Multihomed options

Unanswered Question

the customer add an addional t1 (ISP2)for redundancy.

the diag is:

cust_rtr-->isp_provided rtr--->T1--->ISP1

cust_rtr-->isp_provided rtr--->T1--->ISP2


ISP1 assign a /27 public address block; ISP2 assign a /28 public address block to the cusomer.

Customer do not have a ASN.

Is it possible to obtain a ASN since the customer do not really "own" the public address, they are provided by the ISP.

If BGP is not possible, i am assuming default routes pointing to each ISP.

i need your expert opinion on this, thanks.

I have this problem too.
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Pavel Bykov Thu, 09/20/2007 - 03:25


Please refer to the following document for multihoming scenarios.

You should use BGP, but with private AS number (e.g. 65000). ARIN/RIPE will not give you public AS number, unless you have large PI address space or you are an ISP (Which you are not).

Even if you will not be able to negotiate same AS number from both ISPs (which is unlikely), you can use "local-as" feature to translate AS number:

Hope this helps.

Richard Burts Thu, 09/20/2007 - 13:15


Since we do not know much about the customer it would be very difficult for us to say whether they could get an ASN or not. But I suspect that they would have a difficult time justifying their own ASN. And frankly I think that there is a solution that does not need their own ASN or BGP. As Pavlo has indicated there is an option to run BGP and to use private ASN.

But in my opinion they do not need to run BGP. I would think that if they have 2 static routes with a static route to each of the ISP they can handle the outbound traffic. They can do NAT on the router that faces the ISP routers so that if traffic is being sent to ISP1 it gets NATed with the block of addresses from ISP1 and if it is being sent to ISP2 then it gets NATed with the block from ISP2.

The main reason to run BGP is if you have some policy that you want to enforce. There is not anything in your post that indicates that the customer has any need for policy enforcement. It sounds like they just need connectivity. The static routes and NAT should do ok for them.



ariesc_33 Thu, 09/20/2007 - 23:45

I think its also depends on how large your network is. If you decided to use BGP, you may use local-as as stated above. The difficult thing about using static routes in a multihomed scenario is load balancing...I still prefer BGP but it also depends on how large your network is..

Pavel Bykov Fri, 09/21/2007 - 00:44

Rick, that is a good idea. When I was going to work this morning i suddenly realized that it was not very efficient to think in BGP circle, because floating static configuration could be enough.

BGP could still be useful though if there is no way to detect ISP failure on link level, or as you mentioned, to enforce a policy.

Thanks for the advises.

I had a /27 subnet but the ISP won't advertise the /27 subnet; they will only a class C or higher subnet. So, looks like static route is the way.

my topology is actually as follows:

ISP1 --> router --> pix1(active)


ISP2 --> router --> pix2(standby)

i am runnig the pix active and standby.

is this config would work? but i need your help.

I can control the outboud traffic as static route. but i am not sure how the inbound traffic being handle here? if it comes through ISP2, looks like it will hit the pix2 and traffic will drop. should i connect a ethernet between the routers and running a routing protocol between the routers so they can redirect to the active link?


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