Cisco Support Community
Community Member

Subnet routing question

I have a theoretical question. For example, one router - two ethernet ports - static routes - with IP addresses for the same subnet on each port. How would routing happen for a specific subnet, same subnet on two ports? Which port would be routed if a packet was destined for a subnet which is attached to two ports?


Re: Subnet routing question


U r question is not clear.can u please eloborate it with the topology design,it will help us to understan it better.



Community Member

Re: Subnet routing question

I will try. There would be a router (IDDN 1) with two ports each connected to a switch (ADLE & ASC) via dsl modems. The switches are also connected via dsl modems. So, it looks like a triangle with a router at the top and each of the other legs to a switch. The question is basically how would a packet from/through one of the switches get to the router since both ports at the router connect to effectively the same subnet? Through the direct path (static route) or through the other switch to the other port on the router. No specific routing protocols are used (RIP or OSPF for example).

Re: Subnet routing question

HI, [Pls Rate if Helps]

Your Question is not Clear but anyhow if my undestanding is not wrong,

2 - Ports in one Router

Both are in Same Subnet

Static Route exist in Router

For eg:

IF you have created 2 static route means there is a possibility of Load Balancing (if per packet or per destination is enabled) / failover techniq...

If only one static route means which port with Higher MAC address will be choosen for Routing of Traffic.....

You can verify the same using "show ip route" command shows the routing table informaiton.

Pls Rate if Helps

Best Regards,

Guru Prasad R

Hall of Fame Super Gold

Re: Subnet routing question


There is a basic consideration of Cisco routers that impacts your question. Cisco IOS will not allow 2 routed LAN interfaces in the same subnet. If you want to connect two LAN interfaces on the same router in the same subnet then you must configure bridging on the interfaces and configure Integrated Routing and Bridging with a BVI to make this happen. What would then happen is that spanning tree would operate on the router interface (at the head of the triangle) and it would put one of the interfaces into blocking mode. As a theoretical question it may be interesting but as a practical matter you can not have 2 active interfaces on that router routing into that subnet.



CreatePlease to create content