A question about Multiple Routers with a Single MLS Catalyst

Discussion in 'Cisco' started by worldwidestar, May 2, 2007.

  1. Hello,

    In <<Cisco Lan Switching>> book, there is a paragraph about the "Using
    Multiple Routers with a Single MLS-Capable Catalyst". I don't
    understand how the "Double lookups" and "Double Rewrite" operate,
    could you take a look at this and give me a instruction?

    Two MLS Routers and One MLS Switch http://www3.freep.cn/photo36/070502/18/0705021818434327.gif

    Description in the book:
    Here, Host-A is still located in the Red VLAN and Host-B is still
    located in the Blue VLAN. However, a new VLAN has been created between
    the two routers (call it the Purple VLAN). Host-A still sends traffic
    destined to Host-B to its default gateway using the Red VLAN. As the
    first packet passes through the Catalyst, the NFFC recognizes it as a
    candidate packet and creates a partial shortcut entry (labeled Step 1
    in the picture). Router-A then forwards the traffic over the Purple
    VLAN to Router-B. As the packet passes back through the Catalyst, the
    NFFC recognizes the packet as an enable packet and completes the
    shortcut entry (Step 2 in the picture). However, it also recognizes
    the destination MAC address as that of Router-B and therefore sees
    this packet as another candidate packet (Step 3 in the picture).
    Router-B then routes the packet normally and forwards it to Host-B
    over the Blue VLAN. As the packet passes back through the Catalyst for
    the third time, it is identified as an enable packet for the partial
    entry created in Step 3. A second shortcut entry is created (Step 4
    the picture).
    ******When additional traffic flows from Host-A to Host-B (Step 5 in
    the picture), two sets of shortcut lookups and rewrite operations are
    performed. ******** As a result, the additional packets are not sent
    to either router.

    I don't quite understand the above sentence besieged by asterisks.
    How the "two sets" work, please? If anyone comprehends very well,
    could you give me a detail explanation?

    worldwidestar, May 2, 2007
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  2. The part between asterisks is easiest part - it's basics of MLS. When you
    have MLS-capable router, and MLS-capable switch, first packet goes through
    the router, and then router sends "instructions" to the switch how to handle
    remaining traffic in the same flow without "asking a router what to do". So,
    all remaining frames in the data flow between two hosts will be switched
    within a switch.

    More tough part is the scenario they draw. Practically I see the only two
    "scenarios" when it's possible - PBR (Policy Based Routing), and two
    Supervisor modules working independently for load-balancing. In all other
    situations you have the only one "router", and you have one "lookup" and

    Good luck,

    CCNP, CCDP, CCSP, Cisco Voice, MCSE W2K, MCSE+I, Security+, etc.
    CCIE R&S (in progress), CCIE Voice (in progress)
    headsetadapter.com, May 2, 2007
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