Information Technology Reference
In-Depth Information
As updates are received from a peer, they are stored in a Routing Information Base (RIB)
for that peer (Adj-RIB-In). The updates are filtered by the Input Policy Engine. A path
selection algorithm is then performed to determine the best path for each prefix, as
discussed in detail in Chapter 2.
The resulting best paths are stored in the local BGP RIB (Loc-RIB) and then are submitted
to the local IP routing table (IP-RIB) for installation consideration. Chapter 2 discusses the
IP-RIB installation process.
When multipath is enabled, the best path plus all equal-cost paths are submitted for IP-RIB
consideration.
In addition to the best paths received from peers, the Loc-RIB also contains BGP prefixes
injected by the current router (called
locally sourced
) that are selected as the best paths. The
content of the Loc-RIB must pass through the Output Policy Engine before being adver-
tised to other peers. The routes that successfully pass through the Output Policy Engine are
installed in the output RIB (Adj-RIB-Out).
This discussion of RIBs is a conceptual overview. Actual update processing can vary
depending on the BGP implementation and configuration. In Cisco IOS, the BGP table or
the BGP RIB (the output of
show ip bgp
) contains all the routes that are permitted by the
Input Policy Engine, including routes that are not selected as the best paths. When the
Inbound Soft Reset IOS feature (soft reconfiguration) is enabled, routes that are denied by
the Input Policy Engine are also retained (marked as
Receive only
) but are not considered
in the path-selection process. The use of soft reconfiguration is discussed in Chapter 3.
Comparing BGP and IGP
When discussing BGP, it is important to understand the difference between an Interior
Gateway Protocol (IGP) and BGP (an example of an Exterior Gateway Protocol). An IGP
is designed to provide reachability information within a single routing domain.
Three types of IGPs are commonly used in networks today:
•
Distance vector protocols such as Routing Information Protocol (RIP) and Interior
Gateway Routing Protocol (IGRP)
•
Link-state protocols such as Open Shortest Path First (OSPF) and Intermediate
System-to-Intermediate System (IS-IS)
•
Hybrid protocols such as Enhanced IGRP (EIGRP)
Although these protocols are designed with different goals and behave differently, the
common goal is path optimization within a routing domain—that is, finding an optimal path
to a given destination.
