Internet Cloud AS Path Prepend functionality Query

[kumarmh91282]

Hi

Please find the attached Diagram for Reference:

We have our own APNIC pool which we have advertised to ISP-1 and ISP-2 as shown in diagram.

On ISR routers we are Advertising 1.1.1.0/24 to ISP-1 and ISP-2 via EBGP and receiving Default route on both ISR routers.

Issue: Majority of our inbound traffic is preferred via ISP-2, even after trying AS path Prepend still the same issue.

Since we have advertised the entire subnet to  ISPs  even though the traffic leave via ISP-1 preferred path is ISP2.

on my Lab its a straight forward with AS-path prepend is working fine.. but since ISR routers are connected to Service providers, how do I influence my Inbound traffic.. (Tried AS path prepend and not working)

I would like to make ISP-1 as primary and ISP-2 as Secondary

Please need your Guidance/Suggestions

Thanks in advance.

[pman]

Hi,

Diagram not attached

[Richard Burts]

How many prepends are you advertising? In a lab a single prepend is usually enough. In a production environment sometimes it might take several prepends to achieve making ISP 1 primary.

It might help if we knew who ISP 1 and ISP 2 are. Is it possible that ISP 1 is a tier 2 provider while ISP 2 is a tier 1 provider?

[kumarmh91282]

Both the ISPs are Tier -1  service provider

[Richard Burts]

It is good to know that both providers are tier 1. That would eliminate one potential cause of the behavior. We know very little about your environment and that makes it difficult to give good advice. Can you provide more information? Is your address block a /24 or some other size? Did you obtain your address block independently from one of the registries? Or did you obtain the block from someone else?

Is it possible that your address block is part of a larger address block that is being summarized somewhere? If you check a couple of looking glass sites do you see one advertisement of your address block, or see both?

If you do 

show ip bgp neighbor advertised

are you advertising the same things to both neighbors?

Could you post the configuration for BGP from both routers?

[Jon Marshall]

Just to add to Rick's post. 

AS path prepending is not an exact science so it may well work perfectly in the lab but not in the real world because it is really only expressing a preference as to what path to use nothing more. 

We use it where I work as we have dual ISPs but, for example, if a customer also has internet connectivity via one of those providers it is highly likely they will come in to us via that provider even if we have used prepending to favour the other one. 

Unfortunately you only have a /24 by the looks of it so you cannot do more and less specific routes to favour one way over the other. 

Jon

[paul driver]

Hello
As stated by others it could be you are performing as-path prepending correctly however there is nothing to stop those ISPs implementing their own bgp policies on any prefixes you advertise to them.

Do you have any agreement with them regards route advertisement?
Are both ISP inks of equal bandwidth or do they differ? 

Are you negating transit routing between both IPS's on your rtrs?

One way you could fix this would be to conditionally stop advertising that /24 subnet to ISP2 so wan traffic would then to return via ISP1, this condition would be based on IPSLA object tracking and EEM scripting and as such if the tracked object failed (ISP1 monitored interface/ip etc..) then the EEM script would initiate pertaining to a IPSLA failure and advertise the /24 subnet via ISP2.