Route Selection
©2008 Allied Telesis Inc. All rights reserved.
25.4
AlliedWare Plus
TM
Operating System Software Reference C613-50003-00 REV E
Software Version 5.2.1
RIB and FIB Routing Tables
Your device maintains its routing information in routing tables that tell your device how to find
a remote network or host. Each route is uniquely identified in a table by its IP address, network
mask, next hop, interface, protocol, and policy. There are two routing tables populated by your
device: the
Routing Infor
m
ation Base
(RIB) and the
Forwarding Infor
m
ation Base
(FIB).
Routing
Information Base
The RIB records
all
the routes that your device has learnt. Your device uses the RIB to
advertise routes to its neighbor devices and to populate the FIB. It adds routes to this table
when:
■
you add a static route using the
ip route
command
■
one or more routing protocols, such as RIP or OSPF, exchanges routing information with
other routers or hosts
■
your device receives route information from a connected interface
■
your device gathers route information from an ICMP redirect message or DHCP message
Forwarding
Information Base
The RIB populates the
Forwarding Infor
m
ation Base
(FIB) with the best route to each
destination. When your device receives an IP packet, and no filters are active that would
exclude the packet, it uses the FIB to find the most specific route to the destination. If your
device does not find a direct route to the destination, and no default route exists, it discards the
packet and sends an ICMP message to that effect back to the source.
Adjusting table
entries
To view the routes in the RIB, use the command:
awplus#
show ip route database [bgp|connected|ospf|rip|static]
To view the routes in the FIB, use the command:
Administrative Distance
When multiple routes are available for the same prefix, the AlliedWare Plus
TM
OS adds the
routes with the lowest
ad
m
inistrative distance
to the FIB. The administrative distance is a rank
given to a route based on the protocol that the route was received from. The lower the
administrative distance, the higher the route preference. For example, if the RIB has these
routes:
then the AlliedWare Plus
TM
OS adds routes 1 and 3 to the FIB. It does not add route 2, as this
has a higher administrative distance than a route with the same prefix.
awplus#
show ip route [bgp|connected|ospf|rip|static|A.B.C.D|A.B.C.D/M]
Route
Prefix
Protocol Distance
1
192.168.1.0/16 eBGP
20
2
192.168.1.0/16 OSPF
110
3
192.168.1.1/24 OSPF
110