BROCADE IP PRIMER

Chapter 4: TRILL—Adding Multi-Pathing to Layer 2 NetworksLink-State ProtocolsAs noted earlier, TRILL will use link-state protocols to form the control plane ofTRILL. The purpose of the control plane is to distribute the VLAN configurationto all the RBridges on the VLAN. Link-state protocols also continuously monitorthe VLAN configuration and adjust the configuration data base in the event ofchanges. The control plane also provides the algorithms used to calculate theshortest path between any two RBridges on the VLAN. Considering the factthat TRILL will be used in converged environments where storage and TCP/IPnetworks are deployed, you can expect that link-state protocols from bothworlds will be utilized by TRILL.Routing BridgesRouting bridges are a new type of L2 device that implement the TRILL protocol,perform L2 forwarding, and require little or no configurations. Using the configurationinformation distributed by the link-state protocol, RBridges discovereach other and calculate the shortest path to all other RBridges on the VLAN.The combination of all calculated shortest paths make up the RBridge routingtable. It is important to note that all RBridges maintain a copy of the configurationdatabase, which helps reduce convergence time. When they discovereach other, RBridges select a designated bridge (DRB), which in turn assigns adesignation for the VLAN and selects an appointed forwarder (AF) for theVLAN. Although the DRB can select itself as an AF, there can only be a singleAF per VLAN. The AF handles native frames on the VLAN.Moving TRILL DataTRILL works by adding a new header to the beginning of an Ethernet frame.This new header is added to the frame when the first RBridge encounters it.Note that this can happen if a host is directly connected to an RBridge or if anon-routing Ethernet segment hands the frame off to an RBridge-either way. Ifthe frame can remain in a non-routing Ethernet segment without ever touchingan RBridge, then no header is added and it isn't really necessary.Using the original destination address as a key, a list of eligible next-hopRBridges is determined. This list contains the RBridges that could be the nextstep along all least-cost paths moving to the final destination. If more than oneRBridge is in the list, a hash is used to distribute the traffic load and guaranteethat all traffic in a single stream stays on the same path to avoid reorderingoverhead. The RBridge that results from this is placed in the TRILL header andthe frame is sent on.94 Brocade IP Primer