Cabletron Systems ETHERNET TECHNOLOGY GUIDE
Notice Cabletron Systems reserves the right to make changes in specifications and other information contained in this document without prior notice. The reader should in all cases consult Cabletron Systems to determine whether any such changes have been made. The hardware, firmware, or software described in this manual is subject to change without notice.
Notice ii
Contents CHAPTER 1 OVERVIEW Purpose of This Manual .........................................................................................................1-1 Who Should Use This Manual................................................................................................1-1 Structure of This Manual ........................................................................................................1-2 CHAPTER 2 INTRODUCTION Ethernet History ................................................
Contents CHAPTER 5 ETHERNET MEDIA ACCESS METHOD Clean Frame Transmission .................................................................................................... 5-1 Packet Involved in a Collision ................................................................................................ 5-2 Collision Detection on Point-to-Point Media .................................................................... 5-3 Out-Of-Window Collision...........................................................
Contents CHAPTER 8 PROPAGATION DELAY Calculating the Delay .............................................................................................................8-1 Propagation Delay Example...................................................................................................8-2 CHAPTER 9 ETHERNET BRIDGE OPERATION Filtering and Forwarding ........................................................................................................9-1 Spanning Tree Algorithm ..............
Contents vi
Chapter 1 Overview Purpose of This Manual Welcome to the Cabletron Systems Ethernet Technology Guide. This guide discusses the aspect of an Ethernet network known as the physical and datalink layer. Although there may be some mention of specific networking products and software, the primary focus is on the understanding, design and implementation of a generic Ethernet Local Area Network (LAN). Throughout this document, references are made to both Ethernet and IEEE 802.3 (CSMA/CD).
Structure of This Manual Structure of This Manual This manual is organized as follows: Chapter 1, Overview - Outlines the purpose of this manual, who should use it, and how it is structured. Chapter 2, Introduction - Introduces Ethernet and gives a brief discussion of the features and characteristics of the Ethernet technology. Chapter 3, Ethernet LAN Standards - Discusses the Open Systems Interconnect Model (OSI).
Chapter 2 Introduction This chapter introduces Ethernet features and describes characteristics that distinguish Ethernet from other Local Area Network (LAN) technologies such as Token Ring or FDDI. Ethernet History Ethernet was developed by Xerox Corporation’s Palo Alto Research Center (PARC) in the mid-1970s. Ethernet was the technological basis for the IEEE 802.3 specification, which was initially released in 1980.
Ethernet Features Ethernet Features The Institute of Electrical and Electronic Engineers (IEEE) is a standards organization that establishes standards for many different technical areas. This broad standards responsibility includes computer networking. IEEE project 802 is responsible for networking standards for all network access methods while project 802.3 specifically defines the Carrier Sense Multiple Access/Collision Detection (CSMA/CD) access method, or Ethernet.
Ethernet Features Transmission Medium Ethernet transmits data frames over a physical medium of coaxial, fiber optic, or twisted pair cable. The coaxial and fiber optic cable typically represents the backbone of an Ethernet LAN while twisted pair is used as a low cost connection from the backbone to the desktop.
Ethernet Features Frame Transmission Ethernet stations encode their data into groups using a method known as Manchester Encoding. These data groups, called frames, can be one of four basic Ethernet frame types: • 802.3 “raw” frame • Ethernet II (DIX) frame • Ethernet 802.2 frame • Ethernet SNAP frame The Ethernet 802.2 and Ethernet SNAP frames are extensions of the 802.3 “raw” frame format, while the Ethernet II frame is formatted differently.
Ethernet Features The most popular topologies used in Ethernet are the bus, star and tree. Even though this discussion is about Ethernet, it is worth spending a few moments on topologies not commonly used with Ethernet. A couple of definitions at this point will assist with understanding the following descriptions. • Point-to-Point: A point-to-point connection is a connection between two and only two network devices (computers, servers, printers, etc.). No taps or daisy chains are allowed.
Ethernet Features Ring Topology A ring topology is a point-to-point topology in which the network devices are connected, device to device, in an unbroken circle. Each signal to be transmitted on the network must be processed by each station on the ring before it is passed (or repeated) to the next station. General Characteristics Ring topologies commonly use an access method that is called token passing.
Ethernet Features Contention Star Topology The contention star is the access method used with Ethernet. Workstations are connected to a hub or concentrator located in a wiring closet. Contention rules dictate that only one station can transmit data at any given time and any station may talk providing the network is quiet. This access method eliminates the need for polling and vastly improves throughput and performance. Hubs can be expanded to handle hundreds of devices without performance degradation.
Ethernet Features 2-8 Introduction
Chapter 3 Ethernet LAN Standards Standards play an important role in modern local area networks. Without standards, users are forced to buy proprietary networking equipment from a single vendor. Companies come and go, and product lines are changed or discontinued. This leads to increased network costs to the users, network down time and network equipment that does not inter-operate if standards are not in place. Standards allow for easy integration of multiple vendor equipment into a common network.
The Open Systems Interconnect (OSI) Model 7 Application 6 Presentation 5 4 3 2 1 Session Transport Network Data Link Physical 1913-02 Figure 3-1. Open Systems Interconnect (OSI) Model Application of the OSI Model The perception of network operation appears as a direct peer-to-peer communication to the user. The user message appears to go from the sending application layer directly to the receiving application layer as if the devices were directly attached.
The Open Systems Interconnect (OSI) Model • LAYER SIX: Presentation Layer–The presentation layer deals with data translation and code conversion between devices with different data formats (e.g., ASCII to EBCDIC). This layer also handles translation between differing device types and file formats, as well as data encryption and decrypting services. • LAYER FIVE: Session Layer–The session layer manages the communication dialogue (the “session”) between two communicating devices.
The Open Systems Interconnect (OSI) Model • LAYER TWO: Data Link Layer–The data link layer is involved with transmission, error detection and flow control of the data. The major function of the data link layer is to act as a shield for the higher layers of the network model, controlling the actual transmission and reception process. Error detection and control of the physical layer are the primary functions of this layer ensuring the upper layers that any data received from the network is error free.
The Open Systems Interconnect (OSI) Model On the receive side, the MAC layer follows the reverse of the above steps. It checks the frame for errors, strips control information then passes the remainder of the packet to the upper layers by way of logical link control. • LAYER ONE: Physical Layer–At this layer, the transmission of data between devices is defined.
The Open Systems Interconnect (OSI) Model 3-6 Ethernet LAN Standards
Chapter 4 Ethernet Data Frames Most computer networks require that information transmitted between two stations be divided into blocks called frames. For these frames to be sent successfully to other devices on the network, certain protocol and routing information must be added to the data. In addition, the way this information is arranged inside the frame must conform to a specific format.
Manchester Encoding In Ethernet, the digital D.C. signal is transformed into discrete time segments called bits by a method called Manchester Encoding. With Manchester Encoding, the incoming digital signal is checked at specific time intervals for its change of state. In other words, the signal is checked to see if it is changing from 0 volts to -1.2 volts or from -1.2 volts to 0 volts during a certain time period.
Ethernet Data Frames Ethernet Data Frames Previously it was shown how Ethernet data signals are transformed into bits. Before these bits are sent onto an Ethernet network, they must be formatted into specific groups called data frames. Data frames are strings of bytes (eight bits equal one byte) which contain addressing, timing, protocol, and error correction information as well as the data being sent. The packet structure used in IEEE 802.3 and Ethernet is shown in Figure 4-2.
Ethernet Data Frames • Data Field: The data field follows the length field. It is 46 bytes minimum to a maximum of 1500 bytes in length. This field contains the actual data being sent across the network along with some control information. If the data to be sent is less than the minimum 46-byte packet size, a special bit pattern called PAD is used to fill in up to the 46-byte minimum. The minimum packet size set by the IEEE 802.3 specification is explained below.
Ethernet Data Frames Due to inherent propagation delays in electronics and cabling it would make sense that within 25.6 µs (half of 51.2 µs) our transmitted signal should have reached the farthest point on the network. If a collision were to happen at the farthest point on the network the collision signal will have the remaining 25.6 µs to travel back to the transmitting node thus alerting the node that its transmission needs to be re-sent. The 25.
Ethernet Data Frames Ethernet II Frame Type In the early days of computer networks, Digital, Intel, and Xerox got together and specified a networking standard that they called Ethernet. This standard included the definition of a data link level access method and a packet format that shared the Ethernet name (it is now called Ethernet II because it is in its second revision). Table 4-1 shows the fields defined for Ethernet II data frames. Table 4-1.
Ethernet Data Frames Table 4-2. Ethernet “Raw” Frame Type Field Name Field Size Field Definition Preamble 7 bytes Signals beginning of the frame. Start Frame Delimiter 1 byte Signals start of data. Destination Address 6 bytes Contains the address of the destination of the frame. Source Address 6 bytes Contains the address of the frame’s origin. Length Field 2 bytes Specifies the length of the data field. Data 46–1500 bytes Contains the data to be transferred.
Ethernet Data Frames Table 4-3. Ethernet 802.2 Frame Type Field Name Field Size Field Definition Preamble 7 bytes Signals beginning of the frame. Start Frame Delimiter 1 byte Signals start of data. Destination Address 6 bytes Contains address of the destination frame. Source Address 6 bytes Contains address of the frame’s origin. Length Field 2 bytes Indicates length of the Data plus LLC fields.
Ethernet Data Frames Ethernet SNAP Frame Type After the 802.2 frame was defined, there was some concern that the one byte DSAP and SSAP fields were not adequate for the number of protocols that eventually needed to be identified. In response from Apple Computer and the TCP/IP community, another frame standard was defined for both Ethernet and Token Ring. It was called the Sub-Network Access Protocol, shown in Table 4-4 below. Table 4-4.
Ethernet Data Frames This frame type adds a five-byte protocol identification field at the end of the 802.2 header, where the protocol is identified. To distinguish an IEEE 802.2 SNAP frame, the value of the DSAP and SSAP fields in the 802.2 header are both set to AA. If a network device finds AA in the DSAP and SSAP fields, it knows this is a SNAP-based frame and it should look for the protocol type in the protocol identification field.
Ethernet Data Frames The 00-00-1D manufacturer ID belongs to Cabletron Systems. When a specific Ethernet address is used as the destination address in a packet, that packet will be decoded only by the station with that specific address. Multicast Addressing At times it is necessary to communicate with many devices on a network simultaneously. For instance, a network management station might poll or query a group of devices to determine their status.
Ethernet Data Frames A broadcast address contains all “F” hexadecimal characters which is equivalent to all bits being set to logic 1 in both the manufacturer ID and sequential number area of the address (see Figure 4-6). FF-FF-FF-FF-FF-FF Manufacturer ID Sequential Address 1913-10 Figure 4-6. Ethernet Broadcast Address In the following chapter we will look at how the packet is transmitted onto the network and the rules that must be followed to ensure a successful transmission.
Chapter 5 Ethernet Media Access Method Ethernet, as stated in Chapter 1, uses a method of access control known as Carrier Sense Multiple Access with Collision Detection, or CSMA/CD. Access to the network media is controlled by the lower half of the Data Link Layer called Media Access Control, or MAC. The following chapter describes the operation of the Ethernet MAC.
Packet Involved in a Collision Interframe Gap Data Packet 9.6 µs .6 µs 1.4 µs 2 µs SQE Test 1913-11 Figure 5-1. Ethernet Interframe Spacing During this time, the station will see the Signal Quality Error (SQE) test signal on its collision detection. When the station sees this signal during the 1.4 µs window, it is informed that the transceiver collision detect circuits are working properly and if a collision occurred, it would be notified. After the remainder of the 9.
Packet Involved in a Collision When a collision is detected, both stations will transmit a jam signal that is long enough to ensure that the collision is detected by all stations on the network. Then, each station involved in the collision will wait for a random period of time and then attempt transmission again. The station will attempt again transmission up to 16 consecutive times before an error is sent to the upper layer protocols notifying the station of a serious communication problem.
Packet Involved in a Collision 5-4 Ethernet Media Access Method
Chapter 6 Ethernet Devices This chapter describes devices that are common to an Ethernet network. All devices attached to an Ethernet bus must comply with the IEEE 802.3 Standard. Typical Ethernet devices include stations, transceivers, repeaters, bridges and routers. Figure 6-1 shows various devices attached to an Ethernet LAN. The following sections provide a description of each of these devices and their network functions.
Ethernet Transceivers Network Server Network Printer Station 1913-13 Figure 6-2. Ethernet Stations Ethernet Transceivers A transceiver (transmitter/receiver) is the device that connects workstations, servers, and other equipment to the Ethernet cabling media being used for network transmissions. For full descriptions of Ethernet networking media, refer to the Cabletron Systems Cabling Guide.
Ethernet Repeaters Multi-port Transceivers A multi-port transceiver or fanout is a transceiver that has one port to connect to a regular transceiver and up to fifteen AUI ports to connect through AUI cables to individual devices. This allows you to connect several addressable devices to one cable tap. IEEE 802.3 standards for transceiver placement and tap spacing specify that only 100 taps will be allowed on a 10BASE5 segment with a distance of 2.5 m between them.
Ethernet Repeaters Figure 6-5 shows a Cabletron Systems LR2000 two-port local Ethernet repeater. LR-2000 1913-16 Figure 6-5. Cabletron Systems Two-Port Local Ethernet Repeater Repeaters and Collisions A collision happens when more than one station transmits on the network at one time. Since the repeater physically separates the two coaxial segments, a collision on one segment cannot be seen by devices on the other segment connected to the repeater.
Ethernet Bridges Multi-port Repeaters A multi-port repeater is a device which has more than two ports that connect to full-length Ethernet Segments. Generally the repeaters are very similar in appearance to the two-port local repeater shown in Figure 6-5, with the exception of the number of ports. These repeaters regenerate the preamble and amplify and re-time a signal from one cable segment to the others.
Routers Unlike a repeater, which sends all frames it receives to all segments it is connected to, a bridge reads the frames it receives and decides whether to filter or forward the frame based on the addressing information contained within it. Bridges can also be used to connect similar networks (networks with the same upper five layers of the OSI model) such as Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI) together.
Chapter 7 Ethernet Network Design Designing an Ethernet network must be approached with care. Many aspects of the network must be considered before actual implementation can begin. A detailed plan must be laid out to ensure that all the goals and obstacles are identified. In this chapter we will discuss many aspects of network design by beginning with a single segment 10BASE5 Ethernet network and gradually building the single segment into a large, multiple segment network.
10BASE5 Ethernet Network Design The coaxial cable can be run in one continuous length or in sections, joined using N-type connectors and N-type barrel connectors. If the cable is installed in segments and connected together, IEEE recommends that the segments should be an odd multiple of 23.4 m in length. These special lengths of cable are used to minimize signal reflections caused by the insertion of connectors and barrel splices.
10BASE5 Ethernet Network Design A maximum length 10BASE5 coaxial cable has 199 annular rings marked off at 2.5 m intervals. Because each transceiver tap introduces noise onto the coaxial cable in the form of a small impedance discontinuity, and contributes to the overall attenuation of the cable, IEEE has specified that only 100 taps will be allowed on a 10BASE5 segment, with each tap separated by a minimum of 2.5 m.
10BASE5 Ethernet Network Design Coaxial Backbone TRANSCEIVER Multi-port Transceiver AC 5 6 7 8 Network 22 22 22 4 22 3 22 2 22 22 22 22 1 To Network Devices 1913-21 Figure 7-4. Multi-port Transceiver Multi-port Transceiver Rules When multi-port transceivers are used, two rules must be observed: 1. Multi-port transceivers can be cascaded by connecting the male connector of one multi-port transceiver to the female connector of a second multi-port transceiver.
10BASE5 Ethernet Network Design Coaxial Backbone TRANSCEIVER Connect up to 8 MP transceivers or devices Accounts for 10 m of AUI cable AC 2 3 4 5 6 7 22 22 1 8 Network Connect up to 8 devices AC 1 2 3 4 5 6 7 2 2 Accounts for 10 m of AUI cable 8 Network 22 To Device These cables add up to a total of 30 meters. 1913-22 Figure 7-5.
10BASE5 Ethernet Network Design Multiple Segment 10BASE5 Ethernet Network We have seen how we can build a single segment 10BASE5 Ethernet network. This is adequate if we only want to span a distance of 500 m. If it is necessary to cover a greater area or to add additional coaxial taps beyond the 100 tap limit, more coaxial cable must be added. To connect the new coaxial segment to the existing backbone, a repeater must be used.
10BASE5 Ethernet Network Design Inter-Repeater Link (IRL) Repeaters can be used to connect up to 3 coaxial segments before special considerations need to be taken into account. If it is necessary to connect more than 3 coaxial segments together, then an Inter-Repeater Link (IRL) must be used (see Figure 7-7). An IRL is a segment that spans between two repeaters with no other devices attached to it.
10BASE2 Ethernet Network Design 10BASE2 Ethernet Network Design NOTE The characteristics and test requirements of 10BASE2 cables are presented in the Cabletron Cabling Guide. Single Segment 10BASE2 Ethernet Network 10BASE2 is the IEEE specification for Ethernet running on RG58 A/U coaxial cable. 10BASE2 coaxial cable is more flexible and less expensive than 10BASE5 coaxial cable while still maintaining the required 50 ohm nominal impedance. The maximum length of a 10BASE2 cable segment is 185 m.
10BASE2 Ethernet Network Design Workstation Connections Workstations may be connected to the BNC T-connector in one of the following two ways: 1. By connecting a transceiver with a BNC connection directly to the T-connector then running up to 50 m of standard AUI cable to the workstation. 2. By connecting the T-connector to the internal transceiver that is built into most network interface cards on the device itself.
10BASE2 Ethernet Network Design By using repeaters, multi-port repeaters, or a combination of both, it is possible to design a 10BASE2 network that has 4 multi-port repeaters and five interconnected segments that span a distance of 5x185 m = 925 m, with a maximum of 1,024 connected devices. As long as the longest possible signal path does not pass through more than four repeaters and five segments, with only three of the segments being populated with devices, the network is within IEEE 802.
Fiber Optic Ethernet Network Design Grounding and Insulation When cascading multi-port repeaters be careful to avoid the creation of ground loops (or multiple paths to ground). If two multi-port repeaters that perform internal grounding are connected using the BNC ports, a ground loop will result (see Figure 7-10). In other words, if one repeater is connected using a BNC port, the other must be connected using the AUI port.
10BASE-T Twisted Pair Ethernet Network Design Fiber Optic Inter-Repeater Link (FOIRL) Fiber concentrator Fiber concentrator Fiber drop to workstation Fiber drop to workstation FOT Fiber transceiver AUI cable 1913-28 Figure 7-11. A Complete Fiber Optic Ethernet 10BASE-T Twisted Pair Ethernet Network Design Unshielded Twisted Pair (UTP) wiring is found in most business environments.
10BASE-T Twisted Pair Ethernet Network Design Twisted Pair Repeater Multiple Twisted Pair Drops to Workstations 1913-29 Figure 7-12.
10BASE-T Twisted Pair Ethernet Network Design 7-14 Ethernet Network Design
Chapter 8 Propagation Delay From the preceding, Chapter 7, Ethernet Network Design, we have seen how it is possible to build a maximum size network using each of the available media. When designing any Ethernet network, it is wise to calculate the maximum round trip propagation delay for the proposed design. The maximum allowable round-trip propagation delay of 51.
Propagation Delay Example Propagation Delay Example To clarify the methods used to calculate the propagation delay of a given network, refer to Figure 8-1 and complete the following steps to calculate the propagation delay of the network shown.
Propagation Delay Example 3. Now that you have the equipment delay times, you must calculate the total delay for each media type. To do this, make a list of each media type found in the signal path and add up the total length, in meters, of each. Now, multiply this number by the appropriate media delay time for each media type found in Table 8-1, to get the delay time for each length of media found in the signal path. NOTE The values contained in Table 8-1 are maximum values.
Propagation Delay Example Table 8-1. Equipment and Cable Propagation Delay Times 8-4 Equipment Type Delay Media Type Delay per Meter Local Repeater 0.65 µs 10BASE5 coaxial 0.00433 µs/m Fiber Optic Repeater 1.55 µs 10BASE2 coaxial 0.00514 µs/m Multi-port Repeater 1.55 µs Shielded Twisted Pair (STP) 0.0057 µs/m Multi-port Transceiver 0.10 µs Unshielded Twisted Pair (UTP) 0.0057 µs/m Standard Transceiver 0.86 µs Fiber Optic 0.005 µs/m Fiber Optic Transceiver 0.
Propagation Delay Example Table 8-2. Equipment Propagation Delay Worksheet Equipment Type A Delay B Quantity (A*B) Total 1 Local Repeater 0.65 µs 2 1.30 µs 2 Fiber Optic Repeater 1.55 µs 1 1.55 µs 3 Multi-port Repeater 1.55 µs - 0 4 Multi-port Transceiver 0.10 µs 2 0.20 µs 5 Standard Transceiver 0.86 µs 6 5.16 µs 6 Fiber Optic Transceiver 0.20 µs 1 0.20 µs 7 Twisted Pair Transceiver 0.27 µs 1 0.27 µs 8 Concentrator 1.90 µs 1 1.90 µs 10.
Propagation Delay Example To calculate the One Way propagation delay time, add the values in the shaded bottom right hand corner of Table 8-2 and Table 8-3 together. Equipment Delay: Cable Delay 10.58 µs 12.87 µs Total One Way Delay 23.45 µs As with designing anything, it is not advisable to design to the limits of the specification. As an electronic device ages, its internal propagation delay may change.
Chapter 9 Ethernet Bridge Operation Bridges are devices that are added to a network to allow expansion beyond the limits of the IEEE 802.3 specification. They are also used to connect two similar networks together, allowing communication between them. Bridges accomplish this by reading in frames and deciding to either filter or forward the frame based on the destination address of the frame. The following sections detail the operation of bridges and their functions.
Filtering and Forwarding When the bridge is first powered up, its SAT is empty: Network 1 Network 2 Assume Station A wants to transmit a frame to Station B. The bridge receives the frame and checks the CRC (Cyclic Redundancy Check) of the frame. The bridge then looks at the source address of the frame and puts that address in the source address table of Network 1 as shown below: Network 1 Network 2 A The bridge then checks the destination of the frame to see if it is located on Network 1.
Spanning Tree Algorithm Assume that station A wants to transmit to Station D. Station A sends the frame to Station D. The bridge reads in the frame and checks the CRC. The bridge reads the source address of the frame (Station A) and makes sure Station A is still in the SAT. The bridge checks the SAT for the destination address, Station D. It is not found to reside on the Network 1 side of the bridge so the frame is forwarded to Network 2. Next, Station D sends its response to Station A.
Spanning Tree Algorithm Configuration BPDU When a bridge is powered up, it goes through a series of self tests to check its internal operation. During this time the bridge is in a standby condition and does not forward traffic. Also during this standby period, the bridge sends out special bridge management frames called Configuration Bridge Protocol Data Units (BPDU). Bridges use the BPDU frames as a way of communicating with each other.
Spanning Tree Algorithm - Message Age: A 2-byte field that contains the age of the configuration BPDU. This parameter allows a bridge to determine if the BPDU is too old and needs to be discarded. - Max Age: A 2-byte field that contains a time-out value initially set by the root bridge. This value is compared to the Message Age to determine the validity of the BPDU. - Hello Time: A 2-byte field that contains the value for the time interval used to generate configuration BPDU’s by the root bridge.
Spanning Tree Operation Spanning Tree Operation In the following explanation we take Spanning Tree through the network shown in Figure 9-2, which consists of two-port bridges. You must understand that the Spanning Tree process is a single operation, combining both root bridge determination and data loop detection and resolution.
Spanning Tree Operation The primary function of the Spanning Tree Algorithm is to ensure that there is only one data path between any two end stations within the bridged Local Area Network. All computations are geared towards the fact that a bridge wants to be considered as the Designated Bridge for any LAN that it is connected to. Upon power up, BPDUs are sent out as multicast frames.
Spanning Tree Operation With the understanding of our shortened BPDU, we now begin our explanation of the Spanning Tree Operation. In Figure 9-3, SAM is sending out BPDUs across all LANs to which it is connected. SAM sends BPDUs out through its port 2 onto LAN C and through port 1 onto LAN A. LAN A ANN JANET Port 1 LAN B SAM Port 2 EVENIN LAN C SAM's BPDUs 1913-33 Figure 9-3.
Spanning Tree Operation Now let’s look at one of the other bridges, in this case ANN. Figure 9-4 shows ANN generating BPDUs from all of its ports, considering itself as root until it finds out differently. LAN A Port 1 ANN JANET Port 2 LAN B SAM EVENIN LAN C ANN's BPDUs 1913-34 Figure 9-4.
Spanning Tree Operation SAM checks the incoming BPDU from ANN and it reflects a different root ID. In the process of checking the incoming data against that which is current at that port, SAM realizes that it has the higher priority root ID (lower number) and does not forward ANN’s BPDU through port 2; it continues to send its own. ANN checks the incoming BPDU from SAM and senses that the BPDU carries a higher priority BPDU (lower number) than its own.
Spanning Tree Operation A similar case can be made for EVENIN, the bridge that spans LAN B and LAN C shown in Figure 9-5. LAN A ANN JANET LAN B SAM BPDU in from ANN Port 1 EVENIN BPDU out Port 2 LAN C 1913-35 Figure 9-5. BPDUs in from ANN to EVENIN EVENIN would have also thought that it was root until it found out otherwise. Let’s look at the progression of BPDUs from ANN.
Spanning Tree Operation Data Loop Resolution The center of our discussion here will be within EVENIN. It received BPDUs from both of its ports. A BPDU from SAM came in on port 2, and a BPDU from ANN came in on port 1. With the reception of these two frames from different ports, each identifying SAM as the root, EVENIN realizes that there is a data loop present.
Spanning Tree Operation The information that both the bridges have in relation to LAN B is as follows: Port Parameters ANNs Port 2 EVENINs Port 1 Port ID 8002 8001 Designated Root SAM SAM Designated Bridge ANN EVENIN Designated Port 8002 8001 Root Path Cost 100 100 The algorithm to determine who breaks the identified data loop is in the following order: a. lowest root path cost b. highest priority designated bridge ID c. highest priority designated port ID d.
Spanning Tree Operation There is one final data loop present. JANET is bridging from LAN A to LAN B. The bridge entities recognize the data loop condition by monitoring the incoming BPDUs. Upon seeing BPDUs coming in through both of its ports, all originating from the root, it realizes that there is more than one path to the root bridge.
Index Numerics D 10BASE2 network design 7-8 10BASE5 network design 7-1 10BASE-T network design 7-12 Data Field 4-4 Data Frame Type 4-5 802.
Index L R Length Field 4-3 Logical Link Control 3-4 Repeater 6-3 Root Identifier 9-4 Root Path Cost 9-4 Routers 6-6 M Manchester Encoding 4-1 Max Age 9-5 Media Access Control 3-4 Media Access Method 2-2, 5-1 Medium Dependant Interface 3-5 Message Age 9-5 Multi-point 2-5 Multi-port Repeaters 6-5 Multi-port Transceivers 6-3 Signal Quality Error 5-2 Source Address 4-3 Source Address Table 9-1 Spanning Tree Algorithm 9-3 Spanning Tree Operation 9-6 Start Frame Delimiter 4-3 Stations 6-1 N T Network Des