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Help Understanding Jumbo Frames

dave cooper
Level 1
Level 1

I am trying to understand jumbo frames and want to understand what happens in the following scenario. I have two VLANs configured on a switch.

 

VLAN 10 has mtu 9216 enabled on the SVI interface. The other VLAN is 11 and it has a mtu of 1518 on the SVI interface. if a host in VLAN 10 sends a jumbo packet of 9216 to a host in VLAN 11 which only supports a mtu of 1518 what happens on the switch. Will the switch fragment the 9216 packet into 1518 byte packets?

 

Thanks

2 Replies 2

ronbuchalski
Level 1
Level 1

You can test it and see what happens, but what I suspect will happen is that the MTU discovery process will find that smaller MTU and fragment the packet.  But if the packet has the DF (Do not Fragment) bit set, the large packet will be dropped.

-rb

Charles Hill
VIP Alumni
VIP Alumni

Hey Dave,

If a jumbo frame is sent from vlan 10 to vlan 11, the packet will be fragmented unless the DF bit is set.

 

When a jumbo frame traverses only layer 2, it will be dropped.

When a jumbo frame is routed, it will be fragmented.(unless the df bit is set, in which case it's dropped)

 

The posting below discusses jumbo frames in detail.

https://supportforums.cisco.com/discussion/11399941/does-jumbo-frames-ever-get-fragmented

 

Hope this helps,

Please rate helpful posts.

Thanks.

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