table size further increases dramatically. TRILL multicast forwarding table size is limited by hardware and L3 multicasting may share the same table with it in hardware implementations. Therefore multicast table entry is a precious resource. This document specifies a VLAN based tree selection mechanism to reduce the TRILL multicast forwarding table size. No data plane change is required.