Information Technology Reference
In-Depth Information
There is no core IGP routing process, because eBGP sessions between autonomous systems
are tied directly to connected interfaces. Each core router has an eBGP peering session only
with each directly connected AS.
Path Selection
In this design, BGP path selection is primarily influenced by two parameters: the AS_PATH
length and the neighbor router ID. Without modifications, the path chosen by the best path-
selection algorithm might not be the optimal path. All BGP routers are configured with the
bgp bestpath compare-routerid
command to ensure deterministic path selection based on
lowest router ID.
The prefix advertisement in this design is best illustrated with an example. In Figure 5-5,
the prefix 10.2.0.0/16 is first injected into BGP in AS 65102 on R4 and R6. Both routers
advertise the prefix to each other via iBGP and to the eBGP peers in AS 65101 and AS 65103,
respectively.
Figure 5-5
Initial Prefix Advertisements for 10.2.0.0/16
EIGRP 102
BGP 65102
R4
R6
10.2.0.0/16
10.2.0.0/16
R8
EIGRP 101
BGP 65101
EIGRP 103
BGP 65103
R3
R9
R11
EIGRP 104
BGP 65104
R3 receives the prefix in AS 65101 via the eBGP session with R4 in AS 65102. R3 installs
the prefix in the routing table because at this point it is the only path for prefix 10.2.0.0/16.
Then R3 advertises the prefix via eBGP to R9 in AS 65103 and R11 in AS 65104. This
process is shown in Figure 5-6.
