Rib Administrative Distance - Cisco ASR 9000 Series Configuration Manual

Implementing and Monitoring RIB
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.
This table lists default administrative distances for the common protocols.
Table 4: Default Administrative Distances
Protocol
Connected or local routes
Static routes
External BGP routes
OSPF routes
IS-IS routes
Internal BGP routes
The administrative distance for some routing protocols (for instance IS-IS, OSPF, and BGP) can be changed.
See the protocol-specific documentation for the proper method to change the administrative distance of that
protocol.
Note Changing the administrative distance of a protocol on some but not all routers can lead to routing loops
and other undesirable behavior. Doing so is not recommended.
RIB Support for IPv4 and IPv6
In Cisco IOS XR software, RIB tables support multicast and unicast routing.
The default routing tables for Cisco IOS XR software RIB are the unicast RIB tables for IPv4 routing and the
multicast-unicast RIB tables for IPv6 routing. For multicast routing, routing protocols insert unicast routes
OL-30423-03
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.1.x
RIB Administrative Distance
Administrative Distance Default
0
1
20
110
115
200
427