Information Technology Reference
In-Depth Information
Routing Information Base
The IP RIB, or IP routing table, is a critical database that provides a vital link between the
control plane and the forwarding plane, as mentioned earlier in the section “Comparing the
Control Plane and Forwarding Plane.” On one hand, various routing sources/protocols such
as BGP and IS-IS populate the RIB with their paths. On the other hand, RIB provides infor-
mation to build the forwarding database (some switching methods use the RIB directly for
forwarding).
As each routing protocol receives updates and other information, it chooses the best path
to any given destination and attempts to install this path into the routing table. When mul-
tiple paths for the same prefix/length exist, the router decides whether to install the routes
based on the administrative distances of the protocols involved. IOS has predefined but con-
figurable administrative distances for various routing protocols/sources. The prefixes from
a routing source that has a lower administrative distance are preferred. Backup routes are
still maintained by the protocol, if supported, and are used as the best routes when existing
best routes fail.
When BGP fails to install a route into the IP RIB, a RIB failure is reported in the route's
BGP RIB. The failure code indicates the reason. Check out Appendix B, “Matrix of BGP
Features and Cisco IOS Software Releases,” for additional information.
NOTE
The IP RIB is organized as a collection of Network Descriptor Blocks (NDBs). Each NDB
is a single entry in the routing table and represents a network prefix obtained via one of
three sources:
An address/mask pair configured on a local interface on the router. This becomes a
connected route, which has the highest preference, or an administrative distance of 0.
A static route configured on the router. A static route has a default administrative
distance of 1.
A dynamic routing protocol such as BGP.
NDBs contain information about the network address, mask, and administrative distance,
as well as information needed for the operation of dynamic routing protocols, such as route
redistribution. Because each prefix in an NDB can be potentially reached through multiple
paths, Routing Descriptor Blocks (RDBs) are also used. One or more RDBs can be linked
to each NDB to store the actual next-hop information. An NDB currently may have up to
eight RDBs, which sets the upper limit to the number of load-shared links per destination
(that is, eight). Note that because NDBs are controlled by individual routing protocols, the
routing protocols determine how many RDBs to associate with an NDB.
 
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