Newsgroups: comp.dcom.sys.cisco
Subject: Cisco 500-CS ignoring MTU on SLIP links
From: don@rata.vuw.ac.nz (Don Stokes)
Date: Mon, 24 Aug 1992 00:47:21 GMT
Organization: Victoria University of Wellington

Hello folks.

What to I have to do to get a Cisco 500-CS to honour its MTU parameters on its
SLIP interfaces?

My test rig is:

         130.195.2.0                         130.195.23.0
       <------+------>                 <-+------------------+->
              |                          |                  |
           500-CS /\/\/\ Line /\/\/\ PC-Router    Laptop (NCSA telnet)
                        Monitor

The PC-Router uses an MTU of 1000 for its SLIP links.  However, despite 
telling every part of the terminal server that would listen to use an MTU 
of 800, large packets are being fragmented at 1500 bytes.  I have verified 
this with the line monitor.  The PC-Router drops packets over the MTU in 
size.

Since one of the major objectives in getting the 500-CS was to replace the 
boat-anchors functioning as PC-routers in our machine room, I'd kinda like 
this to work...  (the boat-anchors at the far ends of the links are _not_
being replaced, alas).

So, what have I missed?


The relevant setups are as follows:
-------------------------------------------------------------------------------
dialgate1#sh version
CS Software (CS500-K), INTERIM SOFTWARE Version 9.1A(0.2)
Copyright (c) 1986-1992 by cisco Systems, Inc.
Compiled Thu 04-Jun-92 18:43

System Bootstrap, Version 4.4A(0.2), INTERIM SOFTWARE

dialgate1 uptime is 1 hour, 40 minutes
System restarted by reload
Running default software

Cisco-500 (68331) processor with 2048K bytes of memory.
1 Ethernet/IEEE 802.3 interface.
16 terminal lines.
32K bytes of non-volatile configuration memory.
Configuration register is 0x101

dialgate1#sh line 2

 Tty Typ    Tx/Rx    A Modem  Roty AccO AccI  Uses    Noise Overruns
S  2 TTY  9600/9600  -    -      -    -    -     0        0        0

Line 2, Location: "Don's test SLIP link", Type: ""
Length: 24 lines, Width: 80 columns
Baud rate (TX/RX) is 9600/9600, no parity, 1 stopbits, 8 databits
Status: Ready, SLIP mode, Hardware XON/XOFF
Capabilities: SLIP allowed, Permanent SLIP
Modem state: Ready
Special Chars: Escape  Hold  Stop  Start  Disconnect  Activation
                ^]x    none   -     -       none         
Timeouts:      Idle EXEC    Idle Session   Modem Answer  Session   Dispatch
               0:10:00        never         0:00:15      not imp   not set
Session limit is not set.
Allowed transports are telnet rlogin.  Preferred is telnet
No output characters are padded
Characters causing immediate data dispatching:
   Char    ASCII
Line is running SLIP version 0 for address 130.195.2.180
Receive state is RECV, Transmit state is IDLE
 Input packet exists. 0 Packets queued.

dialgate1#sh int async 2

Async 2 is up, line protocol is up 
  Hardware is Async Serial
  MTU 800 bytes, BW 9 Kbit, DLY 100000 usec, rely 255/255, load 1/255
  Encapsulation SLIP, loopback not set, keepalive set (10 sec)
  Last input 0:00:46, output never, output hang never
  Last clearing of "show interface" counters never
  Output queue 0/3, 0 drops; input queue 0/75, 0 drops
  Five minute input rate 0 bits/sec, 0 packets/sec
  Five minute output rate 0 bits/sec, 0 packets/sec
     8 packets input, 480 bytes, 0 no buffer
     Received 0 broadcasts, 0 runts, 0 giants
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
     0 packets output, 0 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface resets, 0 restarts
     0 carrier transitions
dialgate1#sh ip int async 2

Async 2 is up, line protocol is up
  Broadcast address is 255.255.255.255
  MTU is 800 bytes
  Helper address is not set
  Outgoing access list is not set
  Proxy ARP is enabled
  Security level is default
  Split horizon is enabled
  ICMP redirects are always sent
  ICMP unreachables are always sent
  ICMP mask replies are never sent
  IP fast switching is disabled
  Gateway Discovery is disabled
  Router Discovery is disabled
  IP accounting is disabled
  TCP/IP header compression is disabled
  Probe proxy name replies are disabled
dialgate1#sh slip   

Slip statistics:
  Rcvd: 134 packets, 0 bytes
        0 format errors, 0 checksum errors, 0 overrun, 0 no buffer
  Sent: 80 packets, 0 bytes, 4 dropped



 --More--  

 Tty Mod   Address      Istate  Ostate   Qd InPack OutPac Inerr Dropped MTU Qsz
   1  -  Dynamic        IDLE    IDLE      0      0      0     0      0 1500   3
*  2  -  130.195.2.180  RECV    IDLE      0    134     80     0      4  800   3
  12  -  130.195.22.2   IDLE    IDLE      0      0      0     0      0 1500   3
dialgate1#
-------------------------------------------------------------------------------

--
Don Stokes, ZL2TNM (DS555)                      don@zl2tnm.gen.nz (home)
Network Manager, Computing Services Centre          don@vuw.ac.nz (work)
Victoria University of Wellington, New Zealand.           +64-4-495-5052
Newsgroups: comp.dcom.sys.cisco,comp.protocols.tcp-ip