Cisco AS5300 Series Universal Gateways

Field Notice: MTU Limited to 1500 Bytes on Cisco AS5350, AS5400, and AS5850

Revised September 18, 2001

Products Affected






Problem Description

The Cisco AS5350, AS5400 and AS5850 platforms are limited to a MTU of 1500 bytes. This will prevent X.75/LAPB-TA from working, because X.75/LAPB-TA by default uses a MTU larger than 1500. The command to change the size of the MTU per interface: MTU xxx is not supported. This limitation is listed as DDTS CSCdv30422.

Customers increasing the MTU size larger than 1500 bytes will see counters for dropped packets increasing, and calls will drop.


The Cisco AS5350, AS5400 and AS5850 HDLC controller chip has only one register for maximum packet size that is shared across all the T1/E1s. This limitation means that IOS cannot honor the "per serial" CLI MTU change command. The default/maximum MTU is 1500 bytes. An example of the MTU change command is shown below:

#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
(config)#interface serial 1:15
(config-if)#mtu 1500

The X.75 protocol supports a MTU size of 2048 bytes. If customers change the MTU size on a client terminal adapter to 2048, the hardware driver for IOS will not be able to handle this size packet. Voice calls will drop because LAP-B frames larger than 1500 bytes are dropped.

If customers suspect they are getting dropped packets larger than 1500 bytes, they can check the "giants" counter under the ISDN interface. To check the counter, issue the command below:

#show interface serial 1:10
 Serial1:10 is up, line protocol is up
   Hardware is DSX1
   MTU 2048 bytes, BW 64 Kbit, DLY 20000 usec,
      reliability 249/255, txload 1/255, rxload 11/255
   Encapsulation LAPB-TA, loopback not set
   DTR is pulsed for 1 seconds on reset
   Time to interface disconnect:  idle 00:01:44
   LAPB dxe/DCE, state REJSENT, modulo 8, k 7, N1 16416, N2 20
       T1 3000, T2 0, interface outage (partial T3) 0, T4 0
       VS 4, VR 7, tx NR 7, Remote VR 4, Retransmissions 0
       Queues: U/S frames 0, I frames 0, unack. 0, reTx 0
       IFRAMEs 20/83 RNRs 0/0 REJs 15/0 SABM/Es 0/1 FRMRs 0/0 DISCs 0/0
   Last input never, output never, output hang never
   Last clearing of "show interface" counters 00:12:03
   Input queue: 0/10/0/0 (size/max/drops/flushes); Total output drops: 0
   Queueing strategy: weighted fair
   Output queue: 0/1000/64/0 (size/max total/threshold/drops)
      Conversations  0/1/16 (active/max active/max total)
      Reserved Conversations 0/0 (allocated/max allocated)
   5 minute input rate 3000 bits/sec, 4 packets/sec
   5 minute output rate 0 bits/sec, 0 packets/sec
      119 packets input, 57300 bytes, 0 no buffer
      Received 0 broadcasts, 0 runts, 42 giants, 0 throttles
      42 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
      52 packets output, 2318 bytes, 0 underruns
      0 output errors, 0 collisions, 0 interface resets
      0 output buffer failures, 0 output buffers swapped out
      1 carrier transitions
   Timeslot(s) Used:11, Transmitter delay is 0 flags

Because of the hardware constraint, these platforms can never support MTU changes per interface. This DDTS is a proposal to support 2048 byte MTU for the entire box via another CLI command.

Problem Symptoms

Calls will be dropped because LAP-B frames larger thatn 1500 bytes are dropped.

Under the ISDN interface, the counter "giants" will be incrementing. See the background description for the IOS command to display the counter.


Change the MTU size to 1500 on the client Terminal Adapter. Please refer to the Terminal Adapter documentation for the correct command.


To follow the bug ID link below and see detailed bug information, you must be a registered user and you must be logged in.



CSCdv30422 (registered customers only)

MTU size 2048 is not supported

CSCdu64782 (registered customers only)

Related DDTS for 5300 platform

For More Information

If you require further assistance, or if you have any further questions regarding this field notice, please contact the Cisco Systems Technical Assistance Center (TAC) by one of the following methods:

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