[quagga-dev 4070] Re: bgpd and pointopoint links

Andrew J. Schorr wrote:
> On Fri, Apr 07, 2006 at 02:20:09PM +0100, Paul Jakma wrote:
> 
>>Just I can imagine a sysadmin borrowing the prefix for the unnumbered 
>>interface from a broadcast network literally:
>>
>>  eth0: 192.168.1.1/24  <etc>
>>  ptp0: 192.168.1.1/24  peer 10.1.1.1
> 
> 
> I guess this is the major point of confusion.  My sense is that such
> a configuration is illegal.  It makes no sense to me to assign the
> same /24 network to more than one interface.  I would argue that
> the ptp0 interface must use the 192.168.1.1/32 address in this situation.
> 
> Are you 100% certain that it is legal to assign the same /24 network
> to multiple interfaces?

If your system can support ECMP, yes.
It means that you can have:
     eth0: 192.168.1.1/24
     eth1: 192.168.1.1/24

If your system cannot support ECMP, yes, but only a mix between ptp and 
broadcast interfaces is allowed.
It means that you cannot have:
     eth0: 192.168.1.1/24  <etc>
     eth1: 192.168.1.1/24  <etc>
     ptp0: 192.168.1.1/24  peer 10.1.1.1

In fact, the connected prefix of a ptp interface is not into the FIB 
(192.168.1.1/24 of ptp0 is not into the kernel). You only put the peer's 
address as the connected entry into the FIB (10.1.1.1 thru ptp0 is into 
the Kernel). It alows to support logics where you can have:
     ptp0: 192.168.1.1/24  peer 10.1.1.1
     ptp1: 192.168.1.1/24  peer 10.1.2.1
     ptp3: 192.168.1.1/24  peer 10.1.3.1

but you are not allowed to have:
     ptp0: 192.168.1.1/24  peer 10.1.1.1
     ptp1: 192.168.2.1/24  peer 10.1.1.1
     ptp3: 192.168.3.1/24  peer 10.1.1.1

Regards,
   Vincent

>  I certainly do not consider myself an
> expert on this topic.  Can anybody else confirm that Paul is correct
> that this is legal?  If so, my patch is certainly flawed.
> 
> Regards,
> Andy
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