[rbridge] [Isis-wg] Why is MTU discovery important?

[Radia Perlman]

No, Vishwas -- merely stating the MTU is not sufficient, because there could
be components between two RBridges (like, for instance, bridges), that 
wind up causing
a big packet not to get through. So to really test the MTU requires a 
padded packet,
(but it need not be (and MUST NOT be, for TRILL) the same, simultaneous 
mechanism
as finding neighbors, such as layer 3 IS-IS which does not see neighbors 
that can't
be reached using the largest packets.)

Radia

Vishwas Manral wrote:
> Hi,
>
> Though the MTU padding of Hello's is one way of getting the MTU right in all
> the machines, a simpler and more efficient way would be to actually exchange
> MTU values in a TLV.
>
> The reason padding of Hellos was done in a layer 3 network, instead of a TLV
> was because if between 2 - Layer 3 peers there was a switched network (not
> just a single link), and the MTU in the links somewhere in between was
> lesser, there would be no way to figure that out. 
>
> For trying to work IS-IS for layer 2 the reason no longer holds good.
>  
> Thanks,
> Vishwas
>
> -----Original Message-----
> From: isis-wg-bounces at ietf.org [mailto:isis-wg-bounces at ietf.org] On Behalf
> Of Tony Li
> Sent: Thursday, April 09, 2009 9:55 PM
> To: Don Fedyk
> Cc: TRILL/RBridge Working Group; Radia Perlman; isis-wg at ietf.org; Dinesh G
> Dutt
> Subject: Re: [Isis-wg] [rbridge] Why is MTU discovery important?
>
> Don Fedyk wrote:
>   
>> I have a different interpretation.   I think you just broke an IS-IS 
>> safeguard by not padding Hellos.
>>     
>
>
> +1
>
> Tony
>
>
>
>