Re: L2TPv3 MTU

Here is what I came up with after referencing Layer 2 VPN Architectures...

FR over L2TPv3
-----------------------
20 bytes IPv4 delivery header
4 bytes L2TPv3 session ID
4 bytes L2TPv3 cookie
4 bytes default L2 specific sublayer

Ethernet over L2TPv3
-------------------------------
20 bytes IPv4 delivery header
4 bytes L2TPv3 session ID
8 bytes L2TPv3 cookie
14 bytes Ethernet frame overhead

Thanks,
Ronnie

On Tue, Mar 23, 2010 at 5:58 PM, sp-ie m <sp.ccie.me_at_gmail.com> wrote:
> i'm trying to understand L2TPv3 header overhead from CCO -
>
> http://www.cisco.com/en/US/products/ps6587/products_white_paper09186a00800a8444.shtml
> it says the following -
>
> The MTU in the IP backbone must be x bytes larger than the MTU that
> operates on the pseudo-wire. The value for x is:
>
> 802.1Q = 50
> Ethernet = 46
> POS = 36
> Frame Relay = 34
> CHDLC = 36
>
> My understanding is for e.g. in Frame-relay , if i have default IP MTU
> = 1500, and i generate an IP packet of 1500bytes,
> 1. 1504-byte frame will be received on a Frame Relay attachment
> circuit including the 4 byte Frame-Relay header
> 2. 20 bytes L2TP IP packet header
> 3. 12 bytes L2TPv3 session header
> 4. 4 bytes - default Layer 2 specific sublayer (considering sequencing
> is enabled)
> Total = 1540 bytes.
>
> Can somebody explain Cisco's breakup? i cant understand how Cisco came
> with th figure of "34 bytes" extra overhead over IP MTU.
>
> Similarly for Ethernet its listed as 46 bytes which is not clear.
>
> This comes from a real OEQ in the SP lab, hence i'm being lil extra meticulous
>
> Thanks.
>
>
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-- 
Ronald Angello
Network Architect
CCIE 17846
CCDP, CCIP, CCNP
Blogs and organic groups at http://www.ccie.net