Avaya Configuring Frame Relay Services Instrukcja Użytkownika Pobierz pdf (Strona 62)

Configuring Frame Relay Services

1-40

308624-14.10 Rev 01

Table 1-9 summarizes the changes you may need to make to the MTU value for

your frame relay configuration. In addition to these adjustments, other router

subsystems using frame relay, such as IP or IPX, may also affect the MTU value.

Those adjustments are based on the particular subsystem configuration.

Based on the values in Table 1-9

, if you configure IP over frame relay on a

synchronous line, the largest IP datagram that the router transmits is 1539. This

number is determined by taking the default MTU value (1600) and subtracting the

overhead packet sizes for WCP (3), WEP (56), and the CRC (2). To ensure that the

router sends an IP datagram size of 1600 bytes, you must set the MTU value to

1661. For more information about the MTU parameter, see Configuring WAN Line

Services.

Table 1-9. MTU Adjustments for Frame Relay Connections

Reason for the

Adjustment

Size of

Overhead

Packets (bytes)

Lines for which the

Adjustment Applies Notes

Data compression

(WCP) overhead

3 Synchronous, MCT1,

MCE1, ISDN BRI

The router always subtracts the WCP

header regardless of whether WCP is

enabled. Consequently, the amount of data

that the router sends across the frame relay

line will be less than you configured for the

MTU value.

Data encryption

(WEP) overhead

56 Synchronous, MCT1,

MCE1, ISDN BRI

The router always subtracts the WEP

header regardless of whether WCP is

enabled. Consequently, the amount of data

that the router sends across the frame relay

line will be less than you configured for the

MTU value.

Cyclic redundancy

check (CRC)

2 or 4 Synchronous, MCT1,

MCE1, ISDN BRI

The router always subtracts the CRC size.

Consequently, the amount of data that the

router sends across the frame relay line will

be less than you configured for the MTU

value. The default CRC value is 2.

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