1、Medium-Sized Switched Network Construction,Improving Performance with Spanning Tree,Interconnection Technologies,Determining Equipment and Cabling Needs,Each link provides adequate bandwidth for the total aggregate traffic over that link.,Advantages of EtherChannel,Logical aggregation of similar lin

2、ks between switches Load-shares across links Viewed as one logical port to STP Redundancy,Redundant Topology,Redundant topology eliminates single points of failure. Redundant topology causes broadcast storms, multiple frame copies, and MAC address table instability problems.,Station D sends a broadc

3、ast frame. Broadcast frames are flooded to all ports except the originating port.,Broadcast Frames,Broadcast Storms,Host X sends a broadcast. Switches continue to propagate broadcast traffic over and over.,Multiple Frame Copies,Host X sends a unicast frame to router Y. The MAC address of router Y ha

4、s not been learned by either switch. Router Y will receive two copies of the same frame.,Host X sends a unicast frame to router Y. The MAC address of router Y has not been learned by either switch. Switches A and B learn the MAC address of host X on port 1. The frame to router Y is flooded. Switches

5、 A and B incorrectly learn the MAC address of host X on port 2.,MAC Database Instability,Provides a loop-free redundant network topology by placing certain ports in the blocking state Published in the IEEE 802.1D specification Enhanced with the Cisco PVST+ implementation,Loop Resolution with STP,Spa

6、nning-Tree Operation,One root bridge per broadcast domain. One root port per nonroot bridge. One designated port per segment. Nondesignated ports are unused.,STP Root Bridge Selection,BPDU (default = sent every 2 seconds) Root bridge = bridge with the lowest bridge ID Bridge ID =,Bridge Priority,MAC

7、 Address,Spanning tree transits each port through several different states:,Spanning-Tree Port States,Describing PortFast,PortFast is configured on access ports, not trunk ports.,Configuring and Verifying PortFast,spanning-tree portfast,SwitchX(config-if)#,Configures PortFast on an interface,spannin

8、g-tree portfast default,SwitchX(config)#,Enables PortFast on all non-trunking interfaces,show running-config interface interface,SwitchX#,Verifies that PortFast has been configured on an interface,OR,Spanning-Tree Operation Example,Spanning-Tree Path Cost,Spanning-Tree Recalculation,Per VLAN Spannin

9、g Tree Plus,PVST+ Extended Bridge ID,Bridge ID without the extended system ID,Extended bridge ID with system ID,System ID = VLAN,Rapid Spanning Tree Protocol,Default Spanning-Tree Configuration,Cisco Catalyst switches support three types of STPs: PVST+ PVRST+ MSTP The default STP for Cisco Catalyst

10、switches is PVST+ : A separate STP instance for each VLAN One root bridge for all VLANs No load sharing,PVRST+ Configuration Guidelines,Enable PVRST+. Designate and configure a switch to be the root bridge. Designate and configure a switch to be the secondary root bridge. Verify the configuration.,P

11、VRST+ Implementation Commands,spanning-tree mode rapid-pvst,SwitchX(config)#,Configures PVRST+,show spanning-tree vlan vlan# detail,SwitchX#,Verifies the spanning-tree configuration,debug spanning-tree pvst+,SwitchX#,Displays PVST+ event debug messages,Verifying PVRST+,The spanning-tree mode is set

12、to PVRST.,SwitchX# show spanning-tree vlan 30 VLAN0030 Spanning tree enabled protocol rstp Root ID Priority 24606 Address 00d0.047b.2800 This bridge is the root Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 24606 (priority 24576 sys-id-ext 30) Address 00d0.047b.2800 Hello T

13、ime 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 Interface Role Sts Cost Prio.Nbr Type - - - - - - Gi1/1 Desg FWD 4 128.1 P2p Gi1/2 Desg FWD 4 128.2 P2p Gi5/1 Desg FWD 4 128.257 P2p,Configuring the Root and Secondary Bridges,Configuring the Root and Secondary Bridges: SwitchA,spanning-tr

14、ee vlan 1 root primary,This command forces this switch to be the root for VLAN 1.,spanning-tree vlan 2 root secondary,This command configures this switch to be the secondary root for VLAN 2.,OR,spanning-tree vlan # priority priority,This command statically configures the priority (increments of 4096

15、).,SwitchA(config)#,SwitchA(config)#,SwitchA(config)#,Configuring the Root and Secondary Bridges: SwitchB,spanning-tree vlan 2 root primary,This command forces the switch to be the root for VLAN 2.,spanning-tree vlan 1 root secondary,This command configures the switch to be the secondary root VLAN 1.,OR,spanning-tree vlan # priority priority,This command statically configures the priority (increments of 4096).,SwitchB(config)#,SwitchB(config)#,SwitchB(config)#,Summary,A redundant