1. Spanning Tree Algorithm
Autumn
Sept 14, 2004
Spanning Tree Algorithm
Network Protocols and Standards
Autumn
Sept 14, 2004
CS573: Network Protocols and Standards
CS573: Network Protocols and Standards

2. CS573: Network Protocols and Standards
Topologies with Loops
Problems
Frames proliferate
Learning process unstable
Multicast traffic loops forever A
LAN 1 B1 B2 B3
LAN 2
Sept 14, 2004
CS573: Network Protocols and Standards

3. CS573: Network Protocols and Standards
Topologies with Loops
Solutions
Require that the topologies be loop-free through careful deployment of segments and bridges
Design Bridges to detect loops and complain and, perhaps, stop working
Not a good idea because loops provide redundancy
Design into the bridges an algorithm that prunes the topology into a loop-free subset (a spanning tree)
Blocking of some ports may be required
Automatically adapt to the changes in topology
Sept 14, 2004
CS573: Network Protocols and Standards

4. Reconfiguration Algorithm
Configures an arbitrary topology into a spanning tree
Automatic reconfiguration in case of topology changes
The algorithm should converge for any size LAN; the stability should be achieved within a short, bounded time
Active topology should be reproducible and manageable
Transparency to end-stations is required
Must not use a lot of bandwidth
Sept 14, 2004
CS573: Network Protocols and Standards

5. Spanning Tree Algorithm
A distributed Algorithm
Elects a single bridge to be the root bridge
Calculates the distance of the shortest path from each bridge to the root bridge (cost)
For each LAN segment , elects a “designated” bridge from among the bridges residing on that segment
The designated bridge for a LAN segment is the one closest to the root bridge
And…
Sept 14, 2004
CS573: Network Protocols and Standards

6. Spanning Tree Algorithm
For each bridge
Selects ports to be included in spanning tree
The ports selected are:
The root port --- the port that gives the best path from this bridge to the root
The designated ports --- ports connected to a segment on which this bridge is designated
Ports included in the spanning tree are placed in the forwarding state
All other ports are placed in the blocked state
Sept 14, 2004
CS573: Network Protocols and Standards

7. CS573: Network Protocols and Standards
Forwarding frames along the spanning tree Forward and Blocked States of Ports
Data traffic (from various stations) is forwarded to and from the ports selected in the spanning tree
Incoming data traffic is always discarded (this is different from filtering frames. Why?) and is never forwarded on the blocked ports
Sept 14, 2004
CS573: Network Protocols and Standards

8. Root Selection: Bridge ID
Each port on the Bridge has a unique LAN address just like any other LAN interface card. Bridge ID is a single bridge-wide identifier that could be:
A unique 48-bit address
Perhaps the LAN address of one of its ports
Root Bridge is the one with lowest Bridge ID B
Port Address
Sept 14, 2004
CS573: Network Protocols and Standards

9. CS573: Network Protocols and Standards
Path Length (Cost)
Path length is the number of hops from a bridge to the root
While forming a spanning tree, we are interested in the least cost path to the root
Cost can also be specified based on the speed of the link
Not fair to treat a 10Mb/s link the same as a 1Gb/s link
A guideline for cost selection is in Table 8.5 of the latest IEEE 802.1D standard
Sept 14, 2004
CS573: Network Protocols and Standards

10. CS573: Network Protocols and Standards
Path cost guidelines
Source: IEEE 802.1D standard
Sept 14, 2004
CS573: Network Protocols and Standards

11. CS573: Network Protocols and Standards
Example Topology 1 4 5 7 8 6 10 11 2
Sept 14, 2004
CS573: Network Protocols and Standards

12. After algorithm executionDP 1 RP DP BP BP 4 5 7 RP RP RP RP DP DP 8 6 10 DP RP RP BP DP 11 2 RP RP
RP: Root Port
DP: Designated Port
BP: Blocked Port DP DP
Sept 14, 2004
CS573: Network Protocols and Standards

13. Configuration Message (BPDU)
Configuration Bridge Protocol Data Unit
Transmitted by Bridges to implement the spanning tree algorithm
Just like any other data link layer frame
Destination Adderss: Special Multicast Address
01-8-c
Source Address: MAC address of the port
DSAP = SSAP =
6 octets
6 octets
2 octets
2 octets
Data Field
destination
source
length
DSAP SSAP
Configuration Message
Sept 14, 2004
CS573: Network Protocols and Standards

14. Configuration BPDU Contents
The Data Field of Config BPDU contains:
Root ID
ID of the Bridge known to be root
Cost to Root
Cost of the known least cost path to the root
Transmitting Bridge ID
ID of the bridge transmitting this message
Representation
<RootID>.<CostToRoot>.<TransmittingBridgeID>
Sept 14, 2004
CS573: Network Protocols and Standards

15. BPDU: Transmission and Processing
When a bridge is first booted up:
It assumes it is the root
Transmits on each port:
<ownID>.<0>.<ownID>
BPDUs are received on each port
For each port, every bridge saves the best BPDU received
Best among the ones received or transmitted on that port
Sept 14, 2004
CS573: Network Protocols and Standards

16. CS573: Network Protocols and Standards
Comparing two BPDUs
Given two BPDUs containing configuration messages C1 and C2:
C1 is better than C2 if rootID in C1 is lower than the rootID in C2
In case of a tie above, C1 is better than C2 if cost in C1 is lower than the cost in C2
In case of a tie above, C1 is better than C2 if transmitting bridge ID in C1 is lower than the transmitting bridge ID in C2
Can we still get a tie?
Consider a bridge with two ports on same segment!!!
Sept 14, 2004
CS573: Network Protocols and Standards

17. CS573: Network Protocols and Standards
Port Identifier
Each bridge has an internal numbering of its ports
These numbers are unique only for a bridge
Bridge 1 2 3
Sept 14, 2004
CS573: Network Protocols and Standards

18. CS573: Network Protocols and Standards
Port Identifier
Port IDs are useful when rootID, cost, and transmittingBridgeID are the same in two configuration messages
In such situations, Port IDs are used as tie-breakers and the BPDU with lower portID is better
Sept 14, 2004
CS573: Network Protocols and Standards

19. CS573: Network Protocols and Standards
Comparing BPDUs
Message C1
<Root>.<Cost>.<Tx>
<29>.<15>.<35>
<35>.<80>.<39>
<35>.<15>.<80>
Message C2
<Root>.<Cost>.<Tx>
<31>.<12>.<32>
<35>.<80>.<40>
<35>.<18>.<38>
(a)
(b)
(c)
In all cases above, C1 is “better” than C2
Sept 14, 2004
CS573: Network Protocols and Standards

20. CS573: Network Protocols and Standards
Designated Bridge
If a bridge receives better BPDU on a port than the one it would transmit, it will no longer transmit BPDUs on that port
As a result, only one bridge called the “designated bridge” transmits configuration BPDUs on each LAN
Sept 14, 2004
CS573: Network Protocols and Standards

21. Root ID and Cost: determining
Consider B with bridge ID 18
Best BPDUs on each of its ports are:
<RootID>.<Cost>.<TransmitterID>
Port 1 <12>.<93>.<51>
Port 2 <12>.<85>.<47>
Port 3 <81>.<0>.<81>
Port 4 <15>.<31>.<27>
Root bridge ID is determined as 12
Least cost is determined as 85+1 = 86
Port 2 is regarded as the Root port …cont…
Sept 14, 2004
CS573: Network Protocols and Standards

22. Root ID and Cost: determining
At next opportunity, B will transmit
<12>.<86>.<18>
Better than the best BPDUs on all ports
Ports 1, 3, and 4 are designated
B considers itself designated for the segments connected to ports 1, 3, and 4
B continues transmitting BPDUs on those segments (through respective ports)
Sept 14, 2004
CS573: Network Protocols and Standards

23. CS573: Network Protocols and Standards
Spanning Tree Ports
For each bridge, what ports are “included” in the spanning tree?
Root Port
On which best of the best BPDUs is received
Designated Port
Connected to segments on which B is designated
Ports selected in the spanning tree are placed in the forwarding state
B will forward data packets to and from those ports
Sept 14, 2004
CS573: Network Protocols and Standards

24. CS573: Network Protocols and Standards
Ports Blocked
Bridge B will block a port if it receives a better BPDU on that port than the one it would transmit (except for the root port)
Bridge B will no longer transmit BPDUs on either root port or blocked ports
Data packets are not forwarded to and received from the blocked ports
Sept 14, 2004
CS573: Network Protocols and Standards

25. CS573: Network Protocols and Standards
Example
Bridge ID 92
Port 1
Port 2
Port 3
Port 4
Port 5
Root Bridge is selected as 41
Least cost to root is 12+1 = 13  Port 4 is selected as root
Now B92 will transmit a BPDU
What does it do with the ports?
Sept 14, 2004
CS573: Network Protocols and Standards

26. How the Ports are treated?
Remember: B92 will transmit a BPDU
Port 1: <  DP
Port 2: <  DP
Port 3: >  BP
Port 4:  Root Port
Port 5: >  BP
Sept 14, 2004
CS573: Network Protocols and Standards