Alcatel-Lucent University Antwerp

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University

(Rapid) Spanning Tree Protocol

Alcatel-Lucent University

University

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IEEE 802.1D (STP) & IEEE 802.1W (RSTP)

802.1D Bridge Protocol Interconnection of IEEE 802 LANs Includes Spanning Tree Protocol (STP)

802.1W Rapid Bridge Protocol Amendment to IEEE Std 802.1D Rapid Spanning Tree Protocol (RSTP)

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Phys1

12

Phys2

Phys MAC addr. Port nbr

Phys MAC addr. Port nbr

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P1 P21

P1 1

P1 1 2

2

P1 P2

3

P1 2 4

P1 P25

P1 P2

LOOP!6

Without STP, redundant paths cause loops Multiple active paths between hosts cause loops

end stations receive duplicate messages switches learn host MAC addresses on multiple

interfaces. This results in an unstable network

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STP prevents loops

In bridged networks, redundant paths can cause loops Spanning Tree Protocol (STP) prevents loops:

STP allows only one single path between any pair of hosts

STP relies on BPDU messages: Bridged Protocol Data Units

Key functions:

find an active topology without loops block and unblock ports discover failures

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STP – Spanning Tree Protocol

IEEE 802.1d bridge protocol

Spanning tree: loop-free subset of a network topology Select root bridge Calculate loop-free path from root to every bridge Block ports to put redundant paths in standby In case of failure, a new spanning tree will be calculated

and activated.

Operation of STP is transparent to hosts

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STP – Spanning Tree Protocol

Bridges exchange BPDU – Bridge Protocol Data Unit BPDU are used to build the spanning tree :

select root bridge (lowest bridge-id) calculate shortest path from each bridge to the root based on

path cost define a designated bridge in each LAN that will forward

frames to the root (based on path cost) on each non-root bridge, select the root port

Only one port can be the designated port.

block ports to be excluded from the spanning tree

the bridge-id is based on one of the MAC addresses Bridge id and path cost are configurable!

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Bridge and port definitions

ROOT

DESIGNATED

Alternate port(Blocked)

Backup port (Blocked)

R R

D

D D

Take over Take over

R

D

If receiving best BPDU on segment

If sending best BPDU on segment

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Spanning tree – Example

0

11 10

12 13 9

2 3

6 7 8

5 1

419

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2 2

4 4

419 19

19

100 100 100

root

path cost

Bridge (identifier)

LANPath cost

10Mbps 100100Mbps 191000Mbps 4

10Gbps 2

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RSTP (Rapid Spanning Tree Protocol)

Limitations of IEEE802.1d STP ? STP recovers connectivity after an outage within 1 minute. L3 routers need less time for recovery (e.g. OSPF)! Cisco added proprietary enhancements (configuration needed)

IEEE802.1w RSTP Evolution of 802.1d STP Most parameters remain the same Capable of reverting back to 802.1d on a per port basis

Enhancements Only 3 port states: discarding, learning and forwarding All bridges send BPDUs periodically i.s.o relaying root-BPDU Rapid transition to forwarding state which (faster convergence -

sometimes within hundreds of ms) New topology change mechanisms

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