Building Cisco Remote Access Networks

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• Building Cisco Remote Access Networks
• Managing Network Performance

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Queuing To do, or not to do

• If there is no congestion on the WAN link, there
  is no reason to implement traffic prioritization.
• Mark McGregor says If you have a T1 or higher,
  you shouldnt be messing with queuing. If you
  link is congested, buy more bandwidth.

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Queuing

• Routers can be configured to prioritize certain
  kinds of traffic based on protocol information,
  such as TCP port numbers
• Such traffic prioritization ensures that packets
  carrying mission-critical data take precedence
  over less important traffic.

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Queuing

• The question of priority is most important on
  routers that maintain a slow WAN connection and
  thus experience frequent congestion.
• If a remote site maintains a single 56-kbps link
  to the outside world, competition for that small
  WAN pipe could get fierce.
• the router must buffer incoming packets and
  schedule them for transmission on the slower
  outgoing port.

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Queuing
Because RTA can accept packets on a 10-Mbps LAN
interface much faster than it can send packets
out on the slow WAN link, an administrator may
configure a special queuing strategy on S0.
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Queuing

• In the previous example, if the router schedules
  these packets for transmission on a first-come,
  first-served basis, users could experience an
  unacceptable lack of responsiveness.
• An end user sending delay-sensitive voice traffic
  may be forced to wait too long while the router
  empties its buffer of another users file upload
  of a long train of packets.

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Queuing

• The Cisco IOS addresses priority and
  responsiveness issues through queuing.
• Queuing refers to the process the router uses to
  schedule packets for transmission during periods
  of congestion.
• By using the queuing feature, you can configure a
  congested router to reorder packets so that
  mission-critical and delay-sensitive traffic gets
  sent out first--even if other low-priority
  packets arrive before them.

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Queuing

• The Cisco IOS supports four methods of queuing
• first-in, first out (FIFO) queuing
• priority queuing
• custom queuing
• weighted fair queuing (WFQ)
• Only one of these queuing methods can be applied
  per interface, since each method handles traffic
  in a unique way.

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Queuing
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Queuing

• Prioritization is most effective on WAN links in
  which the combination of bursty traffic and
  relatively lower data rates can cause temporary
  congestion.
• Depending on the average packet size,
  prioritization is most effective when applied to
  links at T1/E1 bandwidth speeds or lower.

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AGAIN To do, or not to do

• If there is no congestion on the WAN link, there
  is no reason to implement traffic prioritization.
• Mark McGregor says If you have a T1 or higher,
  you shouldnt be messing with queuing. If you
  link is congested, buy more bandwidth.

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FIFO

• As its name implies, FIFO queuing does not
  prioritize packets according to type of traffic
  or protocol.
• FIFO merely queues packets for transmission
  according to the order in which they arrive.
• Because of its simplicity, FIFO is the fastest of
  the four queuing methods, and should be used on
  all non-congested interfaces.
• It is the Cisco default queuing mode for all
  interfaces faster than E1 (2.048 Mbps).

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Priority Queuing Overview

• You can use priority queuing to prioritize one
  type of traffic over others, so that the
  highest-priority traffic always gets dispatched
  before any other packets.
• To implement priority queuing, you classify
  traffic according to various criteria, including
  protocol type, and then assigns that traffic to
  one of four output queues
• high
• medium
• normal
• low priority.

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Priority Queuing Overview
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Priority Queuing Operation

• When the router is ready to send a packet, it
  checks the high queue first.
• Once the high queue is empty, the router checks
  the medium queue, and so on.
• This process is repeated every time the router is
  ready to send a packet.

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Priority Queuing Operation

• An incoming packet is compared with the priority
  list to select a queue.
• If there is room, the packet is buffered in
  memory and waits to be dispatched after the queue
  is selected.
• If the queue is full, the packet is dropped.
• For this reason, controlling queue size is an
  important configuration task.

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Priority Queuing Operation
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Configuring PQ - Priority Lists

• You can set a priority queue either by protocol
  type or by incoming interface type
• Router(config)priority-list list-number protocol
  protocol-name high medium normal low
  queue-keyword keyword-value
• Router(config)priority-list list-number
  interface interface-type interface-number high
  medium normal low
• Use this command to set queuing priorities for
  all traffic arriving on an incoming interface

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Configuring PQ - Default Queue

• The default queue is normal, if you want
  something else
• Router(config)priority-list list-number default
  high medium normal low

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PQ - priority-list examples

• This command assigns a high priority to traffic
  that matches IP access list 10
• priority-list 1 protocol ip high list 10
• This command assigns a medium priority level to
  Telnet packets
• priority-list 4 protocol ip medium tcp 23
• This command assigns a medium priority level to
  UDP Domain Name service packets
• priority-list 4 protocol ip medium udp 53
• This command assigns medum priority to traffic
  arriving in on ethernet 0
• priority-list 1 interface ethernet 0 medium
• NOTE When using multiple rules for a single
  protocol, remember that the order of the rules
  matters.

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Default PQ Queue Sizes
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Configuring PQ - Queue Sizes

• You can specify the maximum number of allowable
  packets in each queue.
• In general, it is recommended that the default
  queue sizes not be changed.
• Router(config)priority-list list-number
  queue-limit high-limit medium-limit normal-limit
  low-limit

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Implementing PQ

• Add the priority-group command to an interface to
  implement a priority-list
• Router(config-if)priority-group list

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PQ Configuration Example
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Custom Queuing

• Whereas priority queuing may result in an
  unacceptable delay of low-priority traffic,
  custom queuing allows an administrator to reserve
  a minimum amount of bandwidth for every kind of
  traffic.
• Each traffic type gets a share of the available
  bandwidth, although these amounts dont have to
  be equal.
• delay-sensitive and mission-critical traffic can
  be assigned a large percentage of available
  bandwidth, while low-priority traffic receives a
  smaller portion.

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Custom Queuing

• An administrator can configure up to 16 queues,
  which makes it ideal for networks that must
  provide a minimum level of responsiveness for
  several different protocols.
• Each queue is serviced sequentially until the
  number of bytes sent exceeds the configurable
  byte count or the queue is empty.

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Custom Queuing
Note Bandwidth allocation not part of this
course.
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Allocating bandwidth to different queues

• This discussion is beyond the scope of this
  course, but for information on this and other
  topics related to queuing, go to
• Congestion Management Overview
• http//www.cisco.com/univercd/cc/td/doc/product/so
  ftware/ios121/121cgcr/qos_c/qcprt2/qcdconmg.htm20
  680

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Allocating bandwidth to different queues

• Basically, the more types of traffic sharing the
  same queue, the less bandwidth that traffic will
  receive.
• If a type of traffic has a queue all to itself it
  will receive more bandwidth.
• This will be even more true if fewer queues are
  used.
• For example, if you only have two queues, with
  telnet (port 23) traffic in one and everything
  else in the other, the telnet traffic will
  receive 50 of the available bandwidth.

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Queue 0

• Queue 0 is a system queue that handles system
  packets such as keepalives.
• Queue 0 is emptied before the other custom
  queues.

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Custom Queuing

• Traffic filteringThe forwarding application
  (e.g. IP) applies a set of filters or access-list
  entries to each message that it forwards. The
  messages are placed in queues, based on the
  filtering.
• Queued message forwardingCQ uses a round-robin
  dispatching algorithm to forward traffic. Each
  queue continues to transmit packets until the
  configured byte limit is reached.
• When the threshold of this queue is reached or
  the queue is empty, the queuing software services
  the next queue in sequence.

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Custom Queuing
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Configuring CQ - queue-list

• Similar to using the priority-list command
• Router(config)queue-list list-number protocol
  protocol-name queue-number queue-keyword
  keyword-value
• Router(config)queue-list list-number interface
  interface-type interface-number queue-number

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• Protocol Type
• The following example assigns traffic that
  matches IP access list 10 to queue number 1
• queue-list 1 protocol ip 1 list 10
• The following example assigns Telnet packets to
  queue number 2
• queue-list 4 protocol ip 2 tcp 23
• The following example assigns UDP Domain Name
  Service (DNS) packets to queue number 3
• queue-list 4 protocol ip 3 udp 53
• Interface Type
• In this example, queue list 4 establishes
  queueing priorities for packets entering on
  serial interface 0. The queue number assigned is
  10.
• queue-list 4 interface serial 0 10

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Configuring CQ - Default queue

• You must explicitly assign a queue for packets
  that were not specified in the queue list.
• The default queue is 1, if you want something
  else
• Router(config)queue-list list-number default
  queue-number

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Configuring CQ - Queue Size
Queue
of packets
1-16 (all)
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• You can designate the maximum number of packets
  that a queue can contain.
• To limit the length of a particular queue
• Router(config) queue-list list-number queue
  queue-number limit limit-number
• Example queue-list 3 queue 10 limit 40

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Default CQ Queue Byte Count- Service Threshold
Queue
of bytes
1-16 (all)
1500

• You can configure the minimum byte count
  transferred from a given queue at a time (service
  threshold).
• This value is set on a per-queue basis
• Router(config) queue-list list-number queue
  queue-number byte-count byte-count-number
• Example queue-list 9 queue 10 byte-count 1400
• NOTE Router will not split packet for the
  purpose of queuing. If a queue threshold is
  reached during the transmission of a packet, the
  whole packet is still transmitted.

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Implementing CQ

• Add the priority-group command to an interface to
  implement a priority-list
• Router(config-if)custom-queue-list list

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CQ Configuration Example
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Weighted Fair Queuing

• Custom queuing (like priority queuing) requires
  that an administrator predefine priorities and
  configure access lists
• you have to know what the priorities will be in
  advance, and have to do some complex
  configuration on the router ahead of time.
• If network conditions change, routers using these
  static queuing methods cant adapt to the
  changes.
• For administrators looking for a dynamic, fair
  method to prioritize traffic on a congested
  interface, WFQ is the answer.
• WFQ automatically allocates bandwidth to all
  types of network traffic, but prioritizes
  delay-sensitive packets so that high-volume
  conversations dont consume all of the available
  bandwidth.

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Weighted Fair Queuing

• By using a complex algorithm, WFQ sorts the
  packets that make up the different conversations
  on an interface, and then assigns a precedence,
  or weight, to traffic so that each conversation
  gets its fair share of bandwidth.
• WFQ breaks up large trains of packets so that
  low-volume conversations dont get overrun by
  large file transfers or any other heavy traffic.
• With WFQ, the router can prioritize traffic based
  on the actual network conditions at the time,
  without complex administration.
• As the only dynamic queuing strategy of the four
  queuing methods, WFQ is the Cisco default on all
  E1 (2.048 Mbps) and slower interfaces.

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(No Transcript)
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• From the earlier suggested Cisco web page
• WFQ classifies traffic into different flows based
  on packet header addressing, including such
  characteristics as source and destination network
  or MAC address, protocol, source and destination
  port and socket numbers of the session, Frame
  Relay data-link connection identifier (DLCI)
  value, and type of service (ToS) value.
• There are two categories of flows high-bandwidth
  sessions and low-bandwidth sessions.
• Low-bandwidth traffic has effective priority over
  high-bandwidth traffic, and high-bandwidth
  traffic shares the transmission service
  proportionally according to assigned weights.
• Low-bandwidth traffic streams, which comprise the
  majority of traffic, receive preferential
  service, allowing their entire offered loads to
  be sent in a timely fashion.
• High-volume traffic streams share the remaining
  capacity proportionally among themselves.

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Configuring WFQ

• Router(config-if)fair-queue

Thats it.
(Almost.)
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Configuring WFQ

• Router(config-if)fair-queue
• congestive-discard-threshold
• The congestive-discard-threshold is the number of
  messages to queue for high-volume traffic.
• In other words, the maximum packets in a
  conversation held in a queue before they are
  discarded.
• Valid values are 1 to 512, inclusive.
• The default is 64 messages.

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Configuring WFQ
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Verifying Queuing
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Verifying Queuing

• show queueing custom
• show queueing priority
• show queueing fair

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Compression

• Link compression (also known as per-interface
  compression)
• Payload compression (also known as
  per-virtual-circuit compression)
• TCP header compression
• Microsoft Point-to-Point Compression (MPPC)

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Compression Overview
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Link Compression

• Link compression (also known as per-interface
  compression) involves compressing both the header
  and payload sections of a data stream.
• Unlike header compression, link compression is
  protocol independent.

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Link Compression

• The link-compression algorithm uses Predictor or
  STAC to compress the traffic in another link
  layer, such as PPP or LAPB, to ensure error
  correction and packet sequencing.
• Cisco HDLC uses STAC compression only.

Hi, Im on the BCRAN test!
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Predictor

• Predictor --Predicts the next sequence of
  characters in the data stream by using an index
  to look up a sequence in a compression
  dictionary.
• FYI
• It then examines the next sequence in the data
  stream to see if it matches. If it does, that
  sequence replaces the looked-up sequence in a
  maintained dictionary.
• If it does not, the algorithm locates the next
  character sequence in the index, and the process
  begins again.

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STAC

• STAC -- Developed by STAC Electronics, STAC is a
  Lempel-Ziv (LZ)-based, compression-based
  algorithm.
• FYI
• searches the input data stream for redundant
  strings and replaces them with what is called a
  token, which turns out to be shorter than the
  original redundant data string.

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Good to know

• Predictor is memory intensive.
• Stacker is CPU intensive.

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Link Compression

• If the data flow transverses across a
  point-to-point connection, use link compression.
• In a link-compression environment, the complete
  packet is compressed and the switching
  information in the header is not available for
  WAN-switching networks like Frame Relay.
• The best applications for link compression are
  point-to-point environments with a limited hop
  path.
• Typical examples are leased lines or ISDN.

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Payload Compression

• Payload compression (also known as
  per-virtual-circuit compression) compresses only
  the data portion of the data stream.
• The header is left intact.
• It uses the STAC compression method.

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Payload Compression
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Payload Compression

• When designing an internetwork, the customer
  cannot assume that an application will go over
  point-to-point lines.
• If link compression is used, the header may not
  be readable at a particular hop, however, the
  customer can use payload compression instead.
• When using payload compression, the header is
  left unchanged and packets can be switched
  through a WAN packet network. Payload compression
  is appropriate for
• X.25
• Switched Multimegabit Data Service (SMDS)
• Frame Relay
• ATM

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TCP Header Compression

• The TCP header compression subscribes to the Van
  Jacobson Algorithm, which is defined in RFC 1144.
  
• TCP/IP header compression lowers the overhead
  generated by the disproportionately large TCP/IP
  headers as they are transmitted across the WAN.

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TCP Header Compression

• TCP/IP header compression is protocol specific
  and compresses only the TCP/IP header, which
  leaves the Layer 2 header intact to allow a
  packet with a compressed TCP/IP header to travel
  across a WAN link.
• It is beneficial on small packets with few bytes
  of data, such as Telnet.
• Cisco header compression supports X.25, Frame
  Relay, and dial-on-demand WAN link protocols.
• Because of processing overhead, header
  compression is generally used at lower speeds,
  such as 64-Kbps links.

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MPPC

• Microsoft Point-to-Point Compression (MPPC)
• means of representing arbitrary PPP packets in a
  compressed form
• a Windows 95 client connecting to an NT server
• The Windows 95 client software supports both MPPC
  and LZS, whereas the Microsoft NT server supports
  only MPPC, which is negotiated during the
  Compression Control Protocol (CCP) process.

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Compression Commands

• Link Compression
• Router(config-if)compress predictor stac
  mppc
• Payload Compression
• Router(config-if)frame-relay payload-compress
• TCP/IP Header Compression
• Router(config-if)ip tcp header-compression
• Remember Never recompress data. Compressed data
  does not compress it actually expands.