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Reconfigurations

This text discusses the reconfiguration process in advanced computer networks, including tree maintenance, message propagation, and handling failures to maintain network stability.

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Reconfigurations

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  1. Reconfigurations Advanced Computer Networks

  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

  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

  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

  5. Failure: example Port 1 Bridge ID 92 Port 5 Port 4 Port 2 Port 3 DP BP 41.13.92 41.13.90 41.12.111 41.12.315 41.13.92 DP BP RP Consider the previous example… What happens if Port 4 times out?

  6. Failure: example Port 1 Bridge ID 92 Port 5 Port 4 Port 2 Port 3 BP 41.13.90 41.13.92 41.12.111 41.12.315 41.13.92 DP DP 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?

  7. Failure: example Port 1 Bridge ID 92 Port 5 Port 4 Port 2 Port 3 41.13.90 41.14.92 41.12.111 41.12.315 New Root Port 41.14.92 DP DP Time out  41.14.92 (DP) 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

  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 DP <Root.cost.B_ID> < B1_ID.0.B1_ID> B B1 New bridge

  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

  10. Spanning Tree Recalculation • When is spanning tree recalculated? • Timer Tick • Age reaches max age on the root port • BPDU received on a port is either better or same with smaller age

  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?

  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?

  13. Example: wait period (BF) Why bridges should wait twice the max delay before switching a port from blocking (B) to forwarding (F)? DP RP DP RP DP RP DP RP 0 2 3 N 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 DP RP DP RP DP RP 0 2 3 N 1

  15. Example: wait period (BF) Why bridges should wait twice the max delay before switching a port from blocking to forwarding? DP DP RP DP RP DP RP DP DP DP DP RP DP RP DP RP 0 2 3 N 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

  16. Example: Temporary Loops How do temporary loops happen? 2 4 6 8 0 1 3 5 7 9

  17. Example: Temporary Loops How do temporary loops happen? First consider the following topology… RP DP RP DP RP DP RP DP 2 4 6 8 DP RP 0 1 BP DP 3 5 7 9 RP DP DP RP DP RP DP RP If a bridge fails, loop will not happen…

  18. Example: Temporary Loops Next consider the following topology… RP DP RP DP RP DP RP DP 2 4 6 8 DP DP RP 0 1 BP DP BP 3 5 7 9 RP DP DP RP DP RP DP RP If bridge 0 fails, what will happen?

  19. Example: Temporary Loops DP RP DP RP DP RP DP RP DP 2 4 6 8 DP DP DP RP 0 1 DPRP BP DP BP DP 3 5 7 9 RP DP DP RP DP RP DP RP DP Bridge 0 has failed  Temporary Loop

  20. Example: Temporary Loops DP RP DP RP DP RP DP RP RP DP RP DP RP DP RP DP 2 4 6 8 DP DP DP 0 1 BP DP DP RP 3 5 7 9 RP DP DP RP DP RP DP RP DP RP RP DP RP DP RP DP Steady state  Temporary loop is gone!

  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

  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

  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 • Unnecessary transmissions

  24. 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

  25. 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

  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 • This is to allow the news of topology change to propagate through the network • IEEE 802.1D recommends 15 seconds

  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

  28. 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

  29. Bridge Message Format Number of octets 2 Protocol Identifier Version 1 Message Type 1 Flags TCA Reserved TC 1 Root ID 8 Cost of Path to Root 4 Bridge ID 8 Port ID 2 Message Age 2 Max Age 2 Hello Time 2 Forward Delay 2

  30. Topology Changes Topology Change Notification Number of octets 2 Protocol Identifier Version 1 Message Type 1 Protocol Identifier = 0 Version = 0 Message Type = 128 (Normally it is 0)

  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

  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

  33. Expedited Traffic Preview • Traffic types • Network Control • Voice • Video • Excellent Effort or CEO Effort • Best Effort • Background

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