home *** CD-ROM | disk | FTP | other *** search
- Newsgroups: comp.dcom.sys.cisco
- Path: sparky!uunet!usc!sol.ctr.columbia.edu!hamblin.math.byu.edu!hellgate.utah.edu!fcom.cc.utah.edu!cosmic.physics.utah.edu!corbato
- From: corbato@cosmic.physics.utah.edu (Steve Corbato)
- Subject: Strange behavior associated with MTU change on Cisco serial ports
- Message-ID: <1992Nov20.222620.5936@fcom.cc.utah.edu>
- Followup-To: comp.dcom.sys.cisco
- Originator: corbato@cosmic.physics.utah.edu
- Sender: news@fcom.cc.utah.edu
- Organization: Cosmic Ray Physics Group, University of Utah
- Distribution: na
- Date: Fri, 20 Nov 92 22:26:20 GMT
- Lines: 49
-
- Hello-
- Recently we've observed Cisco box `crashes' after modifying the MTU
- values on both ends of a serial line. Below I will describe the problem.
- In an effort to improve telnet and rlogin performance on heavily used
- (for ftp and decnet copies) 56 kb lines to our remote experimental sites,
- we have made several modifications to the configuration files on our two
- Ciscos (gw-umc and gw-hires1). A networking colleague here suggested one
- possible improvement -- reducing the MTU value on the serial interfaces
- from 1500 to ~500 bytes:
-
- int ser N
- mtu 536
- ip mtu 536
-
- As an initial experiment, I have twice attempted this change on both
- serial interfaces comprising the line between gw-hires1 and gw-umc. Each
- time several minutes after the change, the gw-umc box stopped forwarding
- packets on all interfaces. The removal of the offending MTU lines was
- required before the gw-umc Cisco worked properly again.
-
- Does anyone have any explanation for this behavior or any other
- suggestions for improving "asynchronous packet" (e.g., telnet, rlogin)
- performance on this line (we've already implemented packet type/size
- prioritization...)?
-
- Many thanks, Steve Corbato
-
- Remote : U of Utah
- E0 (MTU=1500) site : main campus
- | :
- +------+ S0(MTU=1500) 56 kb S2(MTU=1500) +----------+ AGS+
- MGS |gw-umc|----------------------------------------|uu-gw-utah| 8.2(7)
- CSC/2 +------+ : +----------+
- 8.3(5) / S1 (MTU=1500->536) : Main campus
- / : gateway
- / 56 kb :
- /
- / S0(MTU=1500->536)
- +---------+
- IGS |gw-hires1|
- 9.0(2)+---------+
- |
- |
- E0(MTU=1500)
- --
- Steve Corbato corbato@cosmic.physics.utah.edu (Internet)
- Cosmic Ray Physics Group utahco::corbato, 47623::corbato (HEPnet, NSI)
- Physics Dept., Univ. of Utah corbato@utahcca (BITnet)
- Salt Lake City UT 84112 USA (801) 581-5053 (office), -4801 (Fax)
-