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Beginner

MTU and WS-C6506-E

Model: WS-C6506-E (R7000) processor (revision 1.1)

IOS: 12.2(33)SXI7, RELEASE SOFTWARE (fc1)

Module: 48-port 10/100/1000 RJ45 EtherModule WS-X6148A-GE-TX

 

Hello! I'm trying to determine how this model handles MTU and I think I'm starting to confuse myself.

 

PROBLEM: The problem is that not large enough frames can pass from point A to point B in the network. And I'm now stuck at analyzing an interface on a WS-C6506-E. The traffic enters the interface via a L2 trunk port into a subinterface on the 6500 with MTU set to 1512, and from there travels over an MPLS network.

 

I don't think the problem lies on this specific interface, but I need to understand how the MTU is handled to say that this is true. And at this I am at a bit of a loss atm.

 

From this document: https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/12-2SX/configuration/guide/book/intrface.html

 

I keep bouncing back to this bit: 

Bridged and Routed Traffic Size Check at Ingress Gigabit Ethernet Ports

 

Gigabit Ethernet LAN ports configured with a nondefault MTU size accept frames containing packets of any size larger than 64 bytes. With a nondefault MTU size configured, Gigabit Ethernet LAN ports do not check for oversize ingress frames.

 

This to me, sounds like as long as any MTU value which differs from the system default is configured, it will cause the interface to accept any frame regardless of the size. Which to me makes no sense, since you specify an actual value for the MTU when configuring.

 

But I'm leaning towards that it actually is this way, since when I send frames of size 1550 for example there is no increase in the counter for giants on the interface. But I'm not quite sure that this is the correct counter to look for. When I use "show counters int x/x" there are no counters which indicates that anything is dropped.

 

However this bit also from the same document, triggers my interest:

Routed Traffic Size Check on the PFC

 

For traffic that needs to be routed, Jumbo frame support on the PFC compares traffic sizes to the configured MTU sizes and provides Layer 3 switching for jumbo traffic between interfaces configured with MTU sizes large enough to accommodate the traffic. Between interfaces that are not configured with large enough MTU sizes, if the "do not fragment bit" is not set, the PFC sends the traffic to the RP to be fragmented and routed in software. If the "do not fragment bit" is set, the PFC drops the traffic.

 

However I'm not quite sure where to find a counter which would correspond to this.

 

Any and all help is much appreciated.

 

1 ACCEPTED SOLUTION

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Cisco Employee

Tob,

I have attached a white paper a colleague of mine wrote many years ago, but it is still the best paper I know for jumbo frames and MTU for Cat 6K (it even has CatOS config examples !)

Many of the links at the end may not work given the age of the paper :-)

Cheers,
Scott

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Highlighted
Cisco Employee

Tob,

I have attached a white paper a colleague of mine wrote many years ago, but it is still the best paper I know for jumbo frames and MTU for Cat 6K (it even has CatOS config examples !)

Many of the links at the end may not work given the age of the paper :-)

Cheers,
Scott

View solution in original post

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VIP Expert

Sometime back, I recall a Cisco SE telling me that many Cisco switches (e.g. a 6500), that can support jumbo frames, will accept jumbo size frames even if not configured to do so. However, those switches will not forward/transmit a jumbo frame unless configured to do so. That appears to agree with the documentation you reference.
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