[j-nsp] Juniper MX240 to Alcatel SAS-M epipe

Saku Ytti saku at ytti.fi
Wed Aug 7 12:08:29 EDT 2013

On (2013-08-07 17:43 +0200), Mark Tinka wrote:

> True, but the OP suggested that some of the web sites he's 
> browsing don't load correctly, which could allude to an MTU 
> issue in the data plane.

Mea culpa, luckily point remains it does not matter what the eline MTU is.

Fully agreed on your suggestion that just guarantee core is large enough,
no point doing 'exactly the right size' but large enough.

For sake of argument 'exactly the right size' in untagged ethernet core

6B DMAC (not calculated by IOS)
6B SMAC (not calculated by IOS)
2B type (not calculated by IOS)
4B IGP label
4B VPN label
4B control word (maybe, maybe not)
6B inner DMAC
6B inner SMAC
2B inner type
xB payload
4B FCS (not calculated by IOS or JunOS)

It might be prudent to figure out customer experienced MTU by doing over
the l2circuit:

a) ping with 1500B DF set
b) ping with 1501B DF not set


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