IP address distribution in a Forwarding Table

Discussion in 'Cisco' started by gullu, Jan 26, 2007.

  1. gullu

    gullu Guest

    Hi,

    Does any one know a resource which provides some statistics about IPv4
    and IPv6 addresses stored in a forwarding table of a router.
    For Example,
    for IPv4, each entry in a forwarding table contains at least the
    destination address and the corresponding next hop. Destination
    addresses might be stored entirely or they can match a set of IP
    addresses. What I am looking for is some statistics that can tell me
    how on the average how many entries are of the form
    127.123.65.42
    136.121.45.12x (where 'x' means a don't care i.e. use the longest
    prefix matching here)
    136.121.45.1xx (where 'x' means a don't care i.e. use the longest
    prefix matching here)
    136.141.54.xxx (where 'x' means a don't care i.e. use the longest
    prefix matching here)
    136.141.5x.xxx (where 'x' means a don't care i.e. use the longest
    prefix matching here)
    and so on.

    I will really appreciate any help.

    Thanks.
    gullu, Jan 26, 2007
    #1
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  2. In article <>,
    gullu <> wrote:

    >Does any one know a resource which provides some statistics about IPv4
    >and IPv6 addresses stored in a forwarding table of a router.
    >For Example,
    >for IPv4, each entry in a forwarding table contains at least the
    >destination address and the corresponding next hop. Destination
    >addresses might be stored entirely or they can match a set of IP
    >addresses. What I am looking for is some statistics that can tell me
    >how on the average how many entries are of the form
    >127.123.65.42
    >136.121.45.12x (where 'x' means a don't care i.e. use the longest
    >prefix matching here)
    >136.121.45.1xx (where 'x' means a don't care i.e. use the longest
    >prefix matching here)
    >136.141.54.xxx (where 'x' means a don't care i.e. use the longest
    >prefix matching here)
    >136.141.5x.xxx (where 'x' means a don't care i.e. use the longest
    >prefix matching here)
    > and so on.


    Your examples hint that you might be misunderstanding the routing
    table. "longest prefix matching" does not use the decimal address
    values: it uses the binary addresses, the "longest" is based upon
    the number of bits matched, not the number of decimal digits matched.

    Cisco routers do not keep statistics about the routing table size
    (and on Cisco routers, the routing process can end up being -much-
    more complex than a simple forwarding table.) However, you can
    generate these kinds of statistics yourself by using snmp to
    periodically read out the forwarding table and doing some simple
    totalling. You want the IP-FORWARD-MIB, base OID 1.3.6.1.2.1.4.24
    In particular, you might want ipCidrRouteTable 1.3.6.1.2.1.4.24.4
    or the inetCidrRouteTable 1.3.6.1.2.1.4.24.7
    http://tools.cisco.com/Support/SNMP...late=Translate&objectInput=1.3.6.1.2.1.4.24.7
    Walter Roberson, Jan 26, 2007
    #2
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  3. You may wish to investigate the IFM IPv6 / IPv4 web page.

    You've heard a lot of talk about IPv6, and you'd like to have a play
    with it, but your ISP doesn't offer it. This article discusses one
    option, 6to4, that you can use.

    http://www.ifm.net.nz/research/ipv6.html

    as well as Microsoft SBS 2003 IPv6 Readiness:

    http://www.ifm.net.nz/research/sbs2003ipv6readiness.html

    and the Cisco IPv6 Config Wizard:

    http://www.ifm.net.nz/cookbooks/ipv6configwizard.html

    Sincerely,

    Brad Reese
    Cisco Resumes
    http://www.bradreese.com/cisco-resumes.htm
    www.BradReese.Com, Jan 26, 2007
    #3
  4. www.BradReese.Com, Jan 26, 2007
    #4
  5. gullu

    gullu Guest

    Hi Walter,

    Thanks for your reply.

    I know that the prefix matching is done on binary addresses. I just
    gave an example so that everyone can understand it.

    I was wondering if CISCO collected some data about the distribution of
    the IP addresses in the forwarding table on order to introduce some
    optimizations in their future products.

    Can you answer one more question? Do CISCO routers use TCAMs for IP
    lookup in the forward table. If so (and if you feel that you are
    allowed to answer this question), Can you provide me with some details
    about how these TCAMs are used in the routers. There are so many
    research papers out on IP lookup and TCAMs and I am just wondering what
    industry has to say about it.

    Thanks.

    On Jan 26, 11:45 am, (Walter Roberson) wrote:
    > In article <>,
    >
    >
    >
    > gullu <> wrote:
    > >Does any one know a resource which provides some statistics about IPv4
    > >and IPv6 addresses stored in a forwarding table of a router.
    > >For Example,
    > >for IPv4, each entry in a forwarding table contains at least the
    > >destination address and the corresponding next hop. Destination
    > >addresses might be stored entirely or they can match a set of IP
    > >addresses. What I am looking for is some statistics that can tell me
    > >how on the average how many entries are of the form
    > >127.123.65.42
    > >136.121.45.12x (where 'x' means a don't care i.e. use the longest
    > >prefix matching here)
    > >136.121.45.1xx (where 'x' means a don't care i.e. use the longest
    > >prefix matching here)
    > >136.141.54.xxx (where 'x' means a don't care i.e. use the longest
    > >prefix matching here)
    > >136.141.5x.xxx (where 'x' means a don't care i.e. use the longest
    > >prefix matching here)
    > > and so on.Your examples hint that you might be misunderstanding the routing

    > table. "longest prefix matching" does not use the decimal address
    > values: it uses the binary addresses, the "longest" is based upon
    > the number of bits matched, not the number of decimal digits matched.
    >
    > Cisco routers do not keep statistics about the routing table size
    > (and on Cisco routers, the routing process can end up being -much-
    > more complex than a simple forwarding table.) However, you can
    > generate these kinds of statistics yourself by using snmp to
    > periodically read out the forwarding table and doing some simple
    > totalling. You want the IP-FORWARD-MIB, base OID 1.3.6.1.2.1.4.24
    > In particular, you might want ipCidrRouteTable 1.3.6.1.2.1.4.24.4
    > or the inetCidrRouteTable 1.3.6.1.2.1.4.24.7http://tools.cisco.com/Support/SNMP/do/BrowseOID.do?local=en&translat...
    gullu, Jan 26, 2007
    #5
  6. gullu

    stephen Guest

    "gullu" <> wrote in message
    news:...
    > Hi Walter,
    >
    > Thanks for your reply.
    >
    > I know that the prefix matching is done on binary addresses. I just
    > gave an example so that everyone can understand it.
    >
    > I was wondering if CISCO collected some data about the distribution of
    > the IP addresses in the forwarding table on order to introduce some
    > optimizations in their future products.


    it wouldnt make sense - there is no way my routers are sending forwarding
    table info to Cisco....

    more importantly - why would that help?

    Cisco routers get used in all sorts of environments, many of which arent
    even directly connected to the Internet
    >
    > Can you answer one more question? Do CISCO routers use TCAMs for IP
    > lookup in the forward table. If so (and if you feel that you are
    > allowed to answer this question), Can you provide me with some details
    > about how these TCAMs are used in the routers. There are so many
    > research papers out on IP lookup and TCAMs and I am just wondering what
    > industry has to say about it.


    depends. low end boxes use software (but may use TCAM table structures).

    Cisco have put TCAM logic into chips for higher performance boxes such as
    the 7609 with a high end processor card.

    the search for "tcam forwarding table" on the cisco web site gets 1680
    hits - without a CCO login....
    >
    > Thanks.
    >
    > On Jan 26, 11:45 am, (Walter Roberson) wrote:
    > > In article <>,
    > >
    > >
    > >
    > > gullu <> wrote:
    > > >Does any one know a resource which provides some statistics about IPv4
    > > >and IPv6 addresses stored in a forwarding table of a router.
    > > >For Example,
    > > >for IPv4, each entry in a forwarding table contains at least the
    > > >destination address and the corresponding next hop. Destination
    > > >addresses might be stored entirely or they can match a set of IP
    > > >addresses. What I am looking for is some statistics that can tell me
    > > >how on the average how many entries are of the form
    > > >127.123.65.42
    > > >136.121.45.12x (where 'x' means a don't care i.e. use the longest
    > > >prefix matching here)
    > > >136.121.45.1xx (where 'x' means a don't care i.e. use the longest
    > > >prefix matching here)
    > > >136.141.54.xxx (where 'x' means a don't care i.e. use the longest
    > > >prefix matching here)
    > > >136.141.5x.xxx (where 'x' means a don't care i.e. use the longest
    > > >prefix matching here)
    > > > and so on.Your examples hint that you might be misunderstanding the

    routing
    > > table. "longest prefix matching" does not use the decimal address
    > > values: it uses the binary addresses, the "longest" is based upon
    > > the number of bits matched, not the number of decimal digits matched.
    > >
    > > Cisco routers do not keep statistics about the routing table size
    > > (and on Cisco routers, the routing process can end up being -much-
    > > more complex than a simple forwarding table.) However, you can
    > > generate these kinds of statistics yourself by using snmp to
    > > periodically read out the forwarding table and doing some simple
    > > totalling. You want the IP-FORWARD-MIB, base OID 1.3.6.1.2.1.4.24
    > > In particular, you might want ipCidrRouteTable 1.3.6.1.2.1.4.24.4
    > > or the inetCidrRouteTable

    1.3.6.1.2.1.4.24.7http://tools.cisco.com/Support/SNMP/do/BrowseOID.do?local=
    en&translat...
    >

    --
    Regards

    - replace xyz with ntl
    stephen, Jan 26, 2007
    #6
  7. In article <>,
    gullu <> wrote:

    >Can you answer one more question? Do CISCO routers use TCAMs for IP
    >lookup in the forward table. If so (and if you feel that you are
    >allowed to answer this question), Can you provide me with some details
    >about how these TCAMs are used in the routers.


    Any relevant NDA I had expired long ago, so I can answer with
    absolute confidence: "I haven't the foggiest idea." ;-)
    Walter Roberson, Jan 26, 2007
    #7
    1. Advertising

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