Frame Relay Virtual Circuit Parameters - Page 3

By Cisco Press | Posted Jan 22, 2002
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PVC DLCIs
Although DLCI values can be 10, 16, or 23 bits in length, 10-bit DLCIs have become the de facto standard for Frame Relay WAN implementations.

The 10-bit DLCI values, as recommended by the Frame Relay Forum, are allocated as Table 15-5 indicates.


Table 15-5: Frame Relay Forum 10 Bit DLCI Recommendations

DLCI Value Function
0 FRF -- In-channel signaling
1 to 15 Reserved
16 to 1007 Available for VC endpoint assignment
1008 to 1022 Reserved
1023 LMI

The 10-bit DLCI values, as recommended by both the ANSI (T1.618) and the ITU-T (Q.922), are allocated as Table 15-6 indicates.

Table 15-6: ANSI (T1.618) and ITU-T (Q.922) 10-Bit DLCI Recommendations

DLCI Value Function
0 In-channel signaling and management (LMI)
1 to 15 Reserved
16 to 991 Available for VC endpoint assignment
992 to 1007 Frame Relay bearer service Layer 2 management
1008 to 1022 Reserved
1023 Reserved for in-channel layer management


NOTE:   The number of DLCIs configurable per port varies depending on the traffic level. All 1,000 DLCIs can be used. However, 200 to 300 is a common maximum. If the DLCIs are used for broadcast traffic, 30 to 50 is a more realistic number due to CPU overhead in generating broadcasts.

Within the Cisco IOS, the number of PVCs that is configurable per interface is limited to 255; this means that a Frame Relay serial interface is limited to 255 subinterfaces. The 255 subinterface limit is dependent on how they are configured; however, no more than 255 point-to-point subinterfaces with one DLCI each can exist.

You must consider and deal with some practical performance issues on an individual case basis. The higher the CIR on the DLCIs, the more impact that the individual interface's ability will have on supporting the traffic flow.

A T1 could certainly be expected to handle 24 56 K DLCIs with little problem. However, substantial broadcast traffic could affect the performance. If, for example, 50 56 K DLCIs are configured into a T1 interface, traffic issues, such as congestion and dropped traffic, will arise. This configuration is referred to as oversubscription. For example, consider the following two scenarios:

A network configuration that consists of 24 (DLCIs/PVCs) 4 56 kbps (CIR per PVC) = 1.344 Mbps is well within the T1 bandwidth limitation of 1.344/1.536 Mbps (depending on physical line coding; AMI = 1.344 Mbps, B8ZS = 1.536 Mbps).

This configuration is not oversubscribing the interface because available bandwidth is sufficient to support the traffic requirement: 1.344 Mbps equal to or less than 1.344/1.536 Mbps.

A network configuration of 50 (DLCIs/PVCs) 4 56 kbps (CIR per PVC) = 2.800 Mbps far exceeds the maximum bandwidth supported by a T1 limitation of 1.344/1.536 Mbps (depending on physical line coding; AMI = 1.344 Mbps, B8ZS = 1.536 Mbps).

This configuration is oversubscribing the interface because the bandwidth available is not sufficient to support the traffic requirement: 2.800 Mbps equal to or greater than 1.344/1.536 Mbps.


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Our next segment from Cisco Press' Network Consultants Handbook will cover polling, error handling, and specification enhancements.

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