Troubleshooting Token Ring . It is still IBM's primary local- area network (LAN) technology, and is second only to Ethernet/IEEE 8. LAN popularity. The IEEE 8. IBM's Token Ring network. In fact, the IEEE 8. IBM Token Ring, and continues to shadow IBM's Token Ring development. The term Token Ring is generally used to refer to both IBM's Token Ring network and IEEE 8.
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IBM's Token Ring network specifies a star, with all end stations attached to a device called a multistation access unit (MAU), whereas IEEE 8. IEEE 8. 02. 5 implementations also are based on a star). Other differences exist, including media type (IEEE 8. IBM Token Ring networks use twisted- pair wire) and routing information field size. Figure 6- 1 summarizes IBM Token Ring network and IEEE 8. Token- passing networks move a small frame, called a token, around the network.
Possession of the token grants the right to transmit. If a node receiving the token has no information to send, it simply passes the token to the next end station. Each station can hold the token for a maximum period of time. While the information frame is circling the ring, there is no token on the network (unless the ring supports early token release), so other stations wishing to transmit must wait. Therefore, collisions cannot occur in Token Ring networks. If early token release is supported, a new token can be released when frame transmission is complete. The information frame continues to circle the ring and is finally removed when it reaches the sending station.
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The sending station can check the returning frame to see whether the frame was seen and subsequently copied by the destination. In other words, it is possible to calculate the maximum time that will pass before any end station will be able to transmit. This feature and several reliability features, which are discussed in the section . Factory automation environments are examples of such applications.
Patch cables connect MAUs to adjacent MAUs. Lobe cables connect MAUs to stations. MAUs include bypass relays for removing stations from the ring.
Token Ring frames have two fields that control priority: the priority field and the reservation field. Once the token is seized and changed to an information frame, only stations with a priority value higher than that of the transmitting station can reserve the token for the next pass around the network. When the next token is generated, it includes the higher priority of the reserving station.
Stations that raise a token's priority level must reinstate the previous priority after their transmission is complete. For example, one station in the Token Ring network is selected to be the active monitor. This station, which can potentially be any station on the network, acts as a centralized source of timing information for other ring stations and performs a variety of ring maintenance functions. One of these functions is the removal of continuously circulating frames from the ring. When a sending device fails, its frame may continue to circle the ring. This can prevent other stations from transmitting their own frames and essentially lock up the network. The active monitor can detect such frames, remove them from the ring, and generate a new token.
Because all information in a Token Ring network is seen by active MAUs, these devices can be programmed to check for problems and selectively remove stations from the ring if necessary. Whenever a station detects a serious problem with the network (such as a cable break), it sends a beacon frame. The beacon frame defines a failure domain, which includes the station reporting the failure, its nearest active upstream neighbor (NAUN), and everything in between. Beaconing initiates a process called autoreconfiguration, where nodes within the failure domain automatically perform diagnostics in an attempt to reconfigure the network around the failed areas. Physically, the MAU can accomplish this through electrical reconfiguration.
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Both formats are shown in Figure 6- 3. This field includes signals that distinguish the byte from the rest of the frame by violating the encoding scheme used elsewhere in the frame. It also contains bits to indicate a damaged frame and a frame that is the last in a logical sequence. Data frames carry information for upper- layer protocols; command frames contain control information and have no data for upper- layer protocols. The frame control byte indicates whether the frame contains data or control information. In control frames, this byte specifies the type of control information. As with IEEE 8. 02.
The length of this field is limited by the ring token holding time, which defines the maximum time a station may hold the token. This field is filled by the source station with a calculated value dependent on the frame contents. The destination station recalculates the value to determine whether the frame may have been damaged in transit. If damage did occur, the frame is discarded. It describes a specific Token Ring symptom, the problems that are likely to cause this symptom, and the solutions to those problems.
Use the show interfaces token command to determine the status of the router's Token Ring interfaces. If the status line indicates that the interface and line protocol are not up, check the cable from the router to the MAU. Make sure that the cable is in good condition. If it is not, replace it.
If you are performing a new installation, make sure that the MAU has been properly initialized. For information on initializing your MAU, refer to the manufacturer's documentation. Check the ring speed specification on all nodes attached to the Token Ring backbone.
The ring speed configured for all stations must be the same (either 4 Mbps or 1. Mbps). Use the show running- config privileged exec command to determine which speed is specified on the router.
If necessary, modify ring speed specifications for clients, servers, and routers. On routers, use the ring- speed interface configuration command to change the ring speed. For more information about ring speed specifications, refer to the hardware installation and maintenance manual for your system.
Use the clear interface privileged exec command to reset the Token Ring interface and reinsert the router into the ring. Use the show interfaces token exec command to verify that the interface and line protocol are up. If the interface is operational, but the . If the message continues to appear, disconnect all devices from the MAU and reset the MAU's relay with the tool provided by the MAU vendor. Reattach the router and determine whether it can connect to the ring.
If resetting the relay does not solve the problem, try replacing the MAU with one that is known to be operational. If the router still cannot connect to the ring, check internal cable connections of the router Token Ring cards. Ensure that cables associated with the respective port numbers are correctly wired and that they are not swapped. If the router still cannot connect to the ring, replace the cables that connect the router to the MAU with working cables.
Use the clear interface command to reset the interface and reinsert the router into the ring. Use the show interfaces token command to verify that the interface and line protocol are up. Alternatively, you can connect the router to a spare MAU to which no stations are connected.
If the router can attach to the ring, replace the original MAU. Use a network analyzer to check the Duplicate Address test frames from a booting station. If the station gets a response, then there is another station already configured with the MAC address of the booting station. If there are two stations with the same MAC addresses, change the MAC address of one of the stations and reinitialize the node. Insert the router during an off- peak period.
If insertion is successful during off- peak periods, but unsuccessful during peak load, segment your internetwork to distribute traffic. Use the no lnm rps interface configuration command to disable the RPS function on the router that you are trying to insert into the ring. Try to insert the router into the ring. If you can insert the router with RPS disabled, there is a conflict between RPS implementations. Contact your technical support representative for more information. This section provides a detailed description of the show interfaces tokenring command and the information it provides in Table 6- 2.
The optional keyword accounting displays the number of packets of each protocol type that have been sent through the interface. Hardware is Token Ring indicates that the board is a CSC- R board. Hardware is 1. 6/4 Token Ring indicates that the board is a CSC- R1.
Also shows the address of the interface. The group address is a multicast address; any number of interfaces on the ring may share the same group address. Each interface may have at most one group address.
Useful for knowing when a dead interface failed. When the number of hours in any of the .
If that field overflows, asterisks are printed. Note that variables that might affect routing (for example, load and reliability) are not cleared when the counters are cleared. Each number is followed by a slash, the maximum size of the queue, and the number of packets dropped due to a full queue. These rates are exponentially weighted averages with a time constant of five minutes.
A period of four time constants must pass before the average will be within 2 percent of the instantaneous rate of a uniform stream of traffic over that period. Compare with ignored count. Broadcast storms on Ethernet networks and bursts of noise on serial lines are often responsible for no input buffer events. On a LAN, this usually indicates noise or transmission problems on the LAN interface or the LAN bus itself. A high number of CRCs is usually the result of a station transmitting bad data. These buffers are different than the system buffers mentioned previously in the buffer description. Broadcast storms and bursts of noise can cause the ignored count to be increased.