From: Andrew Dempsey (apdccie@gmail.com)
Date: Mon Sep 29 2008 - 15:58:31 ART
Lets say you have HOSTA on the 10.0.0.0/24 subnet that has its ip set to
10.0.0.2/8 (maybe it doesn't support classless addressing) and it is trying
to reach HOSTB 10.10.10.2/24 . There is a router with ethernet connections
to both of these subnets with IP's 10.0.0.1/24 and 10.10.10.1/24 both hosts
have these addresses set up as their default gateway. When HOSTB tries to
reach HOSTA the packet is sent to the default gateway as the destination is
remote. However when HOSTA tries to reach HOSTB it will not send the
traffic to its default gateway as it believes HOSTB is on the same network
it will actually arp locally. When the router with proxy-arp enabled on its
incoming interface and has a route for the destination it will forward the
packet out the correct interface.
One of the things people will misconfigure is lets say you have a
fastethernet connection to your provider they will route 0.0.0.0
0.0.0.0FastEthernet0/0 and because of proxy arp everything seems to
work unless you
really look at whats happening. Now because the interface is multipoint you
will be arping for every new destination, you will quickly see your arp
table fill up with seemingly random entries, delay traffic waiting for arp
responses, and also you will be using more CPU than would be otherwise
Andrew
On Mon, Sep 29, 2008 at 4:41 AM, <mark.chandra@gmail.com> wrote:
> Hi all,
>
> I actually have the same curiousity here, so anyway what is proxy arp used
> for and in what environment we can use this command useable?
>
> Thx a lot guys
> Sent from my BlackBerry. wireless device from XL GPRS/EDGE/3G network
>
> -----Original Message-----
> From: "Ahsan Mohiuddin" <ahsan.mohiuddin@gmail.com>
>
> Date: Sun, 28 Sep 2008 23:48:52
> To: Ali Mousawi<mousawi.ali@gmail.com>
> Cc: <ccielab@groupstudy.com>
> Subject: Re: Proxy-arp
>
>
> Hello Ali,
>
> when IP Routing is enabled on R1, it looks up the route to 155.1.0.0/24 in
> its IP Route table. Upon not finding the desired network in the routing
> table, the ping packets to destination 155.1.0.4 are dropped by R1.
>
> However, when you turn off IP Routing on R1, the router essentially behaves
> like a Host (similar to a PC connected to a LAN). It assumes that
> the destination in question (155.1.0.4) is a directly attached host out
> one
> of its LAN interfaces. So, it sends out ARP requests out all FastEthernet
> interfaces for resolving the unknown IP address to a Layer 2 MAC address.
> You can see this in "debug arp" output:
>
> *Mar 1 00:10:14.071: IP ARP: creating incomplete entry for IP address:
> 155.1.0.4 interface FastEthernet0/0
> *Mar 1 00:10:14.075: IP ARP: sent req src 155.1.146.1 cc00.0f44.0000,
> dst 155.1.0.4 0000.0000.0000 FastEthernet0/0
> *Mar 1 00:10:14.079: IP ARP: sent req src 155.1.12.1 cc00.0f44.0020,
> dst 155.1.0.4 0000.0000.0000 FastEthernet0/1
> *Mar 1 00:10:14.099: IP ARP: rcvd rep src 155.1.0.4 cc02.0f44.0000, dst
> 155.1.146.1 FastEthernet0/0
> You can see that no such activity takes place when IP Routing is enabled on
> R1, because router is in fact behaving like a router and thinking, "Well, I
> do not know the IP network to which this host belongs, why should I bother
> looking up for its Layer 2 address?"
>
> Of course, if R1 did have a route to 155.1.0.4, it will also know the
> outbound interface for that route. So, accordingly the ping
> packet's destination Layer 2 address will be set to that of the directly
> attached neighbor on that link.
>
> Hope this helps,
> ~Ahsan
> On Sun, Sep 28, 2008 at 11:15 PM, faisal saleem <faisal2heaven@live.com
> >wrote:
>
> > Hi Ali,
> > You can check my this post
> > ==> http://www.sadikhov.com/forum/index.php?showtopic=126696
> > Here ^^ i made same scenario with 3 routers and there has been enough
> > debate
> > on these simple concepts...
> >
> > HTH.
> >
> > > Hello GS,
> > >
> > > I am testing proxy-arp with the following scenario: R1
> > > fa0/0-------155.1.146.0--------fa0/0 R4 s0/0-------155.1.0.0------s0/0
> R5
> > >
> > > R4 has proxy-arp enabled on int Fa0/0.I am able to ping 155.1.0.4 from
> > R1
> > > only if ip routing is disabled. Would you please explain how proxy arp
> > works
> > > ?
> > >
> > > Regards
> > > Ali
> > >
> > >
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