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5
Troubleshooting Fiber Distributed Data Interface
The Fiber Distributed Data Interface (FDDI) standard was produced by the ANSI X3T9.5 standards
committee in the mid-1980s. During this period, high-speed engineering workstations were beginning
to tax the capabilities of existing local-area networks (LANs)—primarily Ethernet and Token Ring. A
new LAN was needed that could easily support these workstations and their new distributed applications.
At the same time, network reliability was becoming an increasingly important issue as system managers
began to migrate mission-critical applications from large computers to networks. FDDI was developed
to fill these needs.
After completing the FDDI specification, ANSI submitted FDDI to the International Organization for
Standardization (ISO). ISO has created an international version of FDDI that is completely compatible
with the ANSI standard version.
Although FDDI implementations are not as common as Ethernet or Token Ring, FDDI has gained a
substantial following that continues to increase as the cost of FDDI interfaces diminishes. FDDI is
frequently used as a backbone technology as well as a means to connect high-speed computers in a local
area.
FDDI Technology Basics
FDDI specifies a 100-Mbps, token-passing, dual-ring LAN using a fiber-optic transmission medium. It
defines the physical layer and media-access portion of the link layer, and is roughly analogous to IEEE
802.3 and IEEE 802.5 in its relationship to the Open System Interconnection (OSI) reference model.
Although it operates at faster speeds, FDDI is similar in many ways to Token Ring. The two types of
networks share many features, including topology (ring), media-access technique (token passing), and
reliability features (redundant rings, for example). For more information on Token Ring and related
technologies, refer to Chapter 6, “Troubleshooting Token Ring.”
One of the most important characteristics of FDDI is its use of optical fiber as a transmission medium.
Optical fiber offers several advantages over traditional copper wiring, including security (fiber does not
emit electrical signals that can be tapped), reliability (fiber is immune to electrical interference), and
speed (optical fiber has much higher throughput potential than copper cable).
FDDI defines use of two types of fiber: single mode (sometimes called monomode) and multimode.
Modes can be thought of as bundles of light rays entering the fiber at a particular angle. Single-mode
fiber allows only one mode of light to propagate through the fiber, whereas multimode fiber allows
multiple modes of light to propagate through the fiber. Because multiple modes of light propagating
through the fiber may travel different distances (depending on the entry angles), causing them to arrive
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at the destination at different times (a phenomenon called modal dispersion), single-mode fiber is
capable of higher bandwidth and greater cable run distances than multimode fiber. Because of these
characteristics, single-mode fiber is often used for interbuilding connectivity, and multimode fiber is
often used for intrabuilding connectivity. Multimode fiber uses light-emitting diodes (LEDs) as the
light-generating devices, whereas single-mode fiber generally uses lasers.
FDDI Specifications
FDDI is defined by four separate specifications (see Figure 5-1):
• Media Access Control (MAC)—Defines how the medium is accessed, including frame format,
token handling, addressing, an algorithm for calculating a cyclic redundancy check value, and error
recovery mechanisms.
• Physical Layer Protocol (PHY)—Defines data encoding/decoding procedures, clocking
requirements, framing, and other functions.
• Physical Layer Medium (PMD)—Defines the characteristics of the transmission medium,
including the fiber-optic link, power levels, bit error rates, optical components, and connectors.
• Station Management (SMT)—Defines the FDDI station configuration, ring configuration, and ring
control features, including station insertion and removal, initialization, fault isolation and recovery,
scheduling, and collection of statistics.
Figure 5-1 FDDI Standards
Physical Connections
FDDI specifies the use of dual rings. Traffic on these rings travels in opposite directions. Physically, the
rings consist of two or more point-to-point connections between adjacent stations. One of the two FDDI
rings is called the primary ring; the other is called the secondary ring. The primary ring is used for data
transmission, and the secondary ring is generally used as a backup.
tr190501.ps
Logical link control
Media access control
Physical layer protocol
Station
management
FDDI
standards
Physical layer medium
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Class B or single-attachment stations (SASs) attach to one ring; Class A or dual-attachment stations
(DASs) attach to both rings. SASs are attached to the primary ring through a concentrator, which
provides connections for multiple SASs. The concentrator ensures that failure or power down of any
given SAS does not interrupt the ring. This is particularly useful when PCs, or similar devices that
frequently power on and off, connect to the ring.
A typical FDDI configuration with both DASs and SASs is shown in Figure 5-2.
Figure 5-2 FDDI Nodes: DAS, SASs, and Concentrator
Each FDDI DAS has two ports, designated A and B. These ports connect the station to the dual FDDI
ring. Therefore, each port provides a connection for both the primary and the secondary ring, as shown
in Figure 5-3.
Figure 5-3 FDDI DAS Ports
Traffic Types
FDDI supports real-time allocation of network bandwidth, making it ideal for a variety of different
application types. FDDI provides this support by defining two types of traffic: synchronous and
asynchronous. Synchronous traffic can consume a portion of the 100-Mbps total bandwidth of an FDDI
network, and asynchronous traffic can consume the rest. Synchronous bandwidth is allocated to those
stations requiring continuous transmission capability. Such capability is useful for transmitting voice
and video information, for example. Other stations use the remaining bandwidth asynchronously. The
FDDI SMT specification defines a distributed bidding scheme to allocate FDDI bandwidth.
FDDI
tr190502.ps
ConcentratorDAS
SAS SAS SAS
tr190503.ps
Primary
Secondary
Primary
Secondary
Port A Port B
FDDI DAS
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Asynchronous bandwidth is allocated using an eight-level priority scheme. Each station is assigned an
asynchronous priority level. FDDI also permits extended dialogues, where stations may temporarily use
all asynchronous bandwidth. The FDDI priority mechanism can essentially lock out stations that cannot
use synchronous bandwidth and have too low an asynchronous priority.
Fault-Tolerant Features
FDDI provides a number of fault-tolerant features, the most important of which is the dual ring. If a
station on the dual ring fails or is powered down or if the cable is damaged, the dual ring is automatically
wrapped (doubled back onto itself) into a single ring, as shown in Figure 5-4. In this figure, when Station
3 fails, the dual ring is automatically wrapped in Stations 2 and 4, forming a single ring. Although
Station 3 is no longer on the ring, network operation continues for the remaining stations.
Figure 5-4 Station Failure, Ring Recovery Configuration
Figure 5-5 shows how FDDI compensates for a wiring failure. Stations 3 and 4 wrap the ring within
themselves when wiring between them fails.
tr190504.ps
Station 1
Station 3
Failed station
Station 2Station 4
Ring wrap Ring wrap
B
B
A
A
B
A
AB
MAC
MACMAC
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Figure 5-5 Failed Wiring, Ring Recovery Configuration
As FDDI networks grow, the possibility of multiple ring failures grows. When two ring failures occur,
the ring is wrapped in both cases, effectively segmenting the ring into two separate rings that cannot
communicate with each other. Subsequent failures cause additional ring segmentation.
Optical bypass switches can be used to prevent ring segmentation by eliminating failed stations from the
ring. This is shown in Figure 5-6.
tr190505.ps
Station 1
Station 3
Station 2Station 4
Ring wrap
Ring wrap
B
B
A
A
B
A
AB
MAC
MAC
MACMAC
Failed wiring
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Figure 5-6 The Use of an Optical Bypass Switch
Critical devices such as routers or mainframe hosts can use another fault-tolerant technique called dual
homing to provide additional redundancy and help guarantee operation. In dual-homing situations, the
critical device is attached to two concentrators. One pair of concentrator links is declared the active link;
the other pair is declared passive. The passive link stays in backup mode until the primary link (or the
concentrator to which it is attached) is determined to have failed. When this occurs, the passive link is
automatically activated.
Frame Format
FDDI frame formats (shown in Figure 5-7) are similar to those of Token Ring.
Station 1
Station 3
Station 2
Optical bypass switch
“normal configuration”
Failed station
Station 4
A
A
B
A
B
B
AB
tr190506.ps
Station 1
Station 3
Station 2
Ring does
not wrap
Optical bypass switch
“bypassed configuration”
Station 4
B
A
B
A
B
A
AB
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Figure 5-7 FDDI Frame Format
The fields of an FDDI frame are as follows:
• Preamble—Prepares each station for the upcoming frame.
• Start delimiter—Indicates the beginning of the frame. It consists of signaling patterns that
differentiate it from the rest of the frame.
• Frame control—Indicates the size of the address fields, whether the frame contains asynchronous
or synchronous data, and other control information.
• Destination address—Contains a unicast (singular), multicast (group), or broadcast (every station)
address. As with Ethernet and Token Ring, FDDI destination addresses are 6 bytes.
• Source address—Identifies the single station that sent the frame. As with Ethernet and Token Ring,
FDDI source addresses are 6 bytes.
• Data—Contains either information destined for an upper-layer protocol or control information.
• Frame check sequence (FCS)—Filled by the source station with a calculated cyclic redundancy
check (CRC) value dependent on the frame contents (as with Token Ring and Ethernet). The
destination station recalculates the value to determine whether the frame may have been damaged
in transit. If it has been damaged, the frame is discarded.
• End delimiter—Contains nondata symbols that indicate the end of the frame.
• Frame status—Allows the source station to determine whether an error occurred and whether the
frame was recognized and copied by a receiving station.
CDDI
The high cost of fiber-optic cable has been a major impediment to the widespread deployment of FDDI
to desktop computers. At the same time, shielded twisted-pair (STP) and unshielded twisted-pair (UTP)
copper wire is relatively inexpensive and has been widely deployed. The implementation of FDDI over
copper wire is known as Copper Distributed Data Interface (CDDI).
Before FDDI could be implemented over copper wire, a problem had to be solved. When signals strong
enough to be reliably interpreted as data are transmitted over twisted-pair wire, the wire radiates
electromagnetic interference (EMI). Any attempt to implement FDDI over twisted-pair wire had to
ensure that the resulting energy radiation did not exceed the specifications set in the United States by the
Federal Communications Commission (FCC) and in Europe by the European Economic Council (EEC).
Three technologies reduce energy radiation:
Preamble
tr190507.ps
Data FCS
Start
delimiter
Frame
control
Frame
status
End
delimiter
Source
address
Data frame
Destination
address
Preamble
Token
Start
delimiter
Frame
control
End
delimiter
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• Scrambling—When no data is being sent, FDDI transmits an idle pattern that consists of a string of
binary ones. When this signal is sent over twisted-pair wire, the EMI is concentrated at the
fundamental frequency spectrum of the idle pattern, resulting in a peak in the frequency spectrum
of the radiated interference. By scrambling FDDI data with a pseudo-random sequence prior to
transmission, repetitive patterns are eliminated. The elimination of repetitive patterns results in a
spectral peak that is distributed more evenly over the spectrum of the transmitted signal.
• Encoding—Signal strength is stronger, and EMI is lower when transmission occurs over
twisted-pair wire at lower frequencies. MLT3 is an encoding scheme that reduces the frequency of
the transmitted signal. MLT3 switches between three output voltage levels so that peak power is
shifted to less than 20 MHz.
• Equalization—Equalization boosts the higher frequency signals for transmission over UTP.
Equalization can be done on the transmitter (predistortion), at the receiver (postcompensation), or
both. One advantage of equalization at the receiver is the ability to adjust compensation as a function
of cable length.
Troubleshooting FDDI
This section provides troubleshooting procedures for common FDDI media problems.
Table 5-1 outlines problems commonly encountered on FDDI networks and offers general guidelines for
solving those problems.
Table 5-1 Media Problems: FDDI
Media Problem Suggested Actions
Nonfunctional
FDDI ring
1. Use the show interfaces fddi exec command to determine the
status of the router’s FDDI interfaces.
2. If the show interfaces fddi command indicates that the
interface and line protocol are up, use the ping command
between routers to test connectivity.
3. If the interface and line protocol are up, make sure the MAC
addresses of upstream and downstream neighbors are as
expected.
4. If all zeros appear in either of the address fields for these
neighbors, there is probably a physical connection problem.
In this case (or if the status line does not indicate that the interface
and line protocol are up), check patch-panel connections or use an
OTDR
1
or light meter to check connectivity between neighbors.
Ensure that signal strength is within specifications.
Upstream
neighbor has
failed and
bypass switch
is installed
Bypass switches can cause signal degradation because they do not
repeat signals as a normal transceiver does.
1. Check upstream neighbor to determine whether it is
operational.
2. If the node is down and a bypass switch is in place, resolve any
problems found in the upstream neighbor.
1. OTDR = optical time-domain reflectometer
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Troubleshooting FDDI
When you’re troubleshooting FDDI media in a Cisco router environment, the show interfaces fddi
command provides several key fields of information that can assist in isolating problems. The following
section provides detailed description of the show interfaces fddi command and the information it
provides.
show interfaces fddi
To display information about the FDDI interface, use the show interfaces fddi exec command:
show interfaces fddi number [accounting]
show interfaces fddi [slot | port] [accounting] (Cisco 7000 series and Cisco 7200 series)
show interfaces fddi [slot | port-adapter | port] [accounting] (Cisco 7500 series routers)
Syntax Description
• number—Port number on the selected interface.
• accounting—(Optional) Displays the number of packets of each protocol type that have been sent
through the interface.
• slot—Refers to the appropriate hardware manual for slot and port information.
• port—Refers to the appropriate hardware manual for slot and port information.
• port-adapter—Refers to the appropriate hardware manual for information about port adapter
compatibility.
Command Mode
exec
Usage Guidelines
This command first appeared in Cisco IOS Release 10.0.
This information was modified in Cisco IOS Release 11.3 to include sample output for FDDI
full-duplex, single-mode, and multimode port adapters (PA-F/FD-SM and PA-F/FD-MM).
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Sample Displays
The following is a sample partial display of FDDI-specific data from the show interfaces fddi command
on a Cisco 7500 series router:
Router> show interfaces fddi 3/0
Fddi3/0 is up, line protocol is up
Hardware is cxBus Fddi, address is 0000.0c02.adf1 (bia 0000.0c02.adf1)
Internet address is 131.108.33.14, subnet mask is 255.255.255.0
MTU 4470 bytes, BW 100000 Kbit, DLY 100 usec, rely 255/255, load 1/255
Encapsulation SNAP, loopback not set, keepalive not set
ARP type: SNAP, ARP Timeout 4:00:00
Phy-A state is active, neighbor is B, cmt signal bits 008/20C, status ILS
Phy-B state is active, neighbor is A, cmt signal bits 20C/008, status ILS
ECM is in, CFM is thru, — is ring_op
Token rotation 5000 usec, ring operational 21:32:34
Upstream neighbor 0000.0c02.ba83, downstream neighbor 0000.0c02.ba83
Last input 0:00:05, output 0:00:00, output hang never
Last clearing of “show interface” counters 0:59:10
Output queue 0/40, 0 drops; input queue 0/75, 0 drops
Five minute input rate 69000 bits/sec, 44 packets/sec
Five minute output rate 0 bits/sec, 1 packets/sec
113157 packets input, 21622582 bytes, 0 no buffer
Received 276 broadcasts, 0 runts, 0 giants
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
4740 packets output, 487346 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets, 0 restarts
0 transitions, 2 traces, 3 claims, 2 beacons
The following is a sample display of the show interfaces fddi command for the full-duplex FDDI port
adapter on a Cisco 7500 series router:
Router# show interfaces fddi 0/1/0
Fddi0/1/0 is up, line protocol is up
Hardware is cxBus FDDI, address is 0060.3e33.3608 (bia 0060.3e33.3608)
Internet address is 2.1.1.1/24
MTU 4470 bytes, BW 100000 Kbit, DLY 100 usec, rely 255/255, load 1/255
Encapsulation SNAP, loopback not set, keepalive not set
ARP type: SNAP, ARP Timeout 04:00:00
FDX supported, FDX enabled, FDX state is operation
Phy-A state is maintenance, neighbor is Unknown, status HLS
Phy-B state is active, neighbor is A, status SILS
ECM is in, CFM is c_wrap_b, — is ring_op,
Requested token rotation 5000 usec, negotiated 4997 usec
Configured tvx is 2500 usec
LER for PortA = 0A, LER for PortB = 0A ring operational 00:02:45
Upstream neighbor 0060.3e73.4600, downstream neighbor 0060.3e73.4600
Last input 00:00:12, output 00:00:13, output hang never
Last clearing of “show interface” counters never
Queueing strategy: fifo
Output queue 0/40, 0 drops; input queue 0/75, 0 drops
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
62 packets input, 6024 bytes, 0 no buffer
Received 18 broadcasts, 0 runts, 0 giants
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
71 packets output, 4961 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
0 output buffer failures, 0 output buffers swapped out
3 transitions, 0 traces, 100 claims, 0 beacon
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Table 5-2 describes the show interfaces fddi display fields.
Table 5-2 show interfaces fddi Field Descriptions
Field Description
Fddi is {up | down |
administratively down}
Gives the interface processor unit number and tells
whether the interface hardware is currently active and can
transmit and receive or whether it has been taken down by
an administrator.
line protocol is
{up | down}
Indicates whether the software processes that handle the
line protocol consider the interface usable.
Hardware Provides the hardware type, followed by the hardware
address.
Internet address IP address, followed by subnet mask.
MTU Maximum transmission unit of the interface.
BW Bandwidth of the interface in kilobits per second.
DLY Delay of the interface in microseconds.
rely Reliability of the interface as a fraction of 255 (255/255
is 100 percent reliability), calculated as an exponential
average of over five minutes.
load Load on the interface as a fraction of 255 (255/255 is
completely saturated), calculated as an exponential
average of over five minutes.
Encapsulation Encapsulation method assigned to interface.
loopback Indicates whether loopback is set.
keepalive Indicates whether keepalives are set.
ARP type Type of Address Resolution Protocol assigned.
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FDX Displays full-duplex information. Values are not
supported and supported. When the value is supported,
the display indicates whether full-duplex is enabled or
disabled. When enabled, the state of the FDX negotiation
process is displayed. The negotiation states only relate to
the full-duplex negotiation process. You must also ensure
that the interface is up and working by looking at other
fields in the show interfaces fddi command such as line
protocol and —. Negotiation states are
• idle—Interface is working but not in full-duplex
mode yet. If persistent, it could mean that the
interface did not meet all negotiation conditions (for
example, there are more than two stations in the ring).
• request—Interface is working but not in full-duplex
mode yet. If persistent, it could mean that the remote
interface does not support full-duplex or full-duplex
is not enabled on the interface.
• confirm—Transient state.
• operation—Negotiations completed successfully,
and both stations are operating in full-duplex mode.
Phy-{A | B} Lists the state the Physical A or Physical B connection is
in; one of the following: off, active, trace, connect, next,
signal, join, verify, or break.
Table 5-2 show interfaces fddi Field Descriptions (continued)
Field Description
continues
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neighbor State of the neighbor:
A—Indicates that the CMT
1
process has established a
connection with its neighbor. The bits received during the
CMT signaling process indicate that the neighbor is a
Physical A type DAS
2
or concentrator that attaches to the
primary ring IN and the secondary ring OUT when
attaching to the dual ring.
S—Indicates that the CMT process has established a
connection with its neighbor and that the bits received
during the CMT signaling process indicate that the
neighbor is one Physical type in a single-attachment
station SAS.
3
B—Indicates that the CMT process has established a
connection with its neighbor and that the bits received
during the CMT signaling process indicate that the
neighbor is a Physical B dual attachment station or
concentrator that attaches to the secondary ring IN and the
primary ring OUT when attaching to the dual ring.
M—Indicates that the CMT process has established a
connection with its neighbor and that the bits received
during the CMT signaling process indicate that the
router’s neighbor is a Physical M-type concentrator
serving as a master to a connected station or concentrator.
unk—Indicates that the network server has not completed
the CMT process and, as a result, does not know about its
neighbor.
cmt signal bits Shows the transmitted/received CMT bits. The
transmitted bits are 0x008 for a Physical A type and
0x20C for Physical B type. The number after the slash (/)
is the received signal bits. If the connection is not active,
the received bits are zero (0); see the line beginning
Phy-B in the display. This applies to FDDI processor FIP
4
interfaces only.
status Status value displayed is the actual status on the fiber. The
FDDI standard defines the following values:
• LSU—Line state unknown, the criteria for entering
or remaining in any other line state have not been
met.
• NLS—Noise line state, entered upon the occurrence
of 16 potential noise events without satisfying the
criteria for entry into another line state.
• MLS—Master line state, entered upon the receipt of
eight or nine consecutive HQ or QH symbol pairs.
Table 5-2 show interfaces fddi Field Descriptions (continued)
Field Description
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status (continued) • ILS—Idle line state, entered upon receipt of four or
five idle symbols.
• HLS—Halt line state, entered upon the receipt of 16
or 17 consecutive H symbols.
• QLS—Quiet line state, entered upon the receipt of 16
or 17 consecutive Q symbols or when carrier detect
goes low.
• ALS—Active line state, entered upon receipt of a JK
symbol pair when carrier detect is high.
• OVUF—Elasticity buffer overflow/underflow. The
normal states for a connected Physical type are ILS
or ALS. If the report displays the QLS status, this
indicates that the fiber is disconnected from Physical
B, or that it is not connected to another Physical type,
or that the other station is not running.
ECM is... ECM is the SMT entity coordination management, which
overlooks the operation of CFM and PCM. The ECM state
can be one of the following:
• out—Router is isolated from the network.
• in—Router is actively connected to the network. This
is the normal state for a connected router.
• trace—Router is trying to localize a stuck beacon
condition.
• leave—Router is allowing time for all the
connections to break before leaving the network.
• path_test—Router is testing its internal paths.
• insert—Router is allowing time for the optical
bypass to insert.
• check—Router is making sure optical bypasses
switched correctly.
• deinsert—Router is allowing time for the optical
bypass to deinsert.
Table 5-2 show interfaces fddi Field Descriptions (continued)
Field Description
continues
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CFM is... Contains information about the current state of the MAC
connection. The configuration management state can be
one of the following:
• isolated—MAC is not attached to any Physical type.
• wrap_a—MAC is attached to Physical A. Data is
received on Physical A and transmitted on Physical
A.
• wrap_b—MAC is attached to Physical B. Data is
received on Physical B and transmitted on Physical
B.
• wrap_s—MAC is attached to Physical S. Data is
received on Physical S and transmitted on Physical S.
This is the normal mode for a SAS.
— is... — (ring management) is the SMT MAC-related state
machine. The — state can be one of the following:
• isolated—MAC is not trying to participate in the
ring. This is the initial state.
• non_op—MAC is participating in ring recovery, and
ring is not operational.
• ring_op—MAC is participating in an operational
ring. This is the normal state while the MAC is
connected to the ring.
• detect—Ring has been nonoperational for longer
than normal. Duplicate address conditions are being
checked.
• non_op_dup—Indications have been received that
the address of the MAC is a duplicate of another
MAC on the ring. Ring is not operational.
• ring_op_dup—Indications have been received that
the address of the MAC is a duplicate of another
MAC on the ring. Ring is operational in this state.
• directed—MAC is sending beacon frames notifying
the ring of the stuck condition.
• trace—Trace has been initiated by this MAC, and the
— state machine is waiting for its completion before
starting an internal path test.
token rotation Token rotation value is the default or configured rotation
value as determined by the fddi token-rotation-time
command. This value is used by all stations on the ring.
The default is 5,000 microseconds. For FDDI full-duplex,
this indicates the value in use prior to entering full-duplex
operation.
negotiated Actual (negotiated) target token rotation time.
Table 5-2 show interfaces fddi Field Descriptions (continued)
Field Description
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ring operational When the ring is operational, the displayed value will be
the negotiated token rotation time of all stations on the
ring. Operational times are displayed by the number of
hours/minutes/seconds the ring has been up. If the ring is
not operational, the message “ring not operational” is
displayed.
Configured tvx Transmission timer.
LER Link error rate.
Upstream | downstream
neighbor
Displays the canonical MAC address of outgoing
upstream and downstream neighbors. If the address is
unknown, the value will be the FDDI unknown address
(0x00 00 f 8 00 00 00).
Last input Number of hours, minutes, and seconds since the last
packet was successfully received by an interface. Useful
for knowing when a dead interface failed.
output Number of hours, minutes, and seconds since the last
packet was successfully transmitted by an interface.
output hang Number of hours, minutes, and seconds (or never) since
the interface was last reset because of a transmission that
took too long. When the number of hours in any of the
“last” fields exceeds 24 hours, the number of days and
hours is printed. If that field overflows, asterisks are
printed.
Last clearing Time at which the counters that measure cumulative
statistics (such as number of bytes transmitted and
received) shown in this report were last reset to zero. Note
that variables that might affect routing (for example, load
and reliability) are not cleared when the counters are
cleared.
*** indicates the elapsed time is too large to be displayed.
0:00:00 indicates the counters were cleared more than 231
ms (and less than 232 ms) ago.
Queueing strategy First-in, first-out queuing strategy (other queueing
strategies you might see are priority-list, custom-list, and
weighted fair).
Output queue, input
queue,
drops
Number of packets in output and input queues. Each
number is followed by a slash, the maximum size of the
queue, and the number of packets dropped due to a full
queue.
Table 5-2 show interfaces fddi Field Descriptions (continued)
Field Description
continues
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Chapter Troubleshooting Fiber Distributed Data Interface
Troubleshooting FDDI
5 minute input rate,
5 minute output rate
Average number of bits and packets transmitted per
second in the past five minutes.
The five-minute input and output rates should be used
only as an approximation of traffic per second during a
given five-minute period. 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.
packets input Total number of error-free packets received by the system.
bytes Total number of bytes, including data and MAC
encapsulation, in the error-free packets received by the
system.
no buffer Number of received packets discarded because there was
no buffer space in the main system. Compare with ignored
count. Broadcast storms on Ethernet networks and bursts
of noise on serial lines are often responsible for no input
buffer events.
broadcasts Total number of broadcast or multicast packets received
by the interface.
runts Number of packets that are discarded because they are
smaller than the medium’s minimum packet size.
giants Number of packets that are discarded because they exceed
the medium’s maximum packet size.
CRC Cyclic redundancy checksum generated by the originating
LAN station or far-end device does not match the
checksum calculated from the data received. 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 collisions or a station
transmitting bad data.
frame Number of packets received incorrectly that have a CRC
error and a noninteger number of octets. On a LAN, this
is usually the result of collisions or a malfunctioning
Ethernet device. On an FDDI LAN, this also can be the
result of a failing fiber (cracks) or a hardware
malfunction.
overrun Number of times the serial receiver hardware was unable
to hand received data to a hardware buffer because the
input rate exceeded the receiver’s ability to handle the
data.
Table 5-2 show interfaces fddi Field Descriptions (continued)
Field Description
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Internetworking Troubleshooting Handbook, Second Edition
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Chapter Troubleshooting Fiber Distributed Data Interface
Troubleshooting FDDI
ignored Number of received packets ignored by the interface
because the interface hardware ran low on internal
buffers. These buffers are different from the system
buffers mentioned previously in the buffer description.
Broadcast storms and bursts of noise can cause the
ignored count to be increased.
packets output Total number of messages transmitted by the system.
bytes Total number of bytes, including data and MAC
encapsulation, transmitted by the system.
underruns Number of transmit aborts (when the router cannot feed
the transmitter fast enough).
output errors Sum of all errors that prevented the final transmission of
datagrams out of the interface being examined. Note that
this might not balance with the sum of the enumerated
output errors because some datagrams can have more than
one error and others can have errors that do not fall into
any of the specifically tabulated categories.
collisions Because an FDDI ring cannot have collisions, this
statistic is always zero.
interface resets Number of times an interface has been reset. The interface
may be reset by the administrator or automatically when
an internal error occurs.
restarts Should always be zero for FDDI interfaces.
output buffer failures Number of no-resource errors received on the output.
output buffers swapped
out
Number of packets swapped to DRAM.
transitions Number of times the ring made a transition from ring
operational to ring nonoperational, or vice versa. A large
number of transitions indicates a problem with the ring or
the interface.
traces Indicates the number of times this interface started a
trace. Trace count applies to both the FCI, FCIT, and FIP.
claims Pertains to FCIT and FIP only. Indicates the number of
times this interface has been in claim state.
beacons Pertains to FCIT and FIP only. Indicates the number of
times the interface has been in beacon state.
1. CMT =connection management
2. DAS =dual-attachment station
3. SAS =single-attachment station
4. FIP =FDDI processor
Table 5-2 show interfaces fddi Field Descriptions (continued)
Field Description
