Information Technology Reference
In-Depth Information
There are multiple aspects to the BGP portion of this architecture. There is a full iBGP
mesh between the core routers. These iBGP sessions are sourced from the loopback
interfaces on the core routers and are configured with
next-hop-self
. The use of
next-hop-
self
removes the need to inject the subnets on the links connecting to the regional border
routers for next-hop resolution. Sourcing the iBGP sessions from the loopback interfaces
allows the sessions to remain active in the case of a core link failure that can be routed
around.
Each region has its own BGP AS, and the core has its own AS. Network prefixes for each
region are injected into the regional BGP process using
network
statements on the regional
border routers. The use of
network
statements allows for controlled injection of prefixes
that can be reached in the IGP. It is possible to directly redistribute from the IGP into BGP
on the regional border routers. Although this practice is discouraged, if the number of pre-
fixes involved makes using
network
statements administratively unfeasible, redistribution
becomes an option. It cannot be stressed enough that prefix filtering should be applied any
time redistribution is performed between protocols. Another option is to aggregate the pre-
fixes with a static route to Null0 and use the
network
command to inject the aggregate.
The regional BGP autonomous systems connect to the BGP core AS through the use of
eBGP. Each regional border router peers via eBGP with all the core routers it is directly
connected to when redundant routers are in place.
Path Selection
The path selection in the core routers is very similar to that in the iBGP-only architecture
scenario. The BGP decision process typically uses the IGP metric to the next hop as the
decision point.
The prefix 10.2.0.0/16 is injected by both R5 and R7 into BGP 65102. The prefix is then
advertised by R5 to both R4 and R6 via eBGP. The prefix is also advertised by R7 to R4
and R6 via eBGP. The routers R4 and R6 both have to make a path selection, which results
in a decision based on router ID, unless the path-selection process is manipulated through
modification of one of the BGP attributes, or the BGP multipath feature is enabled. The
prefix advertisements are shown in Figure 5-11. Examples 5-16 and 5-17 show the path
selection. Both R4 and R6 select the path from R5.
