|
| |
Instability
The routing tables managed by a BGP implementation are adjusted continually to
reflect actual changes in the network, such as links breaking and being restored
or routers going down and coming back up. In the network as a whole it is normal
for these changes to happen almost continuously, but for any particular router
or link changes are supposed to be relatively infrequent. If a router is
misconfigured or mismanaged then it may get into a rapid cycle between down and
up states. This pattern of repeated withdrawal and reannouncement, known as
route flapping, can cause excessive activity in all the other routers that know
about the broken link, as the same route is continuously injected and withdrawn
from the routing tables.
A feature known as route flap damping (RFC 2439) is built into many BGP
implementations in an attempt to mitigate the effects of route flapping. Without
damping the excessive activity can cause a heavy processing load on routers,
which may in turn delay updates on other routes, and so affect overall routing
stability. With damping, a route's flapping is exponentially decayed. At first
instance when a route becomes unavailable but quickly reappears for whatever
reason, then the damping does not take effect, so as to maintain the normal
fail-over times of BGP. At the second occurrence, BGP shuns that prefix for a
certain length of time; subsequent occurrences are timed out exponentially.
After the abnormalities have ceased and a suitable length of time has passed for
the offending route, prefixes can be reinstated and its slate wiped clean.
Damping can also mitigate denial of service attacks; damping timings are highly
customizable.
However, subsequent research has shown that flap damping can actually lengthen
convergence times in some cases, and can cause interruptions in connectivity
even when links are not flapping. Moreover, as backbone links and router
processors have become faster, some network architects have suggested that flap
damping may not be as important as it used to be, since changes to the routing
table can be absorbed much faster by routers. This has lead the RIPE Route
Working Group to write that "with the current implementations of BGP flap
damping, the application of flap damping in ISP networks is NOT recommended. ...
If flap damping is implemented, the ISP operating that network will cause
side-effects to their customers and the Internet users of their customers'
content and services ... . These side-effects would quite likely be worse than
the impact caused by simply not running flap damping at all." Improving
stability without the problems of flap damping is the subject of current
research.
| |
|
