Layer 2 Problem

Discussion in 'Cisco' started by Michael Müller, Feb 21, 2006.

  1. Hi NG,

    i have a strange problem in my network. Sometimes one device (maybe
    a workstation or printer or even a server) is unreachable by 50% of all
    other devices in the network.A Ping fails and a traceroute stops at the core
    router.
    They are in different ip segments but all in a broadcast domain. I have to
    do an "clear arp-cache"
    on my core router in order to get everything fine.
    What in hell could that be ?
    The log on the router shows nothing weird....

    Thx in advance

    Michael
     
    Michael Müller, Feb 21, 2006
    #1
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  2. Michael Müller

    Guest

    More information about how your network is configured would be needed
    to make any educated guesses. Do you have any HSRP in the part of the
    network under consideration? Do you have NAT running in conjunction
    with HSRP-based load balancing perhaps? A combination like that could
    cause problems like this though that only one device becomes
    unreachable seems a little weird.

    To see more about ARP interaction with HSRP+NAT issues look at

    http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122newft/122t/122t4/ftnthsrp.htm

    for instance.

    Anyway, this is only a wild guess since more detailed information is
    missing. Maybe some other posters will have other ideas.

    Cisco da Gama
    http://ciscostudy.blogspot.com
     
    , Feb 21, 2006
    #2
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  3. Hi,

    we do have a cisco 6000 with 4 msfc modules as core router.
    hsrp is configured, but no nat routing.

    Michael

    <> wrote in message
    news:...
    > More information about how your network is configured would be needed
    > to make any educated guesses. Do you have any HSRP in the part of the
    > network under consideration? Do you have NAT running in conjunction
    > with HSRP-based load balancing perhaps? A combination like that could
    > cause problems like this though that only one device becomes
    > unreachable seems a little weird.
    >
    > To see more about ARP interaction with HSRP+NAT issues look at
    >
    >

    http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122newft/122t/122t4/ftnthsrp.htm
    >
    > for instance.
    >
    > Anyway, this is only a wild guess since more detailed information is
    > missing. Maybe some other posters will have other ideas.
    >
    > Cisco da Gama
    > http://ciscostudy.blogspot.com
    >
     
    Michael Müller, Feb 21, 2006
    #3
  4. "Michael Müller" <> wrote in message
    news:dtek01$s5$...
    > Hi,
    >
    > we do have a cisco 6000 with 4 msfc modules as core router.
    > hsrp is configured, but no nat routing.
    >
    > Michael


    Hi.

    Has MLS been enabled on your MFC/SUPERVISOR?

    Let me know


    Bye

    Carlo
     
    Carlo Poggiarelli, Feb 21, 2006
    #4
  5. Hi,

    no, MLS is not enabled.
    Any idea ??
    The Problem drives me crazy....
    someone things about a spanning-tree loop....

    thx


    "Carlo Poggiarelli" <> wrote in message
    news:43fb5389$0$5995$...
    >
    > "Michael Müller" <> wrote in message
    > news:dtek01$s5$...
    > > Hi,
    > >
    > > we do have a cisco 6000 with 4 msfc modules as core router.
    > > hsrp is configured, but no nat routing.
    > >
    > > Michael

    >
    > Hi.
    >
    > Has MLS been enabled on your MFC/SUPERVISOR?
    >
    > Let me know
    >
    >
    > Bye
    >
    > Carlo
    >
    >
     
    Michael Müller, Feb 22, 2006
    #5
  6. Michael Müller

    Merv Guest

    Capture the contents of the arp cache befre and after you do a clear.

    Determine if the MAC address for the device that has the reachability
    problem is the same
    in both cases..
     
    Merv, Feb 22, 2006
    #6
  7. thats the point....
    it IS the same mac adress. But obviously no one knows how to
    deliver the packet to the mac. maybe still a loop in the network ?
    but why do other clients reach that ip ?


    "Merv" <> wrote in message
    news:...
    > Capture the contents of the arp cache befre and after you do a clear.
    >
    > Determine if the MAC address for the device that has the reachability
    > problem is the same
    > in both cases..
    >
     
    Michael Müller, Feb 22, 2006
    #7
  8. Michael Müller

    Merv Guest

    a loop would typically affect a lot more than one IP address

    so please provide the router & switch details:
    1. hardware models
    2. IOS / CATOS versions
    3. topology for a recent failure case - which switch and port is the
    unreachable device connected to, IP addresses htat can reach devices,
    IP addresses that cannot reaach device.
     
    Merv, Feb 22, 2006
    #8
  9. that will be a hell of information...
    the network has about 10 routers and over 20 switches....

    "Merv" <> wrote in message
    news:...
    > a loop would typically affect a lot more than one IP address
    >
    > so please provide the router & switch details:
    > 1. hardware models
    > 2. IOS / CATOS versions
    > 3. topology for a recent failure case - which switch and port is the
    > unreachable device connected to, IP addresses htat can reach devices,
    > IP addresses that cannot reaach device.
    >
     
    Michael Müller, Feb 22, 2006
    #9
  10. Michael Müller

    Merv Guest

    Perhaps, but if you want to get to the pbottom of this you have to
    start characterizing the nature of the problem in more detail

    How many subnets (routes) are there in your network?

    Does this problem happen on every one of those subnets?

    Any there any observable patterns whatsoever - time of day, etc, etc.

    Have you kept a lot of every IP address for which this problem has
    occurred; if not highly recommend this be done.

    How long has this problem being occuring ?

    Recent router or switch upgrades, new switches added, topology change,
    sighnficant configuration changes ...

    If you want to get assistance from this group, this type of info will
    be required.
     
    Merv, Feb 22, 2006
    #10
  11. There are about 10 subnets. The Problem was until now noticed on three of
    them.
    No observable patterns...different time, different subnets, different
    unreachable ips
    different clients who are able to reach that "unreachable" ip

    yes, i wrote down all Ips and the time it occured. turned the debugging on,
    checked the log....
    This appears now for 3-4 months, sometimes once
    a day, sometimes even 4-5 times.

    as far as i know there were no significant changes in the
    topology or the configs


    "Merv" <> wrote in message
    news:...
    >
    > Perhaps, but if you want to get to the pbottom of this you have to
    > start characterizing the nature of the problem in more detail
    >
    > How many subnets (routes) are there in your network?
    >
    > Does this problem happen on every one of those subnets?
    >
    > Any there any observable patterns whatsoever - time of day, etc, etc.
    >
    > Have you kept a lot of every IP address for which this problem has
    > occurred; if not highly recommend this be done.
    >
    > How long has this problem being occuring ?
    >
    > Recent router or switch upgrades, new switches added, topology change,
    > sighnficant configuration changes ...
    >
    > If you want to get assistance from this group, this type of info will
    > be required.
    >
     
    Michael Müller, Feb 23, 2006
    #11
  12. Michael Müller

    Merv Guest

    1. When the problem occurs is it a single or multiple IP's that becomes
    unreachable?

    2. Per occurrence are these IPs on a single subnet or on all three of
    the subnets?

    3. Are each of the three subnets on differnet switches or the same
    switch ?

    4. Has the problem ever gone away without you have to clear the arp
    cache ?

    5. Please provide the switch models and IOS version for the devices
    supporting the three affected subnets
     
    Merv, Feb 23, 2006
    #12
  13. Hey...first of all, thx for your assistance
    ;-)

    1. It´s a single IP
    2. The unreachable IP is everytime on a single subnet (Class C), the clients
    who can/cannot reach
    it are on different subnets (think so, still have to check this)
    3. They are on different switches
    4. No, only a clear arp cache on the core router solved the Problem
    or what i testet if that ip belongs to for example to a printer, I
    disconnected it from
    the net, connected the cable to a laptop, gave it the same ip it was
    reachable.
    Plugged in the printer again >> reachable....
    5. Core routers (2 cat 6000 cross linked with 2 msfc each, HSRP configured)
    The switches : Mostly 2950, 35XX, one cat6000) I will provide specified
    info as soon as possible (Currently not in office)


    "Merv" <> wrote in message
    news:...
    > 1. When the problem occurs is it a single or multiple IP's that becomes
    > unreachable?
    >
    > 2. Per occurrence are these IPs on a single subnet or on all three of
    > the subnets?
    >
    > 3. Are each of the three subnets on differnet switches or the same
    > switch ?
    >
    > 4. Has the problem ever gone away without you have to clear the arp
    > cache ?
    >
    > 5. Please provide the switch models and IOS version for the devices
    > supporting the three affected subnets
    >
     
    Michael Müller, Feb 23, 2006
    #13
  14. Michael Müller

    Merv Guest

    For the IP that becomes unreachable, can it be pinged from other PCs
    that are connected to the SAME access switch ?

    Are the access switches dual homed to each of the 6500 switches ?
     
    Merv, Feb 23, 2006
    #14
  15. Michael Müller

    Merv Guest

    The next time problem occcurs, from the PC that becomes unreachable,
    try the following ping tests:

    1. ping HSRP virutal IP address for subnet (default gateway)

    2. ping physical IP address of 6500 A

    3. ping phsical IP address of 6500 B
     
    Merv, Feb 23, 2006
    #15
  16. have to check...

    most of the access switches are dual homed.


    "Merv" <> wrote in message
    news:...
    > For the IP that becomes unreachable, can it be pinged from other PCs
    > that are connected to the SAME access switch ?
    >
    > Are the access switches dual homed to each of the 6500 switches ?
    >
     
    Michael Müller, Feb 23, 2006
    #16
  17. will post results asap


    "Merv" <> wrote in message
    news:...
    >
    > The next time problem occcurs, from the PC that becomes unreachable,
    > try the following ping tests:
    >
    > 1. ping HSRP virutal IP address for subnet (default gateway)
    >
    > 2. ping physical IP address of 6500 A
    >
    > 3. ping phsical IP address of 6500 B
    >
     
    Michael Müller, Feb 23, 2006
    #17
  18. Michael Müller

    Merv Guest

    Additionally when problem occurs, capture the CAM or MAC-address table
    on the access switch as well as on both 6500 switches (from the switch
    sup cards).
    also capture the output of show standby
     
    Merv, Feb 23, 2006
    #18
  19. Michael Müller

    Merv Guest

    Please post "show cdp nei detail " from each of the three access
    switches on which the IP connectivity issue has occurred
     
    Merv, Feb 23, 2006
    #19
  20. Michael Müller

    Merv Guest

    How are you making our gathering info ?
     
    Merv, Feb 24, 2006
    #20
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