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Community Member

RIB & FIB

Hi everybody,

Can someone help me in understanding the difference between Forwarding information base (FIB) and Routing Information Base (RIB).

I have gone through various documents, but i am not getting clear understanding how the FIB table looks like

R.B.Kumar

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Re: RIB & FIB

Hello R.B Kumar,

the FIB contains also information that is used for the actual forwarding of packets: for example the MAC address of the ip next-hop is needed to perform the packet rewrite function.

So you can imagine a FIB entry as:

IP subnet base address

IP subnet mask

ip address of next hop

outgoing interface encapsulation information (how to build the correct frame on the outgong interface)

for LAN interfaces you need the MAC address of the next hop, the source MAC address to be used that of the outgoing interface

for example with CEF you can see entry using sh ip cef prefix mask detailed

sh ip cef 10.34.26.64 255.255.255.240 det

10.34.26.64/28, version 744312, epoch 0, per-destination sharing

0 packets, 0 bytes

tag information set, all rewrites owned

local tag: 1031

via 10.98.192.209, FastEthernet1/0, 0 dependencies

traffic share 1

next hop 10.98.192.209, FastEthernet1/0

valid adjacency

tag rewrite with Fa1/0, 10.98.192.209, tags imposed {}

via 10.98.192.213, FastEthernet2/0, 0 dependencies

traffic share 1

next hop 10.98.192.213, FastEthernet2/0

valid adjacency

tag rewrite with Fa2/0, 10.98.192.213, tags imposed {}

0 packets, 0 bytes switched through the prefix

tmstats: external 0 packets, 0 bytes

internal 0 packets, 0 bytes

two next hops are available to reach the prefix out two different interfaces where also is used

sh ip arp 10.98.192.209

Protocol Address Age (min) Hardware Addr Type Interface

Internet 10.98.192.209 143 000e.847e.87c0 ARPA FastEthernet1/0

RT-RM-TLD066-RG-BB-E-6#sh ip arp 10.98.192.213

Protocol Address Age (min) Hardware Addr Type Interface

Internet 10.98.192.213 165 000e.843b.e3c0 ARPA FastEthernet2/0

the RIB for this prefix is

sh ip route 10.34.26.64

Routing entry for 10.34.26.64/28

Known via "isis", distance 115, metric 10011000, type level-2

Redistributing via isis

Last update from 10.98.192.213 on FastEthernet2/0, 1d20h ago

Routing Descriptor Blocks:

* 10.98.192.209, from 10.80.0.101, via FastEthernet1/0

Route metric is 10011000, traffic share count is 1

10.98.192.213, from 10.80.0.101, via FastEthernet2/0

Route metric is 10011000, traffic share count is 1

Hope to help

Giuseppe

Community Member

Re: RIB & FIB

The MPLS course provided by Global Knowledge goes over this in detail. If your company will allow you to do so, I would suggest taking this course.

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