Recomended prefix to advertise to ISPs in BGP Multihomed scenario

[mvsheik123]

Hi all,

I have ex: 60.20.48.0/20 (255.255.240.0) from ARIN, and in production we may use only 15-20 IP addresses(60.20.48.1 -.20). In this scenario, advertising 60.20.48.0/24 to both ISPs is fine (to achieve reachability to our servers incase of any ISP having sisues) or any other recomended method (longer prefix to another ISP)?. The examples I read recomending longer prefixes to one ISP and lesser prefix to 2nd on but in my case I do not have any traffic to other subnet.Both ISPs are of good capacity links.

Thanks in advance

MS

[u2637ps · Accepted solution]

If you send a less specific route to one isp then you need to be aware that all traffic for your /24 will come down one link although traffic going back is likely to go out of either links depending the destinations.

[mvsheik123]

Thank you for the reply. What is the recomended way when the not may IPs being used? Use only one linkadvertise AS prepend on another? Also, if I have /24 advertised to both vendor with no preference, -as BGP deciding the path what would be maximum time it takes for BGP to failover to other ISP incase one ISP has any issues?

Thank you

MS

[samavedula_rama · Accepted solution]

If you manipulate using AS-PREPEND or any other BGP atribute for that matter, outside world will not see a big difference since the route will be visible from both providers on the global table but it is just not the best.

So, I would say AS-PREPEND is a very good option. But keeping future expansion in mind, I would also advise you to advertise an aggregate route to atleast one provider.

hope this helps.

[mvsheik123]

Thanks Rama Mohan. AS-prepend may not work always correct. And as far as the growth... not anticipating use of more than 40 IPs in next few years.  So for now I will let the /24 goes to both providers and incase of any issues try to manipuate AS path prepend.

Thanks again

MS

[Kattubava Edaisal Shahul Hameed]

hi sheik,

would you like to use failover mode of operation or loadbalancing b/w the ISP?? If you need failover mode you can go for as-path prepend option