When a packet arrives on a router’s interface, a protocol-independent switching process handles
it. The router then switches the traffic to the outgoing interface buffer. An example of a protocolindependent
switching process is first-in, first-out (FIFO), which is the original algorithm for
packet transmission. FIFO was the default for all routers until weighted fair queuing (WFQ) was
developed. The problem with FIFO is that transmission occurs in the same order as messages are
received. If an application such as Voice over IP (VoIP) required traffic to be reordered, the network
engineer needed to establish a queuing policy other than FIFO queuing.
Cisco IOS software offers three queuing options as an alternative to FIFO queuing:
Weighted fair queuing (WFQ) prioritizes interactive traffic over file transfers to ensure
satisfactory response time for common user applications.
Priority queuing ensures timely delivery of a specific protocol or type of traffic that is transmitted
before all others.
Custom queuing establishes bandwidth allocations for each type of traffic.
We will discuss these three queuing options in detail in the “IOS Queuing Options” section later
in this chapter.
IT Certification CCIE,CCNP,CCIP,CCNA,CCSP,Cisco Network Optimization and Security Tips
Queuing and Compression
THE CCNP EXAM TOPICS COVERED IN THIS
CHAPTER INCLUDE THE FOLLOWING:
Determine why queuing is enabled, identify alternative
queuing protocols that Cisco products support, and
determine the best queuing method to implement.
Specify the commands to configure queuing.
Specify the commands and procedures used to verify proper
queuing configuration.
Specify the commands and procedures used to effectively
select and implement compression.
Describe traffic control methods used to manage traffic flow
on WAN links.
Plan traffic shaping to meet required quality of service on
access links.
Troubleshoot traffic control problems on a WAN link.
This chapter teaches you how to use both queuing and compression
to help maintain a healthy network, which is important because
user data consists of many types of data packets roaming the internetwork,
hungering for and consuming bandwidth.
Queuing
is the act of sequencing packets for
servicing—similar to a line at an amusement park with a FastPass or “go to the front” ability.
Compression is the ability to communicate a piece of information with fewer bits, typically by
removing repetitions within the data.
As a network administrator, you can help save precious bandwidth on WAN links, the largest
bottlenecks in today’s networks. With Gigabit Ethernet running the core backbones and 10-gigabit
Ethernet networks just now being deployed, a 1.544Mbps T-1 link is painfully slow. By implementing
both queuing and compression techniques, you can help save bandwidth and get the most for
your money.
In addition, this chapter teaches you the three
queuing
techniques available on the Cisco
router: weighted fair queuing (WFQ), priority queuing, and custom queuing. You will learn
when to use each type, as well as how to configure each type on your router. We also present
an overview of newer queuing and policing technologies, including low latency queuing (LLQ),
class-based weighted fair queuing (CBWFQ), and committed access rate (CAR).
Finally, this chapter provides the information you need to both understand and configure the
types of compression on Cisco routers. The types of compression techniques covered in this
chapter include header, payload, and link compression.
CHAPTER INCLUDE THE FOLLOWING:
Determine why queuing is enabled, identify alternative
queuing protocols that Cisco products support, and
determine the best queuing method to implement.
Specify the commands to configure queuing.
Specify the commands and procedures used to verify proper
queuing configuration.
Specify the commands and procedures used to effectively
select and implement compression.
Describe traffic control methods used to manage traffic flow
on WAN links.
Plan traffic shaping to meet required quality of service on
access links.
Troubleshoot traffic control problems on a WAN link.
This chapter teaches you how to use both queuing and compression
to help maintain a healthy network, which is important because
user data consists of many types of data packets roaming the internetwork,
hungering for and consuming bandwidth.
Queuing
is the act of sequencing packets for
servicing—similar to a line at an amusement park with a FastPass or “go to the front” ability.
Compression is the ability to communicate a piece of information with fewer bits, typically by
removing repetitions within the data.
As a network administrator, you can help save precious bandwidth on WAN links, the largest
bottlenecks in today’s networks. With Gigabit Ethernet running the core backbones and 10-gigabit
Ethernet networks just now being deployed, a 1.544Mbps T-1 link is painfully slow. By implementing
both queuing and compression techniques, you can help save bandwidth and get the most for
your money.
In addition, this chapter teaches you the three
queuing
techniques available on the Cisco
router: weighted fair queuing (WFQ), priority queuing, and custom queuing. You will learn
when to use each type, as well as how to configure each type on your router. We also present
an overview of newer queuing and policing technologies, including low latency queuing (LLQ),
class-based weighted fair queuing (CBWFQ), and committed access rate (CAR).
Finally, this chapter provides the information you need to both understand and configure the
types of compression on Cisco routers. The types of compression techniques covered in this
chapter include header, payload, and link compression.
Frame Relay Exam Essentials
Understand Frame Relay and its history. Frame Relay is a streamlined version of X.25 without
the windowing and retransmission capabilities. Frame Relay is a layer 2 protocol that was defined
as a network service for ISDN by the CCITT (now ITU-T). The Group of Four extended Frame
Relay in 1990 to allow for a Local Management Interface (LMI) to assist in PVC management.
Understand the two types of virtual circuits (VCs). Know what a switched virtual circuit
(SVC) is used for and what a permanent virtual circuit (PVC) is used for. Understand why you
would use one type over another.
Know what a DLCI is and how it is mapped to Network layer protocols. The data link
connection identifier (DLCI) is used to identify a PVC in a Frame Relay network. The DLCIto-
Network-layer mapping can be statically configured by an administrator using the framerelay
map command or can be dynamically set by using Inverse ARP (IARP).
Understand the Local Management Interface (LMI). LMI was an extension to Frame Relay
to manage the virtual circuits on a connection. LMI virtual circuit status messages provide communication
and DLCI synchronization between Frame Relay DTE and DCE devices.
Know how to configure Frame Relay and what it uses for congestion control. The
encapsulation frame-relay command is used to configure an interface for Frame Relay
operation. The frame-relay intf-type dce command is used to configure an interface
for DCE operation, but by default, the Frame Relay interface type is DTE. Frame Relay uses
backward explicit congestion notification (BECN) and forward explicit congestion notification
(FECN) messages to control congestion on a Frame Relay switch.
Understand the options for traffic shaping on a Frame Relay interface. There are many
options for traffic shaping to enable a Frame Relay network to operate more efficiently. You
can have the router slow traffic on a VC in response to BECNs received. You can set up queuing
on a per-VC basis and limit traffic going out of a VC. You need to know what the committed
information rate (CIR) and excess information rate (EIR) are.
Know what Cisco IOS commands are used to verify and troubleshoot a Frame Relay connection.
The commands show interface, show frame-relay pvc, show frame-relay map, show
frame-relay lmi, and debug frame-relay lmi are all used to see and verify the operation of
Frame Relay. The command clear frame-relay-inarp is used to delete the dynamic PVC-to-
Network-Layer addressing entries.
the windowing and retransmission capabilities. Frame Relay is a layer 2 protocol that was defined
as a network service for ISDN by the CCITT (now ITU-T). The Group of Four extended Frame
Relay in 1990 to allow for a Local Management Interface (LMI) to assist in PVC management.
Understand the two types of virtual circuits (VCs). Know what a switched virtual circuit
(SVC) is used for and what a permanent virtual circuit (PVC) is used for. Understand why you
would use one type over another.
Know what a DLCI is and how it is mapped to Network layer protocols. The data link
connection identifier (DLCI) is used to identify a PVC in a Frame Relay network. The DLCIto-
Network-layer mapping can be statically configured by an administrator using the framerelay
map command or can be dynamically set by using Inverse ARP (IARP).
Understand the Local Management Interface (LMI). LMI was an extension to Frame Relay
to manage the virtual circuits on a connection. LMI virtual circuit status messages provide communication
and DLCI synchronization between Frame Relay DTE and DCE devices.
Know how to configure Frame Relay and what it uses for congestion control. The
encapsulation frame-relay command is used to configure an interface for Frame Relay
operation. The frame-relay intf-type dce command is used to configure an interface
for DCE operation, but by default, the Frame Relay interface type is DTE. Frame Relay uses
backward explicit congestion notification (BECN) and forward explicit congestion notification
(FECN) messages to control congestion on a Frame Relay switch.
Understand the options for traffic shaping on a Frame Relay interface. There are many
options for traffic shaping to enable a Frame Relay network to operate more efficiently. You
can have the router slow traffic on a VC in response to BECNs received. You can set up queuing
on a per-VC basis and limit traffic going out of a VC. You need to know what the committed
information rate (CIR) and excess information rate (EIR) are.
Know what Cisco IOS commands are used to verify and troubleshoot a Frame Relay connection.
The commands show interface, show frame-relay pvc, show frame-relay map, show
frame-relay lmi, and debug frame-relay lmi are all used to see and verify the operation of
Frame Relay. The command clear frame-relay-inarp is used to delete the dynamic PVC-to-
Network-Layer addressing entries.
The Lowdown on Committed Information Rate (CIR)
We’ve talked formally about CIR—we even presented a calculation—but what does it really mean?
As we have said, the acronym itself stands for committed information rate, which really doesn’t
seem that difficult to understand. But there seems to be widespread misinterpretation of this concept,
especially by some service providers, so let’s attempt to figure the whole thing out.
We’ve discussed terms such as burst rate and phrases such as bursting above your CIR, but
these terms can be misleading. They were devised by network engineers who assumed—you
know what that leads to—that we wouldn’t understand hardcore network-engineering concepts,
so they tried to put them in layman’s terms and botched the whole thing up. In reality,
you’re always “bursting” to your line speed because Frame Relay is an HDLC protocol and
there’s no other way to make it work.
HDLC is a synchronous protocol (which means that the data is synchronized to a clock) that
sends data with a standardized framing and checksum technique. When a frame is transmitted,
the data must be contiguous; that is, there cannot be any holes or spaces between bytes of
data. So if you’re transmitting 500 bytes of data, you can’t send 250 and then wait for a while
and then send the rest. It has to go out as one big chunk. The Frame Relay expression bursting
over your CIR comes into play because there is no way to slow down the data or to change the
length of the chunk after you start transmitting; you just send until you are finished. If you happen
to send too much data because your data chunk is larger than the allotment, you’ve
bursted over your CIR.
So what is the big deal about CIR, then? And why, when you buy Frame Relay from a company
like Qwest, do they quote you a CIR? CIR is the “worst-case” throughput that the Frame Relay
network provider attempts to guarantee. It’s like a restaurant guaranteeing that you’ll always
be able to eat a certain amount of food from its buffet. Like the restaurant, the Frame Relay network
provider can’t guarantee that you’ll always be able to transmit at the CIR (take the case
when everyone on the network happens to be transmitting at once), but they can guarantee it
over a reasonable time span (usually over a span of seconds). Basically, the network backbone
is engineered to handle reasonable loads—just like the number of lanes in a highway. Given a
certain amount of traffic, the data should flow through the backbone without delay. At times,
when unusually heavy traffic exists, you have what is called congestion.
As we have said, the acronym itself stands for committed information rate, which really doesn’t
seem that difficult to understand. But there seems to be widespread misinterpretation of this concept,
especially by some service providers, so let’s attempt to figure the whole thing out.
We’ve discussed terms such as burst rate and phrases such as bursting above your CIR, but
these terms can be misleading. They were devised by network engineers who assumed—you
know what that leads to—that we wouldn’t understand hardcore network-engineering concepts,
so they tried to put them in layman’s terms and botched the whole thing up. In reality,
you’re always “bursting” to your line speed because Frame Relay is an HDLC protocol and
there’s no other way to make it work.
HDLC is a synchronous protocol (which means that the data is synchronized to a clock) that
sends data with a standardized framing and checksum technique. When a frame is transmitted,
the data must be contiguous; that is, there cannot be any holes or spaces between bytes of
data. So if you’re transmitting 500 bytes of data, you can’t send 250 and then wait for a while
and then send the rest. It has to go out as one big chunk. The Frame Relay expression bursting
over your CIR comes into play because there is no way to slow down the data or to change the
length of the chunk after you start transmitting; you just send until you are finished. If you happen
to send too much data because your data chunk is larger than the allotment, you’ve
bursted over your CIR.
So what is the big deal about CIR, then? And why, when you buy Frame Relay from a company
like Qwest, do they quote you a CIR? CIR is the “worst-case” throughput that the Frame Relay
network provider attempts to guarantee. It’s like a restaurant guaranteeing that you’ll always
be able to eat a certain amount of food from its buffet. Like the restaurant, the Frame Relay network
provider can’t guarantee that you’ll always be able to transmit at the CIR (take the case
when everyone on the network happens to be transmitting at once), but they can guarantee it
over a reasonable time span (usually over a span of seconds). Basically, the network backbone
is engineered to handle reasonable loads—just like the number of lanes in a highway. Given a
certain amount of traffic, the data should flow through the backbone without delay. At times,
when unusually heavy traffic exists, you have what is called congestion.
Frame Relay Summary
Frame Relay is one of the most popular WAN protocols in the world. This technology
will become even more critical as corporations stretch their networks globally and the Internet
becomes more pervasive.
To become a successful CCNP, you need to understand the Frame Relay protocol. This technology
makes up the majority of the world’s non-dedicated circuits, and its importance cannot
be underestimated.
Frame Relay is the distant cousin to X.25, without some of X.25’s overhead. It does provide
congestion notification, which can be used with traffic shaping to help traffic response. Like
X.25, Frame Relay provides for permanent and switched virtual circuits.
LMI is an extension to the Frame Relay protocol developed by the Frame Relay Forum and
is used to provide management for virtual circuits. This makes the management of DLCI information
easier for the network administrator.
Cisco provides for a mechanism to enable a multipoint interface such as Frame Relay to look
like multiple virtual point-to-point or multipoint interfaces called subinterfaces. Point-to-point
subinterfaces can be used to solve problems caused by distance-vector routing protocols running
over multipoint interfaces.
Setting up a Cisco router as a Frame Relay switch is not something that you would do often,
but it is a useful feature when you are working in a lab environment. There are many troubleshooting
commands that can be used to verify the configuration of Frame Relay on a Cisco
router. They can be used to see the DLCI-to-Network-layer-address mapping and the current
state of LMI on the router. Frame Relay is a technology used in many networks, and mastering
its configuration and operation will take you far in your networking career.
will become even more critical as corporations stretch their networks globally and the Internet
becomes more pervasive.
To become a successful CCNP, you need to understand the Frame Relay protocol. This technology
makes up the majority of the world’s non-dedicated circuits, and its importance cannot
be underestimated.
Frame Relay is the distant cousin to X.25, without some of X.25’s overhead. It does provide
congestion notification, which can be used with traffic shaping to help traffic response. Like
X.25, Frame Relay provides for permanent and switched virtual circuits.
LMI is an extension to the Frame Relay protocol developed by the Frame Relay Forum and
is used to provide management for virtual circuits. This makes the management of DLCI information
easier for the network administrator.
Cisco provides for a mechanism to enable a multipoint interface such as Frame Relay to look
like multiple virtual point-to-point or multipoint interfaces called subinterfaces. Point-to-point
subinterfaces can be used to solve problems caused by distance-vector routing protocols running
over multipoint interfaces.
Setting up a Cisco router as a Frame Relay switch is not something that you would do often,
but it is a useful feature when you are working in a lab environment. There are many troubleshooting
commands that can be used to verify the configuration of Frame Relay on a Cisco
router. They can be used to see the DLCI-to-Network-layer-address mapping and the current
state of LMI on the router. Frame Relay is a technology used in many networks, and mastering
its configuration and operation will take you far in your networking career.
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