MTU size on dynamic XCF

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Tags:
IPv4
OS/390/z/OS
How does one set the MTU size on dynamic XCF, on Z/os 1.4 ?
ASKED: May 18, 2004  6:32 AM
UPDATED: November 2, 2004  5:03 AM

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Use the BSDROUTINGPARMS statement to define the characteristics of each link defined at the host over which OROUTED will send routing information to adjacent routers running the RIP protocol and which NCPROUTE will send transport PDUs to client NCPs.

Syntax

>>__BSDRoutingparms__ _TRUE__ ___________________________________________________________>
|_FALSE_|

<_________________________________________________________________
>____link_name__ _DEFAULTSize_ __cost_metric__subnet_mask__dest_addr_|___________________>
|_mtu_________|

>__ENDBSDRoutingparms___________________________________________________________________><

The TCPIP stack uses the usemaxpkt_tos flag to determine whether
to use the default MTU of 576 or the BSDROUTINGPARMS MTU when
sending packets to a remote host. The usemaxpkt_tos flag should
have the same setting as the TRUE/FALSE parameter coded in the
BSDROUTINGPARMS profile statement. However, when DynamicXCF or
VIPADYNAMIC is specified, the usemaxpkt_tos flag is always set
to false. This causes the default MTU to be used on all
interfaces which can result in significant performance
degradation.
+————————————————————-+
+ Please check our Communications Server for OS/390 homepages +
+ for common networking tips and fixes. The URL for these +
+ homepages can be found in Informational APAR II11334. +
+————————————————————-+
Problem conclusion
TCPIP has been changed so that neither a dynamicXCF statement
nor a VIPADYNAMIC statement will change the TRUE or FALSE
setting in the BSDROUTINGPARMS.
In addition, for OS390 Releases 7 and 8, the dynamically
generated BSD MTU used for dynamicXCF has been changed from 576
to 65535 for same-host links and 55296 for inter-host links.

The following documentation changes have been made:

OS/390 Communications Server
IP Configuration
Document Number SC31-8513
The “Configuring the TCPIP Address Space” Chapter

and

OS/390 Communications Server
IP Configuration Reference
Document Number SC31-8726
The “TCP/IP Profile Statements (PROFILE.TCPIP)” Chapter

should be changed as follows:

Section: IPCONFIG Statement

Add the following note after the DYNAMICXCF parameter:

NOTE: Since the dynamically generated BSD MTU used for
dynamicXCF is 65535 for same-host links and 55296 for
inter-host links, it might be desirable to change the MTU to a
lower value to prevent packet fragmentation when the MVS host
is acting as a router in a sysplex. If you wish to change the
MTU once the dynamicXCF link is created, issue a NETSTAT
DEvlinks command. This will display the link name that was
generated for dynamicXCF. Use that link name within a
BSDROUTINGPARMS statement to set the new MTU value. Issue a
vary obeyfile command to implement the desired MTU.

For example, the NETSTAT DEvlinks output for the same host
device is:

DEVNAME: IUTSAMEH DEVTYPE: MPC DEVNUM: 0000
DEVSTATUS: READY
LNKNAME: EZASAMEMVS LNKTYPE: MPC LNKSTATUS:
NETNUM: 0 QUESIZE: 0 BYTEIN: 0000000000 BYTEOUT:
BSD ROUTING PARAMETERS:
MTU SIZE: 65535 METRIC: 00
DESTADDR: 0.0.0.0 SUBNETMASK: 255.255.255.128
MULTICAST SPECIFIC:
MULTICAST CAPABILITY: YES
GROUP REFCNT
—– ——
224.0.0.1 0000000001

Issue a VARY TCPIP,,O,obeyfile.dsn command where the
obeyfile.dsn data set contains a new BSDROUTINGPARMS statement
with the EZASAMEMVS link added. For example:

;
; BSD Routing Parms
;
; link maxmtu metric subnet mask dest addr
BSDROUTINGPARMS false
OSATR 2000 0 255.255.255.128 0
VIPA 4000 0 255.255.255.128 0
EZASAMEMVS 4000 0 255.255.255.128 0
ENDBSDROUTINGPARMS

Hope this helps.

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  • Nettecdk
    Hi. Thanks for your answer. It nice to hear from a HOST network tec. Now I can se that my question was to short. I am using OSPF and OMPROUTE, there for I am not able to use BSDroutingparms. When viewing the manual on the topic "OSPF interface" in OMPROUTE there is an MTU to be set, but it has no effect.
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