Sept 21, 2004CS573: Network Protocols and Standards1 Reconfigurations Network Protocols and Standards Autumn 2004-2005

Sept 21, 2004 CS573: Network Protocols and Standards

1

Reconfigurations

Network Protocols and Standards

Autumn 2004-2005

Sept 21, 2004 CS573: Network Protocols and Standards 2

Tree maintenance: Age Field Bridges store configuration messages

for each port Another field called Age Field is also stored Age Field incremented by 1 every unit time

When the value in age field reaches “Max Age”, configuration message at that port is discarded and STA run fresh on that port – assuming no BPDU was ever received on that port


Message Propagation Root bridge generates and

transmits configuration BPDUs At every “hello time” With Age Field = 0

When any Bridge in the downstream receives message coming from the root port, it transmits a BPDU on designated ports with age field = 0


Failures If root bridge fails or the path to the root

bridge becomes unavailable, a bridge stops receiving “fresh” messages and eventually times out When time out occurs (age field = max

age), the stored configuration message is discarded.

Everything is recalculated Root bridge Cost to root Root port


Failure: example

Bridge ID 92Port 1

Port 2 Port 3 Port 4

Port 5

41.13.92 41.12.315 41.12.11141.13.9041.13.92

Consider the previous example…What happens if Port 4 times out?

DP

DP BP RP

BP


Failure: example

Bridge ID 92Port 1


Port 5

41.13.9241.13.92 41.12.315 41.12.111

41.13.90

Time out 41.13.92 (DP)

New Root Port

What if Port 4 times out? (it was root port!)The root port is switched from Port 4 to Port 3What if Port 3 also times out?

DPDP

BP


Failure: exampleBridge ID 92

Port 1


Port 5

41.14.9241.14.92 41.12.315 41.12.111

41.13.90


New Root Port

What if Port 3 also times out? (it was new root port!)The root port is switched from Port 3 to Port 5

What if port 5 also times out at a later point????Ans: B92 will assume itself to be the root


DPDP


Reconfiguration: adding a bridge

Assume B is the designated bridge on a segment on which B1 comes up

Also assume that the age field in B’s internal timer for root port was X

B B1

< B1_ID.0.B1_ID><Root.cost.B_ID> DP

New bridge


Reconfiguration: adding a bridge If B’s BPDU is better than B1’s BPDU

B should not ignore B1’s message B informs B1 by transmitting a BPDU on

that segment that someone else is root Transmits even if a fresh BPDU has not been

received from the root. Why? The BPDU is transmitted with Age field X.

why? IF B1’s BPDU is better than B’s BPDU

B recalculates the spanning tree


Spanning Tree Recalculation When is spanning tree

recalculated? Timer Tick

Age reaches max age on the root port Timing out on blocked ports is slightly

different Should we recalculate the root? Perhaps Not!

BPDU received on a port is either better or same with smaller age


Avoiding Temporary Loops Topology changes that will require

reconfiguration may lead to: Temporary Loops

A bridge hasn’t yet found out that a particular port needs to be changed from Forwarding to Blocking state

Temporary Disconnections A bridge hasn’t yet found out that a particular port

needs to be changed from Blocking to Forwarding state

Temporary disconnections are better than temporary loops in bridged networks. Why?


Avoiding Temporary Loops How to avoid loops?

Force ports in the blocking state to wait for some amount of time before switching to the forwarding state

Wait period should be large enough to allow the topology change information to propagate through the network

Twice the maximum transit time. Why?


Example: wait period (BF)

Why bridges should wait twice the max delay before switching a port from blocking (B) to forwarding (F)?

0 2 3 N 1DP RP DP RP DP RP DP RP



Why bridges should wait twice the max delay before switching a port from blocking to forwarding?

0 2 3 N 1DP DP DP RP DP RP DP RP



Why bridges should wait twice the max delay before switching a port from blocking to forwarding?

0 2 3 N 1DP DP DP RP DP RP DP RP

DP DP RP DP RP DP RP DP

It took bridge 2 twice the max delay, after it believedit is the root, to find out that someone else is the root


Example: Temporary Loops

How do temporary loops happen?

0

2

3

4 6 8

1

5 7 9



How do temporary loops happen?First consider the following topology…

0

2

3

4 6 8

1

5 7 9

DP

DP

RP DP RP DP RP DP RP DP

BP

RP

DPRPDPRPDPRPDPRP

If a bridge fails, loop will not happen…



Next consider the following topology…

0

2

3

4 6 8

1

5 7 9

DP

DP


BP

RP

DPRPDPRPDPRPDPRP

If bridge 0 fails, what will happen?

BP

DP



0

2

3

4 6 8

1

5 7 9

DP

DP


BP

RP

DPRPDPRPDPRPDPRP

Bridge 0 has failed Temporary Loop

BP

DP

DP

DP

DP

DP

DPRP



0

2

3

4 6 8

1

5 7 9

DP

DP


DPRPDPRPDPRPDPRP

Steady state Temporary loop is gone!

RP

DP

DPRPDPRPDPRPDP

DP RP DP RP DP RP DP RP

RP

DP

DPBP


Avoiding Temporary Loops IEEE 802.1D defines two

intermediate states: Listening Intermediate state

Bridges do not learn station addresses Learning Intermediate state

Bridges start learning station addresses Packets are not forwarded, however


1) Port Enabled Management2) Port Disabled Management

or failure3) Selected as RP or DP4) Ceases to be RP or DP5) One forward delay elapses

Avoiding Temporary Loops


IEEE 802.1D Reading Ch 7 – sections 7.1 through 7.9.2 Annex H2 Ch 8 – sections 8.1 through 8.7;

8.10 Ch 9 Ch 10 and 12 Annex H1


FDB Timeout Values Configuration changes

Stations move Topology changes

FDB entries must be refreshed/purged Choosing a suitable timeout period

Too long Traffic lost for a long time (forwarded to the

wrong port) Too short

Unnecessary transmissions


FDB Timeouts Two circumstances requiring timeouts

Station Moving (15 minutes) Network getting reconfigured (15 seconds)

Two values for FDB timeouts A long value, in the usual case A short value when spanning tree changes

STA enhancement Notify all bridges of spanning tree

reconfiguration Notify root which sets “topology change” flag in

its configuration message


Bridge Settable Parameters Bridge Priority

2-octet value that allows network manager to influence choice of root and designated bridges

Port Priority 1-octet value that allows network manager to

influence the choice of port Hello Time

Time between generation of configuration messages by the bridge when it is root bridge

Recommended Time: 2 seconds


Bridge Settable Parameters Max Age

Age at which a stored config message at a port is discarded because it is “too old”

IEEE 802.1D recommends 20 seconds Forward Delay

Time during which a bridge is prevented to forward packets to and from a link

This is to allow the news of topology change to propagate through the network

IEEE 802.1D recommends 15 seconds


Bridge Settable Parameters Long FDB Timer

IEEE 802.1D recommends 5 minutes Path Cost

Value individually settable at each port Cost to be added to the root path cost field

in a BPDU received on a port to determine the cost to root through that port

Large value port more likely to be a leaf


Network Wide Parameters Max Age:

Time at which a configuration message is discarded

Hello Time Time interval between issuing configuration

messages Forward Delay

Amount of times spent in “listening” and “learning”

Root bridge these parameters in BPDUs Designated bridges copy values from root


Bridge Message Format

Protocol Identifier

Version

Message Type

Root ID

ReservedTCA TC

Cost of Path to Root

Bridge ID

Port ID

Message Age

Max Age

Hello Time

Forward Delay

Number of octets

2

1

1

1

8

4

8

2

2

2

2

2

Flags


Topology Changes

Protocol Identifier

Version

Message Type

Number of octets

2

1

1

Topology Change Notification

Protocol Identifier = 0Version = 0Message Type = 128 (Normally it is 0)


Topology Changes When a bridge notices that STA has

caused a port into or out of blocked state, it transmits topology change notification on its root port

A bridge receiving topology change notification on one of its designated ports will: Set the TCA (ack) flag in the next BPDU on

that port Send a topology change notification on its

own root port


Topology Changes When the root bridge receives

topology change notification, it sets the TC flag in its BPDUs for a time period equal to forward delay + max age

Bridges receiving BPDUs with TC flag set should use short timers instead of long timers to expire the FDB entries


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Sept 21, 2004CS573: Network Protocols and Standards1 Reconfigurations Network Protocols and Standards Autumn 2004-2005