Re: BGP TE attr last call by softwires WG

[Yakov Rekhter <yakov@juniper.net>]

Adrian,

> Hello Don,
> 
> Thanks for the response.
> 
> >> 1. A CE is attached by parallel links to a single PE
> >>
> >>    Suppose the links have the same switching types, but
> >>    different bandwidths. Link-a has plenty of bandwidth
> >>    available with an MTU size of x. Link-b has not much
> >>    bandwidth available with an MTU size of y > x.
> >
> > Interface MTU is part of the PSC TE attributes in the our spec. If the
> > MTUs are different they should not be aggregated.
> 
> Right. Aggregation in this case is hard to make representative of reality.
> 
> > Also In L1VPN (LSC and TDM) networks the Switching type
> > covers Switching granularity (which is like an MTU).
> 
> Sure. But I wanted to keep in PSC, because that problem represents a single 
> layer network.
> 
> > I'm trying to understand if you are saying the following text would
> > fix the problem or you see a larger issue:
> > "Therefore, such routes and the corresponding TE information
> > SHALL NOT be aggregated unless they share identical Traffic
> > Engineering Attributes".
> 
> The question I am asking is - is the solution to this problem to advertise 
> two routes?  (Clearly two is a special case of many.)
> 
> I don't believe that in the general (non-TE case) we would consider the case 
> of two parallel links between CE and PE as route aggregation. The PE would 
> simply advertise reachability (through itself) to the CE.

Let's put the general (non-TE) case aside, and focus on the L1VPN
scenario. 

According to the bgp-auto-discovery and l1vpn-basic-mode drafts in
the context of L1VPN what is advertised in the BGP auto-discovery
information is the information about *ports*.  Therefore, in the
context of L1VPN TE information advertisements are associated with
individual *ports*, not with a particular CE.  Which means that if
a CE is connected to a PE by N ports, then the BGP auto-discovery
information has to advertise all these N ports, and associated with
each such advertisement may be the TE information.

> Hence my question below.
> 
> >>    Normally, we would expect the PE to advertise a single
> >>    piece of reachability information for the CE leaving
> >>    the actual choice of links to be made by the PE.
> >>
> >>    What would the PE advertise in this case?
> >>    - It cannot aggregate the TE information because that
> >>      would imply that there was lots of bandwidth at the
> >>      larger MTU size.
> >>    - It could include duplicate Switching Capability
> >>      Descriptors, but that would need some clarification as
> >>      duplicate descriptors are intended for different
> >>      switching caps in the IGP RFCs.
> >>    - It could advertise two pieces of reachability
> >>      information, but that would be a change to how the
> >>      implementation currently works.

The answer to your question is that if the implementation follows
the spec, it would advertise information about *ports*.  So, if a
CE has two ports that connect it to a PE, then the PE originates
two L1VPN advertisements, one per port, each with its own TE
attribute.
  
Yakov.