Information Technology Reference
In-Depth Information
An important point to note from this section is that not taking advantage of an IGP for its
ability to react to network changes quickly has a slight effect on the speed at which the net-
work can reconverge. The amount of time required to reconverge can also be additive. This
additive effect is a result of each BGP speaker upon receiving the new path information hav-
ing to run the path-selection process and then withdraw and advertise based on the outcome.
Administrative Control
This architecture provides clear points at which administrative authority can be divided.
The easiest way to divide administrative control from a routing perspective is to introduce
eBGP sessions. When eBGP is used, the next hop on advertised prefixes is changed to the
address of the advertising router. Only a single BGP session is required at each intercon-
nection point. Not all the autonomous systems need an eBGP session directly between
them—only those with a direct physical connection.
Routing Policy
It is sometimes desirable to prevent two regions from communicating with each other. In
this design, however, every core router must have full routing information, because it might
be acting as a transit router between two other regions. This disallows the use of route
filtering to block connectivity between two regions. The best method of limiting connec-
tivity is inbound packet filtering on the core router interfaces connecting with the regional
network.
Internal/External BGP Core Architecture
The internal/external BGP core architecture employs an iBGP core, with external BGP as
the mechanism by which regions attach to the core. Figure 5-10 shows an example. This
architecture provides prefix reduction in regional IGP processes, clear delineation of
administrative boundaries, and flexible policy control. It also bounds the scope of regional
IGP instabilities.
The internal/external BGP scenario at first appears to be the most complex scenario because
of the number of components. However, the end result is a BGP architecture that is easier
to work with when defining policy, troubleshooting, or expanding the network.
The regional IGP process provides reachability throughout the entire regional network.
This process carries full routing and topological information for the region. The
regional IGP process also provides next-hop resolution for the iBGP-learned prefixes
between the regional border routers in addition to reachability for the iBGP peers, as
