[c-nsp] static route to null0 not visible (BGP)

[Tony Li]

> > I have a couple of 3550s with already existing bgp sessions 
> with some
> > upstream providers and with iBGP running between them.  Im
> > advertising a /24 at present and now want to advertise a 
> /23 netblock
> > in addition.  I 
> > use static routes to null0 with metric of 254 for the 
> existing /24 (on
> > both routers) but when I add new routes to null0 for the 
> /23, only one
> > of my routers installs the route - the other learns the /23 
> via iBGP.
> > If I remove the 254 metric on the offending router (and use the
> > default 
> > metric), the route is installed and advertised.  When the /23 is
> > learned 
> > by iBGP it has a metric of 200 so does this mean it is 
> preferred over
> > the static route ?? - If so why doesn't the /24 installed with a 254
> > metric on both routers also behave this way.
> 
> Both the /23 and /24 should behave this way, i.e. one of the 
> two routers
> install the route as static and advertises it via BGP, the other will
> learn the route via iBGP and will remove the floating static as the
> static has a higher admin distance (254 vs. 200).
> Not sure why your /24 doesn't behave this way, "show ip route 
> 83.245.6.0
> 255.255.255.0" as well as "show ip bgp 83.245.6.0 255.255.255.0" would
> help.. Maybe you learn the /24 via an IGP (OSPF) as well, so the OSPF
> route wins over both the iBGP and the static route, so BGP will also
> advertise the /24 on this router.


BGP should only be picking up routes that are actually in your routing
table.  If you learn the router via BGP or OSPF, it will have a lower
(better) administrative distance than the static routes that you've
described and the static routes won't be in the table for BGP to
redistribute.

If your goal is to advertise those routes always, then they should have
a lower administrative distance.  Otherwise you'll only see them in the
routing table as a backup.  Which router gets to advertise the routes
into BGP could end up being whichever router was rebooted least
recently.

Tony