4. Route Reflection

The basic idea of route reflection is very simple. Let us consider the simple example depicted in Figure 1 below.

                +-------+        +-------+
                |       |  IBGP  |       |
                | RTR-A |--------| RTR-B |
                |       |        |       |
                +-------+        +-------+
                      \            /
                  IBGP \   ASX    / IBGP
                        \        /
                         +-------+
                         |       |
                         | RTR-C |
                         |       |
                         +-------+

                 Figure 1: Full-Mesh IBGP

In ASX, there are three IBGP speakers (routers RTR-A, RTR-B, and RTR-C). With the existing BGP model, if RTR-A receives an external route and it is selected as the best path it must advertise the external route to both RTR-B and RTR-C. RTR-B and RTR-C (as IBGP speakers) will not re-advertise these IBGP learned routes to other IBGP speakers.

If this rule is relaxed and RTR-C is allowed to advertise IBGP learned routes to IBGP peers, then it could re-advertise (or reflect) the IBGP routes learned from RTR-A to RTR-B and vice versa. This would eliminate the need for the IBGP session between RTR-A and RTR-B as shown in Figure 2 below.

               +-------+        +-------+
               |       |        |       |
               | RTR-A |        | RTR-B |
               |       |        |       |
               +-------+        +-------+
                     \            /
                 IBGP \   ASX    / IBGP
                       \        /
                        +-------+
                        |       |
                        | RTR-C |
                        |       |
                        +-------+

             Figure 2: Route Reflection IBGP

The route reflection scheme is based upon this basic principle.