Re: BGP query

ashishpanda · ‎08-13-2006

Hi,

I am working on a redundant internet setup. I have got 2 ISPs for ease of understanding lets say ISP A and ISP B.

I dont have my public IP segment registered. I have got a /24 subnet from ISP A. Can I run the BGP session with ISP A and ISP B with private AS number and will be able to advertise the subnet through both the ISPs?

Thanks in advance

Ashish

vijayasankar · ‎08-13-2006

Hi Ashish,

This is not possible. Refer to similar posts in this forum.

As per norm, in any case ISP B will not be able to route the ISP A's IP Subnets. This is the design.

If you want to Use ISP A and ISP B's link for fault tolerance/redundancy and with out having a registered IP Address/AS, then you have to resort to some other multihoming solutions( there are some hardware products, which does this) You can check in net for more on this.

HTH

-VJ

vijayasankar · ‎08-13-2006

Hi Ashish,

This is not possible. Refer to similar posts in this forum.

As per norm, in any case ISP B will not be able to route the ISP A's IP Subnets. This is the design.

If you want to Use ISP A and ISP B's link for fault tolerance/redundancy and with out having a registered IP Address/AS, then you have to resort to some other multihoming solutions( there are some hardware products, which does this) You can check in net for more on this.

HTH

-VJ

mheusinger · ‎08-13-2006

Hi,

As the previous poster wrote, it is not likely. There are examples, however, where ISP B agreed to your suggested setup. So you might or might not be able to have redundancy like you proposed. Just ask ISP B and see what the answer is.

Depending on the service you like to implement, a second subnet from ISP B might be sufficient as well.

Hope this helps! Please rate all posts.

Regards, Martin

ashishpanda · ‎08-13-2006

Hi Martin,

I have checked this with ISP B. They are ready for the setup.But ISP A is not convinced. According to them if the Subnet is not originated from the RIPE AS records the advertisement will be dropped.I am not sure whether they are right. I was going through the following post in this forum

http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Network%20Infrastructure&topic=WAN%2C%20Routing%20and%20Switching&CommCmd=MB%3Fcmd%3Ddisplay_location%26location%3D.1ddbe331

If I go with this post the setup i am looking at is quite possible. Kindly send your views on this.

Thanks

Ashish

sundar.palaniappan · ‎08-13-2006

Hi,

It's definitely possible. Prepend your AS-path more times when advertising the prefix to the ISP you want to use as your backup path. This would work as long as both ISPs advertise your network as it is and do not aggregate it. Verify the routing is working as intended by doing a route look up on the looking glass.

Hope that helps!

Regards,

Sundar

ashishpanda · ‎08-13-2006

Thanks sundar

I am looking at having a private AS number and advertise the subnet to both the ISPs. ISP A is not willing to do a peering with me saying that the advertisment by ISP B will be dropped in the internet as the subnet belongs to ISP A and the prefix is not originated from the AS# recorded in RIPE database. I need to convince them, but i dont know how to. Is ISP A right in saying so??????What is the exact critaira in a path selection and BGP advertisment propagation and how do the local registries like RIPE control this????

Kindly let me know your views on the same.

Thanks

Ashish

ashishpanda · ‎08-13-2006

Hi,

Can anyone give some more inputs on this??

Thanks in advance

Ashish

sundar.palaniappan · ‎08-14-2006

Ashish,

When you are peering with two or more ISPs and since you want both to route your IP space you should get your own unique AS #. Then you can prepend AS path to achieve desired routing.

Hope that helps!

Regards,

Sundar

globalnettech · ‎08-13-2006

Hello,

in addition to the other posts, in general, your ISP A is right when saying that subnets originated in private AS's are dropped. You might want to try and configure the 'remove-private-as' eBGP neighbor command, which effectively removes the private AS from advertisements to your ISP. There are, however, some restrictions with regard to how this feature works. Check out this link to the command reference:

remove-private-as

http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124tcr/tirp_r/rtebght2.htm#wp1114402

Regards,

GNT

ashishpanda · ‎08-13-2006

Hi

I have client which is working with the same private-as removal feature. Apart from this restriction is there any other point which justifies the point ISP A is maiking?????

Thanks

Ashish

mheusinger · ‎08-14-2006

Hello,

the "private-as removal" is something the ISP A and B have to configure. If your BGP router has only a private AS number then it can not be removed in eBGP updates. If you get two different private AS numbers from ISP A and ISP B there is a solution called "local AS". This allows a router placing one private AS in the eBGP updates though another one is configured with "router bgp ".

The idea that routes are dropped, because the AS is not the registered one, might or might not be true. In case an ISP is announcing a network to its internet peers, it is not so likely that filters are in place preventing it. Without specific filters the updates are accepted. Checking every AS/IP network combination out there for all 200k prefixes would be a huge effort for any ISP and thus usually is not done. This is the reason you see so many "inconsistent AS" entries in the BGP table.

There is a good chance you get things working once both ISPs agree to it.

Still it is not "nice" as there will be two ASes (ISP A and ISP B) announcing the same prefix, as if it is theirs, leading to inconsistent AS information. This complicates troubleshooting.

Another issue you might encounter is, that ISP B will only announce the network given to you by ISP A. The latter however might announce the registered IP address block as such, usually larger than your network. This would lead to the situation, that all traffic would go through ISP B to your site, because he has the longest match. There are f.e. all sorts of NAT problems arising from it.

So the "cleanest" setup would be your own registered AS and official IP addresses. You need to justify, however, why other solutions are not possible. It also involves IP readdressing and BGP redesign.

One more thing: once you have your redundant setup in place, make sure you implement proper filters not to become a transit AS between ISP A and ISP B. They should prevent this in any case, still you never know (after all they are humans making config changes ;-)

Regards, Martin

sundar.palaniappan · ‎08-14-2006

Asish,

To add to my previous post, ISP A is correct that ISP B can't be the originating AS for your IP space as it belongs to ISP A. However, as stated in my previous post if you use a public (unique) AS then your AS would be the one originating and ISP B would be come a transit AS in the path. You should be able to register your own AS just based on the fact you are multihomed to two ISPs.

Hope that helps!

Regards,

Sundar