Cisco Router ISR

Unanswered Question
Sep 26th, 2010

How does this router allows host with IP addresses of same subnet to connect to router's interface and communicate?

When I connect two hosts present on different VLAN with different IP addresses to the router, how are they communicating?

In which case bridging is taking place and which case routing is taking place?

I apologize if the question seems to be very trivial.


Sagar K.

I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (1 ratings)
Mahesh Gohil Sun, 09/26/2010 - 22:59

Hi sagar,

Router is having a one routing table only no matter it belongs to which vlan they all are inter communicating with each other.

this is the reason we integrate router for inter vlan communication.

If you want to keep broadcast domain different for two vlan on router level you can use vrf-lite concept. It is like virtual routing table for two vpn.



Sagar Karale Mon, 09/27/2010 - 23:25

Thanks Mahesh for the reply.
But my question is somewhat different.

As I understand each interface of router has a different subnet connected to it and router does not accept IPs of same subnet connected on two different interfaces.

In article:

They have shown in the IRB sample configuration, Network Diagram, PC1 and PC2 have IPs & respectively. How is that possible on router's interface?

My second is question is altogether a different scenario.

I have two VLAN 1 and VLAN 2. Host from VLAN 1 is connected to one interface of ISR router and host from VLAN 2 is connected to another interface on ISR. I am able to ping from PC1 and PC2.

Now this is because of which behavior of router? Is bridging taking place here or routing? What is exactly happening in side router whn my ping reaches router? Is the VLAN header, MAC header are stripped and new headers are appended to the IP packet?

Sagar K.

Mahesh Gohil Tue, 09/28/2010 - 00:55

Hi Sagar


Yes it is entirely possible to use sabe subnet on two different interface with help of bridging concept.

bridging take place when you communication happens between same subnet and

routing take place between two different subnet within router.

As you must have read in this document they created bridge group 1 and assigns this group to interface E0 and E1 to enable bridging between

two interface. also please see the BVi interface where IP subnet is given which is to be shared between E0 and E1 .

If you are not using this IRB functionality router will give you error message of duplicacy of IP's



Lei Tian Tue, 09/28/2010 - 03:42


Regarding your 2nd question.

Assuming both PCs have gateway configured properly, and have the arp entry for gateway.

1,PC1 will send ICMP with destination IP to PC2 and destination MAC to gateway.

2,Router receive this ICMP from vlan1, it will do routing lookup (will be CEF lookup most time, but the concept is same). The destination subnet is a connected route in routing table, the router will route the ICMP to vlan 2, and do L2 rewrite.

3.1,If router has the arp entry for PC2, the router will send out ICMP source IP from PC1, source MAC from vlan 2, destination IP to PC2, destination MAC to PC2.

3.2 if router doesn't have the arp entry for PC2, it will first send ARP to vlan 2 asking who is PC2 tell me your MAC. After it receives the reply, it will follow same process in 3.1.

4. Once receive the packet, PC2 will send ICMP reply back to PC1 with destination IP to PC1, destination MAC to gateway. The ICMP reply packet will follow same steps to reach PC1.


Lei Tian


This Discussion