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Re: Re: OID's for Cisco equipment?  runts, giants, CRC's, collisions etc

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I've gone around the loop here several times about the MTU settings, and it's important to specify where the settings are being configured and what the effect actually is on the device.

 

The layer-2 interfaces should probably be set to 9216, but the layer-3 interfaces to 9000. It is not obvious on much equipment what you are actually configuring when you say MTU.

 

a) layer-2 maximum frame size

juniper: set interfaces ge-0/0/0 mtu 9216

5406zl: jumbo max-frame-size 9216

 

Cisco 76xx routers:

Router(config)# interface gigabitethernet 1/2

Router(config-if)# mtu 9216

 

b) layer-3 maximum packet size.

juniper: set interfaces ge-0/0/0.0 mtu 1500

juniper: set interfaces ge-0/0/0.0 mtu 9000

5406zl: jumbo ip-mtu 1500

5406zl: jumbo ip-mtu 9000

Router(config)# interface vlan200

Router(config-if)# ip mtu 1500 (or 9000)

 

Layer-3 MTU size MUST be smaller than the layer-2 MTU; on some systems setting the layer-3 MTU implicitly sets the layer-2 MTU, and other like windows setting the MTU in the layer-2 adapter driver setting sets it implicitly for layer-3]

 

If a frame larger than the permitted maximum frame size is received on an ethernet interface the device (switch or router) MAY discard it.

If a frame larger than the permitted maximum frame size is to be transmitted on an ethernet interface the device MUST discard it

 

If an IP packet larger than the permitted maximum frame size is transmitted on an ethernet interface the router MUST fragment it UNLESS the DF bit is set, in which case it discards the packet and sends an ICMP Fragment required notice.

 

there is all sorts of subtle breakage possible because network devices are not simply switches OR routers now, they are some kind of hybrid.

 



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