BGP Fundamentals Help

[conor.maton]

Hi guys,

I am currently studying up on BGP and have a few queries of three scenarios which I cannot understand. Can you help with below?

Scenario 1: There are two networks. Network 1 within the 192.168.1.0/24 subnet and Network 2 within the 192.168.2.0/24 subnet. Both have a single connection to SP1 which  runs BGP within AS 1000

Network 1's CE Router has a default route to the SP1 PE Router as does Network 2 which are redistributed to their respective IGPs

If Network 1 wants to talk to IP addresses within Network 2, how does this happen?

Scenario 2: There are two networks. Network 1 and Network 2 are both within the 192.168.1.0/24 subnet Both have a single connection to SP1 which  runs BGP within AS 1000

Network 1's CE Router has a default route to the SP1 PE Router as does Network 2 which are redistributed to their respective IGPs

If Network 1 wants to talk to IP addresses within Network 2, how does this happen when they are within the same subnet?

Scenario 3: There are two networks. Network 1 within the 192.168.1.0/24 subnet and Network 2 within the 192.168.2.0/24 subnet. Both have two connctions to SP1 which  runs BGP within AS 1000

Network 1 and Network 2 just so happen to share the same AS number (65530) and taks to SP1 via BGP

If Network 1 wants to talk to IP addresses within Network 2, how does this happen when the AS numebrs are the same?

Thanks in advance :)

[Julio E. Moisa]

Hi

Apologies if I understanding incorrectly but Networks 1 and 2 are on the same customer side configured on the CE router or are they different routers? could you please share the topologies? If you are have an eBGP with the ISP, basically the ISP must provide the AS number to be used on the client side. 

Thank you in advance

:-)

[conor.maton]

Thank you for the reply.

Imagine they are two completely different sites connected to two completely different PE routers.

[Julio E. Moisa]

Thank you for the clarification, 

Now if you have 2 switches connected to the same ISP through BGP, probably the ISP is using MPLS to move the traffic from the site A to site B, now it should be transparent for you, now the ISP will be involved on the routing procedures. Site A and the site B will be advertising their networks through BGP to their BGP peers (in this case to the ISP), take in consideration if both sites (A and B) have the same AS number, for example 1000, the ISP must be informed to apply AS-Override in order to enable the flow of the networks otherwise the site A and B will not be able to reach the networks from each other because the iBGP rule (Split Horizon rule).

:-)