1. CIT 384: Network Administration
Spanning Tree Protocol
CIT 384: Network Administration

2. CIT 384: Network Administration
Topics
Redundant LAN Problems
802.1d Spanning Tree
CIT 384: Network Administration

3. Redundant LAN Problems
Broadcast Storms
Forwarding of frame repeatedly on same links, consuming most bandwidth.
MAC Table Instability
Continual updating of a switch’s MAC address table with incorrect entries in reaction to looping frames, resulting in frames being sent to the wrong locations.
Multiple Frame Transmission
Looping frames result in multiple copies of same frame being received by destination host, confusing some apps.
CIT 384: Network Administration

4. CIT 384: Network Administration
Broadcast Storms
Bob sends broadcast from
SW3 forwards broadcast to SW1 and SW2
SW3 learns that Bob is on port Fa 0/13
SW2 forwards broadcast to SW1 and SW3
SW3 learns that Bob is on port Gi 0/2
CIT 384: Network Administration

5. Blocking and Forwarding Ports
STP prevents loops by placing switch ports that could cause loops in a Blocking State.
Blocking State ports only process STP pkts.
Forwarding State ports act normally.
CIT 384: Network Administration

6. CIT 384: Network Administration
Spanning Tree
A tree is a type of graph, such that there is one and only one path joining any 2 vertices
A spanning tree is a tree that spans all vertices of the original graph.
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CIT 384: Network Administration

7. CIT 384: Network Administration
Forwarding Criteria
STP elects a root switch. STP puts all working interfaces on root switch in Forwarding State.
Each nonroot switch considers one of its ports to have least administrative cost between itself and root switch. STP puts this least root cost interface, called the switch’s root port, in Forwarding State.
For switch’s attached to the same Ethernet segment, the switch with the lowest admin cost is placed in Forwarding State. The lowest-cost switch on each segment is called the designated bridge and that port is the Designated Port.
All other ports are in Blocking State.
CIT 384: Network Administration

8. Algorhyme by Radia Perlman
I think that I shall never see A graph more lovely than a tree. A tree whose crucial property Is loop-free connectivity. A tree which must be sure to span So packets can reach every LAN. First the Root must be selected By ID it is elected. Least cost paths from Root are traced In the tree these paths are placed. A mesh is made by folks like me Then bridges find a spanning tree.
CIT 384: Network Administration

9. Bridge ID and Hello BPDU
Bridge ID: 8-byte value unique to switch
2 byte priority field + 6 byte MAC address
Bridge Protocol Data Units (BPDU)
Hello BPDU sends switch’s bridge ID and other values:
Field
Description
Root Bridge ID
Bridge ID of switch that sender believes is the current root bridge.
Sender Bridge ID
Bridge ID of sender of Hello BPDU
Cost to reach root
STP cost between this switch and root
Timer values
Hello, MaxAge, Fwd Delay timer values
CIT 384: Network Administration

10. CIT 384: Network Administration
Root Bridge Election
Root Bridge has lowest BridgeID.
Since first digits are priority, lowest pri wins.
If priority ties, then lowest MAC address wins.
Election process
Switches claim to be root, sending Hello BPDUs with their own ID as the root ID.
If switch hears a Hello with a lower root ID, then it starts forwarding that ID as root ID.
After a root is chosen, only root sends Hello BPDUs, but other switches forward them.
CIT 384: Network Administration

11. CIT 384: Network Administration
Root Bridge Election
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CIT 384: Network Administration

12. CIT 384: Network Administration
Choosing a Root Port
Root port is port with lowest STP cost to reach the root switch.
Cost = Hello cost + STP port cost
If multiple ports have least cost, choose:
Port with lowest configured port priority, or if that ties
Port with lowest internal port number
CIT 384: Network Administration

13. CIT 384: Network Administration
Choosing Root Ports
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CIT 384: Network Administration

14. Choosing a Designated Port
Choose a DP on each LAN segment.
DP is switch port that advertised lowest Hello on segment.
All DPs are placed into Forwarding State.
Default port costs based on Ethernet speed:
Ethernet Speed
Original Default Cost
Revised IEEE Cost
10 Mbps
100
100 Mbps 10 19
1 Gbps 1 4
10 Gbps 2
CIT 384: Network Administration

15. CIT 384: Network Administration
Final Port States
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CIT 384: Network Administration

16. Final STP State of Example
Interface
State
Reason
SW1, Gi0/1
Forwarding
Interface on root switch
SW1, Gi0/2
SW2, Gi0/2
Root port
SW2, Gi0/1
Designated port on LAN segment to SW3.
SW3, Gi0/1
SW3, Gi0/2
Blocking
Not a root port or a designated port
CIT 384: Network Administration

17. Reacting to Network Changes
Root switch sends Hello BPDU every 2 sec.
Switches forward Hello BPDUs from root.
Changing root cost and sender bridge ID.
If switch does not receive Hello BPDU for Max Age, it will start changing STP topology.
Max Age is 10 times Hello timer by default.
Switch will decide if it’s the new root bridge.
If not, will recalculate RP and DPs.
CIT 384: Network Administration

18. Reacting to Network Changes
SW3 fails to receive Hello BPDUs on Gi0/1.
SW3 decides SW1 is still root based on forwarded Hello from SW2.
SW3 re-evaluates RP, decides on Gi0/2.
CIT 384: Network Administration

19. Intermediate STP states
Direct Block to Forward transition can temporarily cause a loop.
STP uses intermediate states.
Listening: Does not forward frames. Old, now incorrect MAC table entries timed out during state, because these entries would cause temporary loops.
Learning: Does not forward frames, but switch begins to learn MAC table entries in this state.
Remains in state for Forward Delay timer.
CIT 384: Network Administration

20. CIT 384: Network Administration
EtherChannel
Combine parallel connections of equal speed between same pair of switches into a single link.
EtherChannel link fails only when all connections fail, avoiding STP convergence unless all fail.
CIT 384: Network Administration

21. CIT 384: Network Administration
EtherChannel
(config)# interface fa 0/3
(config-if)# channel-group 1 mode on
(config)# interface fa 0/4
EtherChannel Modes
on: both sides must be an EtherChannel
auto: wait for other side to negotiate EtherChannel (other side must be on or desirable)
desirable: attempt to negotiate EtherChannel
CIT 384: Network Administration

22. PortFast and BPDU Guard
A PortFast port is always in Forwarding State.
Bypasses any choices about STP topology.
Only safe for ports that do not connect to other bridges or switches.
BPDU Guard disables port if BPDUs received.
Useful in combination with PortFast.
Prevents end-user from introducing a switch with low bridge ID that becomes root switch.
CIT 384: Network Administration

23. CIT 384: Network Administration
Rapid STP (802.1w)
RSTP reduces convergence time
STP takes MaxAge + 2*ForwardDelay=50s
RSTP typically converges under 10s
RSTP categories physical connectivity
Link-type point-to-point
Link-type shared
Edge type
RSTP will not improve convergence in networks that use hubs (shared connections.)
CIT 384: Network Administration

24. RSTP Link and Edge Types
CIT 384: Network Administration

25. CIT 384: Network Administration
RSTP Port Roles
Alternate: best alternative to current RP.
Backup: second link to same LAN segment.
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26. CIT 384: Network Administration
RSTP Changes
Edge-Type
Immediately places edge ports in Forwarding state like Cisco’s PortFast feature.
Link-Type Shared
Behave identically to STP.
Link-Type Point-to-Point
MaxAge = 3 * Hello timer = 6s
Does not use Listening state when changing.
Reduces time required in Learning state by sending RSTP messages to negotiate.
CIT 384: Network Administration

27. RSTP Convergence Example
SW4 selects port on new link as root port.
SW4 temp blocks all other link-type ports to avoid loops.
SW4 negotiates with SW1 using RSTP proposal and agreement messages, allowing SW1 and SW4 to place ports in Forwarding state immediately.
CIT 384: Network Administration

28. RSTP Convergence Example
SW4 forwards new root BPDU to SW3
SW3 changes root port to port to SW4.
SW3 blocks all other link-type ports.
CIT 384: Network Administration

29. RSTP Convergence Example
SW3 and SW4 negotiate.
SW4 forwards to root through SW3.
SW3 maintains blocking on port to SW2.
CIT 384: Network Administration

30. CIT 384: Network Administration
VLANs and STP
Cisco proprietary per-VLAN STP variants
PVST+ for STP, PVRST for RSTP
Multiple Instances of Spanning Trees (MIST)
IEEE 802.1s standard supports only RSTP
CIT 384: Network Administration

31. CIT 384: Network Administration
Bridge IDs with VLANs
STP System ID extension
16-bit priority = 4-bit priority + 12-bit VLAN ID
Example: priority = 8
VLAN 1 = 32,769, VLAN 2 = 32,770, etc.
CIT 384: Network Administration

32. STP Defaults and Options
Setting
Default
Commands
Bridge ID
Priority = VLAN ID
System: Burned in MAC
spanning-tree vlan vlan-id root [ primary | secondary ]
spanning-tree vlan vlan-id priority priority
Port Cost
100 for 10Mbps, 19 for 100Mbps, 4 for 1 Gbps,
2 for 10 Gbps
spanning-tree vlan vlan-id cost cost
PortFast
Not enabled
spanning-tree portfast
BPDU Guard
spanning-tree bpduguard enable
CIT 384: Network Administration

33. CIT 384: Network Administration
STP Status Example
SW1#show spanning-tree vlan 3
VLAN0003
Spanning tree enabled protocol ieee
Root ID Priority
Address e
Cost
Port (FastEthernet0/16)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority (priority sys-id-ext 3)
Address e86a.6f80
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
Fa0/ Desg FWD P2p
Fa0/ Root FWD P2p
Fa0/ Altn BLK P2p
SW1#show spanning-tree root
Root Hello Max Fwd
Vlan Root ID Cost Time Age Dly Root Port
VLAN e Fa0/16
VLAN e Fa0/16
VLAN e Fa0/16
VLAN e Fa0/16
CIT 384: Network Administration

34. CIT 384: Network Administration
References
James Boney, Cisco IOS in a Nutshell, 2nd edition, O’Reilly, 2005.
Cisco, Cisco Connection Documentation,
Cisco, Internetworking Basics,
Matthew Gast, Wireless Networks: The Definitive Guide, O’Reilly, 2005.
Wendell Odom, CCNA Official Exam Certification Library, 3rd edition, Cisco Press, 2007.
CIT 384: Network Administration