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

From: Xu Xiaohu <[email-masked]>
Date: Mon, 27 Aug 2007 16:50:05 +0800

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?

Best regards,
Steven XU

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