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Webcast-Catalyst9k
New Member

Basic Question

Hi, my question was on basic lan communication and how a workstation talks to a workstation on its local subnet and if the host it wants to talk to is not local what would it do. Can someone give a detailed explanation on this?

Than my next question was if we have 2 workstations that are hanging of the same ethernet interface of a router and the router interface is configured with 2 ip addresses (primary (let's say: 10.1.1.1/24)and a secondary (30.1.1.1/24)) and one workstation has a address in the primary ip subnet (let's say: 10.1.1.10/24) and the second workstaion has a ip in the secondary subnet (30.1.1.10/24) and the workstations do not have a default gateway defined would they be able to talk to each other. Appreciate the help. Thank you

3 REPLIES
Cisco Employee

Re: Basic Question

It should...due to proxy-arp

http://www.cisco.com/warp/public/105/5.html

New Member

Re: Basic Question

What if there was no proxy arp configured. Also could you please explain the process by which the workstation would determine how to get to the segment. I mean arping for the mac or sending a broadcast and when would it do so. I mean after it has determined if the subnet is local or not by performing the boolean with the mask and the ip address. Thank you

New Member

Re: Basic Question

Strict IP should work as follows, assuming the workstation is configured according to the subnet it actually resides on (No supernetting or anything).

When Workstation attempting to send to destination.

1) Determine if Dest is local (Using IP and Mask)

2) If Dest is local (according to IP and Mask)

a) the workstation will look to its local Arp cache and if it finds the Dest IP it will construct the packet and send it.

b) if the Dest IP is not in the cache the workstation will ARP for the Dest, if it receives a reply it will add the Dest to its cache and construct the packet with the new information it received.

c) if the Dest IP is local (IP and Mask) and no ARP information can be obtained the process ends and a Destination Unreachable message may be returned from the protocol stack.

3) If the Dest is not local (IP and Mask) the packet is forwarded to the defined gateway. If no gateway is defined and the Dest is not local the process will end at this point.

That is how it is supposed to work in theory. So in your scenerio, the two workstations would be unable to "see" each other using IP.

TH

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