Simple question on advertisement by two ISP's

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Sep 9th, 2010
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Imagine I have ISP1, managed router1, serving my company. ISP1 advertises let's say my prefix a.b.c.d/16.

Then I want to install another router2, managed by another service provider. That will become my production router, therefore it will also advertise a.b.c.d/16.

Later ISP1 connection and router1 will go away and router disconnected.


Is there any problem to have both ISP's advertising a.b.c.d/16 concurrently for a while? Or should I plan a coordination?

If that is OK, which route would be preferred to get to my company, since both ISP's would be advertising it?

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marikakis Thu, 09/09/2010 - 17:56
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It all depends on what "my prefix" actually means. Formally, prefixes are a public resource and they "belong" to nobody. They are "assigned". Was this prefix assigned to you directly by a RIR or was it assigned to ISP1? Also, have you been assigned an AS number?

a. If this block and an AS was assigned to your organization, I see no problem. The temporary situation is also a valid multi-homing setup.
b. If this block was assigned to you, but no AS, then the temporary situation will cause a single block to be advertised to the Internet by two different AS's (ISP1 & ISP2). This is not the most beautiful BGP setup (looks the same as an announcement made by mistake), so I would recommend talking with your ISPs about policy issues to avoid surprises. I would also recommend you politely ask the customer support of each ISP to confirm details with the engineers that will actually perform the related tasks (i.e. escalate your issue).
c. If this block was not assigned to you and no AS, prepare to fight and perhaps lose with dignity. ISPs don't like holes in the blocks that were assigned to them. It's cumbersome, breaks their aggregation and makes them look bad to the rest of the Internet.

Since you mentioned a /16, you are probably not in case c above, and I'm optimistic about you chances. In any case, discussing such details with the ISPs is never a bad idea.

Regarding your second question: generally, you can control your outgoing traffic to use one of the 2 ISPs, but for your incoming traffic, as long as both advertisements are made, you will most likely receive traffic from both ISPs. Which one is chosen for an arbitrary remote IP address on the Internet will depend on the location of the remote IP address (e.g. how many AS hops away) and the peerings that each ISP has (e.g. with a customer of ISP1 you will probably communicate directly via ISP1). There exist ways to try to receive most of your traffic from one ISP (e.g. AS path prepend on the least preferred connection), but since this situation will only be temporary, it's probably not worth getting into all the trouble.

news2010a Fri, 09/10/2010 - 10:40
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So in this case it was the ISP1 which gave us the address space. Actually it is a.b.c.d/23.

If it was ISP1 which gave us the prefix and AS# is that still OK if now I ask the ISP2 to advertise a.b.c.d/23 as well?

Giuseppe Larosa Fri, 09/10/2010 - 12:15
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Hello Marlon,

>> If it was ISP1 which gave us the prefix and AS# is that still OK if now I ask the ISP2 to advertise a.b.c.d/23 as well?

No, as Maria has explained in her complete answer if the prefix belongs to ISP1, only ISP1 is wiling to accept it and only ISP1 is going to advertise it over the internet.

ISP2 can give you ANOTHER public IP address block taken from its own public space, and it is left to you how to handle the transition and NAT becomes a major player in this scenario.

The move involves changes in public DNS too,

There is a whitepaper about enterprise multihoming with NAT that is really complete.


Hope to help



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