Draft revision of RIPE-210: Coordinated Route Flap Damping Parameters
Philip Smith
pfs at cisco.com
Tue Apr 24 16:35:20 UTC 2001
Hi,
bcc: nanog, apops, APNIC routing SIG, afnog
Please accept my apologies for any duplicates.
The attached is a proposed update and revision of the RIPE-210 Coordinated
Route Flap Damping Parameters document. The authors would welcome any
comments and feedback regarding the content of this document, either to the
RIPE Routing Working Group, or at the RIPE meeting in Bologna, Italy, on
w/b 30th April.
We would also welcome links/sample configurations/examples for other router
platforms (specifically Juniper/Foundry/Extreme and any other platforms
which may be found in today's Internet) to augment the Cisco IOS
configuration examples given in the Appendix.
thanks, and kind regards,
philip
--
>>>>begin
RIPE Routing-WG Recommendation for coordinated route-flap damping parameters
Philip Smith
Daniel Karrenberg
Christian Panigl
Joachim Schmitz
This document Obsoletes: ripe-210, ripe-178
-----------------------------------------------------------------
Document status Version 1.2, April 24th, 2001
Abstract
This paper recommends a set of route-flap damping parameters which should
be applied by all ISPs in the Internet and should be deployed as default
values by BGP router vendors.
Table of Contents
1. Introduction
1.1 Motivation for route-flap damping
1.2 What is route-flap damping ?
1.3 "Progressive" versus "flat&gentle" approach
1.4 Motivation for coordinated parameters
1.5 Aggregation versus damping
1.6 "Golden Networks"
2. Recommended damping parameters
2.1 Motivation for recommendation
2.2 Description of recommended damping parameters
2.3 Example configuration for Cisco IOS
3. Other Features contributing to Internet Stability
3.1 BGP Route Refresh
3.2 Soft Reconfiguration (Cisco IOS)
3.3 Fast-External-Fallover (Cisco IOS)
4. Potential problems
4.1 Multiplication of flaps between ASes with multiple interconnections
4.2 Non-recommended flap damping parameters
5. References
6. Acknowledgements
Appendices
A.1 "Golden Networks"
A.2 Sample Cisco IOS Configuration
A.3 Study of Flap Damping Operation
1. Introduction
Route-flap damping is a mechanism for (BGP) routers which is aimed at
improving the overall stability of the Internet routing table and reducing
the load on the CPUs of the core routers.
1.1 Motivation for route-flap damping
In the early 1990s the accelerating growth in the number of prefixes being
announced to the Internet (often due to inadequate prefix-aggregation),
the denser meshing through multiple inter-provider paths, and increased
instabilities started to cause significant impact on the performance and
efficiency of the Internet backbone routers. Every time a routing prefix
becomes unreachable because of a single line-flap, the withdrawal has to
be advertised to the whole core Internet and dealt with by every single
router which is carrying the full Internet routing table.
To overcome this situation a route-flap damping mechanism was invented in
1993 and has been integrated into several router software implementations
since 1995 (for example, Cisco, Merit/RSd, GateD Consortium). The
implementation is described in detail in RFC2439. The flap damping mechanism
is now widely used to help keep severe instabilities under control and
more localised in the Internet.
And there is a second benefit: it is raising the awareness of the
existence of instabilities because severe route/line-flapping problems
lead to permanent suppression of the unstable area by means of holding
down the flapping prefixes.
Route-flap damping has its greatest and most consistent value if it
is applied as near to the source of the problem as possible. Therefore
flap-damping should be applied both at peering and upstream boundaries,
as well as at customer boundaries (see 1.4 and 1.5 for details).
1.2 What is route-flap damping ?
When BGP route-flap damping is enabled in a router the router starts
to collect statistics about the announcement and withdrawal of
prefixes. Route-flap damping is governed by a set of parameters with
vendor-supplied default values which may be modified by the router
manager. The names, semantic and syntax of these parameters differ
between the various implementations; however, the behaviour of the
damping mechanism is basically the same.
Each time a prefix is withdrawn, the router will increment the damping
penalty by a fixed amount. When the number of withdrawals/announcements
(=flap) is exceeded in a given time frame (cutoff threshold) the
path is no longer used and not advertised to any BGP neighbour for a
predetermined period starting from when the prefix stops flapping. Any
more flaps happening after the prefix enters suppressed state will
attract additional penalty. Once the prefix stops flapping, the penalty
is decremented over time using a half-life parameter until the penalty is
below a reuse threshold. Once below this reuse threshold the suppressed
path is then re-used and re-advertised to BGP neighbours.
Pointers to some more detailed and vendor specific documents:
Cisco BGP Case Studies: Route Flap Damping
http://www.cisco.com/warp/public/459/16.html
ISI/RSd Configuration: Route Flap Damping
http://www.isi.edu/div7/ra/RSd/doc/dampen.html
GateD Configuration: Weighted Route Damping Statement
http://www.gated.org/gated-web/code/doc/manuals/config_guide/bgp/
weighted_route_dampening.html
See also "5. References"
1.3 "Progressive" versus "flat&gentle" approach
One easy approach would be to just apply the current default-parameters
which are treating all prefixes equally ("flat&gentle") everywhere. However,
there is a major concern to penalise longer prefixes (=smaller aggregates)
more than well aggregated short prefixes ("progressive"), because the
number of short prefixes in the routing table is significantly lower and
it seems in general that those are tending to be more stable and also are
tending to affect more users.
Another aspect is that progressive damping might increase the awareness
of aggregation needs. However, it has to be accompanied by a careful
design which doesn't force a rush to request and assign more address space
than needed.
A significant number of important services is sitting in long prefixes
(e.g. root name servers), so the progressive approach has to exclude the
strong penalisation for these so-called "golden" prefixes.
With this recommendation we are trying to make a compromise and it is
therefore called "graded damping".
1.4 Motivation for coordinated parameters
There is a strong need for the coordinated use of damping parameters for
several reasons:
Coordination of "progressiveness":
If penalties are not coordinated throughout the Internet, route-flap damping
could lead to additional flapping or inconsistent routing because longer
prefixes might already be re-announced through some parts of the Internet
where shorter prefixes are still held down through other paths.
Coordination of hold-down and reuse-threshold parameters between ISPs:
If an upstream or peering provider would be damping more aggressively
(e.g. triggered by less flaps or applying longer hold-down timers) than an
access-provider towards his customers it will lead to a very inconsistent
situation, where a flapping network might still be able to reach "near-line"
parts of the Internet. Debugging of such instabilities is then much harder
because the effect for the customer leads to the assumption that there
is a problem "somewhere" in the "upstream" Internet instead of making him
just call his ISPs hot-line and complain that he can't get out any longer.
Further, after successful repair of the problem the access-provider
can easily clear the flap-damping for his customer on his local router
instead of needing to contact upstream NOCs all over the Internet to get
the damping cleared.
Vendor Defaults:
As with most software implementations, there need to be some default values
set when route-flap damping is enabled on routers. Vendors choosing different
default values will result in a similar situation to that described above,
where the more aggressive values will result in "black spots" in the
Internet. Coordinated values will ensure consistency in dealing with
instabilities.
1.5 Aggregation versus damping
If a customer of an ISP is only using Provider Aggregated addresses,
the aggregating upstream provider doesn't need to apply damping on these
prefixes towards his customer, because instabilities of such prefixes
will not propagate into the Internet. However, if a customer insists on
announcing prefixes which can't be aggregated by its provider, damping
should be applied. Reasons for leaking prefixes might include dual-homing
(to different providers) of a customer, or customer's reluctance to renumber
into the provider's aggregated address range.
1.6 "Golden Networks"
Even though damping is strongly recommended, in some cases it may make sense
to exclude certain networks or even individual hosts from damping. This is
especially true if damping would cut off the access to vital infrastructure
elements of the Internet. A most prominent example are the root name servers.
At least in principle, there should be enough redundancy for root name
servers. However we are still facing a situation where, at least outside USA,
large parts of the Internet are seeing all of them through the same one or
two backbone/upstream links (sea cable) and any instability of those links
which is triggering damping would unnecessarily prolong the inaccessibility
of the root name servers for an hour (at least those sitting in a /24 or
longer prefix).
Other examples of inclusions in the "Golden Networks" might be the Global
Top Level Domain (gTLD) name servers, and possibly overseas or "special"
networks the local ISP wishes to have continued connectivity to regardless
of the instability of the infrastructure inbetween.
Appendix 1 gives an example of the Golden Networks at the time
of publication of this document. Before implementation of route flap
damping using these Golden Networks, network managers are strongly
encouraged to check that the networks listed are indeed still being
announced, and are home to the servers mentioned. This can be done by
matching BGP table announcements with the published addresses for the
listed servers.
These exceptions must only be made if there are strong and identifiable
needs for them - the rule should be to apply coordinated route flap
damping throughout.
2. Recommended damping parameters
2.1 Motivation for recommendation
At RIPE26 and 27 Christian Panigl presented the following network backbone
maintenance example from his own experience, which was triggering flap
damping in some upstream and peering ISPs routers for all his and his
customers /24 prefixes for more than 3 hours because of too "aggressive"
parameters:
scheduled SW upgrade of backbone router failed:
- reload after SW upgrade 1 flap
- new SW crashed 1 flap
- reload with old SW 1 flap
------
3 flaps within 10 minutes
which resulted in the following damping scenario at some boundaries with
progressive route-flap damping enabled:
Prefix length: /24 /19 /16
suppress time: ~3h 45-60' <30'
Therefore, in the Routing-WG session at RIPE27, it was agreed that
suppression should not start until the 4th flap in a row and that the maximum
suppression should in no case last longer than 1 hour from the last flap.
It was agreed that a recommendation from RIPE would be desirable. Given that
the current allocation policies are expected to hold for the foreseeable
future, it was suggested that all /19's or shorter prefixes are not
penalised harder (longer) than current Cisco default damping does. More
recently, this recommendation has been altered so that only prefixes longer
than a /21 are now damped more aggresively. The registries' minimum
allocation is currently a /20, and a /21 announcement is quite feasible for a
multihoming situation.
With these suggestions in mind, Tony Barber designed the following set of
route-flap damping parameters which have proved to work smoothly in his
environment for a couple of months prior to the publication of RIPE-178 (the
original version of this document).
2.2 Description of recommended damping parameters
Basically the recommended values do the following with harsher treatment
for /24 and longer prefixes:
* don't start damping until the 4th flap
* /24 and longer prefixes: max=min outage 60 minutes
* /22 and /23 prefixes: max outage 45 minutes; min outage of 30 minutes
* all other prefix lengths: max outage 30 minutes; min outage 10 minutes
If a specific damping implementation does not allow configuration of
prefix-dependent parameters the least aggressive set should be used:
* don't start damping before the 4th flap in a row
* max outage 30 minutes; min outage 10 minutes
A sample configuration for Cisco IOS is provided in Appendix 2. This sample
can be used as a basis for a configuration on other router platforms.
3. Other Features contributing to Internet Stability
3.1 BGP Route refresh
RFC2918 describes a Route Refresh Capability for BGP-4. Prior to this, there
was no mechanism to reset or refresh a BGP peering session without tearing
it down and waiting for it to re-establish. This process is destructive -
prefixes being exchanged between the two peering routers are withdrawn from
their respective ASes, and this withdrawal can potentially pass through
the whole Internet causing the burden and increased instability discussed
earlier. Usually all that an ISP wishes when reseting a BGP session is to
implement new or revised policy - destroying a BGP session carrying a large
or the full routing table has severe impact on the ISP and his neighbours
on the Internet. Furthermore, reset of a BGP session means the withdrawal
of reachability information from the ISP's customers, and they have the
perception that the Internet has "vanished" - the impression left with
the end-user is that of an unreliable network.
Route Refresh implements a messaging system whereby a router wishing to
refresh or reset its BGP peering with its neighbour simply has to send the
notification. When the neighbour receives the notification, it will send
its entire announcement to its peer (obtained from BGP best path table and
applicable outbound policy).
To find out if your neighbour supports Route Refresh, using Cisco IOS as an
example, enter:
Router# sho ip bgp neigh w.x.y.c | include refresh
Received route refresh capability(new) from peer
Route refresh request: received 0, sent 0
If your router and your peer router support Route Refresh, you can use:
Router# clear ip bgp w.x.y.c in
for requesting a route refresh without clearing the BGP session.
For an outbound route refresh without clearing the BGP session use
Router# clear ip bgp w.x.y.c out
It is recommended that all users of BGP use the route refresh capability
when implementing new BGP policy.
3.2 Soft-Reconfiguration (Cisco IOS)
Where the neighbour does not support RFC 2918 Route Refresh, there is a
functionality in Cisco IOS called "Soft Reconfiguration". This reserves
additional memory in the router to store the BGP table exactly as it was
received from the peer, prior to any inbound policy being applied. The
advantage of this is that the ISP can then change any inbound policy on the
router without reseting the BGP session - the router simply uses the "raw"
BGP table it has received from its peer. Disadvantage is that this
functionality could potentially consume almost twice the amount of memory
required for the BGP table heard from the peer.
To configure soft-reconfiguration in IOS, simply add the extra line to
the BGP peer configuration as below. Soft-reconfiguration is configured
on a per-neighbour basis.
!
router bgp 65501
neighbor 10.0.0.2 remote-as 65502
neighbor 10.0.0.2 soft-reconfiguration inbound
!
Without the keyword "soft" a "clear ip bgp x.x.x.x" will completely reset
the BGP session and therefore always withdraw all announced prefixes from/to
neighbour x.x.x.x and re-advertise them (= route-flap for all prefixes which
are available before and after the clear). With "clear ip bgp x.x.x.x
soft out" the router doesn't reset the BGP session itself but sends an
update for all its advertised prefixes. With "clear ip bgp x.x.x.x soft
in" the router just compares the already received routes (stored in the
"received" data structures) from the neighbour against locally configured
inbound policy statements.
It is recommended to use soft-reconfiguration with all peers which do not
support RFC2918 Route Refresh Capability.
3.3 Fast-External-Fallover (Cisco IOS)
Cisco IOS by default implements a feature known as "fast-external-fallover".
This feature immediately clears the BGP session whenever the line-protocol
to the external neighbour goes down. This feature is desirable so that
there is fast failover in case of link failures - the router can withdraw
paths as soon as the line goes down, rather than waiting for BGP keepalive
timers. The drawback of this, however, is that circuits which are prone
to unreliability will cause BGP sessions to drop and return (i.e. flap),
resulting in instability within the ISP's network, and the potential for
flap damping by upstreams or peers.
If fast-external-fallover is turned off, the BGP sessions will survive
these short line-flaps as it will use the longer BGP keepalive/hold timers
(default 60/180 seconds). The drawback of turning it off - and currently
it has to be done for a whole router and can not be selected peer-by-peer -
is that the switch-over to an alternative path will take longer.
We recommend turning off fast-external-fallover whenever possible:
!
router bgp 65501
no bgp fast-external-fallover
!
Alternatively it might be considered acceptable to retain
"fast-external-fallover" and to turn off "interface keepalives" on unreliable
circuits to overcome the immediate BGP resets on any significant CRC
error period.
Another potentially more satisfactory alternative would be to use a shorter
per-neighbour BGP keepalive timer which has to be applied on both routers
(e.g. 10 seconds which gives a hold-timer of 30 seconds):
!
router bgp 65501
neighbor w.x.y.z timers 10
!
4. Potential problems
4.1 Multiplication of flaps between ASes with multiple interconnections
Christian Panigl experienced the following during a circuit upgrade of an
Ebone customer:
- Only ONE flap was generated as a result of the upgrade process (disconnect
router-port from modem A, reconnect to modem B). Nevertheless the
customer's prefix was damped in all ICM routers.
- The flap statistics in the ICM routers stated *4* flaps !!!
- The only explanation would be that the multiple interconnections
between Ebone/AS1755 and ICM/AS1800 did multiply the flaps
(advertisements/withdrawals arrived time-shifted at ICM routers through
the multiple circuits).
- This would then potentially hold true for any meshed topology because
of the propagation delays of advertisements/withdrawals.
There are two potential solutions to workaround this problem. The first
one is operational, the second one is a software configuration feature
(for Cisco IOS and possibly other implementations as well).
* Schedule a downtime for at least 3-5 minutes which should be enough
time for the prefix withdrawals to have propagated through all paths
before reconnection and re-advertisement of the prefix. Avoid clearing
BGP sessions as this also could generate a 30 minute outage through
flap damping!
* Configure a permanent static route pointing to the customer interface.
Even if the interface goes down, there is still an entry in the routing
table for the customer network, and BGP will therefore still announce
the prefix. Example:
!
router bgp 65500
network 169.254.0.0 mask 255.255.0.0
ip route 169.254.0.0 255.255.0.0 serial 5/0 permanent
!
If migrating the customer from one router port to another, simply enter
the second static route pointing to the new interface. Move the cable
between ports - BGP continues to announce the prefix as the entry is
still in the routing table.
Note: this solution only applies to customers who connect using
static routes. If the customer connects using BGP, first disable
fast-external-fallover on both the customer and ISP router, and then
move the cable in a time period less than the BGP hold-timer.
4.2 Non-recommended flap damping parameters
There are situations where service providers would like to design their
own route flap damping parameters for local needs or conditions. If this is
really desired, then it is important to pay attention to how flap damping
parameters are configured, whether the values are feasible or not, etc.
For example, in Cisco IOS, it is perfectly possible to configure flap damping
parameters which do nothing.
* One example might be the configuration "set dampening 15 500 3000
30". Here the reuse limit is 500, maximum suppress time is 30 minutes
and the half life is 15 minutes. Using these three parameters gives a
maximum possible penalty value of 2000, well below the suppress limit of
3000. So even though this can be successfully configured on the router,
no damping will take place.
* Another example might be the configuration "set dampening 15 750 3000 30".
Here the reuse limit is 750, maximum suppress time is 30 minutes and the
half life is 15 minutes. Using these three parameters gives a maximum
possible penalty of 3000, exactly the same as the suppress-limit. In
Cisco IOS, the penalty is decayed every 5 seconds, so flap damping will
only be take place if the update follows the withdraw within that 5
second time frame. 99% of the time no flap damping will take place.
5. References
RIPE/Routing-WG Minutes dealing with Route Flap Damping:
http://www.ripe.net/ripe/meetings/archive/ripe-24/ripe-m-24.txt
http://www.ripe.net/ripe/meetings/archive/ripe-25/ripe-m-25.txt
http://www.ripe.net/wg/routing/r25-routing.html
http://www.ripe.net/wg/routing/r26-routing.html
http://www.ripe.net/wg/routing/r27-routing.html
Curtis Villamizar, Ravi Chandra, Ramesh Govindan
RFC2439: BGP Route Flap Damping (Proposed Standard)
ftp://ftp.ietf.org/rfc/rfc2439.txt
Enke Chen
RFC2918: Route Refresh Capability for BGP-4 (Proposed Standard)
ftp://ftp.ietf.org/rfc/rfc2918.txt
Merit/IPMA: Internet Routing Recommendations
http://www.merit.edu/ipma/docs/help.html
Cisco BGP Case Studies: Route Flap Damping
http://www.cisco.com/warp/public/459/16.html
Cisco Documentation: Configuring BGP / Route Dampening
http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/
121cgcr/ip_c/ipcprt2/1cdbgp.htm
Cisco Documentation: BGP Soft Reset Enhancement
http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/
120newft/120t/120t7/sftrst.htm
ISI/RSd Configuration: Route Flap Damping
http://www.isi.edu/div7/ra/RSd/doc/dampen.html
GateD Configuration: Weighted Route Damping Statement
http://www.gated.org/gated-web/code/doc/manuals/config_guide/bgp/
weighted_route_dampening.html
6. Acknowledgements
Thanks to all the contributors to this updated version.
7. Changes over previous versions
This document is a significant rewrite and update of RIPE-210. The "Golden
Networks" have now been moved to an Appendix as they are frequently changing
and the authors have come across several instances of providers implementing
the recommendations without actually checking that the Root Nameserver
networks were still as listed in the document.
Updates to the Cisco IOS configuration have been made, and the parameters
chosen for /24 networks have been corrected to make them feasible. Examples
of flap damping in operation have been added to Appendix 3.
The sample configuration has been moved to Appendix 2 - again IOS is used
as a known working example.
Appendices
A.1 "Golden Networks"
This appendix lists some examples of Golden Networks which could be used
when applying route flap damping at the edge of an ISP's network. The
example uses Cisco IOS configuration syntax. The list was correct at time
of writing (April 2001).
ip prefix-list golden-networks description Golden Networks
! Root Nameservers
ip prefix-list golden-networks permit 198.41.0.0/24 ! A & J
ip prefix-list golden-networks permit 128.9.0.0/16 ! B
ip prefix-list golden-networks permit 192.33.4.0/24 ! C
ip prefix-list golden-networks permit 128.8.0.0/16 ! D
ip prefix-list golden-networks permit 192.203.230.0/24 ! E
ip prefix-list golden-networks permit 192.5.4.0/23 ! F
ip prefix-list golden-networks permit 192.112.36.0/24 ! G
ip prefix-list golden-networks permit 128.63.0.0/16 ! H
ip prefix-list golden-networks permit 192.36.148.0/24 ! I
ip prefix-list golden-networks permit 193.0.14.0/24 ! K
ip prefix-list golden-networks permit 198.32.64.0/24 ! L
ip prefix-list golden-networks permit 202.12.27.0/24 ! M
! Global Top Level Domain Servers
ip prefix-list golden-networks permit 192.5.6.0/24 ! A
ip prefix-list golden-networks permit 203.181.96.0/19 ! B
ip prefix-list golden-networks permit 192.26.92.0/24 ! C
ip prefix-list golden-networks permit 192.31.80.0/24 ! D
ip prefix-list golden-networks permit 192.12.94.0/24 ! E
ip prefix-list golden-networks permit 192.35.51.0/24 ! F
ip prefix-list golden-networks permit 192.42.93.0/24 ! G
! ip prefix-list golden-networks permit ???? ! No H
ip prefix-list golden-networks permit 192.36.144.0/24 ! I
ip prefix-list golden-networks permit 210.132.96.0/19 ! J
ip prefix-list golden-networks permit 213.177.192.0/21 ! K
ip prefix-list golden-networks permit 192.41.162.0/24 ! L
ip prefix-list golden-networks permit 202.153.112.0/20 ! M
A.2 Sample Cisco IOS Configuration
! Parameters are :
! set damp <half-life-time> <reuse-at> <suppress-at> <max-suppress-time>
! There is a 1000 penalty for each flap
! There is a 500 penalty for change in attribute
! Penalty decays at granularity of 5 seconds, decay starts as soon as
! prefix is withdrawn
! Unsuppressed at granularity of 10 seconds
! damping info kept until penalty becomes < half of reuse limit.
!
! Cisco/IOS value-ranges:
!
! <half-life-time> (range is 1-45 minutes).
! <reuse-value> (range is 1-20000).
! <suppress-value> (range is 1-20000).
! <max-suppress-time> (range is 1-255 minutes ).
!
!-----------------------------------------------------------------------
! ENABLE BGP DAMPenING using "graded" route-map
!-----------------------------------------------------------------------
router bgp 65500
NO bgp damp
bgp damp route-map graded-flap-damping
!
!-----------------------------------------------------------------------
! DEFINE "graded" route-map
!-----------------------------------------------------------------------
NO route-map graded-flap-damping
!
! don't damp Candidate Default Routes
! OPTIONAL (not part of recommendation)
! prefix-list default-networks lists the Candidate Default Routes
!
!route-map graded-flap-damping deny 5
! match ip address prefix-list default-networks
!
! don't damp Golden Networks
!
route-map graded-flap-damping deny 10
match ip address prefix-list golden-networks
!
! - /24 and longer prefixes: max=min outage 60 minutes
!
route-map graded-flap-damping permit 20
match ip address prefix-list min24
set damp 30 820 3000 60
!
! - /22 and /23 prefixes: max outage 45 minutes but potential for
! less because of shorter half life value - minimum of 30 minutes
! outage
route-map graded-flap-damping permit 30
match ip address prefix-list max22-23
set damp 15 750 3000 45
!
! - all else prefixes: max outage 30 minutes min outage 10 minutes
!
route-map graded-flap-damping permit 40
set damp 10 1500 3000 30
!
!-----------------------------------------------------------------------
! DEFINE PREFIX-LISTS
!-----------------------------------------------------------------------
!
! OPTIONAL default-networks
!no ip prefix-list default-networks
!ip prefix-list default-networks description Candidate Default Routes
!
no ip prefix-list golden-networks
ip prefix-list golden-networks description Golden Networks
! See 7.1 above for sample list
!
no ip prefix-list min24
ip prefix-list min24 description Apply to /24 and longer prefixes
ip prefix-list min24 permit 0.0.0.0/0 ge 24
!
no ip prefix-list max22-23
ip prefix-list max22-23 description Apply to /22 and /23 prefixes
ip prefix-list max22-23 permit 0.0.0.0/0 ge 22 le 23
!
A.3 Study of Flap Damping Operation
It is instructive to observe how route flap damping actually works on
a router - doing so will help the reader understand how the particular
values described above were chosen.
The test bed had two Cisco routers connected to each other. One router
originated prefixes, the other one had the flap damping parameters described
above in the text. The router originating the prefixes would withdraw a
prefix, then reannounce, then withdraw, reannounce, etc. The BGP process
in IOS checks every 60 seconds for any new or withdrawn prefixes in the
local configuration - so on the source router, the withdraw and announce
was done by removing and adding the BGP network statement for the prefix
in question. The router monitoring the flaps would see the prefix being
withdrawn and then announced 60s later.
A.3.1 For /24s
Parameters used are "set dampening 15 820 3000 30"
1st flap 1000 decay to 966, 982 at update
2nd flap 1966 decay to 1894, 1926 at update
3rd flap 2894 decay to 2787, 2846 at update
4th flap 3280 decay to 3165, 3226 at update
Maximum possible penalty is 3280 as defined by the flap parameters, so
the penalty at the 4th flap was only incremented from 2787 to 3280, not
3787 as might have been expected. At the 4th flap the prefix was marked as
being suppressed for 59 minutes when the update message was received. If
the update after the 4th flap was not received within 4 minutes and 20
seconds, the penalty dropped below 3000, and the prefix was not suppressed.
A.3.2 For /22s, /23s
Parameters used are "set dampening 15 750 3000 45"
1st flap 1000 decay to 921, 960 at update
2nd flap 1921 decay to 1777, 1850 at update
3rd flap 2777 decay to 2583, 2671 at update
4th flap 3583 decay to 3311, 3451 at update
Maximum possible penalty is 6000. At the 4th flap the prefix was marked as
being suppressed for 33 minutes when the update message was received. If
the update after the 4th flap was not received within 4 minutes and 40
seconds, the penalty dropped below 3000, and the prefix was not suppressed.
A.3.3 For remaining prefixes
Parameters used are "set dampening 10 1500 3000 30"
1st flap 1000 decay to 889, 946 at update
2nd flap 1889 decay to 1679, 1781 at update
3rd flap 2679 decay to 2367, 2526 at update
4th flap 3367 decay to 3019, 3176 at update
Maximum possible penalty is 12000. At the 4th flap the prefix was marked as
being suppressed for 10 minutes when the update message was received. If the
update after the 4th flap was not received within 2 minutes and 5 seconds,
the penalty dropped below 3000, and the prefix was not suppressed.
A.3.4 Summary
When analysing flap damping performance on the router or across the network,
network managers should compare with the above lab tests. Note especially
that slowly flapping prefixes are unlikely to be suppressed even though
they show significant flapping history. A future version of this document
may consider what to do in this instance.
>>>>end
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