BGP network for two sites

[bensonlei]

Our company has two geographical separated sites, with the following BGP network infrastructure:
1. siteA has two BGP routers(RouterA1-primary, RouterA2-secondary), siteB has also two BGP routers (RouterB1-primary, RouterB2-secondary).
2. siteA has been running the BGP network properly in production, while siteB is intending to set up another BGP network.
3. Four BGP routers are connected to different ISP lines ( RouterA1 - ISP01, RouterA2 - ISP02; RouterB1 - ISP03, RouterB2 - ISP04 ).

Questions:
1. siteA and siteB are running the same AS number, if the siteA advertises an IP subnet 10.10.10.0 /24, while siteB advertises an IP subnet 10.20.20.0 /24 ( siteA and siteB have their owner IP subnets under the same AS number ); 
Do these four BGP routers have to have iBGP configuration ?
2. Do siteA and siteB have to establish an network connection ( private link ) ?

Any idea or recommendation.

Many thanks

[Giuseppe Larosa]

Hello @bensonlei ,

you can make each router able to accept advertisements that include their own AS number using

RA1

router bgp <yourASN>

neighbor ISP01-adress allowas-in   [ n]

where the last parameter is the number of times the own AS can appear in AS path. If you are using AS path prepending you need to use a n value appropriate for this.

Of course this has to be done on all four routers

Hope to help

Giuseppe

[bensonlei]

Hi, Giuseppe,

Thanks so much for your input, your advice is for incoming network to our router, right ?

How about our IP subnet broadcast, SiteA and SiteB have different IP subnet broadcast, any recommendation ?

Many thanks

[Giuseppe Larosa · Accepted solution]

Hello @bensonlei ,

IP subnet broadcast should not be an issue they just are the last host in each subnet with all 1 in the host portion.

You can enable or disable propagation of subnet broadcast on a per destination router interface basis.

Hope to help

Giuseppe

[bensonlei]

Hi,  Giuseppe,

Thanks for your great knowledge, my concept is now clear.

Cheers

Benson

[MHM Cisco World]

what you meaning about IP subnet broadcast ? can you more explain this point ?

[bensonlei]

BGP announces your IP subnet to the world.

[Sergey Lisitsin]

bensonlei,

Are all 4 ISP links belong to different ISPs? If that is the case, then I doubt that you will have it working, as ISPs normally don't accept private IP ranges in peering with clients. If it is the same ISP or a pair of ISPs providing MPLS VPN service, then what Giuseppe suggested will work just fine. Is there a hard requirement to use the same AS number?

[bensonlei]

Giuseppe gives me the clear concept.

[MHM Cisco World]

sorry, your subnet is unique for you AS so not problem with flood them to other site.
do I need to config iBGP between the both site?
it depend 
for the Service provider redundancy if your both site connect to two differs ISP router then iBGP is config between the both ISP router.

for your case if one edge router-ISP router link down, instead of loss connectivity to other site we can shift the traffic to other site through iBGP  
if edge router-ISP router Site A down the traffic will go through the edge router-ISP router Site B