[c-nsp] Tracking a dynamic default - supported?

[David Freedman]

I can't find any information which would suggest that such a thing is an
unsupported configuration. Nonetheless, am unable to maintain a stable
track on a BGP default in 7200 12.2(33)SRC or 12.2(33)SRD (tested with
SRC2 and SRD2a)

#sh run | in track
track 20 ip route 0.0.0.0 0.0.0.0 reachability

#sh track 20
Track 20
  IP route 0.0.0.0 0.0.0.0 reachability
  Reachability is Down (BGP)
    5 changes, last change 4d14h
  First-hop interface is unknown

#sh ip ro 0.0.0.0
Routing entry for 0.0.0.0/0, supernet
  Known via "bgp 1234", distance 200, metric 0, candidate default path,
type internal
  Last update from 192.168.100.100 2d20h ago
  Routing Descriptor Blocks:
  * 192.168.100.100, from 192.168.100.100, 2d20h ago
      Route metric is 0, traffic share count is 1
      AS Hops 0

#sh ip ro 192.168.100.100
Routing entry for 192.168.100.100/32
  Known via "isis", distance 115, metric 300, type level-2
  Redistributing via isis
  Last update from 192.168.10.1 on FastEthernet0/0, 2d20h ago
  Routing Descriptor Blocks:
  * 192.168.10.1, from 192.168.100.100, via FastEthernet0/0
      Route metric is 300, traffic share count is 1

#sh ip cef 0.0.0.0 0.0.0.0 inte
0.0.0.0/0, epoch 0, RIB[B], refcount 5, per-destination sharing
  sources: RIB, DRH
  feature space:
   NetFlow: Origin AS 0, Peer AS 0, Mask Bits 0
   IPRM: 0x00018000
  ifnums:
   FastEthernet0/0(3): 192.168.10.1
  path 65FF4264, path list 65FEA258, share 1/1, type recursive, for
IPv4, flags eos indirection, neos indirection
  recursive via 192.168.100.100[IPv4:Default], fib 66033BB0, 1 terminal fib
    path 65FF38E0, path list 65FE9C1C, share 1/1, type attached nexthop,
for IPv4
      MPLS short path extensions: MOI flags = 0x0 label implicit-null
    nexthop 192.168.10.1 FastEthernet0/0, adjacency IP adj out of
FastEthernet0/0, addr 192.168.10.1 665F0B00
  output chain:
    loadinfo 65FD6C64, per-session, 1 choice, flags 0003, 164 locks
    flags: Per-session, for-rx-IPv4
    1 hash bucket
      < 0 > IP adj out of FastEthernet0/0, addr 192.168.10.1 665F0B00
    Subblocks:
     None


The track is down most of the time, but changes state to up randomly
when there is no change in the network and for short periods of time.

If you track another prefix with a mask longer than /0 with the same
nexthop, stays up continually.

When labbing, I can demonstrate the issue in 12.2(33)SR but not in , for
example 12.3(M) in which the track works but is labelled "unsupported"

#sh track
Track 20
  IP route 0.0.0.0 0.0.0.0 reachability
  Reachability is Up (unsupported)
    2 changes, last change 00:00:55
  First-hop interface is FastEthernet1/0


As usual, bugtool unhelpful.

Any ideas appreciated.