[c-nsp] l2tp xconnect problem with packets > 1430

Antonio Querubin tony at lavanauts.org
Thu Dec 8 00:09:35 EST 2011

On Wed, 7 Dec 2011, Peter Rathlev wrote:

> If the L2TPv2 packets cannot exceed 1500 bytes then the ICMP payload
> size of an encapsulated packet cannot exceed 1430 bytes, assuming you're
> not transporting a 802.1Q frame.
> An ICMP packet with 1430 bytes payload, 8 bytes ICMP header and 20 bytes
> IP header is 1458 bytes. Next comes the Ethernet frame header of 14
> bytes. Add to this the 8 bytes L2TPv3 header and 20 bytes outer IP
> header and you're at the 1500 bytes.
> Removing PMTUD and via DF-bit allowing fragmetation greatly increases
> the CPU load on the devices terminating the tunnels.

Understood.  What I don't understand is why the large pings (in this case 
large is anything > 1430) will sometimes work and sometimes not.  There is 
no packet loss between the two routers and pmtud works between them.  They 
can ping each other with large packets all day.  Not so the xconnected 

Antonio Querubin
e-mail:  tony at lavanauts.org
xmpp:  antonioquerubin at gmail.com

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