[c-nsp] phantom BGP route

[Brad Henshaw]

I've stumbled across [what I think is] a strange issue involving the
presence of a non-existent BGP route.

Prefix 10.31.251.128/28 used to be advertised into BGP by a remote site
in this private network. A few days ago the subnet was removed and the
route advertisement withdrawn. (it's no longer in the routing table or
BGP table of the remote site)

Despite this, the prefix still seems /somewhat/ present in BGP and is
subsequently being redistributed into our IGP locally:

SBNE002#sh ip ro 10.31.251.128
Routing entry for 10.31.251.128/28
  Known via "bgp 64720", distance 130, metric 0
  Tag 65530, type external
  Redistributing via ospf 1
  Advertised by ospf 1 subnets tag 64700
  Last update from 10.31.253.254 00:00:06 ago
  Routing Descriptor Blocks:
  * 10.31.253.254, from 10.31.253.254, 00:00:06 ago
      Route metric is 0, traffic share count is 1
      AS Hops 5
      Route tag 65530
 
SBNE002#sh ip bgp 10.31.251.128 255.255.255.240
BGP routing table entry for 10.31.251.128/28, version 79099
Paths: (0 available, no best path)
Flag: 0x820
  Not advertised to any peer

AS 64720 is the local AS
AS 65530 is another service provider's AS through which we reach some of
our remote sites

A 'clear ip route' on the offending subnet only caused it to reappear
after a few seconds. I haven't done a soft clear on the neighbour yet.

Any idea why the route is present in BGP but with 0 paths available?

Regards,
Brad