Question

A large corporation owning IP address range of 192.168.2.0/25 has its own ISP. Assume that an office building within the corporation using address range of 192.168.2.0/28 desires a connection with its own autonomous ISP in addition to the connection with its corporation. The building is connected to corporation edge router R1 through its edge router R2, and is also connected to its own ISP through R3. We would like to configure connecting routers according to BGP. (a) Show the configuration of R1 and R2 and specifically demonstrate what R1 and R2 should advertise.

Answer 1

Consider 3 routers : R1--------R2 ------ R3, and all of them are running iBGP:

-    if you advertise the both interconnects networks R1R2 and R2R3 from router R2 it will work

-    if you advertise those networkers from the edge routers ( router R1 an R3) , the you will have a issue - iBGP loop prevention : do not advertise a prefixes received from a iBGP neighbour to another iBGP neighbour. This means that R1 will not receive the R2R3 network and C will not receive the R1R2network

If you advertise the routers R1 and R2 then the Building can use the internet services from Isp of corporation and internet from Isp of autonomous building. More services can be utilized by the building .

BGP is usually used for specific services. It was initialy designed for the internet prefix exchange and currently there are some other services that require BGP : like MPLS VPN.