BGP: bestpath needs to prefer confed-external over confed-internal

BGP: bestpath needs to prefer confed-external over confed-internal

Topology:
                    +-----------------------------------------+
                    |                                         |
                    |                 AS 100                  |
                    |                                         |
                    |  +----------------+                     |
  +-----------+     |  |                |                     |
  |           |     |  |   SubAS 65001  |                     |
  |   AS 90   |     |  |                |    +-------------+  |
  |    r9----------------r1---------r2----\  |             |  |
  |     |     |     |  |  |         |   | |  | SubAS 65002 |  |
  +-----|-----+     |  |  \--- r3 --/   | \-------r4       |  |
        \---------------------/  \---------------/ |       |  |
                    |  |                |    |     |       |  |
                    |  +----------------+    |     |       |  |
                    |                        |     |       |  |
                    |  +----------------+    |    r5       |  |
  +-----------+     |  |                |    |     |       |  |
  |           |     |  |   SubAS 65003  |    +-----|-------+  |
  |   AS 80   |     |  |                |          |          |
  |    r8----------------r7--------r6--------------/          |
  |           |     |  |                |                     |
  +-----------+     |  +----------------+                     |
                    +-----------------------------------------+

Important info:
- r8 originates 8.8.8.8/32
- r1, r2, r3 -> r7 are 10.0.0.1, 10.0.0.2, etc
- 'bgp bestpath compare-routerid' is configured everywhere (we could still hit
  the problem without this though)

Bestpath selection for 8.8.8.8/32 on r2 and r3 is inconsistent. Here r4
advertised the 8.8.8.8/32 to r2 first, r2 then advertised it to r3, r3 selected
the path from r2 as the bestpath due to lowest router-id.

r2
BGP routing table entry for 8.8.8.8/32
Paths: (1 available, best #1, table Default-IP-Routing-Table)
  Advertised to non peer-group peers:
  10.0.0.1 10.0.0.3 10.0.0.4
  (65002 65003) 80
    10.0.0.7 (metric 50) from 10.0.0.4 (10.0.0.4)
      Origin IGP, metric 0, localpref 100, valid, confed-external, best
      Last update: Fri May  1 14:46:57 2015

r3
BGP routing table entry for 8.8.8.8/32
Paths: (2 available, best #1, table Default-IP-Routing-Table)
  Advertised to non peer-group peers:
  10.0.0.4 90.1.1.6
  (65002 65003) 80
    10.0.0.7 (metric 50) from 10.0.0.2 (10.0.0.2)
      Origin IGP, metric 0, localpref 100, valid, confed-internal, best
      Last update: Fri May  1 14:46:58 2015

  (65002 65003) 80
    10.0.0.7 (metric 50) from 10.0.0.4 (10.0.0.4)
      Origin IGP, metric 0, localpref 100, valid, confed-external
      Last update: Fri May  1 14:46:57 2015

Here r4 advertised the 8.8.8.8/32 to r3 first, r3 then advertised it to r2, r2
selected the path from r3 as the bestpath due to lowest router-id.

r2
BGP routing table entry for 8.8.8.8/32
Paths: (2 available, best #2, table Default-IP-Routing-Table)
  Advertised to non peer-group peers:
  10.0.0.4
  (65002 65003) 80
    10.0.0.7 (metric 50) from 10.0.0.4 (10.0.0.4)
      Origin IGP, metric 0, localpref 100, valid, confed-external
      Last update: Fri May  1 15:37:27 2015

  (65002 65003) 80
    10.0.0.7 (metric 50) from 10.0.0.3 (10.0.0.3)
      Origin IGP, metric 0, localpref 100, valid, confed-internal, best
      Last update: Fri May  1 15:37:27 2015

r3
BGP routing table entry for 8.8.8.8/32
Paths: (1 available, best #1, table Default-IP-Routing-Table)
  Advertised to non peer-group peers:
  10.0.0.1 10.0.0.2 10.0.0.4 90.1.1.6
  (65002 65003) 80
    10.0.0.7 (metric 50) from 10.0.0.4 (10.0.0.4)
      Origin IGP, metric 0, localpref 100, valid, confed-external, best
      Last update: Fri May  1 15:37:22 2015

The fix is to have bestpath prefer a confed-external path over a confed-internal
path.  I added this just after the "nexthop IGP cost" step because some confed
customers will have one IGP covering multiple sub-ASs, in that case you want to
compare nexthop IGP cost.