Re: [Idr] draft-walton-bgp-add-paths-06.txt as IDR WG document

[Robert Raszuk <[e-mail]>]

Hi Uli,

I was assuming that RR would advertise all paths not just best and 
second best.

But you are in general very correct that regardless of what RR 
propagates via IBGP domain the rtr_id of the advertising peer is still 
subject to be compared before the attr_set hence we have an issue.

In full mesh ibgp that would not be the problem, but in the RR case it 
still  think is.

I propose we talk offline and try to come up together on the best 
solution considering your work already in this space.

Cheers,
R.


> Hi Robert,
> 
> I think I did not get your point yet. Why do you think that a
> misconfiguration is needed for the example?
> Client C-ID.5 keeps two eBGP sessions (to R-ID.6 and R-ID.3) and thus
> receives q1 and p1. Client C-ID.8 also holds two external sessions (to
> R-ID.9 and R-ID.2) and receives p2 and q2. The Route Reflector of
> clients C-ID.5 and C-ID.8 receives all paths, and re-advertises its
> first and second best (which are q1 and q2).
> Thus, client C-ID.5 is provided with q1, q2, and p1. Knowing q2 has no
> effect on its routing decision. Path p2 is not visible at C-ID.5 at all.
> 
> Can you please explain I detail from where client C-ID.5 should learn
> path p2?
> 
> Thank you and Regards
> 
> Uli
> 
> Robert Raszuk schrieb:
>> Hi Uli,
>>
>>> Client C-ID.5 receives the paths q1 and p1.
>> This is already not the correct observation. In the correct
>> architecture of this network (no misconfiguration) C-ID.5 would
>> receive all 4 paths. Therefor p2 will win.
>>
>> Cheers,
>> R.
>>
>>> Hi Robert,
>>>
>>> here should be a simple example for the problem regarding Add-path, Best
>>> / Second Best, and Border Router Attr Set I mentioned:
>>>
>>> -------------------------------------
>>>
>>> ---------+        RR        +--------
>>>       ASx|   d1 .    . d1   |ASy
>>>          |     .      .     |
>>>  R-ID.6.....C-ID.5  C-ID.8.....R-ID.2
>>>      q1->|    .        .    |<-q2
>>>          |    .        .    |
>>> ---------+    .        .    +--------
>>>               .        .
>>>          MED2 .        . MED1
>>>        +------.--------.----+
>>>        |ASz p1.        .p2  |
>>>        |      .        .    |
>>>        |   R-ID.3    R-ID.9 |
>>>
>>> -------------------------------------
>>> q1, q2, p1, p2: Paths advertised from neighboring systems.
>>>      R: Routers in neighboring systems, Route Reflector or Clients
>>>      C: Client in own System
>>>     RR: Route Reflector in own System
>>> *-ID.x: Router has BGP ID .x
>>>     d1: Links with a distance of 1
>>>    ASa: AS with ID a
>>>
>>> Client C-ID.5 receives the paths q1 and p1. Path p1 is the best (lower
>>> BGP ID, .3) and path q1 is the second best.
>>> Client C-ID.8 receives the paths q2 and p2. Path q2 is the best (lower
>>> BGP ID, .2) and path p2 is the second best.
>>>
>>> Both clients advertise both path to the reflector RR of the system.
>>>
>>> Comparison of p1, p2, q1, q2 at RR:
>>> p1 is discarded due to the higher MED (2, path p2 has a MED of 1)
>>> p2 and q2 are discarded due to the higher BGP ID (.8)
>>> --> Path q1 is the "first" best on RR.
>>> --> Path p1 is also removed (due to the same BGP ID .5 (or even
>>> Nexthop), cf. Draft)
>>>
>>> Comparison of p2, q2 at RR:
>>> p2 is discarded due to the higher BGP ID in the Border Router
>>> Attribute Set
>>> --> Path q2 is the second best.

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