Figure 19-29: A network that could significantly benefit from pruning in Objective-C

Printer PDF 417 in Objective-C Figure 19-29: A network that could significantly benefit from pruning

Figure 19-29: A network that could significantly benefit from pruning
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VLAN STP Considerations
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Another issue with VLANs is how they make use of STP Like any switched network, as long as redundant links are present, STP needs to be enabled The problem with VLANs is that STP actually has multiple broadcast domains to contend with Three methods exist for STP functionality on VLANs: Common Spanning Tree (CST), Per VLAN Spanning Tree (PVST), and Per VLAN Spanning Tree Plus (PVST+) CST is used with the IEEE's 8021q protocol, and it simply sets up a single spanning tree for the entire switching fabric, regardless of the number of VLANs supported A single STP topology is chosen for the entire physical network, and all VLANs are forced to use this topology The up side is that this method takes minimal switch resources to compute the STP
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topology Only one STP root exists, and STP recomputations should be minimal The down side is that all VLANs must use the same (typically huge) topology, leading to the possibility that less than optimal paths will be chosen, as shown in Figure 19-30 In a large environment, the topology may also take a long time to converge
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Figure 19-30: Nonoptimal paths with CST PVST is used with Cisco's ISL protocol, and it uses a separate STP topology for each VLAN This method has the advantage of optimal path selection (as shown in Figure 19-31) and minimal convergence time The down side is that multiple STP roots must be used, multiple STP topologies must be calculated, and BPDUs must be sent for each topology, all of which lead to increased switch resource and bandwidth consumption
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Figure 19-31: Optimal paths with PVST PVST+ is an enhancement to PVST that allows interoperability between a CST network and a PVST network Basically, PVST+ maps PVST spanning trees to the CST spanning tree, sort of like a "trunking gateway" This method can lead to some weird STP topologies, and it is generally much easier and more efficient to stick with PVST if you use ISL or CST if you use 8021q
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VLAN Configuration
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The first step in configuring VLANs is to define the logical topology you require Configuring your VLANs is a fairly simple process if you have a good design to work from Although network design is beyond the scope of this book, once you have decided on a design, implementation consists of following four basic steps: 1 Enable and configure VTP
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2 Define the VLANs 3 Assign VLAN membership 4 Configure trunking You should enable and configure VTP before you define VLANs, because VLANs cannot be defined unless the switch is in VTP server or transparent mode When installing other VTPcapable switches into the network, be extremely careful By default, switches are typically configured for VTP server mode with a blank default domain name If no domain name is assigned to a switch, the switch automatically uses the first domain name it hears in VTP advertisements (From then on, however, it retains this name) This functionality can lead to an issue where you install an additional switch into a network that just happens to ship from the factory with a very high configuration revision number Remember that VTP uses this number to figure out what the most current revision of the VTP database is Because the configuration for this switch is blank and it defaults to server mode, it may begin advertising a higher revision number than the one currently used, overwriting your VTP configuration on all switches in the network with its "newer," blank configuration To eliminate this problem, before installing a new switch into a production VTP-enabled network, perform the following steps: 1 Clear the switch configuration using the command clear config all (a CatOS switch), erase config (a standard IOS switch), or delete nvram (a 1900/2820 series switch) 2 Power cycle the switch 3 Verify that the switch's VTP configuration revision number is zero by using the show vtp domain or show vtp status commands Repeat steps if the number is higher than zero To enable VTP, define the VTP domain name and enable the VTP mode on the switch On a CatOS-based switch, set the domain using the set vtp domain [domain name] command; and on a standard IOS switch, use the vtp domain [domain name] command At this point, you should also set a VTP password for the domain If you set a password, for a switch to participate in VTP for that domain, it must have the correct password configured Setting the domain password ensures that rogue switches do not enter the topology and overwrite the VTP configuration To set the password, use the set vtp password [password] (CatOS) or vtp password [password] (standard IOS) command Note To set VTP parameters on a standard IOS-based switch, you must be in VLAN configuration mode To enter this mode, type vlan database from a privileged mode prompt Also note that to apply any changes made in this mode, you must exit from the mode using the exit command The following output shows these commands:
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>3500 Configuration< 3508# vlan database 3508(vlan)# vtp domain Thisone password secret Domain name set to Thisone Setting device VLAN database password to secret 3508(vlan)# vtp server Setting device to VTP SERVER mode 3508(vlan)#exit
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APPLY completed Exiting >5500 Configuration< Cat5K (enable) set vtp domain Mydomain mode server passwd knowme pruning enable v2 enable Generating MD5 secret for the password This command will enable the pruning function in the entire management domain All devices in the management domain should be pruning-capable before enabling Do you want to continue (y/n) [n] y This command will enable the version 2 function in the entire management domain All devices in the management domain should be version2-capable before enabling Do you want to continue (y/n) [n] y VTP domain Mydomain modified Cat5K (enable)
If you decide not to use VTP, you can skip the domain name and password configuration, but you still need to set the switch's VTP mode to transparent to ensure that you can create VLANs as necessary without causing any undue configuration overwrites on other switches Setting the mode on all switches to transparent effectively disables VTP To set the VTP mode, use the set vtp mode [client | server | transparent] (CatOS) or vtp [client | server | transparent] commands Once the mode is set, you will typically want to set the VTP to version 2 if all of your switches support version 2 To do this, use the command set vtp v2 enable (CatOS) or vtp v2mode (standard IOS) Remember If you set v2 on any switch in the domain, you must make sure to set it on all switches in the domain to ensure proper VTP functionality Finally, you will typically want to enable VTP pruning if you enable VTP There are really no drawbacks to using pruning, so unless all of your switches are set to transparent mode, you should probably enable pruning To enable pruning on the entire management domain, from the VTP server switch, issue the command set vtp pruneeligible [vlan list] (CatOS) or vtp pruning (standard IOS) You normally shouldn't need to do this because, by default, switches are typically set to prune VLANs 2-1000 To check which VLANs are eligible for pruning, use the show vtp domain or show vtp status commands Once you have configured VTP, you need to define your VLANs To perform this task, set the VLAN names and numbers using the set vlan [number] name [name] (CatOS) or vlan [number] name [name] (standard IOS) command If you are using VTP, remember to perform this task on the VTP server Once you have defined your VLANs, you will then need to assign members to the VLANs To assign static VLAN members (based on port number), use the set vlan [vlan number] [mod/port list] (CatOS) or switchport access [vlan number] (standard IOS) command Both of these commands for 5500 and 3500 switches are shown in the following output:
>3500 Configuration< 3508(vlan)#vlan 200 name Classroom VLAN 200 added: Name: Classroom 3508(vlan)#exit APPLY completed Exiting 3508#configure terminal Enter configuration commands, one per line End with CNTL/Z 3508(config)#interface GigabitEthernet 0/2 3508(config-if)#switchport access vlan 200 >5500 Configuration< Cat5K (enable) set vlan 200 name Classroom Vlan 200 configuration successful Cat5K (enable) set vlan 200 3/1-10 VLAN 200 modified VLAN 1 modified VLAN Mod/Ports ---- ----------------------200 3/1-10 Cat5K (enable)
To assign VLAN membership dynamically (based on MAC address), you must use a VLAN Membership Policy Server (VMPS) Unfortunately, VMPS configuration could be a chapter in and of itself and is beyond the scope of this book Note For more information on VMPS configuration, visit the following web addresses:http://wwwciscocom/univercd/cc/td/doc/product/lan/c2900xl/29_35xu/scg/kivlanhtm http://wwwciscocom/univercd/cc/td/doc/product/lan/c2900xl/29_35xp/eescg/mascvmpshtm Finally, you must configure trunking for the switch Before getting into the specific command required for this, however, you need to learn about trunk negotiation Trunk negotiation using Dynamic ISL (DISL) or Dynamic Trunking Protocol (DTP) allows two ports to negotiate whether to become trunking ports This negotiation is used to allow you to configure only one side of the trunk The other side can autoconfigure itself to the required parameters Personally, I prefer to manually configure my trunk ports, because autonegotiation typically doesn't save enough time to be worthwhile (and has been known to cause major headaches) Autoconfiguration works by using negotiation frames between two ports If the settings on the ports are compatible, the ports will enter a trunking state The ports can be set to one of the following five states:
On This state cuts trunking on In this state, negotiation does not occur, but DTP frames are still issued (meaning that if both sides do not support DTP, errors may occur) Off This state cuts trunking off Auto This it the default setting The port will negotiate trunking status and encapsulation (ISL or 8021q) The port will become a trunk if the other side is set to either on or desirable Desirable With this state, the port will become a trunk if the other side is set to on, auto, or desirable
Nonegotiate With this state, the port will be a trunk, and it will not issue DTP frames to negotiate any parameters This state is useful when switches do not support the same Dynamic Trunking Protocol
Typically, it is easiest to just set both sides to either on or off However, if you set one side to either on or desirable, the other side will automatically start trunking, because the default setting for all trunk-capable (100 Mbps or faster Ethernet) ports is auto Note As of this writing, 2900 and 3500 series switches (ie, the standard IOS switches) do not support autonegotiation For these switches to connect to a 1900 series or 5500 series switch, you will need to set the trunking mode on the 1900 or 5500 to nonegotiate If you choose to use autoconfiguration, set the trunk port on one side to desirable, nonegotiate, or on To do this for a 1900 series switch, use the command trunk [desirable | on | auto] from interface config mode on either of the Fast Ethernet interfaces On a 5500 series switch, use the set trunk [module/port] [desirable | on | auto] command The following output shows these commands:
>1900 Configuration< 1900F(config-if)#trunk on >5500 Configuration< Cat5K (enable) set trunk 3/1 desirable isl Port(s) 3/1 trunk mode set to desirable Port(s) 3/1 trunk type set to isl Cat5K (enable) clear trunk 3/1 Port(s) 3/1 trunk mode set to auto Port(s) 3/1 trunk type set to negotiate
To eliminate negotiation (and the possibility of autonegotiation failure), use the trunk nonegotiate command on a 1900 switch, or the set trunk [module/port] nonegotiate command on a 5500 switch The 2900 and 3500 series will never negotiate, so the command to enable trunking on these switches is switchport mode trunk To set the VLANs transported across your trunk links, use the command trunk-vlan [vlan list] (1900 series), set trunk [module/port] [vlan list] (CatOS), or switchport allowed vlan [add | remove] [vlan list] (2900 or 3500 series) Once these parameters are configured correctly on both sides, your switches should trunk correctly Use the show trunk command to verify trunking functionality, as shown here:
Cat5K (enable) show trunk detail * - indicates vtp domain mismatch Port Mode Encapsulation Status Native vlan -------- ----------- ------------- ------------ ----------4/1-2 on lane trunking 1 Port Vlans allowed on trunk -------- ---------------------------------------------------------4/1-2 1-1005 Port Vlans allowed and active in management domain -------- ----------------------------------------------------------
4/1-2 Port Vlans in spanning tree forwarding state and not pruned -------- ---------------------------------------------------------4/1-2
Note For trunked connections to routers, additional configuration on the routers is necessary 22 covers router trunk configuration
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