sept 21, 2004cs573: network protocols and standards1 reconfigurations network protocols and...
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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
Sept 21, 2004 CS573: Network Protocols and Standards 3
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 4
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
Sept 21, 2004 CS573: Network Protocols and Standards 5
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
Sept 21, 2004 CS573: Network Protocols and Standards 6
Failure: example
Bridge ID 92Port 1
Port 2 Port 3 Port 4
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
Sept 21, 2004 CS573: Network Protocols and Standards 7
Failure: exampleBridge ID 92
Port 1
Port 2 Port 3 Port 4
Port 5
41.14.9241.14.92 41.12.315 41.12.111
41.13.90
Time out 41.14.92 (DP)
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
Time out 41.14.92 (DP)
DPDP
Sept 21, 2004 CS573: Network Protocols and Standards 8
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
Sept 21, 2004 CS573: Network Protocols and Standards 9
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 10
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 11
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?
Sept 21, 2004 CS573: Network Protocols and Standards 12
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?
Sept 21, 2004 CS573: Network Protocols and Standards 13
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
Sept 21, 2004 CS573: Network Protocols and Standards 14
Example: wait period (BF)
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
Sept 21, 2004 CS573: Network Protocols and Standards 15
Example: wait period (BF)
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
Sept 21, 2004 CS573: Network Protocols and Standards 16
Example: Temporary Loops
How do temporary loops happen?
0
2
3
4 6 8
1
5 7 9
Sept 21, 2004 CS573: Network Protocols and Standards 17
Example: Temporary Loops
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…
Sept 21, 2004 CS573: Network Protocols and Standards 18
Example: Temporary Loops
Next 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 bridge 0 fails, what will happen?
BP
DP
Sept 21, 2004 CS573: Network Protocols and Standards 19
Example: Temporary Loops
0
2
3
4 6 8
1
5 7 9
DP
DP
RP DP RP DP RP DP RP DP
BP
RP
DPRPDPRPDPRPDPRP
Bridge 0 has failed Temporary Loop
BP
DP
DP
DP
DP
DP
DPRP
Sept 21, 2004 CS573: Network Protocols and Standards 20
Example: Temporary Loops
0
2
3
4 6 8
1
5 7 9
DP
DP
RP DP RP DP RP DP RP DP
DPRPDPRPDPRPDPRP
Steady state Temporary loop is gone!
RP
DP
DPRPDPRPDPRPDP
DP RP DP RP DP RP DP RP
RP
DP
DPBP
Sept 21, 2004 CS573: Network Protocols and Standards 21
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
Sept 21, 2004 CS573: Network Protocols and Standards 22
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
Sept 21, 2004 CS573: Network Protocols and Standards 23
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 24
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
Sept 21, 2004 CS573: Network Protocols and Standards 25
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
Sept 21, 2004 CS573: Network Protocols and Standards 26
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
Sept 21, 2004 CS573: Network Protocols and Standards 27
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
Sept 21, 2004 CS573: Network Protocols and Standards 28
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 29
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
Sept 21, 2004 CS573: Network Protocols and Standards 30
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
Sept 21, 2004 CS573: Network Protocols and Standards 31
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)
Sept 21, 2004 CS573: Network Protocols and Standards 32
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 33
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