How to disable Path MTU Discovery in TCP/IP Services?

[Jeremy Begg]

Hi,

I have a customer running OpenVMS VAX V7.3 with TCP/IP Services V5.3 ECO 4.

Recently they made some changes to their network infrastructure and we're now seeing a lot of dynamic routes showing up in TCP/IP Services.  For example,
 
Type           Destination                           Gateway
 
AN    0.0.0.0                               192.168.50.253
DH    119.252.17.13                         192.168.50.254
AH    127.0.0.1                             127.0.0.1
DH    172.16.0.2                            192.168.50.254
DH    172.16.0.4                            192.168.50.254
DH    172.16.0.5                            192.168.50.254
DH    172.16.1.1                            192.168.50.252
AN    192.160.0.0/16                        192.168.50.252
DH    192.160.20.2                          192.168.50.252
DH    192.160.20.5                          192.168.50.252
DH    192.160.20.8                          192.168.50.252
DH    192.160.20.11                         192.168.50.252
etc.

The problem is that sometimes the dynamic route is via the wrong gateway, e.g.

DH    192.160.24.102                        192.168.50.252
DH    192.160.24.103                        192.168.50.254

The second route entry is wrong; that gateway is the Internet router yet both of those hosts are on the same internal subnet.

The puzzle is, "what is creating these routes?"

Apparently the new routers are sending out RIP updates so I checked TCPIP$CONFIG.COM and confirmed that dynamic routing (GATED or ROUTED) is disabled, and I don't see any GATED or ROUTED processes on the system.

I've seen routing tables like this in MultiNet which uses them to set up different MTU sizes for different hosts, and I suspect the same thing is happening in TCP/IP services since both stacks are derived from the BSD networking code.   (Mind you, I've never seen MultiNet set up the wrong gateway for one of these routes.)

In order to test this I'd like to disable path MTU discovery in TCP/IP services but I can't work out how.  Can someone point me to the appropriate command or documentation page?

Thanks,

Jeremy Begg

[Buenaventura Acera]

Hi Jeremy,

you have that information in the Tuning and Troubleshooting manual but, basically to disable Path MTU discovery you must set the pmtu_enabled parameter to 0 in sysconfig.

TCPIP> sysconfig -c inet --> To check parameter value

TCPIP> sysconfig -r inet pmtu_enabled=0 --> To disable (enabled = 1 by default)

If this change works for your problem, you can make the modification permanent including the sysconfig command in TCPIP$SYSTARTUP.COM

Regards,

Ventura

[Jeremy Begg]

Thanks Ventura, that's the command I was looking for.  I set the parameter and then manually removed the host-specific routes using TCPIP SET NOROUTE ip-address.

It's certainly reduced the number of host-specific routes being added automatically, but I'm not sure it's completely solved my problem.  I'll create a new topic when I've got some more details.

Thanks,

Jeremy Begg