BGP Advertisement

Answered Question
Apr 13th, 2009

Hi,

I request you to clarify the following concepts in BGP Protocols.

"Minimum time between advertisement runs is 30 seconds"

"keepalive interval is 60 seconds"

I am aware that keepalive is sent between the peers to identify the presense of the peer. But how about advertisement / update.

I am sure that update is triggered only when changes are there in the network.

Then what is the time 30 seconds mentioned in the advertisement timer. I noticed that even though a prefix is removed from routing table, BGP table is not getting changed for a while. It takes some time to get changed.

Please clarify the concept

RBK

I have this problem too.
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Correct Answer by Giuseppe Larosa about 7 years 7 months ago

Hello R. B. Kumar,

as I wrote this is the price to pay for scalability.

in an iBGP context in addition to timers tuning, you can take advantage of some advanced features like next-hop tracking.

see

http://www.cisco.com/en/US/docs/ios/iproute/configuration/guide/irp_bgp_adv_features_ps6350_TSD_Products_Configuration_Guide_Chapter.html#wp1056214

this can speed up BGP path removal when the next-hop route disappears in the IGP.

Hope to help

Giuseppe

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Giuseppe Larosa Mon, 04/13/2009 - 23:46

Hello R.B. Kumar,

BGP is designed with scalability as the first objective.

To be able to handle 300,000 of routes the BGP operations are scheduled over time:

there are different timers used internally in addition to those on each BGP session.

Among the most important of these internal timers are:

the BGP scan interval : this is the process that verifies the BGP paths in the BGP table to see if there is any path that is not valid anymore. BGP scan interval runs every 60 seconds.

The advertisement interval:

even in the case of a new prefix to advertise or a prefix to be withdrawn the BGP update is not sent immediately: the new advertisement is placed in a sort of queue and up to 30 seconds are waited so that if other prefixes need to be included in the update this is possible.

For the router it would be more heavier to send each update in a separate packet as soon as the need to update is found.

This is for scalability.

You can see this timer in effect and others like BGP scan in a lab test in which you try to measure convergence time for example for MPLS VPN with a multi homed site and BGP route reflectors.

The time it takes for the backup advertisement to be installed in a remote PE is determined also by the advertisement interval.

Hope to help

Giuseppe

hclisschennai Tue, 04/14/2009 - 01:06

Hi Giuseppe,

Thanks for your comments. As you said, I experimented this intervals in my lab before i write this post. I Found a a delay whenever a new prefix is added or one is removed. It takes 30 seconds to get refelected in all routers.

I am worried this will create a considerable delay in live network and real time traffic. Is it so?

RBK

Correct Answer
Giuseppe Larosa Tue, 04/14/2009 - 01:35

Hello R. B. Kumar,

as I wrote this is the price to pay for scalability.

in an iBGP context in addition to timers tuning, you can take advantage of some advanced features like next-hop tracking.

see

http://www.cisco.com/en/US/docs/ios/iproute/configuration/guide/irp_bgp_adv_features_ps6350_TSD_Products_Configuration_Guide_Chapter.html#wp1056214

this can speed up BGP path removal when the next-hop route disappears in the IGP.

Hope to help

Giuseppe

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