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BGP on internet with two ISPs

dhiraj_g23
Level 1
Level 1

Hi,

I have a situation here. I am hosting two subnets, one from each ISP.
I want one subnet A out ISP A, and subnet B out ISP B
On Router exiting ISP A
I advertised both subnets with route-maps allowing A directly, and B with five times the AS path prepend
On Router exiting ISP B
I advertised both subnets with route-maps allowing B directly, and A with five times the AS path prepend
Now if one of ISP goes down, the other subnet is not working by default. I need to shutdown BGP completely on link down interface
The defect I am assuming is that five times AS path is not sufficient longpath to consider it as a backup path. Hence ISP A treating subnet A and B as best path and ISP B treating subnet A and B as best path. Is it possible for both ISPs to assume that it is hosting the best path for these subnets...or what could be the problem.

I just traced from Router A to Router B via internet and it takes almost 8 hops

I want to try giving 10 AS path-prepend on same AS and try. Let me know if there is anyother solution too

 

Thanks,

Raj

9 Replies 9

Kenneth Knapp
Level 1
Level 1
Have you verified the Providers are not striping the AS pre-pend?

It is a long process to get hold of ISPs.

When one of the links is down, we are advertising both through the second links, and traffic is reaching us on both subnets on the second links, which means ISP are not stripping anything. Is that your question ?

The problem is not on prepending. If one link is down then only one advertisement of this prefix exist so traffic should automatically flow to.from the active ISP. 

@ e102154emp

 

didnt get you. We are allowing both the prefixes on both links with route-maps prepending ASs for secondary subnet.

In that case both prefixes should be working from active link,     but until unless I shutdown everything on secondary(not working)link, this one does not actively take both the prefixes

Thanks,

Raj

Tagir Temirgaliyev
Spotlight
Spotlight

you need one Provider Independent Subnet at least /24

 

to be able to advertise it to both providers

We are advertising two /24..one from each ISP

It is given provider, as a part of circuit offered, but I do not think it has to be advertised on the same link.

It can sent via alternate provider is well.

Hi Everyone,

I also have a similar issue. We have an internet connection from ISP A and the assume that the subnet provided is 192.168.10.0/24.

Now for ISP backup we have to apply for ISP B but we want the same subnet from ISP B also, as we cannot migrate the whole traffic to different subnet for some reasons.

After studying a lot i found that you have to have PE address space and you can advertise you PE address to any ISP.

But the problem is now IANA has stopped issuing /24 subnets, so are there any chances that we can request our ISP A to give the ownership of 192.168.10.0/24 subnet to us.

Are there any solutions to such requirements ? Kindly suggest.

Regards

@Mohammed

Hello,

You can consult with ISPs to see if they allow this to happen between them, however I would be extremely surprised if they do this, also an ISP may not be willing to sell off or give the /24 subnet, chances of this are extremely low unless it was critical and major like the Government departments.

PI address space would allow advertisement of routes to different ISPs, still a problem whether the ISPs you peer with accept /25 (splitting, using x2 /25's at each location). Or just have one /24 in active standby fashion.

PA address space is much more difficult, they probably own the /16 in most cases, why would they make such major routing changes to not advertise that one single /24 if they've given it to you. The only option here is to dual home to a single provider that has different POPs for each circuit and that will guarantee that your traffic will be diversely routed to different tier 1 ISPs.

Hope this helps.

Please rate useful posts & remember to mark any solved questions as answered. Thank you.

Hi Bilal,

Thanks for your prompt reply. We had already contacted both ISP's but they did not agree to do so. However we are in talks with ISP A to transfer the ownership of that subnet to us and as you said i also feels that the possibilities are less.

About having PI address space i am not at all sure whether IANA is still issuing or not IPv4 as i cannot see any progress on this from our side. I think they can now provide IPv6 only, can you check if you have any resources regarding the same.

If we get the IPv4 from IANA i think we can advertise our PI address with two ISP's by forming neighbor-ship. And i know how to tackle them for allowing this to happen. Kindly feed in as many as suggestion please.

Regards

@Mohammed

 

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