Re: [RAM] How to avoid black-hole in LISP-CONS with aggregation mechanism?

[Dino Farinacci <dino at cisco.com>]

> Hi all,
> I have a doubt about how to avoid black-hole in LISP-CONS with  
> aggregation
> mechanism? My doubt is explained as follows:
>                           +---------+   1.0.0.0/8  CDR-1
>                           |  CDR-3  |
>                           +----+---\+   1.1.0.0/16 CDR-2
>                          /          \\
>     Push 1.0.0.0/8  /                    \ Push 1.1.0.0/16
>                //                            \\
>        +----/---+ 1.1.0.0/16 CAR-1           +--\-----+
>        | CDR-1  | 1.2.0.0/16 CAR-2           | CDR-2  | 1.1.0.0/16  
> CAR-3
>        +---+--\-+                            +---+----+
>            |    \\                               |
>            |       \\ Push 1.2.0.0/16            |Push 1.1.0.0/16
>    Push 1.1.0.0/16   \\                          |
>            |            \\                       |
>        +---+----+      +---\------+         +----+----+
>        |  CAR-1 |      |  CAR-2   |         |  CAR-3  |
>        +--------+      +----------+         +---------+
>       1.1.0.0/24         1.2.0.0/24         1.1.2.0/24
>       1.1.1.0/24         1.2.1.0/24         1.1.3.0/24
>
> As shown in the above figure, CAR-1 has two EID-prefixes,  
> 1.1.0.0/24 and
> 1.1.1.0/24, and it sends an aggregated EID-prefix 1.1.0.0/16 to  
> CDR-1. CAR-2
> also sends an aggregated EID-prefix 1.2.0.0/16 to CDR-1. CDR-1  
> sends an
> aggregated EID-prefix 1.0.0.0/8 to its parent CDR, CDR-3.
> CAR-3 has two EID-prefixes, 1.1.2.0/24 and 1.1.3.0/24, and it sends an
> aggregated EID-prefix 1.1.0.0/16 to CDR-2. CDR-2 sends a EID-prefix
> 1.1.0.0/16 to its parent CDR, CDR-3.
> Now CDR-3 has two EID-prefixes, one is 1.0.0.0/8 with a nexthop of  
> CDR-1,
> the other is 1.1.0.0/16 with a nexthop of CDR-2. If CDR-3 receives  
> a mapping
> request for longest-matching entry for 1.1.1.1, it will result in a
> black-hole.
>
> How to avoid it? Use the same aggregation granularity within the  
> same level?
> Or aggregation will not be available until all the component EID- 
> prefixes
> exists in the EID-prefix database of the aggregation attempter?

The diagram has two problems:

1) You are aggregating to the same level (to CDR-3) with different  
mask-lengths.

2) CDR-1 and CDR-2 need to be part of the same CDR-mesh. That is all  
the CARs 1-3
   are authoritative for the same length prefix and hence need to  
advertise to
   the same mesh upstream.

Now having said that, you could have CAR-1 and CAR-3 advertise to the  
same CDR-1 mesh, but since CAR-2 has 1.2 as it's prefix, it could  
advertise into CAR-3 which is a different CDR-mesh. And then both  
CDR-1 and CDR-2 can advertise 1.0.0.0/8 upstream to CDR-3.

Dino

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