[j-nsp] L2VPN MTU Issue

[Harry Reynolds]

I missed the details. Not clear if using vlan tagging for example. Here is a stab.

IIRC, for l2vpn you have:

1. 4 bytes control word (can be disabled, on by default)
2. 12 bytes SA/DA MAC
3. 2 bytes type code/length

Subtracting the 16 above from 1528 leaves 1512. IIRC, family mpls subtracts 12 bytes from the device mtu by default to accommodate a 3 level label stack, which would then yield a payload length of 1500.

Either that or perhaps vlan tagging also in effect.


HTHs

Regards


 
 

-----Original Message-----
From: juniper-nsp-bounces at puck.nether.net [mailto:juniper-nsp-bounces at puck.nether.net] On Behalf Of Eric Van Tol
Sent: Wednesday, September 29, 2010 11:57 AM
To: juniper-nsp
Subject: Re: [j-nsp] L2VPN MTU Issue

> -----Original Message-----
> From: Dermot Williams [mailto:Dermot.Williams at imaginegroup.ie]
> Sent: Wednesday, September 29, 2010 10:43 AM
> To: Eric Van Tol; juniper-nsp
> Subject: RE: [j-nsp] L2VPN MTU Issue
> 
> Hi Eric,
> 
> Unless it's sensitive, would you mind sharing how you arrived at that 
> number please?
> 
> Thanks,
> 
> Dermot

Well, primary credit goes to JTAC, but doing some packet captures, I found that the packets entering the customer interface were not leaving the router.  JTAC suggested 1520, but that only allowed 1498-bytes through.  I bumped it up to 1524, which still did not work, so then I bumped it up to 1528.  I am still not too keen on the math, but JTAC hasn't broken it down for me yet.  If anyone out there knows, please point me to a reference.  I googled like mad yesterday, but either overlooked the obvious, or wasn't searching for the right info.

-evt

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