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Forum Replies

  1. andrew says:

    CAM: High speed memory that is primarily used for a switch's layer 2 lookup information. This information allows the switch to decide which port to send a packet to (a known MAC address) or whether to flood it to all ports (unknown MAC address).

    TCAM: Not all switches have this. Think of this as an extension of CAM. It is used for very rapid decisions on ACLs and Quality of Service. On high end layer 3 switches, the TCAM can also contain the FIB, again, so specialized hardware can making routing decisions without interrupting the central CPU of the switch.

    FIB: When you think of the FIB vs the RIB, or routing table, the difference is where they "live" on the hardware. The RIB lives in the control plane, while FIB lives in the data plane. Any decisions made at the FIB level are fast and do not require an interrupt (and therefore time) from the device itself.

    So the CAM/TCAM and FIB aren't directly related other than the FIB may or may not be held in the TCAM depending on your switch model.

    So, in the example of a packet traveling from source to destination, if a packet's destination is on a remote subnet, the packet's destination MAC would be set to the gateway's MAC. The switch would use the CAM to determine in which port the gateway resides, and it would send it there. Depending on the model hardware involved, the layer3 switch or router would use the FIB to decide what the next hop needs to be (and since the FIB might be in the TCAM--again depending on the model--the TCAM might be used in this process). This would continue until the packet arrives at the destination subnet, where the final switch would again use the CAM table to determine the destination's MAC and corresponding port.

  2. Hi Andrew!,

    Glad to see you around and yea! thanks for confirming my doubt.

    I have a few more doubts that i have no where to turn to and i hope you will enlightened me..

    My coreswitch has ip cef turn on my default.

    q1) I have been reading up abit on cisco about IP CEF and it seems like to enable/disable IP CEF, you have to do in at the ingress interface as the decision (e.g. load balance is done there). -- is it right ?

    Assuming i have "no ip cef" and only wish to turn on ip cef on certain interfaces and ->
    q2) if i want to do to packet loadbalancing, should "ip load-share per packet" command be issue on the ingress interface as well ?

    q3) Reading How to Verify Cisco Express Forwarding -> "http://www.cisco.com/c/en/us/support/docs/ios-nx-os-software/ios-software-releases-120-mainline/47205-cef-whichpath.html"

    Use the show interface x/x stat command and determine the number of packets and bytes that the router forwarded through "Processor" instead of "Route cache." Note that "Route cache" includes both fast-switched and CEF-switched packets.

    router#show interface stats     
    Switching path Pkts In  Chars In Pkts Out Chars Out 
    Processor 95084 26211621 33493 3386174 
    Route cache 24581 1132797 24542 13297583 
    Distributed cache 0 0 0 0 
    Total  119665 27344418 58035 16683757

    Should this command be issue on the ingress or egress interface ?
    if it is to be issue on the INGRESS interface -> What does the "Pkts Outs" under Route cache means then ?

    Really hope to hear from you soon.


  3. andrew says:
    1. The ARP table is stored in RAM, and not in either CAM or TCAM

    2. Here is the Cisco definition of Epoch and how it is used:

    The term "epoch" refers to a period of time. A new epoch for a Cisco Express Forwarding table begins when a table rebuild is initiated. The time after this instant is in an epoch different from the time before, and the different epochs are numbered between 0 and 255. Through the use of epochs, the software can distinguish between old and new forwarding information in the same database structure and can retain the old Cisco Express Forwarding database table while the software builds a new table. This is called epoch tracking and it allows Cisco Express Forwarding forwarding to continue uninterrupted while new Cisco Express Forwarding tables are being constructed, and it makes possible a seamless switchover when the new table becomes active

    When you issue a show ip cef <route>, the dependencies output means how many other routes depend (are resolvable through) the <route> you looked up via the CEF command.

  4. Hello Chandrakant.

    I believe you are correct. Actually, those two commands should be on for each of the R1 and R2 routers like so:

    R2(config)#ip route

    R1(config)#ip route

    I will let Rene know so it can be corrected.

    Thanks very much!


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