Bursting is one of the features that has made Frame Relay so popular. Bursting enables a user
to transmit data faster than the CIR for a short period of time. Figure 29.5 shows the difference
between the CIR and the access rate and how the burst traffic rate can increase beyond the CIR.
The network controls this bursting capability, and it usually does not result in any additional fees
on the user. There is a catch, though. Some burst traffic has the Discard Eligibility (DE) bit turned
on, indicating excess traffic above CIR. If a Frame Relay switch becomes congested, traffic with
the DE bit set (excess burst traffic) is the first to be dropped.
You will see the following symbols: Bc, Be, and Tc. Committed burst size (Bc)
and excess burst size (Be) are the two types of burst sizes. Each of these sizes is measured over
the committed rate measurement interval (Tc). Bc is the maximum amount of data that the network
can guarantee will be delivered during the time Tc. Be is the amount of traffic by which the
user can exceed the committed burst size.
For example, take a user who buys a Frame Relay circuit with the following characteristics:
1,544Kbps access rate
256Kbps committed information rate
Four-second committed time interval
The user is guaranteed a CIR of 256Kbps over a four-second period. The user could transmit
256Kbps for four seconds, and the network would ensure delivery. The user could alternately
send 1,024Kbps for one second, representing the committed burst. However, for the remaining
three seconds, there would be no guarantee of delivery for the excess burst traffic.
IT Certification CCIE,CCNP,CCIP,CCNA,CCSP,Cisco Network Optimization and Security Tips
Committed Information Rate (CIR) network
The committed information rate (CIR) is the rate at which the provider guarantees to deliver
network traffic. The CIR is always less than or equal to the access rate. The CIR is advertised
in Kbps and is actually averaged over a specified time period, referred to as committed rate
measurement interval (Tc). This is what the cost of the Frame Relay connection is normally
based upon.
network traffic. The CIR is always less than or equal to the access rate. The CIR is advertised
in Kbps and is actually averaged over a specified time period, referred to as committed rate
measurement interval (Tc). This is what the cost of the Frame Relay connection is normally
based upon.
Access Rate
Access rate is the maximum speed at which data can be transferred to the Frame Relay network.
This number denotes the actual line speed of the connection to the provider. In a dedicated circuit,
you would consider this the actual data rate. However, in a Frame Relay network, this is
considered the maximum data rate.
This number denotes the actual line speed of the connection to the provider. In a dedicated circuit,
you would consider this the actual data rate. However, in a Frame Relay network, this is
considered the maximum data rate.
Factors Affecting Performance
Network performance at the router level is affected by three primary factors:
Access rate
Committed information rate (CIR)
Bursting
Each of these has an effect on Frame Relay.
Access rate
Committed information rate (CIR)
Bursting
Each of these has an effect on Frame Relay.
Frame Relay Congestion Control
Frame Relay is optimized for speed and contains little error control. Frame Relay will discard
errored frames and will not attempt to recover from the error either through retransmission or
repair. Ideally, users can send as much data as they want across the network without interference.
However, because user requests for bandwidth often outstrip the network’s capability to
provide bandwidth, a mechanism is needed to handle congestion in the frame switch.
In this section, you will learn about the factors that affect network performance, as well as
methods for handling Frame Relay congestion. The two primary methods of congestion handling
use Frame Relay switches and routers.
errored frames and will not attempt to recover from the error either through retransmission or
repair. Ideally, users can send as much data as they want across the network without interference.
However, because user requests for bandwidth often outstrip the network’s capability to
provide bandwidth, a mechanism is needed to handle congestion in the frame switch.
In this section, you will learn about the factors that affect network performance, as well as
methods for handling Frame Relay congestion. The two primary methods of congestion handling
use Frame Relay switches and routers.
Configuring Frame Relay cisco
The first step in configuring Frame Relay is to select the interface and then enable the Frame
Relay encapsulation on the serial interface. You do this with the
encapsulation frame-relay
command. As you will notice in the following router configuration commands, there are two
options:
cisco
and
ietf
.
Router#
config t
Router(config)#interface serial 0
Router(config-if)#encapsulation frame-relay [cisco or ietf]
Cisco is the default encapsulation, which means that you have another Cisco router on the
remote end with which your router will communicate. You will use the IETF encapsulation
when communicating with a remote router that is not a Cisco device.
After you configure the encapsulation to the serial interface, you then need to add the Network
layer address, DLCI number, and LMI type. Cisco’s capability to autosense the LMI type has
greatly simplified configuration. The following router configuration shows the process of specifying
the IP address and DLCI number, but not the LMI type because it is automatically detected:
Router(config-if)#ip address 172.16.10.1 255.255.255.0
Router(config-if)#frame-relay interface-dlci 16
Router(config-if)#no shutdown
Relay encapsulation on the serial interface. You do this with the
encapsulation frame-relay
command. As you will notice in the following router configuration commands, there are two
options:
cisco
and
ietf
.
Router#
config t
Router(config)#interface serial 0
Router(config-if)#encapsulation frame-relay [cisco or ietf]
Cisco is the default encapsulation, which means that you have another Cisco router on the
remote end with which your router will communicate. You will use the IETF encapsulation
when communicating with a remote router that is not a Cisco device.
After you configure the encapsulation to the serial interface, you then need to add the Network
layer address, DLCI number, and LMI type. Cisco’s capability to autosense the LMI type has
greatly simplified configuration. The following router configuration shows the process of specifying
the IP address and DLCI number, but not the LMI type because it is automatically detected:
Router(config-if)#ip address 172.16.10.1 255.255.255.0
Router(config-if)#frame-relay interface-dlci 16
Router(config-if)#no shutdown
Frame Relay Local Management Interface (LMI)
In 1990, the Group of Four developed extensions to the Frame Relay standard to help ease the
management and configuration burden. One of these extensions was the
Local Management
Interface (LMI)
. LMI provides for virtual circuit status messages and multicasting.
Cisco routers support three versions of the LMI standard: Cisco, ANSI, and ITU-T (Q.933a).
LMI autosense, the automatic detection of the LMI type, was introduced in IOS version 11.2. LMI
autosense determines the LMI type by rapidly trying each of them in order: ANSI, ITU-T (Q.933a),
and then Cisco. If it cannot determine the LMI type within 60 seconds, it will terminate the
autosense process and revert to the Cisco LMI type.
After LMI is established between the router and the switch, the next stage is DLCI determination
and IARP. The router will query the switch, asking what the DLCI(s) is/are for this
circuit. The router will configure itself with that DLCI(s) and query the switch to determine
the status of the circuit.
This query is the first stage of discovery. The query that is sent includes the local router’s network
information. The remote router will record the network information and reply in kind.
The local router will map the DLCI it learned from static or dynamic addressing to other network
addresses it discovered from queries.
When an IARP is made, the router updates its map table with one of three possible LMI connection
states:
Active
The connection is active, and the routers can exchange data through the PVC.
Inactive
The local connection to the Frame Relay switch is working, but the remote end of the
PVC is not communicating to the Frame Relay switch.
Deleted
No LMI keepalive information from the switch to the router is being received for this
PVC. This could be because no LMI is actually being exchanged or because the DLCI is not configured
on the ingress switch.
Figure 29.4 shows that the Chicago office PVC to Miami is deleted because the Miami office
is not receiving keepalives from the Frame Relay switch. Neither this inactive state nor this deleted
state affect the other connections (PVCs) that Chicago might have to other locations.
management and configuration burden. One of these extensions was the
Local Management
Interface (LMI)
. LMI provides for virtual circuit status messages and multicasting.
Cisco routers support three versions of the LMI standard: Cisco, ANSI, and ITU-T (Q.933a).
LMI autosense, the automatic detection of the LMI type, was introduced in IOS version 11.2. LMI
autosense determines the LMI type by rapidly trying each of them in order: ANSI, ITU-T (Q.933a),
and then Cisco. If it cannot determine the LMI type within 60 seconds, it will terminate the
autosense process and revert to the Cisco LMI type.
After LMI is established between the router and the switch, the next stage is DLCI determination
and IARP. The router will query the switch, asking what the DLCI(s) is/are for this
circuit. The router will configure itself with that DLCI(s) and query the switch to determine
the status of the circuit.
This query is the first stage of discovery. The query that is sent includes the local router’s network
information. The remote router will record the network information and reply in kind.
The local router will map the DLCI it learned from static or dynamic addressing to other network
addresses it discovered from queries.
When an IARP is made, the router updates its map table with one of three possible LMI connection
states:
Active
The connection is active, and the routers can exchange data through the PVC.
Inactive
The local connection to the Frame Relay switch is working, but the remote end of the
PVC is not communicating to the Frame Relay switch.
Deleted
No LMI keepalive information from the switch to the router is being received for this
PVC. This could be because no LMI is actually being exchanged or because the DLCI is not configured
on the ingress switch.
Figure 29.4 shows that the Chicago office PVC to Miami is deleted because the Miami office
is not receiving keepalives from the Frame Relay switch. Neither this inactive state nor this deleted
state affect the other connections (PVCs) that Chicago might have to other locations.
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