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

2. Sept 21, 2004CS573: Network Protocols and Standards2 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. Sept 21, 2004CS573: Network Protocols and Standards3 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. Sept 21, 2004CS573: Network Protocols and Standards4 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. Sept 21, 2004CS573: Network Protocols and Standards5 Failure: example Bridge ID 92 Port 1 Port 2Port 3 Port 4 Port 5 41.13.92 41.12.315 41.12.111 41.13.9041.13.92 Consider the previous example… What happens if Port 4 times out? DP BPRP BP

6. Sept 21, 2004CS573: Network Protocols and Standards6 Failure: example Bridge ID 92 Port 1 Port 2Port 3 Port 4 Port 5 41.13.92 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 3 What if Port 3 also times out? DP BP

7. Sept 21, 2004CS573: Network Protocols and Standards7 Failure: example Bridge ID 92 Port 1 Port 2Port 3 Port 4 Port 5 41.14.92 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) DP

8. Sept 21, 2004CS573: Network Protocols and Standards8 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 BB1 DP New bridge

9. Sept 21, 2004CS573: Network Protocols and Standards9 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. Sept 21, 2004CS573: Network Protocols and Standards10 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

11. Sept 21, 2004CS573: Network Protocols and Standards11 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. Sept 21, 2004CS573: Network Protocols and Standards12 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. Sept 21, 2004CS573: Network Protocols and Standards13 Example: wait period (B  F) Why bridges should wait twice the max delay before switching a port from blocking (B) to forwarding (F)? 023N1 DP RP DP RP

14. Sept 21, 2004CS573: Network Protocols and Standards14 Example: wait period (B  F) Why bridges should wait twice the max delay before switching a port from blocking to forwarding? 023N1 DP DP DP RP DP RP DP RP

15. Sept 21, 2004CS573: Network Protocols and Standards15 Example: wait period (B  F) Why bridges should wait twice the max delay before switching a port from blocking to forwarding? 023N1 DP 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 believed it is the root, to find out that someone else is the root

16. Sept 21, 2004CS573: Network Protocols and Standards16 Example: Temporary Loops How do temporary loops happen? 0 2 3 468 1 579

17. Sept 21, 2004CS573: Network Protocols and Standards17 Example: Temporary Loops How do temporary loops happen? First consider the following topology… 0 2 3 468 1 579 DP RP DPRPDPRPDPRP DP BP RP DP RPDPRPDPRPDPRP If a bridge fails, loop will not happen…

18. Sept 21, 2004CS573: Network Protocols and Standards18 Example: Temporary Loops Next consider the following topology… 0 2 3 468 1 579 DP RP DPRPDPRPDPRP DP BP RP DP RPDPRPDPRPDPRP If bridge 0 fails, what will happen? BP DP

19. Sept 21, 2004CS573: Network Protocols and Standards19 Example: Temporary Loops 0 2 3 468 1 579 DP RP DPRPDPRPDPRP DP BP RP DP RPDPRPDPRPDPRP Bridge 0 has failed  Temporary Loop BP DP DP  RP

20. Sept 21, 2004CS573: Network Protocols and Standards20 Example: Temporary Loops 0 2 3 468 1 579 DP RP DPRPDPRPDPRP DP RPDPRPDPRPDPRP Steady state  Temporary loop is gone! RP DP RPDPRPDPRPDP RPDPRPDPRPDP RP DP BP

21. Sept 21, 2004CS573: Network Protocols and Standards21 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. Sept 21, 2004CS573: Network Protocols and Standards22 1)Port Enabled  Management 2)Port Disabled  Management or failure 3)Selected as RP or DP 4)Ceases to be RP or DP 5)One forward delay elapses Avoiding Temporary Loops

23. Sept 21, 2004CS573: Network Protocols and Standards23 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

24. Sept 21, 2004CS573: Network Protocols and Standards24 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

25. Sept 21, 2004CS573: Network Protocols and Standards25 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

26. Sept 21, 2004CS573: Network Protocols and Standards26 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

27. Sept 21, 2004CS573: Network Protocols and Standards27 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

28. Sept 21, 2004CS573: Network Protocols and Standards28 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

29. Sept 21, 2004CS573: Network Protocols and Standards29 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

30. Sept 21, 2004CS573: Network Protocols and Standards30 Bridge Message Format Protocol Identifier Version Message Type Root ID ReservedTCATC 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

31. Sept 21, 2004CS573: Network Protocols and Standards31 Topology Changes Protocol Identifier Version Message Type Number of octets 2 1 1 Topology Change Notification Protocol Identifier = 0 Version = 0 Message Type = 128 (Normally it is 0)

32. Sept 21, 2004CS573: Network Protocols and Standards32 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

33. Sept 21, 2004CS573: Network Protocols and Standards33 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

34. Sept 21, 2004CS573: Network Protocols and Standards34 Expedited Traffic Preview Traffic types Network Control Voice Video Excellent Effort or CEO Effort Best Effort Background