Example Monitoring Route Dampening

The example shows how, after the first flap (when a route becomes unreachable), the router withdraws the route but keeps it in its own database to keep track of the penalty. The route enters the history state.

7-76 Configuring BGP on Cisco Routers (BGP) v3.2 © 2005, Cisco Systems, Inc.

The route is gone, but the history I : entry is retained in the BGP table.

BGP: charge penalty for 12.0.0.0/8 path 387 462 with halflife-time 15 reuse/suppress 750/2000

BGP: flapped 1 times since 00:00:00. New penalty is 1000 router# show ip bgp 12.0.0.0

BGP routing table entry for 12.0.0.0/8, version 7 Paths: (2 available, best #2, advertised over EBGP)

387 462 (history entry) 4-

Origin IGP, localpref 90, external w

Dampinfo: penalty 992, flapped 1 times in 00:00:10

1.1.0.4 (metric 41024000) from 1. 0 . 0.1 (11.0.0.1) 8

Origin IGP, metric 0, localpref 100, valid, internal, best :

Dampening Information

Using show ip bgp displays suppressed prefixes with the state "h."

Using show ip bgp prefix displays suppressed prefixes that are marked with "history entry."

If a prefix is in the history state, it means that it is currently unreachable but that the information has been kept in the BGP table to keep track of the penalty.

© 2005, Cisco Systems, Inc. Optimizing BGP Scalability 7-77

Penalty (in Thousands)

-

Penalty goes over suppress limit after several flaps.

nJ 1

-

N

N

-

The example here shows how, after the third flap, the penalty of the route exceeds the suppress limit, and the route could be suppressed. When the route exceeds the suppress limit, one of two things could happen:

■ The router will put the route in the history state if the route is currently unreachable.

■ The router will suppress the route if the route is currently reachable.

7-78 Configuring BGP on Cisco Routers (BGP) v3.2 © 2005, Cisco Systems, Inc.

Route flaps, penally goes over suppress limit.

router*

BGP: charge penalty for 12,0,0,0/8 path 387 462 with halflife-time 15 reuse/suppress 750/2000 BGP: flapped 2 times since 00:05:37. New penalty is 1776

router*

BGP: charge penalty for 12.0.0.0/8 path 387 462 with halflife-time 15 reuse/suppress 750/2000

BGP: flapped 3 times since 00:06:54. New penalty is 2681' router#

BGP: suppress 12.0.0.0/8 path 387 462 for 00:27:00 (penalty 2629) halflife-time 15, reuse/suppress 750/2000

Route is damped after it reappears in the BGP table.

The figure displays the debugging messages that accompany the three route flaps that were illustrated in the previous figure. After the 12.0.0.0/8 prefix flaps for the third time, the router assigns a new penalty of 2681 to the route. Because the new penalty exceeds the suppress limit of 2000, the 12.0.0.0/8 prefix is suppressed by the router for 27 minutes (if it remains stable).

Using the show ip bgp command displays all suppressed prefixes that have their state marked as dampened ("d").

© 2005, Cisco Systems, Inc. Optimizing BGP Scalability 7-79

routert show ip bgp dampened-paths

BGP table version is 20, local router ID is 10,1.1.1

Status codes: s suppressed, d damped, h history, * valid, > best

Network From Reuse Path

*d 12.0.0.0 1.3.0.3 00:23:5 387 462 i routert show ip bgp 12.0.0.0

BGP routing table entry for 12.0.0.0/8, version 7 Paths: (2 available, best #2, advertised over EBGP)

Origin IGP, localpref 90, valid, external

Dampinfo: penalty 2278, flapped 3 times in 00:10:32, reuse in 00:24:00

1.1.0.4 (metric 41024000) from 1.0.0.1 (11,0.0.1) g

Origin IGP, metric 0, localpref 100, valid, internal, best g1

The route is available but not used or propagated to other neighbors.

The show ip bgp dampened-paths command is used to list all networks that are currently suppressed because of dampening.

The show ip bgp prefix command displays detailed information among other paths about suppressed prefixes. These prefixes are marked with the words "suppressed due to dampening."

7-80 Configuring BGP on Cisco Routers (BGP) v3.2 © 2005, Cisco Systems, Inc.

The penalty of the route is decreased following an exponential curve. After a while, the penalty drops below the suppress limit, but the route is not yet released—the route is released only after the penalty drops further below the reuse limit. In the example here, the route flaps again, further increasing the penalty.

© 2005, Cisco Systems, Inc. Optimizing BGP Scalability 7-81

In the example here, the route has stabilized, and the penalty gradually drops below the reuse limit. At that point, the BGP router releases the route, and it can now be selected as the best BGP path. If the released route is selected by the router as the best BGP path, it is also propagated to BGP neighbors and used for data forwarding.

When the penalty that is associated with a route drops below half of the reuse limit, the penalty and the flap history that are associated with the route are cleared by the router.

7-82 Configuring BGP on Cisco Routers (BGP) v3.2 © 2005, Cisco Systems, Inc.

Was this article helpful?

0 0
Search Engine Optimization Overview

Search Engine Optimization Overview

This is the 2nd volume of a 9 volume series called the Webmasters Toolbox package. Search engines are the number one way that internet users find websites. In most cases, a listing in a search engine is free. So, it's no surprise that Search Engine Optimization SEO is often the first priority when marketing a website.

Get My Free Ebook


Post a comment