Reconfigurations
Network Protocols and Standards
Autumn 2004-2005
Sept 21, 2004
CS573: Network Protocols and Standards
1
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
Sept 21, 2004
CS573: Network Protocols and Standards
2
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
Sept 21, 2004
CS573: Network Protocols and Standards
3
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



Sept 21, 2004
Root bridge
Cost to root
Root port
CS573: Network Protocols and Standards
4
Failure: example
Port 1
Bridge ID 92
Port 2
Port 3
Port 5
Port 4
DP
BP
41.13.92
41.13.90
41.13.92
DP
41.12.315
BP
41.12.111
RP
Consider the previous example…
What happens if Port 4 times out?
Sept 21, 2004
CS573: Network Protocols and Standards
5
Failure: example
Port 1
Bridge ID 92
Port 2
41.13.92
41.13.92
DP
DP
Port 3
Port 5
Port 4
41.12.315
41.12.111
BP
41.13.90
New Root Port
Time out 
41.13.92 (DP)
What if Port 4 times out? (it was root port!)
The root port is switched from Port 4 to Port 3
What if Port 3 also times out?
Sept 21, 2004
CS573: Network Protocols and Standards
6
Failure: example
Port 1
Bridge ID 92
Port 2
41.14.92
41.14.92
DP
DP
Port 3
Port 5
Port 4
41.12.315
Time out 
41.14.92 (DP)
41.13.90
41.12.111
New Root Port
Time out 
41.14.92 (DP)
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
Sept 21, 2004
CS573: Network Protocols and Standards
7
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
DP
<Root.cost.B_ID>
B
Sept 21, 2004
< B1_ID.0.B1_ID>
B1
CS573: Network Protocols and Standards
New bridge
8
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
Sept 21, 2004
CS573: Network Protocols and Standards
9
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
Sept 21, 2004
CS573: Network Protocols and Standards
10
Avoiding Temporary Loops

Topology changes that will require
reconfiguration may lead to:

Temporary Loops


Temporary Disconnections


A bridge hasn’t yet found out that a particular port needs
to be changed from Forwarding to Blocking state
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?
Sept 21, 2004
CS573: Network Protocols and Standards
11
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

Sept 21, 2004
Twice the maximum transit time. Why?
CS573: Network Protocols and Standards
12
Example: wait period (BF)
Why bridges should wait twice the max delay before
switching a port from blocking (B) to forwarding (F)?
DP
0
Sept 21, 2004
RP
DP
2
RP
DP
3
RP
DP
N
CS573: Network Protocols and Standards
RP
1
13
Example: wait period (BF)
Why bridges should wait twice the max delay before
switching a port from blocking to forwarding?
DP
0
Sept 21, 2004
DP
DP
2
RP
DP
3
RP
DP
N
CS573: Network Protocols and Standards
RP
1
14
Example: wait period (BF)
Why bridges should wait twice the max delay before
switching a port from blocking to forwarding?
DP
DP
0
DP
DP
RP
DP
2
DP
RP
RP
DP
3
DP
RP
RP
DP
N
DP
RP
1
It took bridge 2 twice the max delay, after it believed
it is the root, to find out that someone else is the root
Sept 21, 2004
CS573: Network Protocols and Standards
15
Example: Temporary Loops
How do temporary loops happen?
2
4
6
8
0
1
3
Sept 21, 2004
5
7
CS573: Network Protocols and Standards
9
16
Example: Temporary Loops
How do temporary loops happen?
First consider the following topology…
RP
2
DP
RP
4
DP
RP
6
DP RP
8 DP
DP
RP
0
1
BP
DP
RP
3
DP RP
5
DP RP
7
DP RP
9
DP
If a bridge fails, loop will not happen…
Sept 21, 2004
CS573: Network Protocols and Standards
17
Example: Temporary Loops
Next consider the following topology…
RP
2
DP
RP
4
DP
RP
6
DP RP
8 DP
DP
DP
RP
0
1
DP
BP
BP
RP
3
DP RP
5
DP RP
7
DP RP
9
DP
If bridge 0 fails, what will happen?
Sept 21, 2004
CS573: Network Protocols and Standards
18
Example: Temporary Loops
DP
RP
2
DP
RP
4
DP
RP
6
DP RP
8 DP
DP
DP
0
1
DPRP
BP
DP
RP
DP
3
DP RP
DP
RP
5
DP RP
7
DP RP
9
BP
DP
DP
Bridge 0 has failed  Temporary Loop
Sept 21, 2004
CS573: Network Protocols and Standards
19
Example: Temporary Loops
DP
RP
2
RP
DP
DP
RP
4
RP
DP
DP
RP
6
RP DP
DP RP
RP
8 DP
DP
DP
0
DP
1
BP
RP
DP
RP
DP
3
DP
DP RP
RP DP
5
DP RP
RP DP
7
DP RP
RP DP
9
DP
RP
Steady state  Temporary loop is gone!
Sept 21, 2004
CS573: Network Protocols and Standards
20
Avoiding Temporary Loops

IEEE 802.1D defines two intermediate
states:

Listening Intermediate state


Bridges do not learn station addresses
Learning Intermediate state


Sept 21, 2004
Bridges start learning station addresses
Packets are not forwarded, however
CS573: Network Protocols and Standards
21
Avoiding Temporary Loops
1)
2)
3)
4)
5)
Port Enabled  Management
Port Disabled  Management
or failure
Selected as RP or DP
Ceases to be RP or DP
One forward delay elapses
Sept 21, 2004
CS573: Network Protocols and Standards
22
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
Sept 21, 2004
CS573: Network Protocols and Standards
23
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

Sept 21, 2004
Unnecessary transmissions
CS573: Network Protocols and Standards
24
FDB Timeouts

Two circumstances requiring timeouts



Two values for FDB timeouts



Station Moving (15 minutes)
Network getting reconfigured (15 seconds)
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
Sept 21, 2004
CS573: Network Protocols and Standards
25
Bridge Settable Parameters

Bridge Priority


Port Priority


2-octet value that allows network manager to
influence choice of root and designated bridges
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
Sept 21, 2004
CS573: Network Protocols and Standards
26
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


Sept 21, 2004
This is to allow the news of topology change to
propagate through the network
IEEE 802.1D recommends 15 seconds
CS573: Network Protocols and Standards
27
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
Sept 21, 2004
CS573: Network Protocols and Standards
28
Network Wide Parameters

Max Age:


Hello Time



Time interval between issuing configuration
messages
Forward Delay


Time at which a configuration message is discarded
Amount of times spent in “listening” and “learning”
Root bridge  these parameters in BPDUs
Designated bridges  copy values from root
Sept 21, 2004
CS573: Network Protocols and Standards
29
Bridge Message Format
Number of octets
2
Protocol Identifier
1
Version
1
Message Type
1
Sept 21, 2004
TCA
Reserved
8
Root ID
4
Cost of Path to Root
8
Bridge ID
2
Port ID
2
Message Age
2
Max Age
2
Hello Time
2
Forward Delay
TC
CS573: Network Protocols and Standards
Flags
30
Topology Changes
Topology Change Notification
Number of octets
2
Protocol Identifier
1
Version
1
Message Type
Protocol Identifier = 0
Version = 0
Message Type = 128 (Normally it is 0)
Sept 21, 2004
CS573: Network Protocols and Standards
31
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
Sept 21, 2004
CS573: Network Protocols and Standards
32
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
Sept 21, 2004
CS573: Network Protocols and Standards
33
Expedited Traffic Preview

Traffic types






Network Control
Voice
Video
Excellent Effort or CEO Effort
Best Effort
Background
Sept 21, 2004
CS573: Network Protocols and Standards
34