Transcript Chapter 03a Frame Relay-pgb

Chapter 3
Frame Relay
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Frame Relay
FR is a WAN Technology.
• Uses (VC) Virtual Circuits to establish
connections across the WAN.
• DLCIs are used to identify Virtual Circuits.
• FR can divide a single Physical WAN
Interface into multiple Subinterfaces.
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Frame Relay is an International
Telecommunications Standards Sector
(ITU-T) and American National Standard
Institute (ANSI) standard.
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Frame Relay is a packet-switched
connection oriented WAN service
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Frame Relay is often used to interconnect
LANs
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The connection through the Frame Relay
network between two DTEs is called a
virtual circuit (VC)
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Switched virtual circuits (SVCs) are
established dynamically by sending
signaling messages to the network.
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Permanent virtual circuits (PVCs) are
preconfigured by the carrier
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Frame Relay is configured on a serial
interface and the default encapsulation
type is the Cisco proprietary version of
HDLC
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By default, a Frame Relay network
provides non-broadcast multi-access
(NBMA) connectivity between remote
sites
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To enable the forwarding of broadcast
routing updates in a hub-and-spoke
Frame Relay topology, configure the hub
router with logically assigned interfaces,
called subinterfaces
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Subinterfaces are logical subdivisions of
a physical interface
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Frame Relay Overview
• CCITT and American National Standards Institute
(ANSI) are standards that define the process for
sending data over a public data network (PDN).
• A data-link technology streamlined to provide high performance
and efficiency.
• Operates - Physical and Data Link Layers
• Relies on TCP for error correction.
• The receiving device drops all error frames with no notification to
the sender
• Uses - Link Access Procedure for Frame Relay (LAPF)
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Frame Relay Overview
• Defines the interconnection process
– CPE (customer premises equipment)
– DCE (data communications equipment ) the
service provider’s local access switching
equipment
– DTE (data terminal equipment) at the CPE
• Computing equipment that is not on a LAN
may also send data across a Frame Relay
network.
– The computing equipment will use a FRAD
(Frame Relay access device) as its DTE.
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Frame Relay Overview
• The SP’s switching equipment maintains table
mapping connection identifiers to outbound
ports.
• When a frame is received, the switching device
analyzes the connection identifier and delivers
the frame to the associated outbound port.
• The complete path to the destination must be
established prior to sending the first frame.
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Frame Relay Overview
• Very cost-effective as:
– FR allows a single interface to support multiple
PVCs (private virtual circuits).
– Less equipment required by the customer
– Only pay for the average bandwidth rather than
the maximum bandwidth requirement.
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DLCI (Data-link connection identifier)
• A number that identifies the logical circuit between the
source and destination device.
• The FR switch maps the DLCIs between each pair of
routers to create a PVC (private virtual circuit).
LMI (Local Management Interface)
• A signaling standard between the CPE device and the
FR switch - responsible for managing the connection
and maintaining status between the devices.
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LMIs may include:
• support for a keepalive mechanism\
• a multicast mechanism that can provide the
network server with its local DLCI
• multicast addressing, providing the ability to
give DLCIs global (whole Frame Relay network)
significance, rather than just local significance
• a status mechanism, providing an ongoing
status on the DLCIs known to the switch
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LMI Types
• Cisco--LMI type defined jointly by Cisco,
StrataCom, Northern Telecom, and DEC
– DLCI 1023 is reserved for cisco type)
• Ansi-– defined by ANSI standard T1.617 Annex D
• ITU-T--Q.933 q933a Annex A
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CIR (Committed information rate)
• The rate, in bits per second, that the
Frame Relay switch service agrees to
transfer data.
• Bc (Committed Burst)
• Tc The maximum number of bits that the
SP agrees to transfer during any
Committed Rate Measurement Interval.
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Excess Burst
• The maximum number of uncommitted
bits that the Frame Relay switch will
attempt to transfer beyond the CIR.
• Depends on the service offerings
available by your vendor, typically
limited to the port speed of the local
access loop.
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FECN (Forward explicit congestion notification)
• When a FR switch recognizes congestion in the network,
it sends a FECN packet to the destination device
indicating that congestion has occurred.
BECN (Backward explicit congestion notification)
• When a FR switch recognizes congestion in the network,
it sends a BECN packet to the source router instructing
it to reduce the packet rate
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Discard Eligibility (DE) Indicator
• When the router detects network
congestion, the FR switch will first drop
packets having the DE bit set.
• The DE bit is set on for oversubscribed
traffic - the traffic that was received after
the CIR was met.
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DLCIs
• Frame Relay virtual circuits are identified by DLCIs
(data link connection identifiers).
• DLCI values are typically assigned by the Frame Relay
service provider
• ONLY local significance, the values themselves are not
unique in the Frame Relay WAN.
• Two DTE devices connected by a virtual circuit
might use a different DLCI value to refer to the same
connection.
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Beginning
of the
frame
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Ending of
the frame
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• Following the leading flags field are two
bytes of address information.
– Ten bits of these two bytes make up the
actual circuit ID, the DLCI. This is the
heart of the Frame Relay header. It
identifies the logical connection that is
multiplexed into a physical channel.
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Three of the remaining bits provide
congestion control.
– The forward explicit congestion notification
(FECN) a bit is set in the Frame to tell the
receiving DTE that congestion is experienced
in the path from source to destination.
– The backward explicit congestion notification
(BECN) bit is set in the Frame traveling in the
opposite direction from frames encountering a
congested path.
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– The DE (discard eligibility) bit is set by
the DTE to tell the network that a frame
has lower importance than other frames
and should be discarded before any
other frames -- if the network becomes
short on resources.
– DE is a very simple priority mechanism.
– This bit is usually set only when the
network is congested.
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Data –
Variable-length field that contains
encapsulated upper-layer data.
FCS –
Frame Check Sequence (FCS), used to
ensure the integrity of transmitted data.
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Assume two PVCs:
– one between Atlanta and Los Angeles,
– one between San Jose and Pittsburgh.
Los Angeles uses DLCI 12 to refer to its PVC with
Atlanta, while
Atlanta refers to the same PVC as DLCI 82.
San Jose uses DLCI 12 to refer to its PVC with
Pittsburgh.
Pittsburgh refers to the same PVC as DLCI 62.
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Reports the
status of PVCs
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LMI Purpose
• PVC status – monitors the operational
status of the various PVCs
• Transmits keepalive packets:
– to insure that the PVC stays up
– to prevent inactivity, which would shut
down the PVC.
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LMI
• The router must be programmed to choose
which LMI type encapsulation will be used.
• Options are (frame-relay lmi-type ?):
ansi | cisco | q933i
–cisco is the default
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Inverse ARP
The Inverse ARP mechanism allows the router
to automatically build the Frame Relay map.
– The router learns the DLCIs that are in use from the
switch during the initial LMI exchange.
– The router then sends an Inverse ARP request to
each DLCI for each protocol configured on the
interface (if the protocol is supported).
– The return information from the Inverse ARP is
then used to build the Frame Relay map.
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Frame Relay Mapping
• The router next-hop address determined from the
routing table must be resolved to a Frame Relay DLCI.
• The resolution is done through a data structure called
a Frame Relay map.
• This data structure may be:
– statically configured in the router, or
– the Inverse ARP feature can be used for automatic setup of
the map.
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Frame Relay Operation - Switching
• The Frame Relay switching table consists of four
entries:
– two for incoming port and DLCI
– two for outgoing port and DLCI
– The DLCI could, therefore, be remapped as it passes
through each switch
– The fact that the port reference can be changed is why
the DLCI is “locally significant."
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1. You order Frame Relay service from:
–
a service provider, or
– you create a private Frame Relay cloud.
2. Each router, either directly or through a CSU/DSU,
connects to the Frame Relay switch.
3. When the CPE router is enabled, it sends a Status
Inquiry message to the FR switch.
–
The message notifies the switch of the router’s status,
–
asks the switch for the connection status of the other remote routers.
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4. When the FR switch receives the
request, it responds with a Status
message that includes the DLCIs of the
remote routers to which the local router
can send data.
5. For each active DLCI, each router:
– sends an Inverse ARP request packet
introducing itself
– asking for each remote router to identify itself
by replying with its network-layer address.
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6. (Inverse ARP happens here)
For each DLCI that each router receives an Inverse ARP
message about, the router will create a map entry in
its Frame Relay map table that includes:
– the local DLCI
– the remote router’s network-layer address,
– as well as the state of the connection
Note: the DLCI is the router’s locally configured DLCI, not
the DLCI that the remote router is using.
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Three possible connection states appear in
the Frame Relay map table:
1. Active state —Indicates that the connection is
active and that routers can exchange data.
2. Inactive state —Indicates that local connection to
FR switch is working, but the remote router’s
connection to FR switch is not working.
3. Deleted state —Indicates that NO LMI is being
received from the FR switch or NO service
between the CPE router and FR switch is
occurring.
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If No Inverse ARP
– If Inverse ARP is not working, or the
remote router does not support Inverse
ARP, you need to configure the routes
(DLCIs and IP addresses) of the remote
routers.
– This configuration is referred to as static
mapping
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7. Every 60 seconds, the routers exchange
Inverse ARP messages.
8. Every 10 seconds or so (this is configurable), the
CPE router sends a keepalive message to the
FR switch.
– The purpose of the keepalive message is to verify
that the FR switch connection is still active.
– The router will change the status of each DLCI,
based on the response from the FR switch.
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Subinterfaces
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Subinterfaces
• More flexible in routing various protocols over partially meshed framerelay networks.
• Fully meshed FR
•
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Partially meshed FR
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Point to Point Subinterfaces
• Subinterfaces can handle issues caused by
split-horizon over NBMA (Non-Broadcast
Multiple Access) networks (like frame-relay)
and (distance-vector) routing protocols.
• Split-horizon says that a routing update
received on an interface cannot be
transmitted out onto that same interface.
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Point to Point Subinterfaces
• This rule holds even if the routing update is received on one FR
PVC and destined to retransmit out onto another FR PVC.
Sites B & C can
exchange routing
information with site A,
but would not be able
to exchange routing
information with each
other. Split horizon
doesn’t allow Site A to
send routing updates
received from Site B
on to Site C and vice
versa.
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Point to Point Subinterfaces
• By dividing the partially-meshed FR network
into a number of virtual, point-to-point
networks using subinterfaces, the splithorizon problem is fixed.
• Each new point-to-point subnetwork is given
its own network number.
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Point to Point Subinterfaces
To the routed protocol, each subnetwork now
appears to be located on separate interfaces.
Routing updates
received from Site
B on one logical
point to point
subinterface can
now be forwarded
to site C on a
separate logical
interface without
hurting split
horizon.
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Point to Point Subinterfaces
• A single subinterface is used to establish one PVC
connection to another physical or subinterface on a
remote router.
• In this case, the interfaces would be:
– in the same subnet and
– each interface would have a single DLCI
• Each point-to-point connection is its own subnet.
• In this environment, broadcasts are not a problem
because the routers are point-to-point and act like a
leased line.
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Multipoint Subinterfaces
• Multipoint subinterfaces are still subject to the splithorizon limitations.
• All nodes attached to a multipoint subinterface
belong to the same network number.
• A multipoint subinterface is used to keep remote
sites on a single network number while slowly
migrating remote sites to their own point-to-point
subinterface network.
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Multipoint Subinterfaces
Here serial 0.1 is a multipoint subinterface that connects to 3
different locations.
All devices on the multipoint subinterface belong to the same
network number.
Site E has migrated off
the multipoint net to its
own point-to-point
network. They can coexist. Site B, C, and D
may move to point to
point and the multipoint
subinterface would now
be point-to-point
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Configuring Frame Relay
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Configuring Basic Frame Relay
• A basic Frame Relay configuration
assumes that you want to:
– Configure Frame Relay on one or more
physical interfaces
– That LMI and Inverse ARP are supported by
the remote router(s).
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Configuring Frame Relay
• Step 1 Select the interface and enter configuration mode.
• Step 2 Configure a network-layer address, (an IP address).
• Step 3 Select the encapsulation type
Router(config-if)#encapsulation frame-relay [cisco | ietf]
– cisco is the default. Use this if connecting to another Cisco
router.
– Ietf —Select this if connecting to a non-Cisco router. - RFC
1490
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Configuring Frame Relay
• Step 4 If using Cisco IOS Release 11.1 or earlier,
specify the LMI-type used by the FR switch:
Router(config-if)#frame-relay lmi-type {ansi | cisco |
q933i}
– cisco is the default.
– With IOS Release 11.2 or later, the LMI-type is
autosensed so no configuration is required.
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Configuring Frame Relay
• Step 5 Configure the bandwidth for the link.
– This command affects routing operation by
protocols such as IGRP, because it is used
to help define the metric of the link.
Router(config-if)#bandwidth kilobits
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Configuring Frame Relay
• Step 6 enable Inverse ARP if disabled
– Inverse ARP is on by default.
Router(config-if)#frame-relay inverse-arp [protocol ]
[dlci]
• protocol - Supported protocols include ip, ipx,
appletalk, decnet, vines, and xns.
• dlci - The DLCI on the local interface that you want to
exchange Inverse ARP messages.
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Optional Commands
Router(config-if)# keepalive number (in seconds)
• Increase or decrease keepalive interval.
• You can extend or reduce the interval at which the router
interface sends keepalive (status inquiry) messages to the
Frame Relay switch.
– This value is usually two to three seconds faster (shorter
interval) than the setting of the Frame Relay switch to ensure
proper synchronization.
• The default is 10 seconds.
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Configuring Subinterfaces
Step 1 Configure the physical interface where you want to create subinterfaces
Step 2 Remove any network-layer address assigned to the physical interface.
–
If the physical interface has an address, frames will not be received by the local
subinterfaces.
Step 3 Configure Frame Relay encapsulation
Router(config-if)#encapsulation frame-relay [cisco | ietf]
Step 4 Select the subinterface you want to configure:
Router(config-if)#interface serial number.subinterface-number {multipoint | pointto-point}
•
subinterface-number - Subinterface number in the range 1 to 429 4967 293.
The interface number that precedes the period (.) must match the interface number to which this
subinterface belongs.
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multipoint - Select this if you:
– want the router to forward broadcasts and routing
updates that it receives
– are routing IP and you want all routers in same subnet
point-to-point - Select this if you:
– do not want the router to forward broadcasts or routing
updates
– want each pair of point-to-point routers to have its own
subnet.
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Configuring Subinterfaces (cont’d)
•
Step 5 Configure a network-layer address on the subinterface.
Router(config-if)#ip address ip-add mask
•
Step 6 If you configured the subinterface as multipoint or point-to-point,
you must configure the local DLCI for the subinterface to distinguish it
from the physical interface:
Router(config-if)#frame-relay interface-dlci dlci-number
dlci-number—Defines the local DLCI number being linked to the
subinterface.
– This is the only way to link an LMI-derived PVC to a subinterface because
LMI does not know about subinterfaces.
MULTI SUB-INTERFACES CANNOT HAVE THE SAME DLCI
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Configuring Subinterfaces (cont’d)
• If you defined a subinterface for point-to-point
communication, you cannot reassign the same
subinterface number to be used for multipoint
communication without first rebooting the router.
• Instead you can avoid using that subinterface
number and use a different subinterface number.
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Frame Relay Subinterfaces
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Configuring Point-to-Point Subinterfaces
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Verifying Frame Relay
• The show interfaces command displays
information regarding the encapsulation and
Layer 1 and Layer 2 status. It also displays
information about the following:
– The LMI type
– The LMI DLCI
– The Frame Relay data terminal
equipment/data circuit-terminating equipment
(DTE/DCE) type
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The show interface Command
LMI Status
LMI DLCI
LMI Type
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The show frame-relay lmi Command
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The show frame-relay pvc Command
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The show frame-relay map Command
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Troubleshooting Frame Relay
The debug frame-relay lmi Command
PVC Status
0x2 – Active
0x0 – Inactive
0x4 – Deleted
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Summary
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Frame Relay is an International
Telecommunications Standards Sector
(ITU-T) and American National Standard
Institute (ANSI) standard.
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Frame Relay is a packet-switched
connection oriented WAN service
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Frame Relay is often used to interconnect
LANs
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The connection through the Frame Relay
network between two DTEs is called a
virtual circuit (VC)
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Switched virtual circuits (SVCs) are
established dynamically by sending
signaling messages to the network.
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Permanent virtual circuits (PVCs) are
preconfigured by the carrier
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Frame Relay is configured on a serial
interface and the default encapsulation
type is the Cisco proprietary version of
HDLC
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By default, a Frame Relay network
provides non-broadcast multi-access
(NBMA) connectivity between remote
sites
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To enable the forwarding of broadcast
routing updates in a hub-and-spoke
Frame Relay topology, configure the hub
router with logically assigned interfaces,
called subinterfaces
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Subinterfaces are logical subdivisions of
a physical interface
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Chapter 3 (end)
Frame Relay
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