Improving performance with spanning tree

Medium-Sized Switched Network Construction Improving Performance with Spanning Tree Interconnection TechnologiesTechnologyUseFast EthernetConnects end-user devices to the access layer switchGigabit EthernetConnects access switch to distribution switch and high use servers to switches 10-Gigabit EthernetProvides high-speed switch to switch links, backbonesEtherChannelProvides high-speed switch to switch links, backbones with redundancyDetermining Equipment and Cabling NeedsEach link provides adequate bandwidth for the total aggregate traffic over that link.Advantages of EtherChannelLogical aggregation of similar links between switchesLoad-shares across linksViewed as one logical port to STPRedundancyRedundant TopologyRedundant topology eliminates single points of failure.Redundant topology causes broadcast storms, multiple frame copies, and MAC address table instability problems.Station D sends a broadcast frame.Broadcast frames are flooded to all ports except the originating port.Broadcast FramesBroadcast StormsHost X sends a broadcast. Switches continue to propagate broadcast traffic over and over.Multiple Frame CopiesHost X sends a unicast frame to router Y.The MAC address of router Y has 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 A and B incorrectly learn the MAC address of host X on port 2.MAC Database InstabilityProvides a loop-free redundant network topology by placing certain ports in the blocking statePublished in the IEEE 802.1D specificationEnhanced with the Cisco PVST+ implementationLoop Resolution with STPSpanning-Tree OperationOne 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 IDBridge ID =BridgePriorityMACAddressSpanning tree transits each port through several different states:Spanning-Tree Port StatesDescribing PortFastPortFast is configured on access ports, not trunk ports.Configuring and Verifying PortFastspanning-tree portfast SwitchX(config-if)#Configures PortFast on an interfacespanning-tree portfast default SwitchX(config)#Enables PortFast on all non-trunking interfacesshow running-config interface interface SwitchX#Verifies that PortFast has been configured on an interfaceORSpanning-Tree Operation ExampleSpanning-Tree Path CostLink SpeedCost (Revised IEEE Specification)Cost (Previous IEEE Specification)10 Gb/s211 Gb/s41100 Mb/s191010 Mb/s100100Spanning-Tree RecalculationPer VLAN Spanning Tree PlusPVST+ Extended Bridge IDBridge ID without the extended system IDExtended bridge ID with system IDSystem ID = VLANRapid Spanning Tree ProtocolDefault Spanning-Tree ConfigurationCisco Catalyst switches support three types of STPs:PVST+PVRST+MSTPThe default STP for Cisco Catalyst switches is PVST+ :A separate STP instance for each VLANOne root bridge for all VLANsNo load sharingPVRST+ Configuration GuidelinesEnable 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.PVRST+ Implementation Commandsspanning-tree mode rapid-pvst SwitchX(config)#Configures PVRST+show spanning-tree vlan vlan# [detail] SwitchX#Verifies the spanning-tree configurationdebug spanning-tree pvst+ SwitchX#Displays PVST+ event debug messagesVerifying PVRST+The spanning-tree mode is set to PVRST.SwitchX# show spanning-tree vlan 30 VLAN0030Spanning tree enabled protocol rstpRoot ID Priority 24606Address 00d0.047b.2800This bridge is the rootHello Time 2 sec Max Age 20 sec Forward Delay 15 secBridge ID Priority 24606 (priority 24576 sys-id-ext 30) Address 00d0.047b.2800Hello Time 2 sec Max Age 20 sec Forward Delay 15 secAging Time 300Interface Role Sts Cost Prio.Nbr Type-------- ----- --- --- -------- ----Gi1/1 Desg FWD 4 128.1 P2pGi1/2 Desg FWD 4 128.2 P2pGi5/1 Desg FWD 4 128.257 P2pConfiguring the Root and Secondary BridgesConfiguring the Root and Secondary Bridges: SwitchAspanning-tree vlan 1 root primaryThis command forces this switch to be the root for VLAN 1.spanning-tree vlan 2 root secondaryThis command configures this switch to be the secondary root for VLAN 2.ORspanning-tree vlan # priority priorityThis command statically configures the priority (increments of 4096).SwitchA(config)#SwitchA(config)#SwitchA(config)#Configuring the Root and Secondary Bridges: SwitchBspanning-tree vlan 2 root primaryThis command forces the switch to be the root for VLAN 2.spanning-tree vlan 1 root secondaryThis command configures the switch to be the secondary root VLAN 1.ORspanning-tree vlan # priority priorityThis command statically configures the priority (increments of 4096).SwitchB(config)#SwitchB(config)#SwitchB(config)#SummaryA redundant switched topology includes multihomed switches and EtherChannel.A redundant switched topology causes looping issues such as broadcast storms. The 802.1D STP establishes a loop-free network. The original STP has been enhanced by PVST+ and RSTP.