Re: diffrent Subnet IPs on same VLAN and Proxy-Arp

From: Fahad Khan (fahad.khan@gmail.com)
Date: Tue Oct 21 2008 - 15:31:27 ARST

Raheel,

I was explaning specifically in the presence of static route. You would not
have any problem. while Huan's point of view is valid as well.

regards,

On 10/21/08, Raheel Itrat <syedraheel@gmail.com> wrote:
>
> Nice explainations Huan!
>
> Fahad !
> Yes I was using dynamips for this c3725-adventerprisek9-mz.124-15.T with
> the
> nm-16 sw module in it for the switch and c3640-jk9o3s-mz.124-5a for the
> routers and what would you say about Huan's explainations?
>
> On Tue, Oct 21, 2008 at 1:03 PM, Huan Pham
> <Huan.Pham@peopletelecom.com.au>wrote:
>
> >
> > Hi Raheel,
> >
> > Marko's soluton are surely valid options.
> >
> > In addition to static ARP, and mobile IP, we can also disable routing
> > all together on both routers, or you can have a secondary IP of each
> > other subnet, and they should be able to ping each other. No need any
> > static route for both cases, obviously, as when you disable routing the
> > routers will only act like hosts, without routing functionality. When
> > you have the secondary IP configure, the routers have the route to each
> > other already.
> >
> >
> > I would just like to add some input as to why these are needed.
> >
> > By default, when routing is enabled, routers will not respond to arp
> > requests from stations whose source IP addresses are not on the IP
> > subnet that receives the request, regardless of whether proxy arp or
> > local proxy arp is enabled or not.
> >
> > When R1 try to ARP for R2 IP address, R2 will see the ARP comes from
> > 101.1.1.1 on the interface that has IP of 102.1.1.1/24, and will ignore
> > it.
> >
> > R2#
> > *Mar 1 06:39:11.166: IP ARP req filtered src 101.1.1.1 0000.1111.1111,
> > dst 101.1.1.2 0000.0000.0000 wrong cable, interface FastEthernet0/0
> >
> > To disable this sanity check, you will have to use any of the three
> > solutions above
> >
> > 1. Static ARP
> >
> > R2#
> > arp 101.1.1.1 0000.1111.1111 arpa
> >
> > 2. Local LAN Mobility
> >
> > R2#
> > interface FastEthernet0/0
> > mac-address 0000.2222.2222
> > ip address 102.1.1.1 255.255.255.0
> > ip mobile arp
> >
> > See the debug message that R2 now does not complain any more about ARP
> > from wrong subnet!
> > R2#
> > *Mar 1 06:43:15.334: IP ARP: rcvd req src 101.1.1.1 0000.1111.1111, dst
> > 102.1.1.1 FastEthernet0/0
> > *Mar 1 06:43:15.334: IP ARP: sent rep src 102.1.1.1 0000.2222.2222,
> > dst 101.1.1.1 0000.1111.1111 FastEthernet0/0
> >
> >
> > 3. Disable routing on both routers.
> >
> > 4. Configure secondary IP address.
> >
> >
> >
> >
> > The Local Proxy Arp or Proxy Arp are not required here.
> >
> > - Local proxy arp is used, so that a router can work as proxy for two
> > hosts on the same subnet, which normally can not communicate directly
> > (e.g. Private VLAN or on protected port).
> >
> > - Proxy arp (on by default) is used so that router responds on any ARP
> > for subnets outside LAN subnet (e.g. when you have a default static
> > route on R1 pointing to e0/0, instead of R2 LAN IP, you will need to
> > enable R2 proxy arp (on by default).
> >
> > As said above, both these options does not work for you, because you
> > have a different IP subnet on the two routers, and by default, routers
> > just ignore ARP request from each other.
> >
> >
> >
> >
> > Hope this helps,
> >
> >
> >
> >
> >
> > -----Original Message-----
> > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> > Marko Milivojevic
> > Sent: Tuesday, 21 October 2008 12:40 PM
> > To: GS CCIE-Lab
> > Cc: Raheel Itrat
> > Subject: Re: diffrent Subnet IPs on same VLAN and Proxy-Arp
> >
> > Enable ARP debugging on the routers or even better, sniff the traffic on
> > both routers and it should be pretty clear why ping fails.
> >
> > As to the solution for your problem, you may want to look into static
> > ARP as a very crude solution, or mobile ip as a relatively neat one
> > :-)
> >
> > --
> > Marko
> > CCIE #18427 (SP)
> > My network blog: http://cisco.markom.info/
> >
> >
> > Blogs and organic groups at http://www.ccie.net
> >
> > _______________________________________________________________________
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-- 
Fahad Khan

Blogs and organic groups at http://www.ccie.net

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