The MPLS-OPS Archive

[Per Carlson <ml@carlson.homeunix.net>]

On 2004-09-18, Garry Glendown wrote:
> How does this help me with the packet transfer? Where would I set it 
> lower in order to fix the problem? Setting it on both sides of the 
> ethernet bridge didn't help me any (set "ip mtu" to 1488) ... should I 
> instead set the MTU to 1488 on our uplink/peering interfaces?

I don't think the router checks the IP MTU when doing bridging (really, 
why should it?). Set the IP MTU on the closest IP-enabled interface at 
both ends of the bridged tunnel, as you suggest yourself.

> POS or E3 or so don't help me any on the Ethernet Bridge ... got the 
> line instead of the E3 in order to try to save money on the equipment 
> and handling (lines cost about the same per month) ...

Sometimes it's better to spend some more money on lines that *can* do 
what you want.... ;-) Ethernet-interfaces has got nice bandwidth/$ 
figures, but that's it.

> So, why does the 7200 FE's say "max mtu 1580" ??
> 
> border-gw.ffm1#show contr fa0/1
> Interface FastEthernet0/1 (idb 0x6274EBD8)
> [..]
>   rx cache size=400, rx cache end=272, rx_nobuffer=0
>   max_mtu=1580

Good question to ask the vendor (your SE or TAC).

On our 7200, max_mtu is 1524 and MPLS VPN works fine.

Per

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