Stupid Question maybe?

Wed Dec 19 21:45:04 UTC 2018

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You could be sure of two things when there were ambiguities in the routing tables:
 1- Every manufacturer knew how to handle them.
2 - Every manufacturer did it a different way.

I suspect that in most cases where two conflicting route entries existed, the router selected the first one that it encountered unless they were advertised using different protocols, then the priority associated with the protocol was used as a tie breaker.

Dave

- -----Original Message-----
From: NANOG <nanog-bounces at nanog.org> On Behalf Of Owen DeLong
Sent: Wednesday, December 19, 2018 3:47 PM
To: Thomas Bellman <bellman at nsc.liu.se>
Cc: nanog at nanog.org
Subject: Re: Stupid Question maybe?

> On Dec 19, 2018, at 12:11 , Thomas Bellman <bellman at nsc.liu.se> wrote:
> 
> On 2018-12-19 20:47 MET, valdis.kletnieks at vt.edu wrote:
> 
>> There was indeed a fairly long stretch of time (until the CIDR RFC 
>> came out and specifically said it wasn't at all canon) where we 
>> didn't have an RFC that specifically said that netmask bits had to be contiguous.
> 
> How did routers select the best (most specific) route for an address?
> If the routing table held both (e.g.) 10.20.30.0/255.255.255.64 and 
> 10.20.30.0/255.255.255.32, then 10.20.30.97 would match both, and have 
> the same number of matching bits.
> 
> 	/Bellman
> 

The institution of the longest match rule came with the prohibition (deprecation) of discontiguous net masks.

Owen
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