Is it necessary to advertise internal routes to ISP through BGP ?

[yangfrank]

Dear All

Please see the picture in attachment and the document below. In production environment, I do not think that it is necessary to advertise the internal routes(192.168.11.0 and 192.168.12.0 between R101 and R102) to ISP through BGP. How do you think about it ? if not, why do they use it? I guess it's just for verification of configuration

Thank you. 

yangfrank

http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13762-40.html

[Jon Marshall · Accepted solution]

That document is just an example of how to do load sharing using private IP addressing.

If you are asking if in a production environment a company advertises it's internal networks using private IPs to an ISP then if the ISP connection is for internet it doesn't because those IPs are not routable on the internet.

If the company was using public IP addressing internally it might but they don't usually have public addressing on their internal devices.

A company may advertise public IP addressing to the ISP but that addressing is usually used on the firewall for NAT purposes.

Finally if the SP is for MPLS then yes a company would advertise it's private addressing because the MPLS network is their WAN and they advertise these networks to all their sites.

Is this what you were asking ?

Jon

[yangfrank]

Hi Jon, Excellent explanation! Thank you

[Joseph W. Doherty · Accepted solution]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

What you need to advertize to your neighbor's AS (unless they are using statics toward you), is whatever network prefixes you want your neighbor to know.

Often multiple prefixes are advertized via a summary prefix.  I.e. you often try to advertize the fewest prefixes (ideally just one prefix).

As Jon noted, in your diagram, your internal IPs are in the private address space.  You could, in fact, pass them to your ISP but your ISP cannot further pass them into the Internet.  Most likely your ISP would just discard them unless there was some reason they had to use them from their AS.

Also in your example diagram, neither AS might be an ISP, so what's passed from one AS to the other, again, depends on what each wants the other AS to know.  I.e. if AS 10 wanted AS 11 to know about the two 192.168.x.x prefixes, yes you would pass them through BGP.  If AS 10 didn't want AS 11 to know of these prefixes it would not advertize them.