Re: bgp

alsayed · ‎12-10-2010

Hi

1)when we should use conditional advertisement in BGP?what is the situation for that?

2)if we have public provider independent and multihoming with two different ISPs,where the conditional advertisement fits in this situation?

Thanks

lgijssel · ‎12-10-2010

Instead of asking all kinds of general questions, you might consider to do some reading yourself.

BGP is extensively documented on CCO.

The link below is a good starting point:

http://www.cisco.com/en/US/docs/ios/11_3/np1/configuration/guide/1cbgp.html

regards,

Leo

Tharak Abraham · ‎12-10-2010

Conditional advertisement allows BGP speaker to advertise a set of BGP prefixes

to a peer only if certain other prefixes are present or not present in the

local BGP table. Thus, the existence (or non-existence) of those “trigger” prefixes

is used as a condition to advertise selected prefixes to a peer. Conditional

advertisements are helpful in situations with multiple uplinks to different ASes.

Example based on this, you want to advertise your local prefixes to the

ISP2 only in case if the primary ISP1 fails. In order to detect the primary

ISP1 failure, you track a certain prefix learned from the primary ISP. This allows

for more sophisticated tracking of the connectivity issue, compared to simply

tracking the interface state. Once the tracked prefix is gone, all local prefixes are

announced to the ISP2, until the moment the tracked prefix appears

again.

k.hariharan1 · ‎12-13-2010

Hi alsayed,

This is really a good question,

2)if we have public provider independent and multihoming with two different ISPs,where the conditional advertisement fits in this situation?

hi tharak,

I have some questions ,

1. i agree that Conditional advertisement allows BGP speaker to advertise a set of BGP prefixes to a peer only if certain other prefixes are present or not present in the local BGP table.

2. But if you peer with two different isp , isp will be advertising the prefixes to you. it may be only the default route or full internet routing table to you.

3. In this case on what basis you will track the prefixes if you receiving the same prefixes from both the service provider ?

For example ,

if you peer with two ISP and both isp is advertising default route , in my router if i give sh ip bgp i will be having two default route in my BGP table so in this case if i am tracking the default route it not going to happen , am i correct???

Regards,

Hariharan k

Tharak Abraham · ‎12-14-2010

Hariharan,

Please find the answers below and hope it helps:-

1. i agree that Conditional advertisement allows BGP speaker to advertise a set of BGP prefixes to a peer only if certain other prefixes are present or not present in the local BGP table.

Conditional advertisements should be viewed as a feature or for flexibility and dont confuse it with Multihoming.

Its like you selling your products to two vendors,

As long as you get money, you prefer Vendor A and when you dont get money; you stop selling it to A and start to sell

it to Vendor B unless untill you get paid again from Vendor A.

2. But if you peer with two different isp , isp will be advertising the prefixes to you. it may be only the default route or full internet routing table to you

Practically put;

In any multihomed scenarios, we define incoming traffic policies and outgoing traffic policies.

Conditional advertisements can influence the incoming traffic policy.

We would always have a primary/best ISP selected out of so many factors and would want to advertise certain of our local prefixes via them which will influence the traffic inbound.

Another way to view is like, You may track an upstream NAP reachability prefix; and based on its presence or absence you may decide to advertise your local prefixes through that particular ISP or not.

3. In this case on what basis you will track the prefixes if you receiving the same prefixes from both the service provider ?

For example ,

if you peer with two ISP and both isp is advertising default route , in my router if i give sh ip bgp i will be having

two default route in my BGP table so in this case if i am tracking the default route it not going to happen , am i correct???

Yes, thats correct.

Tracking based on default route doesn't seems to be a valid option.

NB: The examples given in Cisco Doc are to be viewed as examples only and for us to understand the features and it may NEVER

be practical in real designs !

Ganesh Hariharan · ‎12-12-2010

Hi
1)when we should use conditional advertisement in BGP?what is the situation for that?
2)if
we have public provider independent and  multihoming with two different
ISPs,where the conditional advertisement fits in this situation?
Thanks

Hi,

Check out the below link for BGP conditional adverstisement

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a0080094309.shtml

Hope to Help !!

Ganesh.H

ismailmohammed · ‎12-12-2010

what is a conditional advertisement

Conditional advertisement allows the BGP Speaker to advertise the aggregate routes to its peer only if there is one or more specific routes available in the route table. This allows the BGP router more flexibility to control the aggregate route advertisement to its peers.

In your example you could do the failover using the conditional advertisement.