IP Networking

Hello,

I would appreciate some help with the following. We have 3 SUN X4450 servers, each of these servers talk to each other, one is an application server the other is a database server and the third is a web server. We also have numerous workstation and ACD connections.

When I snoop the application server I can see the following message a lot and I mean a lot

(warning) packet length greater than MTU in buffer offest

The NIC cards on the servers are all set as 1500 and the customer ensures us each router/switch/firewall has the same MTU.

When I snoop a particular packet I can see the Packet size as 1967 which based on the MTU doesn't seem possible. Also the do not fragment bit it set to 1.

So much questions are this;

1) Would there be a network device that is advertising an MTU greater than 1500
2) Would there be a problem with the Solaris server config, the e1000g.conf file hasn't be changed from the default

Any help would be greatly appreciated as finding where the issues lies is proving rather difficult.

Thanks,
Giles

Sometimes localhost, a software device, has a big MTU, like 8K, and it may apply for all local packets. Ethernet is 1500 total, and the payload is less IPV4 40 bytes plus 4 x any options. It seems like the app should be allowing frag if using non-tcp, and tcp is a stream, so the app has no business fussing with packet bits and lengths.

It may be PATH MTU testing, where the routing tables is extended with dynamic host route lines and the MTU of path to host. If ICMP says the packet does not fit, it will retry with a lower size, hence it does not want fragmentation, it wants PATH MTU information. Black holes are when firewalls block that ICMP informations, and the PATH MTU has to time out and assume it went into a black hole, and try a lower packet size to ensure a route exists.

Not Solaris, but you should see similar:

Code:

$ netstat -rn
Routing tables
Dest/Netmask          Gateway            Flags  Refs       Use  Interface  Pmtu
127.0.0.1             127.0.0.1          UH       0   64980021  lo0        4136
191.136.182.236       191.136.182.236    UH       0   23515341  lan7       4136
191.136.182.0         191.136.182.236    U        2          0  lan7       1500
127.0.0.0             127.0.0.1          U        0          0  lo0        4136
default               191.136.182.1      UG       0          0  lan7       1500
$