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938 Views 12 Replies Latest reply: May 11, 2012 2:09 PM by Cristovao RSS

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Arp doubt

May 8, 2012 1:38 PM

Cristovao 6 posts since
Jul 24, 2011

Hi,

 

I feel so lame to have this doubt, so there is the question : Why I haven't connectivity ?

 

The case is simple, I have configured the topology on the picture under and have connectivity between each PC  and respective gateway.

 

The problem is when I tried to ping a device in another network like PC1 to PC0 the next event occur

 

PC1 broadcast arp to know MAC gateway of R0

R0 reply arp to PC1

PC1 send icmp packet R0

R0 broadcast arp to know MAC of PC0

PC0 don't reply because "The ARP request's sender IP address is in a different network than the receiving port."

 

So I want to know why PC0 don't reply ? this is a base functionality of ARP ?

2012-05-08 10.20.49 pm.png

Obviously I've configured 2 static route (one on each router) to route packets to another network on the same interface.

on R1 : ip route 192.168.1.0 255.255.255.0 FastEthernet0/0

on R0 : ip route 172.16.200.0 255.255.255.0 FastEthernet0/0

 

packet tracer file attach.

 

Many thanks for an answer !

Attachments:
  • Rajiv Kumar 217 posts since
    Jan 24, 2010
    Currently Being Moderated
    1. May 8, 2012 2:30 PM (in response to Cristovao)
    Re: Arp doubt

    How can you connect two separate networks with a switch???

  • Jason Robbins 2 posts since
    Sep 18, 2008
    Currently Being Moderated
    2. May 9, 2012 5:37 PM (in response to Cristovao)
    Re: Arp doubt

    You would need a L3 capable device to route between the two networks. 

  • Paul Stewart  -  CCIE Security, CCSI 6,966 posts since
    Jul 18, 2008
    Currently Being Moderated
    3. May 9, 2012 6:30 PM (in response to Cristovao)
    Re: Arp doubt

    Look at the routing table in all of the devices. If no secondary IP addresses are being used, the Layer 3 Routers don't know how to reach each others (and certainly the hosts don't).

  • Pedro 7 posts since
    Dec 18, 2011
    Currently Being Moderated
    7. May 10, 2012 3:56 AM (in response to Cristovao)
    Re: Arp doubt

    Funny scenario.

    First some considerations:

    - ARP is a Frame (layer 2).

    - Broadcasts are stopped by routers.

    - In this scenario the routers are not directly connected therefore "in the middle" the switches MAC's are used.

     

    My explanation is that at this point the router is still trying to figure out the MAC of the other router. So they aren't looking for the IP header yet. They're just looking for layer2 information so any static routes or routing protocolos that may be used are useless.
    You can ignore the PCs and try pinging from one router to the other and check that it won't work either.
    I think the  essence is that the ARP is layer 2, therefore it has no IP address. If there's no IP address no layer 3 routing is done.


    On a network design perspective this a flaw. You are CONNECTING LAYER 3 devices of diferent networks on the same physical network! cant work!!

  • Rajiv Kumar 217 posts since
    Jan 24, 2010
    Currently Being Moderated
    9. May 10, 2012 6:27 AM (in response to Cristovao)
    Re: Arp doubt

    Never seen this topology before. I think pedro is right.

     

    By the way, I don't think switch 2950 is a layer 3 switch.

     

    Anyways, let us know what you find. Good Luck!

  • Michael Taylor 3 posts since
    Jun 26, 2008
    Currently Being Moderated
    10. May 10, 2012 11:28 AM (in response to Cristovao)
    Re: Arp doubt

    The network is behaving exactly the way it is designed. RO's show ip route does not have a route to network 200.  It is dropping the packet. R1 never even sees it.   You could fix it with VLANs and subinterfaces, using dual gateway addresses for each VLAN.

  • Michael Taylor 3 posts since
    Jun 26, 2008
    Currently Being Moderated
    11. May 10, 2012 11:36 AM (in response to Michael Taylor)
    Re: Arp doubt

    Nevermind, I now see that you have static routes. However, still may require a VLAN implementation to work.  Have you tried this on real equipment?

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