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Pre-fragmentation and Link fragmentation and Interleaving questions

I'm going to study two features on security and network performance,  which are:

1- Pre-fragmentation (look ahead fragmentation)

2- Link fragmentation and Interleaving

I have some questions that need your help to answer the following:

> 1- Which cisco  series supports the two features?

> 2- Does Cisco 7206 IP router support any of these features?

> 3- How links and other devices which are not support pre-fragmentation would deal with the pre-fragmented IPsec  traffic?

> 4- Are there any difference between Cisco 7206 IP router and Cisco 7206 VXR?

> 5- I would going to simulate a network to study the above features. Could you have any support ( Ex:Disscus with you and experts the network model suggestions , features parameters and protocols' configurations, affects on performance and others)!


Hi ;

"I'm completely lost with what you want to achieve. It makes no sense to me to want at the same time pre-fragmentation and PMTUD because the goal of PMTUD is to avoid fragmentation and save router resources !!!"

Laurent, Please refer to

PMTU function doesn't mean to prevent fragmentation but just adjusting the MTU to avoid packet drop if fragmentation is in need; i know that the default setting for the DF is 1 when PMTU is enabled but you can clear the DF to allow fragment (like example 1 in the reference doc. at the above link)

" Actually the tunnel PMTUD will copy the DF bit of the inner packet to the GRE header so it may not be always 1."

Please refer to Senario 5 of the above reference: It mentions that PMTU is enabled and the DF bit is not copied when the IP packet is encapsulated by GRE.




I'm sorry but I disagree. PMTUD will compute the MTU with which no fragmentation will occur on the path between a source and a destination. Packets drop + ICMP too big packet are just the way to determine this MTU value.

From the link you found :

" PMTUD was developed to avoid fragmentation in the path between the endpoints. It is used to dynamically determine the lowest MTU along the path from a packet's source to its destination."

In Scenario 5, the DF bit of the original packet is set to 0 so there is no PMTUD for data packet.

Scenario 6 describes PMTUD for both data and GRE in action: " with the tunnel path-mtu-discovery command, and the DF bit is copied from the original IP header to the GRE IP header"



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