BGP Configuration with two ISP

[narendrakurajak1]

Hi,

Please help in achiving the scenario discribed below:

We want to configure BGP with two different ISP.

What would be the prerequisite for this?

Our requiremnet is to load balance the load of internal traffic too.

We have IP block of 16 IPs from one ISP and need the reachablility of these public IPs from different ISP.

Would Each ISP provide their Private AS no. or only AS no from One ISP can work.

Else we have to go with Public AS no.

I am not able to work on this scenario due to not very well knowledge of BGP.

Your help is really appreciated.

Please ask if any other information is required.

[Francesco Molino]

Hi

First thing is that on internet you can't advertise a subnet smaller than /24. In your case, you only 16 IPs.
Then, these IPs are given by your ISP 1 and you won't be able to ask ISP 2 to advertise these same IPs.
In your case, there is no need of public ASN. Each ISP will give you the information about their ASN and on your router, you can use no matter what ASN as soon as it's on a private ASN range (see RFC 6996).

Now, by doing a peering BGP with your ISP, and learning the full BGP table, you will be able to some load-balancing (in different ways) from internal to outside.
But from outside to inside, you'll need a different design:
- ask for your public IPv4 that you can advertise and not getting IPs from your ISP
- or ask ISP 2 to get some public IP and having a public DNS load balancer
- Or getting public IPs from ISP 2, create A DNS record for internal services with IP used from IPS1 and ISP2. Public DNS will do round-robin to send the traffic to 1st or 2nd IP. But you need to ensure that traffic won't be asymmetric.
.....

There are many ways to handle it but with the information given, in your case it would be complicated as you just have ISP 1 public IP and these IPs will be advertised by ISP1

[narendrakurajak1]

Hi,
Thanks a lot.
so minimum subnet is /24 to advertise on internet so if we go for Public AS No. who will provide that AS No. ISP1 or ISP 2?
want to know why we cann't advertise these IP from different ISP i.e. /28 which I have?
Is it possible to advertise these IPs only if possible by mutual understanding of ISPs?
as someone told me that it is possible only If ISP agree upon it.
One more thing how we can load share to two ISP can you post some example.
One more thing we have link on router and then it comes to firewall and we have done PAT over firewall so if we go for load sharing how we can achieve this.
Note: I have all these device from HP (RouterA MSR50-40, Firewall A f-1000.)
any other information you required please ask.
And thanks again please reply AS soon as possible i need it.

[Francesco Molino]

Hi

If you talk with your ISP and agree with them, sure you can announce your /28 and they will announce it out but not as /28 because the announcement will be dropped as smaller than full class C /24.
In addition to that, if these IPs are from ISP B, ISP A won't accept to advertise them as on internet they're advertised by ISP B and these IPs are their properties.
To request public ASN, you can check with RIPE, ARIN, APNIC.... Here is a map with the right Internet Registry agency based on your location: https://www.arin.net/knowledge/rirs.html

[narendrakurajak1]

Hi, Thanks for replying.
I have more queries to ask please help.
1. What issue would be if ISP announces our /28 Block as their /24 IP block.
2. Will it requires a public AS No.
3. If possible to use private As no, who will provide that.
4. If ISP agree to provide then would we need two different Private AS no from both The ISP.
5. What should I have to ask from ISP to config on their end.
And Last how we can do load sharing without becoming our router as transit router.

Please help
and once again thanks you doing good for me to understand this.

[Francesco Molino]

Hi

1. IPs are from ISP B. They will announce their larger subnet in which your /28 is included. When the traffic hits their backbone routers, they have more specific routes to redirect the traffic for your /28 to your router. ISP A won't announce it as this is not their IP range.
2. No public ASN is required for that, unless you request at the same time a /24 (for example) and explain to Internet registry agency that you really need it.
3. You can use any private ASN on your site and add your ISPs router as ebgp because they won't be in the same ASN as you.
4. 1 private ASN on your side and peer with both ISP as eBGP. They will give you the peering IP, ASN and password on their side
5. As quite all ISP, you will fill a form to know what you will get on your BGP: I mean will you get just a default route or the full BGP table, will you announce something, .... They know how to configure it but they will ask you what do you want for that.

[narendrakurajak1]

Hi,
Thanks a ton and I really appreciate your efforts making this discussion healthy.
but please bear with me., I am totally novice for this BGP concept.
Now till this point I am able to understand how to work with bgp and what are the prerquisite for it.
But as I have some Web portals hosted in private network i.e. internal Network and NATTED with ISP1 IP and working fine.So if ISP 1 announces their /24 Network. Will I able to access these public portals from internet using ISP2 if ISP 1 Link goes down.
If yes how else what has to be done if we are not going as per the solution given above by you to use two IP block from different ISP and use load-balancer.

Thanks I think this answer Will resolve my query finally.
I need this to be answered in context to client.
1. What issue would be if ISP announces our /28 Block as their /24 IP block.
is there any issue for our network Infra.

[Francesco Molino · Accepted solution]

What ISP advertise, doesn't impact your network. At the end, as he's giving you the /28 he knows where to forward the traffic for your IPs. If you have your own IP block, it is even simpler because you managed what to advertise and where the primary link is,

Now, in your case, you will do some active/standby design meaning ISP1 will be primary and ISP2 secondary. In that case, as you will have 2 different public IPs, you will need a DNS failover system to update DNS record when ISP 1 goes down with new ISP2 IPs.

If you want to do active/active, you will need a DNS load-balancing or keep a standard round-robin DNS algorithm (add 2 IPs on the same record for example). But in that case, you will need to ensure that traffic is going out from the same link as inbound traffic. It could be complex.

[narendrakurajak1]

Hi,

Thanks for reply,
Suppose we do not want IP pool from second ISP we need the IP from ISP 1 reachable from ISP 2 when ISP 1 link goes down.
what we have to do?

[Francesco Molino]

You need to contact them and see what they gonna answer. Honestly, you have 99% of chance that they gonna tell you it is not possible. Or they will ask you to purchase public IP for you directly instead of getting ISP1 block. Or they will maybe say, they will give you a 2nd internet link from ISP 1 and then they will be able to failover the traffic. In that last case, just make sure you really have 2 different links path to not have SPOF.