Routing flaps, was Re: Ping flooding

Craig A. Huegen c-huegen at
Fri Jul 12 21:22:11 UTC 1996

On Fri, 12 Jul 1996, Forrest W. Christian wrote:

==>If the line between routers A&C or B&D or between either C or D and their 
==>respective static downstreams die, there should be NO external route 
==>flap.  However, if C&D are incapable of 'Null0' routing, it may be 
==>beneficial to run dynamic routing between A&C and between B&D so that 
==>A&B discard packets instead of causing a routing loop.  This "internal 
==>routing flap" should not be visible to the outside world.

It can be, though.  Or at least from what I've seen.

If you have a 'floating static' to null0, it won't take over until the
dynamic holddown timer expires (whatever it happens to be for the
particular IGP you're using).

For example, a situation like this:

Router A <----> Router B <----> TermServ (with ISDN)<--DIAL-UP-LINK-->Router C

Router A is the access-point router doing BGP.  The customer on router C
has a set of addresses assigned "way-back-when" by the InterNIC, and
router A is advertising that set via BGP.

Routing is done between A & B using OSPF.  The LAN between B & TermServ is
running RIP (because certain manufacturers' boxen can't do anything BUT

The organization is designed in such a way that their static dial-up
address is portable among all POPs in their provider (so the B & TermServ
could be in any of their POPs).

Router A has a static route to null0 to hold the BGP route in place.  When
router C is dialed up and present, the route gets propogated from TermServ
to B via RIP, which redists into the OSPF area.  Router A picks up this
route via OSPF.

The dynamically-learned route for customer 'C' now takes precedence over
the static route to null0 on A.

If for some reason, C drops the dial-in links and the TermServ goes
through a few RIP update cycles, the route will be marked as
'inaccessible', but will still be in the routing table until hold-down
expires.  When this route is marked as 'inaccessible', the static route
does not take precedence and the route is withdrawn from BGP--and
consequently re-introduced when the hold-down expires and the null0 route
takes precedence.

Now, if there's something in the config of router A that can be used that
will prevent this flapping, or if it was just a freak coincidence that
routers saw a flap after dropping the connection, then please correct me.

Craig A. Huegen                                   ||        ||
Network Analyst, IS-Network/Telecom               ||        ||
cisco Systems, Inc., 250 West Tasman Drive       ||||      ||||
San Jose, CA  95134, (408) 526-8104          ..:||||||:..:||||||:..
email: chuegen at                    c i s c o  S y s t e m s

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