528 Chapter 7: Link-Efficiency Tools
Foundation Summary
The “Foundation Summary” is a collection of tables and figures that provide a convenient review of many key concepts in this chapter. For those of you already comfortable with the topics in this chapter, this summary could help you recall a few details. For those of you who just read this chapter, this review should help solidify some key facts. For any of you doing your final prep before the exam, these tables and figures are a convenient way to review the day before the exam.
Figure 7-19 shows the fields compressed by payload compression, and by both types of header compression. (The abbreviation “DL” stands for data link, representing the data-link header and trailer.)
Figure 7-19 Payload and Header Compression
Payload Compression
DL IP TCP |
Data |
DL |
TCP Header Compression
RTP Header Compression
DL IP UDP RTP Data DL
Table 7-16 outlines the main points of comparison for the three payload compression tools.
Table 7-16 Point-to-Point Payload Compression tools—Feature Comparison
Feature |
Stacker |
MPPC |
Predictor |
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Uses Lempel-Ziv (LZ) compression algorithm |
Yes |
Yes |
No |
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Uses Predictor public domain compression algorithm |
No |
No |
Yes |
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Supported on High-Level Data Link Control (HDLC) |
Yes |
No |
No |
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Supported on X.25 |
Yes |
No |
No |
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Supported on Link Access Procedure, Balanced (LAPB) |
Yes |
No |
Yes |
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Supported on Frame Relay |
Yes |
No |
No |
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Supported on Point-to-Point Protocol (PPP) |
Yes |
Yes |
Yes |
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Supported on ATM (using multilink PPP) |
Yes |
Yes |
Yes |
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530 Chapter 7: Link-Efficiency Tools
Table 7-18 Configuration Command Reference for TCP and RTP Header Compression (Continued)
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Command |
Mode and Function |
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frame-relay map ip ip-address dlci |
Interface or subinterface configuration mode; enables TCP |
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[broadcast] tcp header-compression |
header compression on the specific VC identified in the |
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[active | passive] [connections number] |
map command. |
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frame-relay map ip ip-address dlci |
Interface or subinterface configuration mode; enables RTP |
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[broadcast] rtp header-compression |
header compression on the specific VC identified in the |
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[active | passive] [connections number] |
map command. |
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Table 7-19 Exec Command Reference for TCP and RTP Header Compression |
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Command |
Function |
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show frame-relay ip rtp header- |
Lists statistical information about RTP header |
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compression [interface type number] |
compression over Frame Relay; can list information per |
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interface |
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show frame-relay ip tcp header- |
Lists statistical information about TCP header |
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compression |
compression over Frame Relay |
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show ip rtp header-compression |
Lists statistical information about RTP header |
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[type number] [detail] |
compression over point-to-point links; can list information |
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per interface |
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show ip tcp header-compression |
Lists statistical information about TCP header |
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compression over point-to-point links |
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LFI tools attack the serialization delay problem by breaking the large packets into smaller pieces (fragmentation), and then sending the smaller frames ahead of most of the new fragments of the original large frame (interleaving). Figure 7-20 outlines the basic process.
Figure 7-20 Basic Concept Behind LFI Tools
Interface Output Queue, no LFI
Delay
Sensitive
60 Byte 1500 Byte Packet
Packet
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Interface Output Queue, with LFI, 300 Byte Fragments |
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300 Byte |
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300 Byte |
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300 Byte |
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300 Byte |
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Delay |
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300 Byte |
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Fragment |
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Fragment |
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Fragment |
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Fragment |
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Sensitive |
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Fragment |
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#5 of |
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#4 of |
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#3 of |
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#2 of |
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60 Byte |
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#1 of |
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Original |
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Original |
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Original |
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Original |
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Packet |
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Original |
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532 Chapter 7: Link-Efficiency Tools
Figure 7-22 Classification Between FRTS LLQ Shaping Queues and Interface Dual FIFO Queues with FRF.12
Shaping Queues Created |
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by LLQ Configuration |
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on a Single VC |
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FRF.12 Dual FIFO Queues |
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Class 1 – |
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LLQ |
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High Queue |
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Class 2 – |
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TX Ring |
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Non-LLQ |
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Shape |
Normal |
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Queue |
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Class 3 – |
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Non-LLQ |
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Class 3 – |
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Non-LLQ |
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Table 7-21 summarizes the queuing tools and identifies when you can use them with FRTS and FRF.12:
Table 7-21 Queuing Tool Support with FRTS and FRF.12 (IOS 12.2 Mainline)
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Queuing Tools Supported on Each VC |
Desired Features |
(Shaping Queues) |
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FRTS only |
FIFO, PQ, Custom Queuing (CQ), Weighted Fair |
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Queuing (WFQ), Class Based Weighted Fair |
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Queuing (CBWFQ), LLQ, IP RTP Priority |
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FRTS with FRF.12 enabled |
WFQ, CBWFQ, LLQ, IP RTP Priority |
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FRTS, FRF.12, with actual interleaving of packets |
LLQ, IP RTP Priority |
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Table 7-22 summarizes the core functions of MLP LFI versus FRF.12, particularly how they each interact with the available queuing tools.
Table 7-22 Comparisons Between MLP LFI and FRF.12
Step in the Process |
MLP LFI |
FRF.12 |
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Configures maximum delay, or |
Maximum delay |
Fragment size |
actual fragment size |
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Classification into the interface |
Based on the queuing tool |
All packets coming from LLQ |
output queues |
enabled on the interface |
or RTP Priority shaping queues |
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placed in higher-priority queue |
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Foundation Summary 533 |
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Table 7-22 Comparisons Between MLP LFI and FRF.12 (Continued) |
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Step in the Process |
MLP LFI |
FRF.12 |
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Number of interface output |
Based on the queuing tool |
2 queues, called Dual FIFO |
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queues |
enabled on the interface |
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How queue service algorithm |
Based on queuing tool’s |
PQ-like algorithm, always |
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causes interleaving to occur |
inherent queue service |
servicing High queue over |
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algorithm; PQ, LLQ, and RTP |
Normal queue |
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Priority most aggressively |
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interleave packets |
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*The popular theory disagrees with this table. The popular theory states that all unfragmented packets end up in the high-priority queue, and all fragments end up in the Normal queue.
MLP, by its very nature, fragments packets. Figure 7-23 shows what really happens.
Figure 7-23 MLP Bundle with 3 Active Links—What Does Happen
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500 (Frag 1) |
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500 (Frag 2) |
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R1 |
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R2 |
100 |
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1500 |
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1500 |
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500 (Frag 3) |
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Tables 7-23 and 7-24 list the pertinent configuration and show commands for MLP interleaving, respectively.
Table 7-23 Configuration Command Reference for MLP Interleaving
Command |
Mode and Function |
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ppp multilink [bap] |
Interface configuration mode; enables multilink PPP on the interface, |
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dialer group, or virtual template |
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ppp multilink interleave |
Interface configuration mode; enables interleaving of unfragmented |
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frames with fragments of larger frames |
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ppp multilink fragment |
Interface configuration mode; enables MLP fragmentation, and defines |
delay time |
fragment size, with formula bandwidth/time |
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ppp multilink fragment |
Interface configuration mode; disables MLP fragmentation |
disable |
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ppp multilink group |
Interface configuration mode; links a physical interface to a dialer group |
group-number |
or virtual template |
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534 Chapter 7: Link-Efficiency Tools
Table 7-24 Exec Command Reference for MLP Interleaving
Command |
Function |
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show ppp multilink |
Lists information about the active links currently in |
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the same MLP bundle |
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show interfaces |
Lists statistics and status about each interface, |
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including multilink virtual interfaces |
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show queueing [interface atm-subinterface |
Lists configuration and statistical information about |
[vc [[vpi/] vci]]] |
the queuing tool on an interface |
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Tables 7-25 and 7-26 list the configuration and show commands for Frame Relay fragmentation, respectively.
Table 7-25 Command Reference for Frame Relay Fragmentation
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Command |
Mode and Function |
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frame-relay traffic-shaping |
Interface subcommand; enables FRTS on the |
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interface. |
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class name |
Interface DLCI subcommand; enables a specific |
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FRTS map-class for the DLCI. |
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frame-relay class name |
Interface or subinterface command; enables a specific |
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FRTS map-class for the interface or subinterface. |
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map-class frame-relay map-class-name |
Global Configuration mode; Names a map-class, and |
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places user in map-class configuration mode |
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frame-relay fragment fragment_size |
Map-class configuration mode; enables FRF.12 for |
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VCs using this class |
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Table 7-26 Exec Command Reference for Frame Relay Fragmentation |
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Command |
Function |
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show frame-relay fragment [interface |
Shows fragmentation statistics |
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interface] [dlci] |
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show frame-relay pvc [interface-type |
Shows statistics about overall performance of a VC |
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interface-number] [dlci] |
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show queueing [interface atm-subinterface |
Lists configuration and statistical information about |
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[vc [[vpi/] vci]]] |
the queuing tool on an interface |
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Figure 7-24 shows the framing when FRF.3 and FRF.11 are used, both for IP telephony traffic and for local voice gateway traffic.
Foundation Summary 535
Figure 7-24 Framing of Voice Traffic with FRF.3 and FRF.11 VCs
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With Data VC |
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IP Telephone Traffic |
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FRF.3 |
IP |
UDP |
RTP |
G.729 |
FRF.3 |
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Header |
Voice |
Trailer |
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Voice Gateway Traffic |
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FRF.3 |
IP |
UDP |
RTP |
G.729 |
FRF.3 |
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IP |
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Header |
Voice |
Trailer |
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SW1 |
R1 |
T/1 R2 |
SW2 |
IP |
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With VoFR VC |
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IP Telephone Traffic |
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FRF.3 |
FRF.11 |
IP |
UDP |
RTP |
G.729 |
FRF.3 |
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Header |
Header |
Voice |
Trailer |
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Voice Gateway Traffic |
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FRF.3 |
FRF.11 |
G.729 |
FRF.3 |
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Header |
Header |
Voice |
Trailer |
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Table 7-27 summarizes some of the key comparison points about FRF.12 and FRF.11-C.
Table 7-27 FRF.11-C and FRF.12 Comparison
Function |
FRF.12 Behavior |
FRF.11-C Behavior |
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Queuing option on the interface |
Dual FIFO |
Dual FIFO |
output queues |
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Classification into the interface |
Based on queuing tool used for |
Voice frames placed in High |
output queues |
shaping, with LLQ and IP RTP |
queue, all others in Normal |
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Priority putting packets into the |
queue, regardless of shaping |
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high-priority queue |
queue configuration |
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Fragmentation based on size, or |
Based only on size; must be |
Nonvoice frames fragmented, |
type of packet |
careful not to fragment voice |
and voice frames are not, |
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packets |
regardless of size |
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Frame Relay network can be |
No |
Yes |
aware of voice vs. nonvoice |
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frames, and acts accordingly |
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Underlying type of VC, and |
FRF.3, available from most if |
FRF.11, not generally available |
general public availability |
not all public Frame Relay |
from public Frame Relay |
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services |
services |
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