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Re: flap flap: AS 10916

  • From: Yu Ning
  • Date: Sun Aug 13 20:49:55 2000


See the following comments.

----- Original Message ----- 
From: "Kai Schlichting" <[email protected]>
To: "John Todd" <[email protected]>
Cc: <[email protected]>
Sent: Saturday, August 12, 2000 5:22 AM
Subject: Re: flap flap: AS 10916

> At Friday 04:59 PM 8/11/00, John Todd wrote:
> >701
> >   \
> >    \____________
> >                |
> >                |   link 2
> >       1239---10916--------6138
> >      /  \________________/
> >     /        link 1
> >13789
> >
> >
> >JT
> >
> >
> Thank you, Todd, this was the scenario I had in mind. Nice to see someone
> still excells in the field of ASCII art, too :)
> The POC of AS10916 has emailed me back since, and is looking into this
> issue.
> I was also intrigued by:
> >  (b) Link that both BGP and traffic pass through is insufficient for continued keepalives once traffic moves in that direction (line becomes preferred by a large amount of traffic, traffic floods line, BGP keepalives fail, BGP session fails, traffic moves away, wash, rinse, repeat - see RED discussion archives some months ago for more detailed discussions on traffic flow dampening with similar patterns.)
> What can be done to prevent flapping in this situation, other than putting
> QoS mechanisms into place to prefer the BGP traffic over everything else?
> Is there a good and automated (Cisco-leaning, sorry) way to keep BGP
> sessions down if they have flapped too often?

In response to your prefered solution, it's natural to turn on bgp dampen.
See the following excerpt:

BGP Dampening
Route flap dampening (introduced in Cisco Internetwork Operating System [Cisco IOS] Release 11.0) is a mechanism for
minimising the instability caused by route flapping. Route flapping is the BGP network prefixes being frequently added
and removed. Whenever a network goes down, the rest of the Internet would like to know about it. Hence, BGP propagates
that state change throughout the Internet. Yet, if this state change is happening from a faulty circuits (frequently going up
and down) or from mis-configured routing (redistributed the IGP into the EGP), the Internet would experience several
hundred BGP state changes a second. For every state change, BGP must allocate time to work to process the work and pass
on the changed to all other BGP neighbours. This places a tremendous strain on the backbone routers. Hence, the tool to
control and minimise the effect of route flaps - BGP Dampening.
The following are the commands used to control route dampening:
bgp dampening [[route-map map-name] [half-life-time reuse-value suppress-value maximum-suppress-

But I wonder why not turn to your "other than" solution? In a Cisco box,
youcan turn on the SPD feature. See the following excerpt:

When a link goes to a saturated state, the router will drop packets. The problem is that the router will drop any type of
packets - including routing protocol packets. Selective Packet Discard (SPD) will attempt to toss non-routing packets
instead of routing packets when the link is overloaded. In releases 11.1CA and 11.1CC, the configuration command:
ip spd enable
will switch on SPD. Selective Packet Discard is enabled by default on 11.2(5)P and more recent releases


Yu Ning
(Mr.) Yu(2) Ning(2)
ChinaNET(AS4134) Backbone Operation Center
Networking Dep.,Datacom Bureau, China Telecom.
Beijing,P.R.C +86-10-66418105/66418121/66418123(fax)