[j-nsp] MPLS issue

[Mark Tinka]

On Monday 03 March 2008, Ying Zhang wrote:

> we are having a wired MPLS problem. Here is how the
> network looks like:
>
> end users ---- M7i ---- M120 ----ISP
>
> The mpls lsp is between M7i and M120. The problem is the
> end users have problems accessing certain websites, very
> slow or not accessible, some websites no problem at all.
> With MPLS disabled (OSPF only), the problem is gone. I
> think it is a MTU issue, tried to lower MTU on M7i user
> side, didn't fix it. Might also try to increase MTU  on
> MPLS interfaces, but not sure. Any thoughts? Very
> appreciated.

Yes, definitely sounds like an MTU issue.

For a single label (MPLS switching only, no VPN's or 
anything kinky like that), you need at least 1504 bytes on 
Ethernet.

With L3VPN's, you need 2 labels (one for the VPN and another 
for MPLS switching). This brings you to 1508 bytes.

To this end, I've normally configured 1524 bytes at a bare 
minimum on the interface. Note, however, that both the 
router port and adjacent Ethernet switch need to be 
configured for higher MTU's, otherwise you'll experience 
the symptoms you describe above.

Cheers,

Mark.
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