Information Technology Reference
In-Depth Information
Example 5-15
R11 Path Information for 10.2.0.0/16 (Continued)
Advertised to non peer-group peers:
172.16.13.17
65103 65101 65102
172.16.13.17 from 172.16.13.17 (172.16.9.1)
Origin IGP, localpref 100, valid, external
65101 65102
172.16.13.9 from 172.16.13.9 (172.16.3.1)
Origin IGP, localpref 100, valid, external, best
The failure of a core router has an effect similar to a core link failure, only amplified. When
the core router fails, every BGP session to that router fails. In Figure 5-9, R9 fails. The
similarity of a router failure to a link failure becomes more obvious when viewed from the
perspective of the other routers. R3 sees a link failure to R9, R11 sees a link failure to R9,
and R8 sees a link failure to R9.
Figure 5-9
Network Topology Core Router Failure
EIGRP 102
BGP 65102
R4
R6
R8
EIGRP 101
BGP 65101
R3
EIGRP 103
BGP 65103
R9
R11
EIGRP 104
BGP 65104
For example, consider 10.2.0.0/16, which originates in AS 65102. Only R11 is actively
using the path received from R9. When R11 detects the failure, the path received from R9
is removed from the BGP RIB, and the path from R3 is used.
A more interesting scenario is the reroute of prefix 10.3.0.0/16, which is originated by AS
65103. When R3 detects the failure of the BGP session to R9, it removes the prefix from its
BGP RIB. Then R3 installs the path received from R4 and advertises this path to R11,
which causes R11 to replace the previous path.
