Information About Rib Configuration - Cisco NCS 6000 Series Configuration Manual

Information About RIB Configuration

Information About RIB Configuration
To implement the Cisco RIB feature, you must understand the following concepts:
Overview of RIB
Each routing protocol selects its own set of best routes and installs those routes and their attributes in RIB.
RIB stores these routes and selects the best ones from among all routing protocols. Those routes are downloaded
to the line cards for use in forwarding packets. The acronym RIB is used both to refer to RIB processes and
the collection of route data contained within RIB.
Within a protocol, routes are selected based on the metrics in use by that protocol. A protocol downloads its
best routes (lowest or tied metric) to RIB. RIB selects the best overall route by comparing the administrative
distance of the associated protocol.
RIB Data Structures in BGP and Other Protocols
RIB uses processes and maintains data structures distinct from other routing applications, such as Border
Gateway Protocol (BGP) and other unicast routing protocols. However, these routing protocols use internal
data structures similar to what RIB uses, and may internally refer to the data structures as a RIB. For example,
BGP routes are stored in the BGP RIB (BRIB). RIB processes are not responsible for the BRIB, which are
handled by BGP.
The table used by the line cards and RP to forward packets is called the Forwarding Information Base (FIB).
RIB processes do not build the FIBs. Instead, RIB downloads the set of selected best routes to the FIB processes,
by the Bulk Content Downloader (BCDL) process, onto each line card. FIBs are then constructed.
RIB Administrative Distance
Forwarding is done based on the longest prefix match. If you are forwarding a packet destined to 10.0.2.1,
you prefer 10.0.2.0/24 over 10.0.0.0/16 because the mask /24 is longer (and more specific) than a /16.
Routes from different protocols that have the same prefix and length are chosen based on administrative
distance. For instance, the Open Shortest Path First (OSPF) protocol has an administrative distance of 110,
and the Intermediate System-to-Intermediate System (IS-IS) protocol has an administrative distance of 115.
If IS-IS and OSPF both download 10.0.1.0/24 to RIB, RIB would prefer the OSPF route because OSPF has
a lower administrative distance. Administrative distance is used only to choose between multiple routes of
the same length.
Routing Configuration Guide for Cisco NCS 6000 Series Routers, IOS XR Release 6.4.x
354
• Cisco IOS XR software
• Base package
Implementing and Monitoring RIB