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Cisco Internetworking Troubleshooting
General Network Management
Criteria:
- Fault management
- Performance management
- Configuration and device
management
- Accounting management
- Security
management
SMT (Station Management: ANSI FDDI specification that defines
how ring stations are managed) entity coordination/management states:
- Responsible for overlooking the operation of CEM (Configuration Management
Elements, not to confuse with Cisco email Manager) and PCM (Physical Connection
Management)
- Out – router is isolated from network
- In –
router is connected to the network
- Trace – router tries to localize a
stuck beacon
- Leave – router allows all connections to break before
leaving the network
- Path_test – router tests its internal
paths
- Insert – router ready for optical bypass process to perform
insert operation
- Deinsert – router ready for the optical bypass
process to perform de-insert operation
- Check – ensures optical
bypasses are switched correctly
Analyzing a Network Problem:
The first step is to make a clear problem statement indicating
symptoms and potential causes. After defining the possibilities, gather facts by
asking users questions and collecting information from sources such as network
management systems, protocol analyzers, output from router diagnostic commands
or software release notes, etc. Then isolate the problem to one device by
changing only one variable at a time until the problem is resolved. If the
changes made do not solve the problem then undo all the changes and redefine the
problem statement.
Typical Troubleshooting Commands for Router / Switch
Show controllers Ethernet [interface number]
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To display statistics like missing datagrams, memory errors,
buffer errors, and overflow errors for an Ethernet interface on a Cisco router
(Please note this command will not give you information on internal hardware
errors)
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Show flash
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To display the router images stored in NVRAM
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Show buffers
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To display statistics for the buffer pools in router (Please
note that the router has one pool of queuing elements and five pools of packet
buffers of different sizes. Network server keeps counts of the number of buffers
outstanding, the number of buffers in the free list, and the maximum number of
buffers allowed in the free list for each pool)
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Show processes
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To display information about the active processes in a router
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Show tech-support
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To show tech support router conditions
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Show version
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To show current status of router
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Debug ip rip
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To check if RIP routing is operating properly
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Show ip route
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To display the entries in the routing table
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Show version
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To obtain a router’s firmware version
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Show running-config
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To obtain a router’s current configuration
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Show interface fddi 3/0
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To display an upstream neighbor value of 0000 0000 0000 for
FDDI (Keep in mind that 0000 0000 0000 means the upstream neighbor is unknown,
indicating that a physical problem is likely to have occurred)
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Debug isdn q921 / debug isdn q931
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To troubleshoot ISDN BRI layer 2 and 3
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Show controller bri
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To view information specifically about the D channel of a BRI
line
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Logging console level
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To send debugging output to the consol
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Show frame-relay
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To obtain statistics about a PVC (Permanent Virtual Circuit)
on all Frame Relay interfaces
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Other commands
- The “trace” command works by using an error message generated
by routers when a datagram exceeds its time-to-live value. It displays the
router round-trip time for each probe.
- The “service timestamps”
command puts a date and time in the log in order to tell how much time has
elapsed between events.
Troubleshooting Tools
Break out boxes / BERTS (Bit Rate Error Testers) / Fox boxes
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Troubleshoot peripheral interfaces by monitoring data line
conditions, analyzing and trapping data, and diagnosing problems common to data
communication systems
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Volt Ohm meter
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Measures the physical properties including current,
resistance, capacitance, and cable continuity
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Cable tester
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Checks physical connectivity on STP, UTP, 10BaseT, coaxial,
and a special cable type called twinax-cable
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Time Domain Reflectors
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Troubleshoots crimps, kinks, impedance, bends, and other
defects in metallic cables by measuring how much time it takes the signal to
reflect and calculating the distance to a fault
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Optical Time Domain Reflectors
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Troubleshoots optical cable
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Network Monitors
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Determines a baseline and establish trends in the
networks
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Network Analyzer
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Decodes the various layers in a frame and presents them as
summaries detailing which layer is involved
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TrafficDirector
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Collects RMON information
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Stack Decoder
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Pastes in the output of a “Show stack” command
after an error. The stack decoder tool will provide meaningful comments in the
stack trace
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Protocol analyzers
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Display packet data
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CCO bug toolkit resources
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Bug Navigator, Bug Alert and Bug Watcher
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NetSYS
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Simulates network changes in a virtual environment
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Cisco website
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The site has a technical database, an open question-and-
answer forum, a mailing-list archive, a troubleshooting assistant, a software
bug toolkit, accesspath configuration tools, an IP subnet calculator, a stack
decoder, a 3600 memory calculator, TAC Case instructions, and Cisco products for
purchase online
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CiscoWorks
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Internetwork management software that works with SNMP. It can
monitor devices for environmental and interface statistics, display information
about the health of a device, view data similar to the output of a “show
exec” command, display and analyze the path between two devices, probe and
extract data about the condition of the network, dynamically monitor and
troubleshoot using graphs of device statistics and comprehensive configuration
information, gather historical data for analysis, and create detailed maps which
you can provide to CiscoTAC for assistance in debugging your network
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CiscoTrafficDirector
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Remote monitoring tools to gather data, monitor activity on
your network and find potential problems, and allows users to monitor all seven
layers of the OSI model
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VLAN Director
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Provides an accurate representation of the physical network,
has the capability to find discrepancy on conflicting ports, can quick detect
changes in VLAN status and switch ports, and has user authentication and write
protection security
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Troubleshooting Engine
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Provides simulations of failures and allows users to test
possible solutions (CSEs (Customer Support Engineers) often connect to the
Troubleshooting Engine to network hardware in the TAC (Technical Assistance
Center) lab)
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Sales Tool Central - Troubleshooting Assistance
Physical Router Hardware Problems
Green LED
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Port is operational
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Orange LED
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Link disabled
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LED flashes orange
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Hardware failure
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Off LED
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No signal
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- Cabling: to eliminate possible cable breaks or cable plant & punch down
connections, try to replace cable with a good external cable.
- Dialing: if
the incorrect cable is used, the router may never attempt to dial; if the speed
is configured incorrectly, the router will dial but not connect.
- Power
system: power supply + wiring + system cables (including all external cables
that connect to the router) + cooling system + blower assembly. Note that some
models have power supply redundancy, and some are even hot
swappable.
- Emulator traps: the processor has executed an illegal
instruction, caused by either the software taking illegal branches or by
hardware failures
- Connection: misconfigured CHAP allows connection but not
authentication; misconfigured route does not allow traffic to reach the remote
end.
- Processor timers: guard against certain types of system hangs. The
watchdog timer must be periodically reset, or a trap will occur (i.e. something
wrong).
- Input errors: error occurred while the data was in
transit.
- Parity errors: internal hardware error checks failed, likely to be
a hardware problem.
- Bus errors: processor tries to use a device or a memory
location that either does not exist or does not respond properly.
- Address
errors: software tries to access data on an incorrectly aligned
boundary.
Configuration Issues
- Bad hop count: when there is a high number of packets with a bad hop count
(i.e., packets were discarded because their hop count exceeded 16), a possible
cause is a backdoor bridge between segments (when Spanning Tree was
disabled).
- Asymmetric VS symmetric flow control: in an asymmetric model the
local port performs flow control of the remote port. If the local port is
congested, it can request the remote port to stop transmitting until the
congestion is clear. In a symmetric model, the local port will perform flow
control only if the remote port can perform flow control.
- Spanning Tree
Protocol: if no information has been received by the end of a forwarding delay,
the port returns to a learning state. As a result of new BPDU information, a
previously blocked port may now be the root port or the designated port for a
given segment. Rather than move directly from the blocked state to the
forwarding state, ports go through two intermediate states – the listening
state and then the learning state. At the end of a second forwarding delay time,
the port switches from the learning state to the forwarding state, thereby
allowing frames to be received and forwarded at the port.
- Booting: a ROM IOS
image is relatively old, so is not desirable. Net booting is acceptable only if
the server is reliable. Booting from Flash is the fastest, but you still need an
alternate boot path setup in the event that your flash becomes corrupt. The
recommended order to boot images is: Flash, network, ROM.
- IRDP (ICMP Router
Discovery Protocol): IRDP uses router advertisements and router solicitation
messages to discover the addresses of routers on directly attached subnets.
Sometimes a host will receive an ICMP redirect to another destination if the
host uses a router with a poor metric to reach a destination.
- Multicasting:
used when a single packet needs to be sent to multiple destinations. Three ways
to multicast are UDP flooding (useful for optimal traffic flow throughout an
Internetwork), subnet broadcast (may lead to packet duplication), and IGMP
(relies on class D IP addresses for the creation of multicast
groups).
- Routing: a route is learned through the wrong interface is caused
by a disabled split horizon. Split horizon allows routes to be propagated
through interfaces other than the one it came from.
- Serial line over
utilization: controlling how the router uses buffers can (to a certain extent)
resolve the problem. Cisco routers allocate different size buffers. When a
buffer is needed and no existing buffer is available in the free list, a new
buffer is created. The count created by the show buffers keeps track of the
number of newly created buffers. Of course, a buffer is a limited resource, so
if the usage is really too heavy, no matter how you adjust it, the buffer
problem will still exist.
- When a router crashes, obtain a full copy of the
core dump and let your technical support representative identify the
cause.
- Critical error messages: displayed to console. If you set “no
logging on” – this will disable logging to all other
destinations.
- Access lists: must be put in place intentionally. Other
possibilities to an access-list problem are addressing and sub-netting problems.
If connection attempts to certain applications succeed while others fail, try
the “show running-config” command and determine which access lists
cause the problems. Finally, disable the troublemakers.
- Novell servers: use
a internal IPX network number that is unique throughout the entire Internetwork.
Novell servers autodetect network numbers and frame types during installation.
Problems may arise when a Novell server is moved from one segment to another.
By default Cisco routers use 802.3 encapsulation for IPX. Watch out for
incorrect encapsulation type settings.
- SNMP: causes significant impact on
the network due to its almost continuous amount of traffic to the management
station.
- LOOPBACK test: to troubleshoot a HDLC (High-level Data Link
Control) or PPP (Point-to-Point Protocol) link, put the CSU/DSU in loop-back
mode and issue a “show interfaces serial” exec command. This command
will check whether the line status changes from “line protocol is
down” to “line protocol is up (looped)” or if it remains down.
The bottom line is the keep-alive counter should increment. Keep in mind though,
there is no loopback in X.25 or Frame-Relay packet- switched
networks.
- System image corruption troubleshooting steps:
- Power cycle the router
- Press the break key within 60 seconds of booting
- In ROM monitor enter “o/r 0x1” to set the configuration register to boot
from ROM (1 to reinitialize the router and obtain the correct system image via TFTP)
- Fix the configuration as necessary
- Enter “boot system flash” to change the configuration register to boot from flash memory
instead of ROM
- Modem configuration: If you are using IOS Release 11.1 or
later, configure your Cisco router to communicate with and configure your modem
automatically ( use the modem “autoconfigure discovery line”
configuration command). To display the list of modems for which the router has
entries, use “show modemcap modem-name.” If you want to change a
modem value, use “modemcap edit modem-name attribute value line”
configuration command.
Counters and Measurements
- Runts: the number of packets discarded because they are smaller than the
medium’s minimum packet size.
- Underruns: the number of times
transmitter has been running faster than the router can handle.
- Ignored -
number of received packets ignored by the interface because the interface ran
low on internal buffers (not system buffer). The usual causes are broadcast
storms and bursts of noise.
- Interface reset: the number of times the
interface has been completely reset. This can happen if packets queued for
transmission were not sent within several seconds, if a serial line a
malfunctioning modem is not supplying a transmit clock signal, or if there is a
cable problem. Sometimes interface resets can occur when an interface is looped
back or shut down.
- Packets pitched: the number of times a router has
received its own broadcast packets
- CRC (Cyclic Redundancy Check): the number
of times the interface receives packets that fail the Cyclic Redundancy
Checksum. On a LAN this usually indicates noise, transmission problems, or a
station transmitting bad data.
- Transition counter: the number of times the
ring made a transition from up to down or vice versa. A large number indicates a
problem with the ring or the interface.
Switching mode and Cisco 7000 series VIP (Versatile Interface
Processor):
- When debugging is on, the mode is process switched
- Based on RISC engine
- One or two port adapters or daughter boards may be attached to a
VIP
- Able to receive route information from the master RSP
- Able to make
its own autonomous, multiplayer-switching decisions for distributed
switching
- Supports mixed media
Troubleshooting for Serial Lines
- Show interfaces serial - displays information specific to serial
interfaces.
- Show controllers - for serial interfaces on Cisco 7000 series
routers use “show controllers cbus.” For the Access series products
use “show controllers.” For the AGS, CGS, and MGS, use “show
controllers mci.”
- Incorrect DSU or CSU configuration can lead to
clocking problem. You can examine the interface and see if CRC, framing errors,
and aborts are exceeding an approximate range of 0.5% to 2.0% of
traffic.
- Excessively high bandwidth utilization (over 70%) results in
reduced performance and possible intermittent failures. Before increasing the
bandwidth, you can adjust the router data buffer to help
temporarily.
Troubleshooting for ISDN
- Commands:
- DEBUG DIALER – debug dial-on-demand routing information.
- DEBUG ISDN EVENT – debug ISDN activity occurring on the
user side of the ISDN interface.
- DEBUG ISDN Q931 – debug information about call setup and teardown of ISDN network connections between local router
and network.
- DEBUG ISDN Q921 – debug data link layer (Layer 2) access
procedures that take place at the router on D-channel.
- DEBUG PPP NEGOTIATION – debug negotiation of Point-to-Point Protocol options and Network Control
Protocol parameters.
- DEBUG PPP AUTHENTICATION - exchange of Challenge
Authentication Protocol and Password Authentication Protocol packets.
- Note: in North America, all ISDN lines are connected to the ISDN switch at the local
Central Office via a U interface, while in Europe you need an S/T
interface.
Troubleshooting ATM
- Allowed encapsulation methods: AAL5, PVC, SVC.
- ISL contains header,
original packet, and FCS fields.
- ISL not directly supported by ATM, but can
be implemented in ATM LANE configuration.
- Virtual LAN ID (called the COLOR)
is 15 bits, which is different for each VLAN.
- Packets on the ISL trunk use:
Debug vlan packet.
Troubleshooting Frame Relay
- Typical problems: frame relay link is down, cannot ping remote router, or
cannot ping end-to-end.
- Output from “show interfaces serial” can
show if the interface and line protocol are down or that the interface is up and
the line protocol is down.
- Ensure that both Cisco devices are using IETF
encapsulation method - check it out with the “show frame-relay map”
command.
- Ensure proper frame relay address mapping by using “show
frame-relay map.”
Troubleshooting FDDI
- FDDI has two rings with data traveling in opposite directions. One ring is
called the primary ring while the other is the secondary ring:
- Primary ring for data transmission, and
- Secondary ring for backup.
- Physical architecture: two or more point-to-point connections between adjacent
stations.
- Four separate specifications:
- MAC (Media Access Control) -
how the medium is accessed including frame format, methods for error detection
(CRC), and error correction.
- PHY (Physical Sub-layer) - data encoding and
decoding procedures, clocking requirements, and framing.
- PMD (Physical
Medium Dependent) - transmission medium characteristics including power levels,
bit error rates and optical components and connectors.
- SMT (Station
Management) - defines FDDI station, ring configuration, and ring
control.
- Ring control functions include station insertion and removal,
initialization, fault isolation and recovery, scheduling, and collection of
statistics.
- Supports both synchronous and asynchronous traffic
management:
- Synchronous - fully utilizes the network by using a reserved
token, best suited for high-demand, low-latency applications such as voice and
video.
- Asynchronous - bandwidth is allocated using an eight-level priority
scheme and using what is left over after all devices have been allocated
synchronous bandwidth. Usually used for continuous stream of data. Please note
that FDDI permits extended dialogs to allow stations to temporarily use all
available asynchronous traffic.
- Dual ring: if a station on the dual ring
fails the dual ring is automatically doubled back on itself into a single ring.
The problem for a big FDDI is multiple failures occur in multiple areas and will
create isolated rings that cannot talk to each other.
- Optical bypass
switches can prevent ring segmentation by eliminating failed stations from the
ring.
- FDDI port statuses:
- ‘A’ - upstream neighbor is a Physical A type DAS.
- ‘B’ - upstream neighbor is a Physical B type DAS.
- ‘S’ - upstream neighbor is a Physical A type SAS.
- ‘M’ - neighbor is a physical M type concentrator serving as
a master to a connection station or concentrator.
- ‘UNK’ - network server has not completed the CMT process – cannot find out about
its neighbor.
- Valid states of the Physical A or Physical B interface are
Off, Active, Trace, Connect, Next, Signal, Join, Verify, or Break.
- To
troubleshoot FDDI ring use the command “show interfaces fddi.” If
both neighbors appear as normal, use ping to test connectivity. If either
neighbor has only zeros in the address field, then try using an OTDR (Optical
Time Domain Reflectometer) or light meter to test for physical
connectivity.
- A fail-over to a bypass switch: bypass switches do not
actually repeat signals.
- Frames: tokens and data/command frames. A token is
not a frame type, but a “field” within a frame. Each token is three
bytes in length with a start delimiter, an access control byte, and end
delimiter. Data / command frames vary in size.
Troubleshoot VLAN
- To configure a VLAN use RSM in the switch or attach a router to a VLAN
trunking port using ISL encapsulation.
- Incorrect VLAN trunking protocol
configuration will cause a VLAN to be slow or non-operational.When a line
protocol Frame Relay is down, check for timing problems with myseq and myseen
keep-alive events, command to show: debug serial
interface.
Troubleshooting AppleTalk
- Debug apple events EXEC command: displays information about AppleTalk
special events or to find out if neighbors become reachable/unreachable or
interfaces go up/down.
- AppleTalk Data Stream Protocol: guarantees that data
bytes are delivered in the same order as they are sent and that they are not
duplicated.
- AppleTalk Session Protocol: establishes and maintains logical
conversations between an AppleTalk client and a server. ASP is considered a
session layer protocol.
- AppleTalk Printer Access Protocol: a
connection-oriented protocol responsible for establishing and maintaining
connections between clients and servers.
- AppleTalk Filing Protocol: allows a
client to share server files across a network.
- AppleTalk Update-Based
Routing Protocol: allows network administrator to connect two or more AppleTalk
Internetworks through a foreign network.
- Datagram Delivery Protocol -
provides connectionless service between network sockets that can be assigned
statically or dynamically. AppleTalk addresses are administered by the DDP and
are made up of two components: a 16-bit network number and a 8-bit node
number.
- AppleTalk’s Name Binding Protocol (NBP): associates AppleTalk
names with addresses. AppleTalk’s node addresses are assigned dynamically.
When a Macintosh starts up it will choose a network layer protocol address and
try to find out whether that address is currently in use. If it cannot the new
node will assign itself an address.
- AppleTalk’s transport layer
consists of:
- Routing Table Maintenance Protocol (RTMP)
- AppleTalk Update-Based Routing Protocol (AURP)
- AppleTalk Echo Protocol (AEP)
- AppleTalk Transaction Protocol (ATP)
- Name Binding Protocol (NBP).
- EtherTalk: AppleTalk over Ethernet.
- TokenTalk: AppleTalk over
Token Ring.
- FDDITalk: AppleTalk over FDDI.
- LocalTalk: AppleTalk’s
proprietary media-access system based on contention access, bus topology, and
baseband signaling running on shielded twisted-pair media at 230.4kbps. Maximum
span of up to 300 meters and support up to 32 nodes.
- Zones: defined by the
AppleTalk network manager during the router configuration process. Every node
belongs to a single specific zone.
- AppleTalk phase II: extended network
having multiple zones.
- Debug apple zip: command to report the discovery of
new zones.
Troubleshooting IPX
- NetWare specifies the upper five layers of the OSI reference
model.
- Remote access transparent to user through remote procedure
calls.
- LAN - runs on Ethernet/IEEE 802.3, Token Ring/IEEE 802.5, Fiber
Distributed Data Interface (FDDI), and ARCnet.
- WAN - Point-to-Point
Protocol.
- IPX uses RIP to route packets in an Internetwork.
- SAP allows
nodes that provide services to advertise their addresses and the services they
provide.
- Supports IBM logical unit (LU) 6.2 network addressable units (NAUs)
for peer-to-peer connectivity across IBM communication environments.
- SPX -
transport layer protocol, reliable and connection-oriented.
- Internet
Protocol is supported in the form of User Datagram Protocol.
- NCP - services
provided include file access, printer access, name management, accounting,
security, and file synchronization.
- NetBIOS session-layer interface
specification is supported!
- Typical problem for IPX network: Misconfigured
client or server, not enough user licenses, mismatched network numbers (all
servers on the same LAN must have the same external network number if they use
the same frame type).
- Other common problems are:
- Router interface is down (use show interfaces)
- Mismatched Ethernet encapsulation methods (use show ipx interface)
- Ring speed specification mismatch (use the show interfaces token command)
- Duplicate node numbers on routers (use ipx routing node)
- Duplicate network numbers (use show ipx servers and show ipx route), and
- Backdoor bridge between segments (use show ipx traffic).
- If you are using NetWare 3.12 or
above and you have LIPX enabled, a client and server could conceivably negotiate
a packet size larger than a router could support, causing intermediate routers
to drop packets.
- RIP timer-value mismatches between routers and servers can
cause connectivity problems – use show ipx interfaces to view the state of
IPX interfaces. Timer values configured on servers and routers should be the
same across the whole IPX network, so use the “ipx update-time”
interface configuration command to change the RIP timer interval.
- When
Novell SAP packets are not forwarded through a router running IPX RIP, it may be
due to a timer mismatch or a server problem / access list
misconfiguration.
Troubleshoot Transparent bridges
- First developed by DEC.
- Their presence and operation are transparent to
network hosts.
- Learn network's topology by analyzing the source address of
incoming frames from all attached networks.
- Sees its table as the basis for
traffic forwarding.
- Without a bridge-to-bridge protocol, the transparent
bridge algorithm fails when there are multiple paths.
- Use “show
bridge" to see whether there is a connectivity problem and to make sure that the
bridging table includes the MAC addresses of attached end nodes.
- Use
“debug spanning-tree” to see whether spanning-tree hello frames are
being exchanged.
Connectionless Network Service - CLNS
- Implemented by using the
Connectionless Network Protocol (CLNP) and Connectionless Network Service
(CLNS).
- ISO 8473
standard.
- CLNP -
network-layer protocol to carry upper-layer data and error indications over
connectionless links. It is the interface between the Connectionless Network
Service (CLNS) and the upper
layers.
- CLNS - does
not perform connection setup or termination, as paths are determined
independently for each packet. It works on best-effort delivery basis
only.
- Does not
exchange control information (handshake) to establish end-to-end
connection before transmitting
data.
- Other
transport-layer protocols will have to take care of error detection and
correction.
- IP is a
connectionless protocol! It relies on protocols in other layers to establish the
connection if connection-oriented services are
required.
- IPX
specifies a connectionless datagram similar to the IP packet of TCP/IP
networks.
- IOS
supports packet forwarding and routing for ISO CLNS on networks using data link
layers: Ethernet, Token Ring, Fiber Distributed Data Interface (FDDI), and
serial.
- CLNS routing
on serial interfaces is possible with High-Level Data Link Control (HDLC),
Point-to-Point Protocol (PPP), Link Access Procedure, Balanced (LAPB), X.25,
Switched Multimegabit Data Service (SMDS), or Frame Relay
encapsulation.
- The
ISO-developed IS-IS routing protocol and Cisco's ISO Interior Gateway Routing
Protocol (IGRP) are designed to include support for dynamic/static routing of
ISO CLNS.
- ISO CLNS
Addressing - addresses in the ISO network architecture are referred to as NSAP
addresses and network entity titles (NETs). Each node in has one or more NETs as
well as many NSAP addresses. Cisco's implementation supports all NSAP address
formats that are defined by ISO 8348/Ad2.
- Key difference
between ISO-IGRP and IS-IS NSAP addressing schemes is in area- addresses
definition: ISO-IGRP NSAP address includes three separate levels for routing:
the domain, area, and system ID, while IS-IS address includes only two fields: a
single continuous area field comprising the domain and area fields defined for
ISO-IGRP and the system
ID.
- The following
link has information on troubleshooting CLNS:
http://www.cisco.com/univercd/cc/td/doc/cisintwk/itg_v1/tr1912.htm#xtocid27250
High-Level Data Link Control - HDLC
- Bit-oriented synchronous
data-link layer
protocol.
- Developed
by ISO.
- Derived from
SDLC (Synchronous Data Link
Control).
- Specifies
data encapsulation method on synchronous serial links using frame characters and
checksums.
- Corresponds
to Layer 2 and is responsible for the error-free movement of data between
network
nodes.
- Perform flow
control to ensure that data is transmitted only as fast as the receiver can
receive it.
- Two
distinct HDLC implementations: HDLC NRM (also known as SDLC) and HDLC Link
Access Procedure Balanced
(LAPB).
- LAPB:
- Complete data
transparency in full-duplex point-to-point
operation.
- Supports
peer-to-peer without the need for permanent master station (NRM does need
designated permanent master
station).
- Very
efficient.
- Frame
window is used to send multiple frames before receiving confirmation that the
first frame has been correctly
received.
- Three
categories of frames:
- Information frames -
transport data across the link and may encapsulate the higher layers.
- Supervisory frames -
perform flow control and error recovery.
- Unnumbered frames -
provide link initialization and termination.
- Maximum
frame size depends on the number of CRC bytes at the end of the
frame.
- Usually used
by
X.25.
- Cisco
router supports point-to-point software compression on serial interfaces that
use HDLC encapsulation to reduce size of a HDLC frame via lossless data
compression, using Stacker (LZS)
algorithm.
- Cisco
MC3810 multiservice access concentrator supports Voice over High-Level Data Link
Control (VoHDLC), a variation of HDLC.
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