BGP, one AS number, multiple sites, one ISP

[ayao]

Hello,

We have a /22 internet address block. We devided it into four /24 address blocks, say, A, B, C, and D.

ISP "Z" is the only ISP we peered with via BGP, and address block A has been distributed thourgh them.

We have only one AS number.

The company plans to build a remote location, with a new BGP router, assign address block B to it, and also peer with ISP "Z" in that location.

(ISP "Z" has many pops accross the country.)

My question would be:

[1] Will this plan work?

[2] Can ISP "Z" exchange IP traffic between address block A and B for us?

[3] If [2] can work, do we still need to set up an iBGP session between our routers for address block A and B? (each router only has default route pointing to ISP "Z", and they don't exchange Internet routes. )

[4] If we'd like to save addres space, would it be okay to ask ISP "Z" to further devide address block B to /26 chunk, so we can save addresses for future sites?

(I know Internet routers would filter out routes smaller than /24, but we kind of delegating ISP "Z" for our /24 routes.)

Thank you for reading my questions!

-Andrew

[bersoare · Accepted solution]

Hi,

As long as your ISP accepts your /26 advertisements, this would work.

How are you advertising the /24? Only a /24 BGP route to the ISP? Or are you advertising the whole block (/22)?

If so, you can still advertise the /24 at the main site, while advertising the more specific (/26) from the remote sites. ISP Z will be responsible for advertising your address block on a /24 or a less specific summary upstream (towards other ASes).

The most important things to consider is:

1- Ask them if you can advertise this public block (/26), so they can route between your more specific blocks.

2- Make sure they will advertise your networks to other ASes

3- If you don't have a backdoor link, there is no need to advertise a /24 or /22 network upstream since you won't know how to route to the more specifics (needing ISP Z to route between them).

See the topo attached, and let me know if I correctly addressed your question.

[ayao]

Hi,

Thank you very much for the quick response and diagram. Yes, the diagram shows what we plan to do correctly!

One more question. Would using /26 block be troublesome down the road if we connect to a 2nd ISP, and become multi-homed?

Thank you!

[bersoare]

Is your /22 block actually yours, or does it belong to the provider Z?

If the /22 block belongs to you (not a PA - provider assigned) you are free to advertise a /22 to both providers - but in order to route between your sites, ISP Z should also know the more specifics. This way the internet will have 2 similar prefixes (both /22) towards your AS using ISP Y and Z as transit.

If this /22 is a PA, you should know beforehand how the internet sees your prefix. Is it summarized down to a, let's say, /16 on ISP Z? if so, advertising a /22 to another provider would generate a more specific prefix in the global routing table point of view and ISP Y would be your primary way in (inbound towards your AS).

In both cases, since only provider Z would know the more specifics (/26) of your other sites, traffic would hit your main office and will be routed across ISP Z all the way to your remote sites.

Anyway, BGP multihoming is a little more complicated design, and there are a few key points to consider in order to prevent future issues.

Here is a very good doc that addresses many concerns related to BGP multihoming design. Pay special attention to item 2.4. There are also practical examples throughout the doc:

http://aspath.net/BGP-MHing-HOWTO-whitepaper.pdf

[ayao]

Hello,

Thank you again for this detailed explanation! Yes, we got this /22 block from APNIC. I'll read the document you attached. Thank you for spending time on this question. Now we have the assurance and will implement site B. Appreciate it!

-Andrew