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  1. Senior Member
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    #1

    Default Routing Table vs RIB - Correct?

    COrrect me if I am wrong:

    The RIB contains all routes from the same routing protocol.
    OSPF has its own RIB. EIGRP has its own RIB etc.

    The "good" routes are taken from the RIB to form the routing table which is in "show ip route".

    Thereafter, the routing table is parsed and the routes are stored in CEF table "show ip cef".

    Am I correct?
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  3. Roaming teh Bytes OfWolfAndMan's Avatar
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    #2
    The RIB is a global routing table with all routes (That are being used that is).
    Dont forget directly connected routes as well. As for the CEF table...

    Usually, at least for IOS, a recursive lookup is required for calculating how to get to a next hop. First there is the route and the next hop, but then the recursion occurs when the router has to find an outgoing interface to get to the next hop. This is where CEF comes in. It pre calculates those values for you and stores it in the FIB.
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    #3
    RIB (Routing Information Base) is just a general term that refers to some kind of storage of routing related data on the router. The routing table is sometimes called "the RIB", but this is not really correct since, as you point out, there are several different RIBs. The routing table in particular is called the "IP RIB" to distinguish it from other RIBs.
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  5. Senior Member
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    #4
    show ip ospf rib -> OSPF RIB
    show ip eigrp rib -> EIGRP rib
    show ip route -> IP RIB
    show ip cef -> CEF table

    What is the for the forward information base and the adjacency table???
    I am pretty confused,
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    #5
    If you've read the CCNP material you should know it.
    The FIB aka CEF table is the layer 3 information CEF uses and the adjacency table is the L2 info it uses to make the "precalculated" forwarding paths.
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  7. Senior Member
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    #6
    So for my posts above, it is correct right?
    So there is no command to display the FIB?
    For the adjacency table, is it show mac-address table?
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    #7
    Quote Originally Posted by dppagc View Post
    So for my posts above, it is correct right?
    So there is no command to display the FIB?
    For the adjacency table, is it show mac-address table?

    Show ip cef is the FIB also known as CEF table
    show adjacency shows the adjacency table of cef.

    I suggest you read more about it. A short nice explanation: https://routing-bits.com/2009/06/02/understanding-cef/
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  9. Senior Member
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    #8
    Yes, each routing protocol running on a router has it's own RIB, and the best routes from all RIB's are selected and placed into the global routing table. As joetest mentioned the FIB and adjacency tables are used for CEF, and while built using the global routing table, works a lot differently under the hood, and is even built on specific hardware AKA TCAM.
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  10. Padawan d4nz1g's Avatar
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    #9
    Take a look at this:

    https://networklessons.com/wp-conten...s-lib-lfib.png

    where:

    -EIGRP RIB = sh ip eigrp topology
    -OSPF RIB = sh ip ospf rib
    -BGP RIB = sh ip bgp

    All of these protocols are "contributors" to the ip rib process, which will be your ip routing table (RIB).

    We now have to understand the difference between control and data planes.
    Control plane is responsible for protocol interaction and information exchange with neighboring routers. RIP, EIGRP, BGP, OSPF and the main RIB are all Control Plane functions.

    As soon as the Control Plane has enough information to move packets, it starts to program the Data Plane, which is responsible for the packet forwarding per se. So we have the Control Plane (route processor, supervisor, MSFC, etc) to program the actually forwarding plane, also called Data Plane (which is composed by ASICS, NPE, DFC, and so on). These Data Plane components usually stores FIB information, which will be your Forwarding Table in a format knows as TCAM.

    Keep in mind that what I mentioned above applies specifically to routers/switches with a distributed architecture (ASR9k, 6500, 7600, etc). Software Based routers (ISR, ASR900, ASR1k) still hold everything on the main processing unit (Main memory or in the Supervisor itself). By the way, data plane and control plane concepts applies for both.


    TL/DR:

    You have Control Plane funcions (Protocols and the RIB which is populated by these protocols) and Data Plane functions (FIB lookups and packet forwarding).

    Show ip route = routing information that will populate your FIB. (Control Plane)
    show ip cef = routing information inside your FIB. (Data Plane)

    FIB will be your actual forwarding path (how you should move packets), and Cisco uses CEF on most of its platforms. CEF stores several information related to packet forwarding, including pre-calculated headers and load balancing/loadsharing computations. As a result, your FIB will contain all the information a router might need to forward a packet.

    I dont know if I was 100% accurate, but I hope this might help you.
    Last edited by d4nz1g; 05-16-2016 at 02:00 PM. Reason: Adjusted a few things
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  11. Senior Member
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    #10
    I will disagree with you on several of your points here, just to try to clarify a few things. First the local RIB for a routing protocol is not equivalent to the OSPF database,EIGRP topology table, or BGP table. The RIB is literally a routing table. The OSPF database and EIGRP topology tables are not routing tables. Each routing protocol builds it's own local RIB based on what routes it has received and stored in its OSPF database and EIGRP topology table, these are the best routes to each network, in the case of OSPF SPF runs on the OSPF database and all of the best routes are placed into the OSPF local RIB,. For EIGRP DUAL runs on the EIGRP topology table, and then determines best routes and places them into the EIGRP local RIB. The global RIB is then built using the local RIB of each routing protocol on the device. This article , helps further explain and provide examples.


    As far as the control plane goes. The PFC would not be part of the control plane, it is strictly used for CEF and or hardware based forwarding, as it's literally a daughter card of the MSFC, and the PFC contains all of the ASICS that do layer 2 and layer 3 forwarding. The MSFC would be the control plane though, as it contains both the layer 2 and layer 3 processors. For reference, this article explains exactly what both do. So the PFC would contain both the FIB and adjacency tables used for CEF, and they would be stored on TCAM, also located on the PFC. Keep in mind that the PFC and MSFC are both part of the Supervisor.

    CEF is not just a way of organizing information, it has it's own logic built into it. For things such as load balancing on a per destination basis by default. CEF is also responsible for making the decisions on which entry in the FIB a packet matches.
    Last edited by james43026; 05-14-2016 at 01:58 AM.
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  12. Padawan d4nz1g's Avatar
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    #11
    Quote Originally Posted by james43026 View Post
    I will disagree with you on several of your points here, just to try to clarify a few things. First the local RIB for a routing protocol is not equivalent to the OSPF database,EIGRP topology table, or BGP table. The RIB is literally a routing table. The OSPF database and EIGRP topology tables are not routing tables. Each routing protocol builds it's own local RIB based on what routes it has received and stored in its OSPF database and EIGRP topology table, these are the best routes to each network, in the case of OSPF SPF runs on the OSPF database and all of the best routes are placed into the OSPF local RIB,. For EIGRP DUAL runs on the EIGRP topology table, and then determines best routes and places them into the EIGRP local RIB. The global RIB is then built using the local RIB of each routing protocol on the device. This article , helps further explain and provide examples.


    As far as the control plane goes. The PFC would not be part of the control plane, it is strictly used for CEF and or hardware based forwarding, as it's literally a daughter card of the MSFC, and the PFC contains all of the ASICS that do layer 2 and layer 3 forwarding. The MSFC would be the control plane though, as it contains both the layer 2 and layer 3 processors. For reference, this article explains exactly what both do. So the PFC would contain both the FIB and adjacency tables used for CEF, and they would be stored on TCAM, also located on the PFC. Keep in mind that the PFC and MSFC are both part of the Supervisor.

    CEF is not just a way of organizing information, it has it's own logic built into it. For things such as load balancing on a per destination basis by default. CEF is also responsible for making the decisions on which entry in the FIB a packet matches.

    Thanks for the correction; it was very clarifying.
    Gonna take a look at those articles.
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