[quagga-users 3427] Re: quagga: ospf and GRE tunnels

Paul Jakma paul at clubi.ie
Wed Dec 15 22:21:50 GMT 2004

On Wed, 15 Dec 2004, Daniel Higgins wrote:

> the two routers are connected using a GRE tunnel, and one side is 
> further connected to the internet with pppoe.

Are the MTUs matched on both sides of the link?

> there seem to be a problem with quagga refusing to send anything 
> over a link with an MTU lower than 1500 (it is at 1468).

You can run into problems with very low MTUs, but you shouldnt run 
into the below problem really. Other MTUs are supported though.

> note that i can NOT increase the mtu to 1500 due to the tunnels, as 
> was suggested in the list archives

However, you can decrease the MTU on the non-pppoe side, if its a 

> what is odd is that i receive this message:
> OSPF: Packet[DD]: MTU is larger than [mtl:]'s MTU
> on only one side (the one that connects to the internet through pppoe)

If its the case that MTUs are mismatched, then that's according to 
the OSPF spec. RFC2328, 10.6 "Receiving Database Description Packets" 

         If the Interface MTU field in the Database Description packet
         indicates an IP datagram size that is larger than the router can
         accept on the receiving interface without fragmentation, the
         Database Description packet is rejected.


> here's the ifconfig on that machine:
> mtl       Link encap:UNSPEC  HWaddr


> as well as the "show interface"
> Interface mtl is up, line protocol detection is disabled
>  index 11 metric 1 mtu 1468 <UP,POINTOPOINT,RUNNING,NOARP,MULTICAST>

> and the one on the other side:
> # ifconfig stjean
> stjean    Link encap:UNSPEC  HWaddr


the MTUs are the same. Strange.

Question, do you change the MTU on one of the links (particularly the 
side that /isnt/ logging the message) after the link is up? ospfd 
doesnt deal with MTU changing on active interfaces.

If that's the case, then that's the problem and you need to set the 
MTU *before* setting the link up - or enable OSPF on that interface 
*after* the interface is fully configured. (former approach is by far 
the easier).

> if more information is needed, just ask.

If previous doesnt explain it, can you try CVS - Andrew has updated 
that log message to be more informative.

> thanks

Paul Jakma	paul at clubi.ie	paul at jakma.org	Key ID: 64A2FF6A
Let the machine do the dirty work.
 		-- "Elements of Programming Style", Kernighan and Ritchie

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