[c-nsp] MTU / BGP

[Mark Tinka]

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On 21/Jul/15 09:16, Gert Doering wrote:
>
> In that case, your L3 MTU on your routers needs to be set to something
> lower or equal to the lowest L2 MTU that your packets might hit.

I generally don't set protocol MTU, only Layer 2 MTU. Normally, your
protocols should be able to infer their MTU from the Layer 2 MTU.

But yes, in some cases, setting the Layer 3 MTU may be necessary.

>
>
> (We have a leased link "ethernet over packet" that loses 8 byte MTU when
> the primary link in their network goes down and switches to backup... bit
> us mightily.  Reduced L3 MTU on our side to take that into account, no
> problems anymore)

Ouch.

>
>
> Best, get your provider to guarantee you a given value, and regularily
> *verify* that.  What we do: ping all routers behind "dubious" links with
> 10k packets - on the way *to* the router, the packet is fragmented to
> 1500 bytes (because the management system doesn't have a bigger uplink),
> so the ping packet will arrive always, no matter what the MTU.  Now the
> return packet will sent fragmented to the L3 MTU of the router being
> pinged - if that is too big for its L2 link, we'll notice and can adjust.
>
>     mgmt -(1500)- R1 -(9000)- R2 -(dubious)- R3
>
> (just to point out the obvious: when the "large fragments" arrive at R1,
> they will be fragmented further, down to 1500.  This feels fairly cruel,
> but it works out nicely)

That's a good workaround.

Mark.
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