From: Ali Mousawi (mousawi.ali@gmail.com)
Date: Tue Sep 30 2008 - 05:45:38 ART
Thanks alot guys that was really good explanation ;) .. my exam in 10 days
(2nd attempt) i am did prepare as i did for the first one just read some
blogs,DocCD minisenarios and this amazing group study...
On Mon, Sep 29, 2008 at 11:58 AM, Andrew Dempsey <apdccie@gmail.com> wrote:
> 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 155.1.0.0/24in
>> 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 155.1.0.4from
>> > R1
>> > > only if ip routing is disabled. Would you please explain how proxy arp
>> > works
>> > > ?
>> > >
>> > > Regards
>> > > Ali
>> > >
>> > >
>> > > Blogs and organic groups at http://www.ccie.net
>> > >
>> > >
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>> > >
>> >
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