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Thread: VLSM & CIDR

  1. Junior Member
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    #1

    Default VLSM & CIDR

    RIPv2, EIGRP, OSPF, are said to support VLSM. BGP is said to support CIDR. CIDR (Classless Inter Domain Routing)... is defined according to Cisco's documentation, is the practice of having multiple "Class C addresses" that can become combined or aggregated, to create a larger classless set of IP addresses...the multiple Class C addresses can then be summarized in routing tables, resulting in fewer route advertisements... the AS-Path attribute can provide autonomous systems that all aggregated routes have passed through. By default, the aggregation will be advertised as coming from the AS that did the aggregation and have the Atomic aggregate attribute set to show that info is missing. Since BGP supports aggregation, in "Theory", BGP supports VLSM (Variable Length Subnet Masks). Infact there is a coomand that BGP can use in a router to advertise classless agregation.Now, in a situation where uare asked to select protocols that support VLSM and you are given all four options to choose two oe three, what do u do? Please i want to know if some protocols have preference above others and if they do, what are the criterias considered for this preference?
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    #2
    A Variable Length Subnet Mask (VLSM) is a means of allocating IP addressing
    resources to subnets according to their individual need rather than some
    general network-wide rule. Of the IP routing protocols supported by Cisco,
    OSPF, Dual IS-IS, BGP-4, and EIGRP support "classless" or VLSM routes.
    Source www.faqs.org
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  4. Johan Hiemstra Forum Admin Webmaster's Avatar
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    #3
    Actually, RIPv2 should be added to that list. BGP support CIDR, which is a bit like VLSM on the Internet. The definition quoted from the cisco doc above is supernetting, which is made possible by CIDR. Classless refers to not having to use class C subnet mask with a class C ip address.

    So if a question would be:

    Which of the following routing protocols support VLSM? (Choose 4)

    a. RIPv1
    b. RIPv2
    c. IGRP
    d. EIGRP
    e. OSPF
    f. IS-IS
    g. BGP

    the correct answers would be b, d, e, f.

    I'm quite sure cisco will never provide BGP as an option if this were an actual exam question.
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  5. Junior Member
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    #4
    Thanks a lot guys. Webmaster your right, that definition is on supernetting(checked it out). Was actually a bit confused.
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    #5
    Are there any good sites for practising VLSM ????
    EMERGENCY!
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