BROCADE IP PRIMER

Chapter 3: Data Center Bridging (DCB)To ensure that all types of traffic are given the appropriate amount of bandwidth,a mechanism called Enhanced Transmission Selection (ETS) is usedwith Priority Flow Control. ETS establishes priorities and bandwidth limitationsso that all types of traffic receive the priority and bandwidth they require forthe proper operation of the server and all applications.ETS establishes Priority Groups (PGs), or traffic class groups. A Priority Groupis a collection of priorities as established in PFC. For example, all of the prioritiesassociated with Inter-Process Communication (IPC) can be allocated toone Priority Group. All priorities assigned to FCoE can be assigned to a secondgroup, and all IP traffic can be assigned to a third group.Each Priority Group has an integer identifier called the Priority Group ID (PGID).The value of the PGID is either 15 or a number in the range of 0 through 7. Ifthe PGID for a Priority Group is 15, all traffic in that group is handled on a strictpriority basis. That is, if traffic becomes available, it’s handled before traffic inall other Priority Groups without regard for the amount of bandwidth it takes.But be cautious. A PGID of 15 should be used only with protocols requiringeither an extremely high priority or very low latency. Examples of traffic in thiscategory include management traffic, IPC, or audio/video bridging (AVB).The other traffic class groups with PGID identifiers between 0 and 7 areassigned a bandwidth allocation (PG%). The sum of all bandwidth allocationsshould equal 100%. The bandwidth allocation assigned to a traffic class groupis the guaranteed minimum bandwidth for that group assuming high utilizationof the link. For example, if the PG for the traffic class group containing all storagetraffic is 60%, it is guaranteed that at least 60% of the bandwidthavailable after all PGID 15 traffic has been processed will be allocated to thestorage traffic Priority Group.The specification for ETS allows a traffic class group to take advantage ofunused bandwidth available on the link. For example, if the storage trafficclass group has been allocated 60% of the bandwidth and the IP traffic classgroup has been allocated 30%, the storage group can use more than 60% ifthe IP traffic class group does not require the entire 30%.Priority Group 1: Storage 60%Priority Group 2: LAN 30%Priority Group 3: IPC 10%82 Brocade IP Primer