11- (rapid) spanning tree protocol

3FL 00327_A AAAA WBZZA Ed 01 1 © 2007 Alcatel-Lucent., All rights reserved

Alcatel-Lucent University Antwerp 1

University

(Rapid) Spanning Tree Protocol

Alcatel-Lucent University Antwerp

University

During class please switch off your mobile, pager or other that may interrupt.

Course objectives:After attending this section, you’ll be able to:> describe the need for a spanning tree protocol in a bridged network> describe how a spanning tree is calculated> describe the improvements of RSTP compared to STP

Entry level requirements:You must have a basic knowledge of bridging (Ethernet networks).

Suggested duration:1 hour


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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)

> The current 802.1d Spanning Tree Protocol (STP) standard was designed at a time where recovering connectivity after an outage within a minute or so was considered adequate performance. With the advent of Layer 3 (L3) switching in LAN environments, bridging now competes with routed solutions where protocols such as Open Shortest Path First (OSPF) and Enhanced Interior Gateway Routing Protocol (EIGRP) are able to provide an alternate path in less time.

> Rapid Spanning Tree Protocol (RSTP; IEEE 802.1w) can be seen as an evolution of the 802.1d standard. The 802.1d terminology remains primarily the same, and most parameters have been left unchanged so users familiar with 802.1d can rapidly configure the new protocol comfortably. 802.1w is also capable of reverting back to 802.1d in order to interoperate with legacy bridges (thus dropping the benefits it introduces) on a per-port basis.


3

Phys1

12

Phys2

Phys MAC addr. Port nbr

Phys MAC addr. Port nbr

12

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

> In order to have a more reliable network, redundant paths are needed. But redundant paths in an Ethernet network cause loops:

• Traffic in a loop might circulate indefinitely (there’s no hop-limiting parameter (“Time to live” or “Hop Count”) in an Ethernet network!)

• In case of broadcast messages causing endless broadcast storms, the impact on the network might be huge!

> STP can prevent this by blocking connections that might cause loops.• STP calculates a forwarding tree without loops.• If there’s a failure in the network, STP will calculate a new spanning tree which will use

certain bridge ports that were blocked in the previous spanning tree.


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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

> Redundant links may be interesting for reliability reasons, but we cannot afford to have loops in the bridged network. Therefore STP will block certain links in order to have only one way to go from node A to node B.


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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

> How does the STP protocol work?• In order to calculate a spanning tree, bridges exchange information using BPDU

messages (Bridged Protocol Data Units).• The first step in the calculation of a spanning tree is to select a root bridge. By default,

this is the bridge with the lowest bridge identifier, but the operator can override this. (The bridge id is configurable.)

• Once a complete spanning tree is calculated, only the bridge ports that are in the spanning tree are in the forwarding state while other ports are blocked. No frames can pass through blocked bridge ports! (No loops are possible!)

• In case the active link fails, STP will detect this and set up a new spanning tree. A blocked link will become active.

• RSTP (Rapid Spanning Tree Protocol) has a mechanism to recover from failure situations much quicker than STP: RSTP converges faster than STP.

> The spanning tree protocol is used between bridges in a bridged network, not between hosts and bridges. The operation of STP is transparent to hosts. Hosts cannot detect whether they are connected to a single LAN segment or a switched LAN with multiple segments.


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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 portOnly 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!

> Tree : hierarchical> The root bridge is the bridge with the lowest bridge-id (=MAC address). This means that the

root bridge needn’t be the bridge with the most links.• A bridge may have different MAC addresses (per port / per LAN segment), but one of them

will be chosen for the bridge-id.> Each port on a bridge is given a cost, e.g. associated with the bandwidth of that port. Typically,

it is preferable to use high bandwidth ports. Therefore they will have a lower cost.> On each non-root bridge, the root port is the port that gives the best path towards the root. One

other port on that bridge can be designated port. All others will be blocked.

> Calculation of a spanning tree:• In STP, each bridge assumes it’s the root and broadcasts its BPDU, containing its bridge

id. The bridges soon find out which one is the real root.• Each bridge calculates the best path (lowest cost) to the root. The resulting paths make

up the spanning tree and the corresponding ports on those paths are enabled (ports are in the forwarding state). Other ports are blocked (standby).

• When a new bridge is added to the network or when there’s a failure, a new spanning tree is calculated. It is possible that ports that were blocked in the previous spanning tree will be in the forwarding state in the new tree.


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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

> Spanning tree works in first instance by selecting a root bridge on the LAN. This particular bridge is elected through the exchange of BPDU. In fact each bridge receives an identifier made part from its MAC address and part from an arbitrary given value. (Priority).

> When the root bridge is selected, then each other bridge selects one of its ports with the least path cost to the root bridge. The least cost path is determined by STP looking at the bandwidth of the link.

> STP continually monitors the network always looking for failures on switch ports or changes in the network topology. If a change is noticed, STP can quickly make redundant ports available and close other ports to ensure the network continues to function

> All ports on the root bridge are known as designated ports and are in what is known as forwarding state. Forwarding state ports can send and receive traffic.

> All of the other bridges present are known as non-root bridges, they choose a port known as a root port which sends and receives traffic.

> On non-root bridges only one port can be designated, all others are blocked. Designated ports forward MAC addresses. Designated ports are selected after the bridge determines the lowest cost path to get to the root bridge.

> By using this method, the redundant links are closed down. They can be opened again if there is a change in network topology and the link is needed once more.


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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 A1 A2

B1

B2 C2

C1

C3

> Propagation delays can occur when protocol information passes through a switched LAN. As a result, topology changes can take place at different times and at different places in a switched network. When a Layer 2 interface transitions directly from non-participation in the spanning tree topology to the forwarding state, it can create temporary data loops. Ports must wait for new topology information to propagate through the switched LAN before starting to forward frames. They must allow the frame lifetime to expire for frames that have been forwarded under the old topology.

> Bridge ports running STP can be in one of four (five) states:• Listening - listens to make sure no loops occur before passing frames, the bridge is

computing the spanning tree protocol to see if this port should be blocked or forwarding• Learning - learns MAC addresses but does not forward frames, transitional state between

listening and learning. The port is still not used but the bridge can already learn addresses from this port.

• Forwarding - sends and receives frames on the port, the port is used to carry user traffic• Blocking - listens but will not forward frames, the port is not used by user traffic• (Disabled – virtually non-operational)

> RSTP ports can be in only three different states. See further. > Spanning-Tree information is shared by exchanging BPDU messages.

The Spanning-Tree Protocol calculation requires that bridges communicate with other bridges in the network that are running the Spanning-Tree Protocol. Each bridge is responsible for sending and receiving configuration messages called bridge protocol data units (BPDUs).BPDUs are exchanged between neighboring bridges at regular intervals (typically 1 to 4 seconds) and contain configuration information that identifies the:• Bridge that is presumed to be the main bridge or root (root identifier)• Distance from the sending bridge to the root bridge (called the root path cost)• Bridge and port identifier of the sending bridge• Age of the information contained in the configuration messageIf a bridge fails and stops sending BPDUs, the bridges detect the lack of configuration messages and initiate a spanning-tree recalculation.


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Spanning tree – Example

0

11 10

12 13 9

2 3

6 7 8

5 14

19

4

2 2

4 4

419 19

19

100 100 100

root

path cost

Bridge(identifier)

LANPath cost

10Mbps � 100100Mbps � 191000Mbps � 4

10Gbps � 2

> In complex networks, you can configure the bridge-id of a certain bridge which you would prefer to have as root bridge (bridge-id = 0).

• Each time a new bridge is added, the spanning tree must be adapted. Maybe the new bridge will become the root bridge. In order to avoid this, you can select the root bridge manually by configuring the bridge-id=0.

> How to calculate the path cost?• Faster links have a lower cost (e.g. a 100 Mbps link has a lower cost than a 10 Mbps link).• Always choose the lowest cost.• In case the cost is equal, the bridge-id will be taken into account (same principle as used

to define the root bridge).


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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

>In STP only the root bridge will generate BPDU messages. The designated bridges will only relay them.

• The recovery time from failure can take 1 minute (time needed to calculate a new spanning tree).

>In RSTP all bridges will send BPDU messages every hello time. This will generate more overhead traffic, but reduce the take-over time!

• The recovery time from failure can be less than a second!

>Port States: There are only three port states left in RSTP:STP RSTP Disabled DiscardingBlocking DiscardingListening DiscardingLearning LearningForwarding Forwarding

>CPE-modem: no loops possible, so CPE mustn’t be able to receive BPDU’s. No STP implemented.


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