Usage Guidelines
The Cisco IOS software uses the following formula to determine the minimum N1 value:
(128
(default
packet
size)
+
LAPB
overhead
+
X.25
overhead
+
2
bytes
of
CRC)
*
8
The Cisco IOS software uses the following formula to determine for the maximum N1 value:
(hardware
MTU
+
LAPB
overhead
+
X.25
overhead
+
2
bytes
of
CRC)
*
8
LAPB overhead is 2 bytes for modulo 8 and 3 bytes for modulo 128.
X.25 overhead is 3 bytes for modulo 8 and 4 bytes for modulo 128.
You need not set N1 to an exact value to support a particular X.25 data packet size. The N1 parameter prevents the processing of any huge frames that result from a “jabbering” interface, an unlikely event.
In addition, the various standards bodies specify that N1 be given in bits rather than bytes. While some equipment can be configured in bytes or will automatically adjust for some of the overhead information present, Cisco devices are configured using the true value, in bits, of N1.
You cannot set the N1 parameter to a value less than that required to support an X.25 data packet size of 128 bytes. All X.25 implementations must be able to support 128-byte data packets. Moreover, if you configure N1to be less than 2104 bits, you receive a warning message that X.25 might have problems because some nondata packets can use up to 259 bytes.
You cannot set the N1parameter to a value larger than the default unless the hardware MTU size is first increased.
The X.25 software accepts default packet sizes and calls that specify maximum packet sizes greater than those the LAPB layer supports, but negotiates the calls placed on the interface to the largest value that can be supported. For switched calls, the packet size negotiation takes place end-to-end through the router so the call will not have a maximum packet size that exceeds the capability of either of the two interfaces involved.
Caution |
The LAPB N1 parameter provides little benefit beyond the interface MTU and can easily cause link failures if misconfigured. Cisco recommends that this parameter be left at its default value.
|
Examples
The following example shows how to use the question mark (?) command to display the minimum and maximum N1 value. In this example, X.25 encapsulation has both the LAPB and X.25 modulo set to 8. Any violation of this N1 range results in an “Invalid input” error message.
router(config)# interface serial 1
router(config-if)# lapb n1 ?
<1080-12056> LAPB N1 parameter (bits; multiple of 8)
The following example sets the N1 bits to 16440:
router(config)# interface serial 0
router(config-if)# lapb n1 16440
router(config-if)# mtu 2048