Spanning-Tree Protocol

Agenda» What Problem is Solved by STP?» Understanding STP Root Bridge Election» BPDU Details and Pathcost» Understanding STP Root and Designated Port Election» Understanding and Configuring STP Timers» STP Port States» Topology Change Process» PVST and PVST+» Load-Balancing Techniques with PVST+

What Problem is Solved by the Spanning-Tree Protocol?

The Problem Resolved by 802.1d STP is a link management protocol that provides path redundancy while preventing undesirable loops in the network 1985: Originally developed by Radia Perlman while working at DEC (Digital Equipment Corporation). 1990: Original publication of IEEE 802.1d Standard DEC and IEEE versions of STP are not compatible.

Spanning Tree Protocol Basics A B 1. Without the spanning-tree protocol in a redundant topology, a broadcast/multicast frame sourced from A would loop endlessly in the network.

Spanning Tree Protocol Basics A X Blocked port B 1. Without the spanning-tree protocol in a redundant topology, a broadcast/multicast frame sourced from A would loop endlessly in the network. 2. The spanning-tree protocol blocks redundant links to prevent frames from looping.

Spanning Tree Protocol Basics A X Remove blocked Blocked port X Bridge fails! B 1. Without the spanning-tree protocol in a redundant topology, a broadcast/multicast frame sourced from A would loop endlessly in the network. 2. The spanning-tree protocol blocks redundant links to prevent frames from looping. 3. The spanning-tree protocol can adjust to changes in the topology by adjusting which ports are blocking and which are forwarding.

Creating a Loop-Free Path

How STP Works» Elect one Root Bridge» Elect one Root Port per bridge» Elect Designated Ports

Root Bridge Election» Switch with lowest Bridge ID in the network becomes Root Bridge» Bridge ID contains Bridge Priority 0-61440 in increments of 4096 System ID Extension 0-4095 MAC Address

Changing the Root Bridge Election» Manually change BID priority spanning-tree vlan [vlan] priority Lower is better» Use root bridge macro spanning-tree vlan [vlan] root [primary secondary] Sets local priority based on current Root Bridge» Verification show spanning-tree vlan [vlan] show spanning-tree root

Root Bridge Are you still out there?» Root Bridge must periodically send Hello frames to maintain its role.» Hello frames sent as BPDUs (Bridge Protocol Data Units)» Sent every 2-seconds by default.» Hold Time/ Dead-Interval = 10x Hello Interval (called M ax-age).

Quiz-1» Given the following command output, if the current Root Bridge for VLAN-1 is removed from this topology, which Switch will become the new Root Bridge? Switch-1 Switch-3 Switch-2

Quiz-2» Based on the output below, which IOS command(s) could be configured on Switch-1 to force it to become the new Root Bridge? Switch-1 A B C D E F Switch-1(config)#spanning-tree vlan 1 priority 32768 Switch-1(config)#spanning-tree vlan 1 priority 30000 Switch-1(config)#spanning-tree vlan 1 priority 36864 Switch-1(config)#spanning-tree vlan 1 priority 28672 Switch-1(config)#spanning-tree vlan 1 root primary Switch-1(config)#spanning-tree vlan 1 root

STP Port Costs» Every switch that transmits/ forwards a BPDU includes its own, local cost to reach STP Root.» 802.1d specified some default port costs but does NOT specify any formula for ports outside of these values.

Root Port Election» RP is upstream facing towards Root Bridge» Elected based on lowest Root Path Cost Cumulative cost of all links to get to the root» Cost based on inverse bandwidth i.e. higher bandwidth, lower cost Not linear» If tie in cost Choose lowest upstream BID Choose lowest upstream Port ID

Quiz-3 =10Mb Ethernet =100Mb FastEthernet =Gigabit Ethernet Bridge-E 1/1 Bridge-B 3/5 1/2 3/6 6/1 5/1 3/1 Bridge-H 5/2 1/1 1/2 3/5 6/2 3/2 Bridge-A 3/5 3/6 Bridge-C 5/1 5/2 Bridge-F 3/6 3/8 3/8 1/1 3/1 3/2 3/1 4/1 4/2 3/9 Bridge-J 3/2 Root Bridge 3/5 1/2 Which port will Bridge-H select as its Root Port? 3/9 Bridge-G 5/1 5/2 3/6

Quiz-4 Which port will Bridge-F select as its Root Port? =10Mb Ethernet =100Mb FastEthernet =Gigabit Ethernet 5/2 Root Bridge 3/1 5/1 32768-000c.8581.a500 3/1 6/1 6/2 32768-000c.8781.a700 3/4 3/3 4/1 4/2 3/2 32768-000c.85c1.a501 3/5 3/3 3/4 3/2 Bridge-F 3/1

Quiz-5 Which port will Bridge-F select as its Root Port? =10Mb Ethernet =100Mb FastEthernet =Gigabit Ethernet 5/2 Root Bridge 3/1 5/1 32768-000c.8581.a500 6/1 6/2 32768-000c.8781.a700 3/4 3/3 4/1 4/2 3/2 32768-000c.85c1.a501 3/5 3/3 3/4 Bridge-F

Designated Port Election» DPs are downstream facing away from Root Bridge» Like Root Port, elected based on Lowest Root Path Cost Lowest BID Lowest Port ID» All other ports go into blocking mode Receive BPDUs Discard all other traffic Cannot send traffic

Changing the Port s Role» Modify the port s cost spanning-tree [vlan] cost bandwidth [bps]» Modify the Bridge ID spanning-tree vlan [vlan] priority» Modify the Port ID spanning-tree vlan [vlan] port-priority» Verification show spanning-tree interface [int] detail show spanning-tree vlan [vlan] detail

Quiz-6 Which of the following ports will be used to transmit data to/from PC-A and PC-B A B C D E 6/2 2/3 1/1 3/3 5/1 5/4 5/3 24576-00ff.8581.a500 B 3/4 3/3 5/2 5/1 32768-000d.8581.a570 2/4 2/3 6/1 6/2 32768-000c.8381.a700 7/4 7/3 4/1 4/2 3/2 24576-00fc.85c1.a501 1/3 1/2 3/1 A 2/1 7/1 7/2 2/2 32768-000c.8581.a500 1/4 1/1 =10Mb Ethernet =100Mb FastEthernet =Gigabit Ethernet

Spanning-Tree Timers and Port States

STP Timers» Timers affect the transition between port states Set only on the Root Bridge» Hello How often configuration BPDUs are sent Defaults to 2 seconds» MaxAge Dead-interval for receiving BPDUs Defaults to 20 seconds» Forward Delay How long to wait in each of the listening and learning phases Defaults to 15 seconds

Changing STP Timers (PVST)» Configuration spanning-tree vlan [vlan] hello-time spanning-tree vlan [vlan] forward-time spanning-tree vlan [vlan] max-age» Verification show spanning-tree vlan [vlan]

Spanning-Tree Diameter» Allows you to safely tune max age, forward delay and hello time» By specifying the network diameter, the switch will determine the most aggressive possible values of the STP parameters to achieve the fastest convergence time» Network diameter is defined as the maximum number of switches between any two end-stations» The default STP timers assume a network diameter of 7 (the maximum recommended by IEEE).

STP Diameter - Examples Switch(config)#spanning-tree vlan 1 root primary diameter 5 Sw it ch((config)#end Sw it ch(#sho spanning-tree vlan 1 VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 1 Address 0013.c3d7.a000 This bridge is the root Hello Time 2 sec Max Age 16 sec Forward Delay 12 sec Switch(config)#spanning-tree vlan 1 root primary diameter 3 Switch(config)#end Switch#sho spanning-tree vlan 1 VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 1 Address 0013.c3d7.a000 This bridge is the root Hello Time 2 sec M ax Age 12 sec Forward Delay 9 sec

STP Port States» Disabled Administratively down, does not participate in frame forwarding or STP» Listening No user traffic through port, and listening to BPDUs» Learning No user traffic through port, and building bridge tables» Forwarding User traffic across port, and transmitting or receiving BPDUs» Blocking No user traffic through port. Receives BPDUs from Designated Port Transmits only management traffic (CDP, VTP, DTP, etc)

Quiz-7 If Link-1 (to the Hub) goes down, how long will PC-A and PC-B be unable to communicate? A B C D E 0-seconds 15-seconds 30-seconds 50-seconds 60-seconds 5/4 5/3 24576-00ff.8581.a400 B 3/4 3/3 3/2 3/1 Hub 32768-000c.8381.a600 4/1 4/2 24576-00fc.85c1.a501 A 7/4 7/3 2/2 7/1 7/2 2/1 32768-000c.8581.a700 =10Mb Ethernet =100Mb FastEthernet =Gigabit Ethernet

The 802.1d Topology Change Process

STP Topology Change Process» Used to notify other switches of a change in the spanning tree topology» Topology Change Notifications (TCNs) are sent: Any time a port transitions to the forwarding state and the bridge has at least one designated port Any time a port transitions from the forwarding or learning state to the blocking or disabled state.» Sent from the bridge that experienced the topology change towards the root bridge» A TCN received on a designated port of a non-root switch is forwarded towards the root

IEEE 802.1d TCN Frame Format 2 1 1 802.3 / 802.2 Headers Protocol Identifier Ver Msg Type Padding FCS 0x80 = TCN BPDU

STP Topology Change Process» TCN is sent every two seconds, until the upstream bridge acknowledges receipt with a TCN ACK flag set in the configuration BPDU» When the root bridge receives the TCN, it sets the TC flag in the next configuration BPDU (it also sets the TCN ACK flag on the port the TCN was received)» When bridges receive a BPDU with the TC flag set, they reduce their MAC aging time to FwdDelay (15 seconds)» The root switch continues to send Config BPDUs with TC flag set for a total of FwdDelay+Max Age seconds (default=35)

Topology Change Process TCN toward root C TCN ACK B Root TCN ACK TCN toward root A A Port moves into forwarding mode 1. PC-A comes online. 2. Bridge B s port moves into forwarding mode. 3. Bridge B generates a TCN and sends it on the root port. 4. Bridge C ACKs the TCN in the next BPDU it sends to Bridge B. 5. Bridge C generates a TCN and sends it on the root port. 6. Root ACKs the TCN in the next BPDU it sends to Bridge C.

Topology Change Process TC flag set=35s Root TC Flag CAM Aging=15s TC Flag CAM Aging=15s TC Flag TC Flag TC Flag CAM Aging=15s CAM Aging=15s CAM Aging=15s 7. Root also sets the topology change (TC) flag in all Config BPDUs 8. Downstream bridges reduce CAM aging time to FwdDelay seconds for duration of the topology change. 9. Root sets TC flag in all BPDUs for MaxAge + FwdDelay seconds, then clears the TC flag. A A

Detecting Topology Changes How do you know if a Topology Change is in progress? Stable Topology TC In-Progress

What Problem is Solved by PVST?

The problem with a CST 802.1d was designed to operate within a single broadcast domain. 802.1d assumes that all bridges, and all ports, are within a single domain. This is called a CST (Common Spanning-Tree) With this design, the operation of STP is independent of any VLAN configuration. Remote subnets Router-A VLAN-2: Default-Gateway F dp Bridge-1 Root Br. F dp Router-B VLAN-3: Default-Gateway VLAN-2 Bridge-2 F rp GigE F dp GigE FastE F rp B Bridge-3 VLAN-3

Cisco s Solution - PVST PVST = Per VLAN Spanning-Tree. Each VLAN runs an independent instance of 802.1d. By default, a CST topology will still be formed. Allows modification of STP topology per-vlan. Remote subnets Router-A VLAN-2: Default-Gateway VLAN-2 Bridge-2 F rp GigE F dp F dp Bridge-1 Root Br. (VLAN-2 & VLAN-3) GigE F dp FastE F rp B Bridge-3 Router-B VLAN-3: Default-Gateway STP interface cost (for VLAN-3 ONLY) lowered to 7. VLAN-3

VLAN Load-Balancing Methods with PVST

PVST Load Balancing Operation (10,10) BID 16384.0000.0000.0002 BID 16384.0000.0000.0022 S2 L1 L3 L2 S1 Root (Red, Blue) BID 8192.0000.0000.0001 BID 8192.0000.0000.0011 Blocked Port (Red, Blue) (10,10) (10,10) BID 32768.0000.0000.0003 S3 BID 32768.0000.0000.0033 Link L1, L2, and L3 are VLAN trunks The port cost is 10 on all ports for all VLANs S1 is the root switch for all VLANs The L1 port on S3 is blocking for all VLANs & therefore cannot carry data traffic.

Load Balancing Operation - Cost BID 16384.0000.0000.0002 BID 16384.0000.0000.0022 S2 (10,10) L3 S1 Root (Red, Blue) BID 8192.0000.0000.0001 BID 8192.0000.0000.0011 L1 L2 Blocked Port (Blue) (10,10) (30,10) S3 BID 32768.0000.0000.0003 BID 32768.0000.0000.0033 Blocked Port (Red) If we change the port cost for the Red VLAN to 30 on S3 s L2 port, then L2 becomes the blocking link for VLAN Red and L1 becomes the forwarding link for VLAN Red

VLAN Load Balancing Configuration» To enable VLAN-based load balancing (by adjusting Port Costs): Switch(config-if)# spanning-tree vlan <x> cost <cost>» Cost is between 1-200,000,000

Load Balancing Operation A Better Method Root (Red, Blue) BID 8192.0000.0000.0001 BID 8192.0000.0000.0011 Blocked Port (Blue) L3 (FastEth) BID 32768.0000.0000.0002 BID 32768.0000.0000.0022 S1 0/1 0/2 L4 (FastEth) (19,19) (19,19) L1 (FastEth) S2 0/1 0/2 Port Priority (Red) = 128 L2 (FastEth) Port Priority (Blue) = 112 Blocked Port (Red) Blocked Port (Red, Blue) (38,38) (38,38) BID 32768.0000.0000.0003 BID 32768.0000.0000.0033 S3

VLAN Load Balancing Configuration (2) To enable VLAN-based load balancing (by adjusting Port Priorities):

VLAN Load Balancing The Best Method (10,10) Root (Red) BID 8192.0000.0000.0002 BID 16384.0000.0000.0022 S2 L1 L3 L2 S1 Root (Blue) BID 16384.0000.0000.0001 BID 8192.0000.0000.0011 Blocked Port (Blue) (10,10) (10,10) S3 BID 32768.0000.0000.0003 BID 32768.0000.0000.0033 Blocked Port (Red) Simpler configuration: Move the root switch for the Red VLAN to S2

PVST and PVST+ PVST Cisco s original implementation of Per VLAN Spanning-Tree Developed for use with ISL Trunking All BPDUs transmitted using 802.1d Destination MAC (0180:C200:0000) PVST+ Cisco s revised implementation of Per VLAN Spanning-Tree Developed for use with 802.1q Trunking BPDUs transmitted on Native VLAN use 802.1d Destination MAC (0180:C200:0000) BPDUs transmitted with 802.1q tags use Cisco-proprietary Destination MAC (0100:0CCC:CCCD)

Quiz-8 In the following topology, all links between switches are operating as VLAN Trunks and carrying VLANs-1 and 2. Which path(s) will be taken by PC-A and PC-B to transmit frames to their default gateway (router)? 5/2 32768-000d.8581.a570 2/4 5/1 6/1 6/2 1.1.1.2 2.2.2.2 24576-000c.8381.a700 7/4 7/3 4/1 4/2 =10Mb Ethernet =100Mb FastEthernet =Gigabit Ethernet 32768-00fc.85c1.a501 1/2 1.1.1.1 A 2/1 VLAN-1 7/1 7/2 32768-000c.8581.a500 VLAN-2 1/1 2.2.2.1 B

Quiz-9 Which of the following commands (or combination of commands) would accomplish the VLAN load-balancing design shown below? A Switch-1(config)#spanning-tree vlan 1 root primary B Switch-3(config)#spanning-tree vlan 1 root primary Switch-1(config)#interface Ethernet6/1 C Switch-1(config-if)#spanning-tree vlan 1 cost 18 D Switch-2(config)#interface Ethernet6/2 Switch-2(config-if)#spanning-tree vlan 1 cost 26 Switch-1 Switch-2 Switch-3 Switch-4

Quiz-10 Which of the following commands (or combination of commands) would accomplish the VLAN load-balancing design shown below? A B C D Switch-1(config)#spanning-tree vlan 1 root primary Switch-2(config)#spanning-tree vlan 1 root primary Switch-2(config)#interface Ethernet7/2 Switch-2(config-if)#spanning-tree vlan 1 port-priority 112 Switch-1(config)#interface Ethernet7/3 Switch-1(config-if)#spanning-tree vlan 1 port-priority 112 E F Switch-1(config)#interface Ethernet7/4 Switch-1(config-if)#spanning-tree vlan 1 port-priority 112 Switch-2(config)#interface Ethernet7/1 Switch-2(config-if)#spanning-tree vlan 1 port-priority 112 Switch-1 Switch-2