Large IP Block = Lots of ARP traffic

Discussion in 'Cisco' started by laxman22, Jul 18, 2005.

  1. laxman22

    laxman22 Guest

    The company I work for owns a /18 - all of which exists at one location
    and is routed by a single router. There is no subnetting or VLAN in
    place, so each host is seeing a TON of ARP traffic. Also the router is
    struggling during peak usage times and I think that if I could cut down
    the amount of ARP it had to do I could stretch the life of the router.
    A big problem is that many of the 200+ machines have IPs on them that
    span many possible subnets, so it would be difficult to use VLANs to
    solve this problem. And would VLANs even really solve this problem?
    Wouldnt the router still have to do a ton of ARPing, only not every
    host would see every ARP? Can someone explain to me how this works?
    Wouldnt every VLAN still see every ARP because they go out to
    255.255.255.255?

    Basically here is the current network design:
    internet - cisco router - cisco L3 switch - <15 24 port unmanaged L2
    switches> - hosts
    I suppose
    internet - cisco router <trunk> cisco L3 switch <trunk> <15 cisco L3
    switches> <access vlan x> host
    would not be out of the question for us to implement, but can someone
    explain to me how a setup like this would work and reduce overall ARPs?

    Any ideas, requests for more info, etc... would be great.

    Thanks!
    laxman22, Jul 18, 2005
    #1
    1. Advertising

  2. laxman22

    aservin Guest

    You have to use vlans, they will resolve the problem for the host but
    you will have to do IP renumering. The broadcast will be segmented in
    per vlan, so, the broadcast from vlan X won't be seen on vlan Y. The
    router will still be seing all the ARPs, so, if it is overload probably
    you will not resolve anything about that with vlans.

    -as
    aservin, Jul 18, 2005
    #2
    1. Advertising

  3. laxman22

    laxman22 Guest

    so do you know of a good way to reduce the arps seen by the router? is
    it just not possible without another router under it? if someone is
    scanning my entire IP range the router is going to always try to arp
    right? I cant pass that off to a L3 switch?
    laxman22, Jul 18, 2005
    #3
  4. laxman22

    Garry Guest

    laxman22 wrote:
    > so do you know of a good way to reduce the arps seen by the router? is
    > it just not possible without another router under it? if someone is
    > scanning my entire IP range the router is going to always try to arp
    > right? I cant pass that off to a L3 switch?
    >

    How about a Firewall between the Internet and your LAN? That way,
    portscans won't even get to your router ... Apart from that, it sounds
    like your best choice would be a re-organization of the whole network, a
    /18 LAN sounds sick ... ;)

    -gg
    Garry, Jul 18, 2005
    #4
  5. laxman22

    Alex Guest

    > Basically here is the current network design:
    > internet - cisco router - cisco L3 switch - <15 24 port unmanaged L2
    > switches> - hosts
    > I suppose
    > internet - cisco router <trunk> cisco L3 switch <trunk> <15 cisco L3
    > switches> <access vlan x> host
    > would not be out of the question for us to implement, but can someone
    > explain to me how a setup like this would work and reduce overall ARPs?
    >
    > Any ideas, requests for more info, etc... would be great.
    >

    You may wish to look at Private VLAN feature together with turning on "ip
    local proxy-arp" on L3 switch.
    "A protected port does not forward any traffic (unicast, multicast, or
    broadcast) to any other port that is also a protected port in the same
    switch and hence providing isolation"
    http://www.cisco.com/warp/public/473/63.html
    Basically, putting each of 15 L2 switches into its own PVLAN on L2 switch
    will drastically reduce ARP traffic - at the expense of L3 switch which
    would have to answer 15 times more ARP requests.
    HTH
    Cheers
    Alex
    Alex, Jul 19, 2005
    #5
    1. Advertising

Want to reply to this thread or ask your own question?

It takes just 2 minutes to sign up (and it's free!). Just click the sign up button to choose a username and then you can ask your own questions on the forum.
Similar Threads
  1. Scott D. Weber For Unuathorized Thoughts Inc.

    Backing Up Large Files..Or A Large Amount Of Files

    Scott D. Weber For Unuathorized Thoughts Inc., Sep 17, 2003, in forum: Computer Support
    Replies:
    1
    Views:
    959
    Joseph
    Sep 19, 2003
  2. -= Hawk =-

    Re: arp traffic?

    -= Hawk =-, Oct 22, 2003, in forum: Computer Support
    Replies:
    0
    Views:
    422
    -= Hawk =-
    Oct 22, 2003
  3. Richard

    Re: arp traffic?

    Richard, Oct 22, 2003, in forum: Computer Support
    Replies:
    2
    Views:
    452
    Meat-->Plow
    Oct 23, 2003
  4. Replies:
    7
    Views:
    762
    Barry Margolin
    Feb 26, 2008
  5. Darren Green

    Arp or Proxy Arp

    Darren Green, Feb 20, 2009, in forum: Cisco
    Replies:
    0
    Views:
    515
    Darren Green
    Feb 20, 2009
Loading...

Share This Page