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New Member

Two VLANs, same Subnet, no IP Routing; Communication dilemma

Here is the scenario:

Part-A: Host 192.168.3.10(vlan 2) is able to ping host 192.168.3.30 (vlan 3) WITHOUT layer-3 routing being enabled on 3560. The reason behind that is when the host 192.168.3.10 generates an ARP request broadcast looking for host 192.168.3.30 as target, the frame is received on VLAN 2 and is broadcasted out on all ports member of VLAN 2 but the receiving port. So, in our scenario, broadcast gets sent out fa0/24 of SW-1 and will be injected into VLAN 3 of Sw-2 via fa0/24 of SW-2. This broadcast will further be forwarded by SW-2 and will eventually reach the target. Target will send ARP req unicast and the return unicast mac of host 192.168.3.10 is being known on SW-2 to be on fa0/24 member of VLAN 3. SW-2 will forward the arp reply unicast out fa0/24 an SW-1 will receive it on its fa0/24 member of VLAN 2 and SW-1 knows that the mac of host is actually on fa0/1 and this completes the switching.

Part-B: Host 192.168.3.10 is not able to communicate with 192.168.3.20 because SW-1 never even sends out the arp broadcast outside of VLAN 2, so fa0/2 a member of vlan 3 on SW-1 will not see ARP request as request is only being broadcasted out ALL vlan 2 ports.

Part-FINAL: Now, here is the confusing part:

Why is the host 192.168.3.10 not able to communicate with int vlan 3 (192.168.3.1/24)  created on SW-2? How is it any different than Part-A? Frames will be broadcasted out fa0/24 of Sw-1 and will be received on fa0/24 of SW-2 a member of VLAN 3 and then SW-2 is forwarding the broadcast out all its physical ports that are member of VLAN 3. Why SVI 3, being like a host on VLAN 3, not ever receive it? I am lost? Why cant host 192.168.3.10/24 communicate with SVI 3 (192.168.3.1/24)?

 

Please help me finding the reason for Part-FINAL. I have also attached packet tracer topology here.

Everyone's tags (4)
2 REPLIES
New Member

Hi,Are you getting ping from

Hi,

Are you getting ping from host 192.168.3.30 to 192.168.3.1..?

 

Regards,

Naisam

 It is due to proxy-arp.

 

It is due to proxy-arp. Below discussion give you details.

 

https://learningnetwork.cisco.com/thread/60350    or

 

http://www.cisco.com/c/en/us/support/docs/ip/dynamic-address-allocation-resolution/13718-5.html

 

hth

MS

 

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