BROCADE IP PRIMER

Console49F Link 50F1 3 5 7 9 1113 15 17 19 21 2325 27 29 31 33 3537 39 41 43 45 47Power1F 2F 3F 4F2 4 6 8 10 1214 16 18 20 22 2426 28 30 32 34 3638 40 42 44 46 48Power49F Link 50FConsole1 3 5 7 9 1113 15 17 19 21 2325 27 29 31 33 3537 39 41 43 45 471F 2F 3F 4F2 4 6 8 10 1214 16 18 20 22 2426 28 30 32 34 3638 40 42 44 46 48Configuring VLANsThe 802.1q header has four parts:Tag Protocol Identifier (TPID). This is a 2-byte (16-bit) field, and it equals0x8100. In Chapter 1, we talked about the Length/Type Field, and how it canbe used to identify the upper-layer protocol that generated the frame (e.g.,0x0800 means the frame was generated by IP). The value “0x8100” tells anyEthernet device receiving it that it is receiving an IEEE 802.1q tagged frame. Itwill then know to expect and process the rest of the 802.1q header beforelooking for the actual Length/Type Field.Priority (802.1p). This is a 3-bit field. It can be a number from 0 to 7. It definesthe Quality of Service priority for the tagged frame. We'll talk more about this inChapter 13.Canonical Format Indicator (CFI). This is one bit. Given that, its value can beeither 0 or 1. This field was largely defined to aid in compatibility betweenEthernet and Token Ring networks. If you're using Ethernet, this value will be 0.VLAN Identifier (VID). This is a 12-bit field. This means that it can be any numberfrom 0 to 4,095. When you create a VLAN, you must give it a number. Itcan be any number between 1 and 4,095 (zero has been reserved to meanthat the frame is not a member of any VLAN). A tagged port can be a memberof more than one VLAN. This field identifies which VLAN this frame belongs to.For tagging to work, both sides of the link must be tagged. This is most commonlyused on links (or trunks) from switch to switch. If we are connectingSwitch A to Switch B, and we want the link between the two switches to bemembers of more than one VLAN, the link's port on Switch A and the link's porton Switch B must be tagged.Switch AtaggedtaggedSwitch BConfiguring VLANsWell, this is all fascinating, but how or when would I ever use this? Let's goback to our H. Pete & Associates, Inc. example. We left off with the Marketingdepartment on the fourth and fifth floor, and the Accounting department onthe fourth floor. Let's say that we're dealing with 48-port switches, and let's saythe Accounting department needs ports 3 to 24 (on the fourth floor switch).The Marketing department needs ports 25 to 48 on the fourth floor switch,and ports 3 to 48 on the fifth floor switch. Finally, let's say that the fourth floorswitch has a two-port trunk configured using ports 1 and 2. This trunk is connectedto the fifth floor switch that also has a trunk configured using ports 1and 2.Brocade IP Primer 173