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Currently we have DC-1 running eBGP with my ISP using AS:1111 and i have prefix 1.1.1.0/24 and using BGP Default route because we have only single ISP.

Now we are opening new Datacenter in remote geolocation and we don't have any dedicated WAN link between datacenter, and same ISP going to provide us Internet connectivity with BGP default route.

Question:

  1. Do i need to get new AS number for new DC-2 ?
  2. If i want to use same AS number then what BGP will understand (or it's impossible to use same AS on two location?)
  3. what are the best practice people do in this scenario?
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    AS numbers are independent of location, and they can be used in multiple sites. You really only need it for advertising your prefixes. You can advertise different prefixes from different sites using the same AS number, or advertise the same prefixes from multiple locations (anycast) using the same AS number.
    – Ron Maupin ♦
    Jun 18, 2020 at 20:49
  • I thought AS somehow part of BGP matrix for path selection and it keep track of prefix which you announcing on with your BGP ASN. (i may be wrong)
    – Satish
    Jun 18, 2020 at 21:36
  • The AS is used in the AS_Path to help with determining the best path. The routing tables will still have the next-hop router as the direction to send the packets. For example, the (extremely large) company where I work has an AS number that we use in places all over the world. The traffic is routed by the destination address, so it arrives at the proper destination, regardless of the AS number used.
    – Ron Maupin ♦
    Jun 18, 2020 at 21:43
  • @RonMaupin problem is of course if the sites split then you have a serious problem
    – Matt Douhan
    Jun 21, 2020 at 14:50
  • @MattDouhan, I'm not sure what you mean. The two sites are separate, and there can be a single AS with no problems.
    – Ron Maupin ♦
    Jun 21, 2020 at 17:06

1 Answer 1

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Since you have network islands with no backbone between them, it's usually best to use a distinct ASN per each distinct island. Just inform your RIR that you have multiple network islands and they will approve your request for the additional ASN(s).

If you decide to use the same ASN at multiple sites:

Because you have default routes only from your transit, you don't need to do anything special.

If you do decide to upgrade from default-only later on, you'll need to:

  • Request your transit providers configure advertise-peer-as on their side
  • Configure loops 2 or similar on your side
  • Ensure you are receiving & accepting routes from your remote sites
  • Double-check your loop-prevention strategy to ensure you're not accidentally, at site B, learning a route for site A from transit and then re-announcing that same route to a second transit provider
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    Thank you for reply, Look like getting new ASN is easy way and same ASN may get tricky at some point. (As you said if i stick to default-only BGP route then i don't need to do anything at present right?)
    – Satish
    Jun 18, 2020 at 21:35
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    One can use the same ASN but different prefixes. As the two regions aren't connected, traffic for one should never be sent to the other. I.e. announce network A for site A from ASN X at site A, and network B for site B from the same ASN X at site B. This is no different from a single site announcing different prefixes to different peers.
    – Ricky
    Jun 18, 2020 at 22:16
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    Care to elaborate on the why it's best to use different ASN? Apart from the fact you need to accept routes from you own AS, I don't see much inconvenience in using the same AS.
    – JFL
    Jun 19, 2020 at 6:06
  • I've operated several large CDN and similar networks with distinct islands using one ASN. Transit providers regularly neglect the advertise-peer-as configuration you request, not only at initial provisioning time, but sometimes during grooming events. For someone who knows what they're doing, this is an inconvenience. For somebody who has to ask these questions, though, they shouldn't be subscribing to these problems for the lifetime of their decision unless they have a compelling reason to use same ASN. That's why it's usually best to use a different ASN per island.
    – Jeff Wheeler
    Jun 19, 2020 at 10:40
  • @JFL if the sites split you have a serious problem with a single AS but not with two
    – Matt Douhan
    Jun 21, 2020 at 14:52

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