Information Technology Reference
In-Depth Information
The BGP architecture for the network reflects that the core is the top of the network
hierarchy. The BGP deployment is based on a route reflector design. The iBGP full mesh is
at the top of the hierarchy in a route reflector design.
The iBGP full mesh consists of all the core routers. Policy is not modified for BGP prefixes
in the network core. A single peer group can be used for the entire iBGP mesh for the core
peering sessions.
The network core is also responsible for providing connectivity to the aggregation layer. A
second set of BGP sessions is required to all directly connected aggregation routers. An
important rule for route reflection is that BGP peering sessions must follow the physical
topology to avoid routing loops.
No connections external to the network are terminated on the core routers. This includes
customer connections and peering or transit connections. The exception to this rule is for
smaller ISPs that might have only a couple of core routers and two or three transit links.
Example 9-1 shows the BGP core router configuration template.
BGP Core Router Configuration Template
Example 9-1
router bgp <ISP ASN>
no auto-summary
no synchronization
bgp log-neighbor-changes
bgp router-id <ROUTER ID>
!
neighbor CORE_ROUTERS peer-group
neighbor CORE_ROUTERS description Core iBGP Full Mesh
neighbor CORE_ROUTERS version 4
neighbor CORE_ROUTERS password <iBGP Password>
neighbor CORE_ROUTERS update-source loopback0
neighbor CORE_ROUTERS remote-as <ISP ASN>
!
neighbor AGG_ROUTERS peer-group
neighbor AGG_ROUTERS description iBGP Sessions for Aggregation Routers
neighbor AGG_ROUTERS version 4
neighbor AGG_ROUTERS password <iBGP Password>
neighbor AGG_ROUTERS update-source loopback0
neighbor AGG_ROUTERS remote-as <ISP ASN>
neighbor AGG_ROUTERS route-reflector-client
...
!
The Aggregation Layer
The aggregation layer, shown in Figure 9-2, is in place primarily to reduce the complexity
of the core routers by providing hierarchy. This includes distributing the circuit aggregation
and reducing the number of BGP peering sessions terminating on the core routers. In
