A.8.a) Reasons to physically aggregate multiple links to the same provider
- It happens often for a customer ISP to upgrade its connectivity to the same upstream provider. The following paragraph concerns multiplications of physical links for an Ebgp session with SDH interfaces.
In case of upgrades, from an ISP to another one, it is common to multiply the Ebgp sessions.
However, some circumstances can avoid it: upgrade from the same customer’s router to its ISP’s router, existing Ebgp session, and links setups on SDH connectivity.
It is always better to avoid multiple Ebgp sessions, because network administrations for load-balancing reasons can be heavy to manage (multiplication of a route-map which will include many as-prepend, LocalPref, communities mechanisms, etc.).
- For instance, let’s say that ISP-A has bought one STM4 link to ISP-C. The router has one Ebgp neighbor, easy to manage traffic engineering and traffic client direction. But now let’s suppose that ISP-A will buy two additional links to ISP-C:
- You have now three Ebgp speakers, and additional unworkable worries. You will need to use communities, as-prepend and also manage load-sharing upload bandwidth.
- The only obligation to bring you to have three Ebgp neighbors, is that the destination of these additional links; lands to another router belonging to ISP-C.
- In the case that all the links land to the same router, even if the physical paths are different, we will prefer to keep one Ebgp session and points all the links to a common IP loopback.
- This scenario is familiar for customers ISPs, who desire to connect to their upstream providers through distinct submarine and regional cables. Nonetheless, it is advised that interval latencies for all the links are not high, and capacity is also the same for each link.
See below Figure A.12: Multihomed Customer connected to an ISP with multiple Ebgp sessions.
See below Figure A.13: Multihomed Customer connected to an ISP through 1 Ebgp session