Ethernet networks can operate in either half-duplex or full-duplex mode. Half-duplex uses CSMA/CD to resolve collisions but only allows data transmission in one direction at a time, while full-duplex allows simultaneous two-way transmission by using point-to-point connections between devices to eliminate collisions. Spanning Tree Protocol (STP) is used to prevent loops in redundant switched networks by providing a single active path and blocking backup paths, while Rapid Spanning Tree Protocol (RSTP) provides much faster convergence times than STP. Virtual LANs (VLANs) allow logical segmentation of networks to isolate broadcast domains while maintaining physical connectivity, with VLAN IDs associating ports on switches to specific

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2. Ethernet Transmission Half-duplex transmission Data sent in one direction at a time Results in collisions Uses CSMA/CD to resolve collisions Hubs are the most common half-duplex devices Full-duplex transmission Data sent in both directions at the same time Requires point-to-point connections No collisions An approach to higher network efficiency Switches are the most common full-duplex devices

3. Half-Duplex Operation (CSMA/CD ) All hosts constantly listen to the line. Host A transmits. Hosts B, C, and D listen to host A and do not transmit. All hosts receive host A’s message. A B C D Hub

4. Half-Duplex Operation (CSMA/CD) All hosts constantly listen to the line. Host A and host B transmit simultaneously. Messages collide. Both hosts back off for a random time interval. Hub A B C D

5. Full-Duplex Operation Attached to a dedicated switched port Requires full-duplex support on both ends Collision-free A B C E D Switch

6. Auto-Negotiation Ethernet’s negotiable operation Speed 10 Mb/s 100 Mb/s 1000 Mb/s Operation mode Half-duplex (CSMA/CD) Full-duplex If auto-negotiation is enabled, Ethernet nodes connected by a twisted pair cable negotiate their speed as well as duplex mode prior to establishing a link.

7. Network Domains 1 Collision Domain, 1 Broadcast Domain Resource 2 Collision Domains, 1 Broadcast Domain Router 3 Broadcast Domains, 3 Collision Domains Bridge Switch

8. Collision Domains In this figure, there are 8 collision domains and 3 broadcast domains. Switch Switch Router Collision Domain Collision Domain Collision Domain Collision Domain Collision Domain Collision Domain Collision Domain Collision Domain Broadcast Domain Broadcast Domain Broadcast Domain Hub Hub Hub Hub Hub Hub

9. Switching Switch 1 2 Host A 00 00 A2 00 00 01 Host B 00 00 A2 00 00 02 Switch Forwarding Table Node MAC Address 00 00 A2 00 00 01 00 00 A2 00 00 02 Interface 1 2

10. Building Up the MAC Forward/Filter Table 1/2 1/1 1/3 1/4 Host A 0000.8c01.000A Host B 0000.8c01.000B Host C 0000.8c01.000C Host D 0000.8c01.000D Step 1: Host A sends a frame to Host B. Step 2: The switch receives the frame on 1/1 and places source in MAC table. Step 3: The destination is not in the MAC table so the switch forwards the frame to all ports except the source. Step 4: Host B responds to Host A. The switch adds the source address of Host B to the MAC table. Step 5: Host A and Host B can now send unicast frames bidirectionally. Step 6: Similarly, Host C and Host D will send frames and populate the MAC table. Step 2 Step 4 0000.8c01.000A 0000.8c01.000B 0000.8c01.000C 0000.8c01.000D Step 6 1/1 1/2 1/3 1/4

11. Spanning Tree Protocol 802.1

12. Spanning Tree Protocol — What Is It? Link management protocol that is part of IEEE 802.1 Spanning tree algorithm provides path redundancy in Ethernet bridge/switch networks Provides 1 active path at a time between 2 bridges or switches Provides backup paths to the active path, should the active path fail Primary function is to avoid looping in redundant path Ethernet networks

13. Redundant Topology — Without STP Redundancy Advantages: Necessary for the link of a switch failover Load balancing Disadvantages: May cause broadcast storms May cause multiple frame copies to be sent May cause FDB table instability Frame looping problems Layer 2 has no mechanism to stop looping as layer 3 has with TTL

14. Receiving Multiple Copies Segment 1 Segment 2 Host X Host Y Switch 1 Switch 2

15. Database Instability Segment 1 Segment 2 Host A Unicast Unicast Port 0 Port 1 Port 0 Port 1 Host B MAC Address DB Host A Port 0 MAC Address DB Host A Port 0 Host A Port 0 Host A Port 1 Switch 1 Switch 2

16. Broadcast Storms Segment 1 Segment 2 Host X Broadcast Host Y Switch 1 Switch 2

17. STP and IEEE 802.1d STP is defined in 802.1d as a link management protocol Initially developed in 1990, based on the ISO/IEC 10038 standard Designed to provide path redundancy in Ethernet bridge/switch-based networks, while preventing loops STP uses a root/branch/leaf model, which determines a single path to each leaf spanning the entire L2 network End hosts (e.g., PCs) are oblivious to STP and instead see a single LAN segment

18. STP Port States All ports in an STP network go through the following states . Initialization Blocking Listening Learning Forwarding Disabled

19. STP Port States and Activities STP port state Part of active topology Learning of MAC addresses Disabled No No Blocking Listening Learning No Yes Forwarding Yes Yes

20. STP in Action: State 2 — Root Bridge/Switch Election Root bridge/switch election calculation: After bridges/switches have initialized and all ports are in blocking mode, root bridge election occurs. Each bridge/switch has a user-assigned bridge priority. The bridge priority ranges from 0 to 65 535 (default is 32 768). Each bridge/switch sends its BID to every other bridge/switch. The BID is 8 bytes: 2 for bridge priority and 6 that contain the MAC address of the bridge/switch. Election of the root bridge is determined using the Bridge ID, which is made up of the Priority and MAC address the switch with lowest Bridge ID value is selected Any subsequent physical change in the network after election of the root bridge will cause an STP recalculation.

21. STP in Action: State 2 B A C D Host A Host B Boot Up Boot Up Boot Up Boot Up State 2 — Blocking BPDU BPDU BPDU BPDU BPDU BPDU BPDU BPDU Root Bridge/Switch Root Bridge/Switch Root Bridge/Switch Root Bridge/Switch

22. STP in Action: State 2 — Root Bridge/Switch Election Host A Host B State 2 — Blocking Root Bridge/Switch Root Bridge/Switch Root Bridge/Switch Root Bridge/Switch Priority - 32 MAC - 00-80-21-00-00-10 Priority - 16 MAC - 00-80-21-00-00-30 Priority - 48 MAC - 00-80-21-00-00-20 Priority - 16 MAC - 00-80-21-00-00-40 B A C D

23. STP in Action: State 2 — Root Bridge/Switch Election Host A Host B Root Bridge/Switch Leaf Bridge/Switch Leaf Bridge/Switch Leaf Bridge/Switch Priority - 32 MAC - 00-80-21-00-00-10 Priority - 16 MAC - 00-80-21-00-00-30 Priority - 48 MAC - 00-80-21-00-00-20 Priority - 16 MAC - 00-80-21-00-00-40 BPDU BPDU BPDU BPDU BPDU BPDU BPDU BPDU B A C D

24. STP in Action: State 2 — Path Calculation Each port on a bridge/switch has a path cost value assigned, depending on bandwidth. The accumulated path cost determines the total cost to reach the root bridge/switch. Path cost values can be found in the IEEE 802.1d standard. Link bandwidth STP cost value 4 Mb/s 250 10 Mb/s 100 16 Mb/s 62 45 Mb/s 39 100 Mb/s 19 155 Mb/s 14 622 Mb/s 6 1 Gb/s 4 10 Gb/s 2

25. STP in Action: State 2 — Path Calculation Host A Host B Root Leaf Leaf Leaf BPDU BPDU BPDU BPDU BPDU BPDU BPDU BPDU B A C D Path Cost 2 Path Cost 10 Path Cost 10 Path Cost 10

26. STP in Action: State 2 — Calculating Forwarding Paths Host A Host B Root Leaf Leaf Leaf Root Port Designated Port Designated Port Designated Ports Root Port Root Port B A C D Path Cost 2 Path Cost 10 Path Cost 10 Path Cost 10

27. STP in Action: State 3 — Listening State Forwarded Traffic BPDUs NM Messages Bridge/Switch

28. STP in Action: State 4 — Learning State Forwarded Traffic BPDUs NM Messages Bridge/Switch

29. STP in Action: State 5 — Final Forwarding Paths Host A Host B Root Leaf Leaf Leaf B A C D Path Cost 2 Path Cost 10 Path Cost 10 Path Cost 10

30. STP in Action — Topology Change (Deleting a Link) Host A Host B Root Leaf Leaf Leaf A B C D Wait 20 seconds (Max age time) BPDU BPDU Designated Ports Root Port Listen (15 seconds) Learn (15 seconds) Path Cost 2 Path Cost 10 Path Cost 10 Path Cost 10

31. STP in Action — Topology Change (Path Cost Change) Host A Host B Root Leaf Leaf Leaf Path Cost 2 Path Cost Path Cost Change 1 — TCN BPDU sent to Root 2 — Reply w/TCA BPDU set 3 — Topology changed 10 BPDU BPDU TBPDU TBPDU BPDU BPDU BPDU TBPDU TBPDU TBPDU TBPDU Listen (15 seconds) Learn (15 seconds) B A D C Path Cost 10 Path Cost 10

32. STP in Action — Topology Change (Adding a Switch) Host A Host B Root Leaf Leaf Leaf Priority - 16 MAC - 00-80-21-00-00-30 Priority - 16 MAC - 00-80-21-00-00-10 BPDU BPDU BPDU B D C E A New Root BPDU BPDU BPDU BPDU Designated Ports Root Port Leaf New switch E added All ports in listening state New BPDUs sent New root switch elected Final topology Path Cost 2 Path Cost 10 Path Cost 10 Path Cost 10 Path Cost 10 Path Cost 10

33. STP and BPDU The root bridge/switch sends STP messages via BPDUs to the branches/leaves. On individual branches and leaves, the user can specify IDs and path costs. The root bridge/switch sets the forwarding delay, hello time, and maximum age. BPDU is sent in Ethernet frame with the port’s address as source and the STP Multicast address 01:80:C2:00:00:00 as destination BPDU Packet Protocol ID (2 bytes) Version (1 byte) Message type (1 byte) Flags (1 byte) Root ID ( 8 bytes) Path cost (4 bytes) Bridge ID (8 bytes) Port ID (2 bytes) Message age (2 bytes) Maximum age (2 bytes) Hello time (2 bytes) Forwarding delay (2 bytes)

34. BPDU Packet Details Configurable on each bridge Configurable on root bridge Protocol ID Always set to 0 Version Always set to 0 Message type Determines which of two BPDU types; configuration or TCN Flags Handle changes in the active topology Root ID Contains the bridge ID of root bridge (after convergence, all BPDUs should contain the same value) Root path cost Cumulative path cost of all links to the root bridge Bridge ID Identifies the bridge that is transmitting the current configuration message Port ID Contains a unique value for each port Message age Time stamp since the root bridge created this BPDU Maximum age Maximum amount of time this BPDU is saved Hello time Time between configuration BPDUs Forwarding delay Time spent in the listening and learning states

35. Spanning Tree Exercise Highlight the steps that will ensure that Switch D is added to the existing Bridge topology using STP Priority - 16 MAC - 00-80-21-00-00-10 Priority - 16 MAC - 00-80-21-00-00-20 Priority - 16 MAC - 00-80-21-00-00-30 Priority - 16 MAC - 00-80-21-00-00-40 A B C D 10 10 10 10 10

36. Rapid Spanning Tree

37. What is RSTP? What is RSTP? Stands for rapid spanning tree protocol An evolution to the loop prevention algorithm (STP) from 802.1d New IEEE specification is 802.1w Achieves rapid failover and convergence times Unlike STP, RSTP is not timer-based Allows backward compatibility with 802.1d STP Why do we need RSTP? Network topology convergence is significantly faster than STP

38. STP vs. RSTP — Port States STP port state RSTP port state Part of active topology Learning of MAC addresses Disabled Discard No No Blocking Listening Learning Learning No Yes Forwarding Forwarding Yes Yes

39. Role — A new variable assigned to a bridge port STP vs. RSTP — Port Roles Port states STP port role (assigned by STP algorithm) RSTP port role (configurable) Forwarding Root Root Designated Designated Blocking Blocked Backup Blocked Alternate

40. Alternate Port Root Root Port Root Port Designated Port Designated Port Designated Port Alternate Port BPDU

41. Backup Port Root Root Port Root Port Designated Port Designated Port Designated Port Alternate Port Backup Port BPDU

42. RSTP BPDU Format Configurable Configurable on root bridge Protocol ID (2 bytes) Version (1 byte) Message type (1 byte) Flags (1 byte) Root ID ( 8 bytes) Path cost (4 bytes) Bridge ID (8 bytes) Port ID (2 bytes) Message age (2 bytes) Maximum age (2 bytes) Hello time (2 bytes) Forwarding delay (2 bytes) Version 1 length (2 bytes) Bit 0 – Topology change Bit 1 – Proposal Bit 2, 3 – Port role 0 0 Unknown 0 1 Alternate/backup 1 0 Root 1 1 Designated Bit 4 – Learning Bit 5 – Forwarding Bit 6 – Agreement Bit 7 – Topology change ACK

43. STP vs. RSTP — BPDUs STP RSTP BPDU handling Non-root bridge only transmits BPDUs when it receives one on the root port Bridge sends BPDU at hello time intervals Aging BPDU is aged after the max-age timer expires (and no BPDU is received on the port) BPDUs are used like keepalive messages (after 3 BPDUs in a row are missed it ages it out) Accepting inferior BPDUs — Inferior BPDU is accepted and previously stored information is replaced Transition to forwarding state Based on timers (Forward Delay and Max-Age) Uses a feedback mechanism (no timers involved)

44. STP vs. RSTP — Topology STP RSTP Topology change notification Sends TCN BPDUs toward root Sends BPDUs (with TC bit set) on all designated and root ports Topology ACKs Replies with BPDU with TCA bit set No acknowledgement (clears MAC addresses on all ports) Topology change First sent to root bridge/switch, then relayed from root all the way to the leaf bridge/switch 1-step process (topology change flooded quickly across the network)

45. Virtual LAN

46. Switches and VLANs A VLAN permits a group of ports to share a common broadcast domain regardless of physical location. A VLAN can reside on 1 switch or on many switches. A port that is not in a specific VLAN is in a default VLAN, and thus in a different broadcast domain. Each VLAN is identified by a VLAN ID. Devices in different VLANs can only communicate with each other if the frame is first sent to a layer 3 device (a router).

47. Why VLANs? T here are two main reasons for the development of VLANs: T he amount of broadcast traffic and increased security. Broadcast traffic increased in direct proportion to the number of stations in the LAN. The goal of the VLAN is the isolation of groups of users so that one group is not interrupted by the broadcast traffic of another. VLANs also have the benefit of added security by separating the network into distinct logical networks. Traffic in one VLAN is separated from another VLAN as if they were physically separate networks. If traffic is to pass from one VLAN to another , it must be routed.

48. How Do VLANs Work? VLAN 101 VLAN 102 VLAN 103 Ethernet switch Internal switch VLAN 101 Internal switch VLAN 102 Internal switch VLAN 103 Port 1 Port 2 Port 3 Port 5 Port 6 Port 7

49. VLAN Exercise VLAN 101 Host 1 sends out a broadcast. Which hosts will receive the broadcast? Switch 1 VLAN 102 VLAN 102 VLAN 101 Host 1 Host 2 Host 3 Host 4 BPDU BPDU

50. VLANs across Multiple Switches Switch 1 Switch 2 Switch 3 VLAN 101 VLAN 102 VLAN 103

51. VLANs over Multiple Switches Switch 1 Switch 2 MAC FDB VLAN 101 MAC FDB VLAN 102 MAC FDB VLAN 103 MAC FDB VLAN 101 MAC FDB VLAN 102 MAC FDB VLAN 103 VLAN 101 VLAN 102 VLAN 103 Separate Physical Interfaces

52. VLAN Trunking Switch 1 Switch 2 VLAN 101 VLAN 102 VLAN 103 MAC FDB VLAN 101 MAC FDB VLAN 102 MAC FDB VLAN 103 MAC FDB VLAN 101 MAC FDB VLAN 102 MAC FDB VLAN 103

53. VLAN Tagging SFD Pre- amble DA SA Length /Type P a y l o a d (46 to 1500 bytes) FCS 802.1q tag type (value 81 00) Tag control information 2 bytes 2 bytes CFI (Canonical format: bit ordering can be different) Length of the MAC frame + 4 bytes VLAN tag 802.1q Ethernet Frame User_priority VLAN_ID 3 bits 1 bit 12 bits

54. Multiple Spanning Tree Protocol

55. Multiple Spanning Tree Protocol (MSTP) What is MSTP? Why do we need MSTP? Differences: MSTP vs. STP Where to use MSTP Example

56. Multiple Spanning Tree Protocol What is MSTP? An IEEE standard that allows more than one instance of STP A natural progression from RSTP, introduced in 2003 as part of 802.1s Why do we need MSTP? Allows load balancing of network between different sets of VLANs Allows a set of VLANs to run a single instance of the spanning tree while another set runs another instance of the spanning tree Some early versions of MSTP, before 802.1s, used a single STP instance per VLAN, which was very CPU-intensive. MSTP lowers CPU usage in these instances. Reduce overhead of BPDUs as otherwise they're sent for every VLAN Interoperability Scalabitility

57. Standard STP Switch A Switch C Switch B R D A VLAN 1-500 VLAN 501-1000 Root Leaf Leaf D R D Port States D - Designated R - Root A - Alternate

58. MSTP Port States Switch A Switch C Switch B R D A VLAN 1-500 VLAN 501-1000 D R D D R D D A R D - Designated R - Root A - Alternate

59. Spanning Tree Protocol Group (STG) Multiple STGs provide multiple data paths, which can be used for load-sharing and redundancy. Enable load sharing between two switches using multiple STGs by configuring each path with a different VLAN and then assigning each VLAN to a separate STG. Each STG is independent. Each STG sends its own Bridge Protocol Data Units (BPDU), and you must independently configure each STG. The tagging for the BPDUs from STG1, or the default STG, is user-configurable (as are tagging settings for all STGs). However, by default STG1 sends only untagged BPDUs to operate with all devices that support only one instance of STP. (By default, STG2 through STG8 are tagged.) The tagging setting for each STG is user-configurable.

60. Spanning Tree Groups and VLANs VLANs are a subset of the STG With the setup below connectivity to VLAN-4 across the switches is lost. Solution is to create a trunk links between the switches STG- 1 STG- 1 Blocked VLAN-3 VLAN-3 VLAN-4 VLAN-4

61. Spanning Tree Groups and VLANs STG-1 STG-1 STG-1 STG-1 SW1 SW2 SW3 SW4 Root VLAN-3 VLAN-3 VLAN-3 VLAN-4 VLAN-4 VLAN-4 VLAN-4 Trunk Link Failure Port Blocked

62. Spanning Tree Groups and VLANs STG-1 STG-1 STG-1 STG-1 SW1 SW2 SW3 SW4 Root VLAN-3 VLAN-3 VLAN-3 VLAN-4 VLAN-4 VLAN-4 VLAN-4 Trunk Link Failure Create VLAN-3 Without Access Ports

63. Why have multiple STG STG-1 STG-1 STG-1 STG-1 SW1 SW2 SW3 SW4 Root Gigabit Link Gigabit Link Gigabit Link Gigabit Link With a single STG configured a Gig port is not utilised as it is in a blocking state VLAN-3 VLAN-3 VLAN-3 VLAN-3 VLAN-4 VLAN-4 VLAN-4 VLAN-4

64. VLAN-3 Why have multiple STG STG-1 STG-1 STG-1 STG-1 Gigabit Link Gigabit Link Gigabit Link Gigabit Link With VLAN-3 in STG1 and VLAN-4 in STG-2 all links in the network are now being utilised STG- 2 STG- 2 STG- 2 STG- 2 blocking In STG-2 blocking in STG-1 VLAN-3 VLAN-3 VLAN-3 VLAN-4 VLAN-4 VLAN-4 VLAN-4

65. Tagged BPDUs In the previous slide BPDU’s were being passed across a tagged link. On the Passport switch : STG–1 BPDU are always untagged. This is necessary inorder for the 8600 to be compatiable with other vendor switches. All other STG BPDU’s when passed across tagged links are tagged

66. Vlan 10 Vlan 20 Vlan 10 Vlan 10 Vlan 20 Vlan 10 Vlan 10 Vlan 20 STG 1 STG 2

67. Spanning Tree Fast Learning Enhanced port mode supported by theNortel. If you enable Spanning Tree Fast Learning on a port with no other bridges, the port starts more quickly after a switch initialization or a spanning tree change. The port passes through the normal blocking and learning states before the forwarding state, but the hold times for these states is the bridge hello timer (2 seconds by default) instead of the bridge forward delay timer (15 seconds by default). The port configured with Fast Learning can forward data immediately, as soon as the switch learns that the port is enabled. Fast Learning is intended for access ports in which only one device is connected to the switch (as in workstations with no other spanning tree devices). For these ports, it is not desirable to wait the usual 30 to 35 seconds for spanning tree initialization and bridge learning.

68. ATTENTION If trunk ports are STP-enabled, ensure that all potential trunk members are connected to their corresponding members; otherwise, STP cannot converge correctly, and traffic loss can result.

69. Troubleshooting Verifiying the STG BPDU’s - show port stat stg Verifying the Vlan settings show vlan info … Displaying the Forwarding DataBase show vlan info fdb-entry or fdb-static… Verifying the STG show stg info config [<sid>] show stg status config [<sid>] Verifying the port status show ports error … or stats … monitor ports error … or stats …