Re: [mpls] Mode negotiation for PSC

["Eric Osborne (eosborne)" <[e-mail]>]

Hi Loa- 

  See inline

> -----Original Message-----
> From: Loa Andersson [mailto:[e-mail]]
> Sent: Monday, August 12, 2013 1:27 AM
> To: Eric Osborne (eosborne)
> Cc: [e-mail]
> Subject: Re: [mpls] Mode negotiation for PSC
> 
> Eric,
> 
> 
> 
> On 2013-08-08 14:34, Eric Osborne (eosborne) wrote:
> <snip>
> 
> > Example of method 2
> > -------------------
> > Both Node A and Node Z announce two things - Capabilities and Mask.
> > Capabilities is a bitmap of the capabilities that are supported.
> > Mask is a mask of don't-care bits against the Capabilities string.  A
> 0 in Mask means "I don't care if we do this capability or not", and a 1
> means "we must (or must not) agree on this capability in order to come
> up".
> >
> > A.capabilities = 00000
> > A.mask         = 00000
> 
> Why is this not
> 
>   A.capabilities = 11111
>   A.mask         = 00000
> 

Althought I didn't put it in my email, a capabilties of 1 and mask of 0 is illegal.  This is partly because it makes no sense ("I require this capability but I don't care if we do it or not") and partly because there's a case which doesn't converge right:

If I have 11111/00000 on one side and 00000/00000 on the other I end up with

A.cap = 11111, A.mask = 00000
Z.cap = 00000, Z.mask = 00000


res = (111111 & 00000) ^ (00000 & 00000)
res = 11111 ^ 00000 = 11111

and if res != 0 then the nodes can't converge on a common subset, even though there are 2^5 combinations that would work for both nodes.



eric

> ?
> 
> /Loa
> 
> >
> > This says "I am capable of supporting all capabilities and I don't
> care if we do any of them or not"
> <snip>
> 
> --
> 
> 
> Loa Andersson                        email: [e-mail]
> Senior MPLS Expert                          [e-mail]
> Huawei Technologies (consultant)     phone: [phone]