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

I believe most people on RAM are also on RRG.
lets carry out this discussion on RRG list only.
(so I removed RAM from cc: list)

Lixia

On Aug 27, 2007, at 10:46 AM, Dino Farinacci wrote:


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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