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vlan and layer 3

Posted on 2013-01-22
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Last Modified: 2013-02-06
I saw 2 configurations that give the same result. I just want your inputs on which one is the best practice.

Scenario 1:
- A Cisco 2921 router (Fa0/1) connects to a 3750 switch (Fa0/2).
- Fa0/1 is configured with 10.10.10.1/24
- Fa0/2 is configured as an access port on vlan 1
- On the switch, vlan 1 is configured with ip address 10.10.10.2/24
- On the switch, default gateway pointed to 10.10.10.1

Scenario 2:
- A Cisco 2921 router (Fa0/1) connects to a 3750 switch (Fa0/2).
- internal subnet is 10.10.10.0/24
- Fa0/1 is configured with 10.10.20.1/30
- Fa0/2 is configured with 10.10.20.2/30
- On the switch, default gateway pointed to 10.10.10.1
- on the router, next hop for 10.10.10.0/24 is 10.10.20.2

Thanks
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Question by:biggynet
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ddiazp earned 668 total points
ID: 38808324
Scenario 1 is what you want. Maybe it's just my preference but working with vlan interfaces is much more flexible, easier and more widely accepted.
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by:rauenpc
rauenpc earned 668 total points
ID: 38808512
If I'm reading that right, the difference between the two is a routed subnet between the switch and router, versus the Router being on the same subnet.

If you only have the single subnet and never go beyond it, then it's a wash because everything talks directly on the same layer two network.

If you end up with more internal subnets, then there can be a difference. With a single subnet, the default gateway for all devices will be the router. The ip on the switch really only provides management. When you have multiple subnets, all devices need to use the switch as a default gateway to get the best performance (the switch will use the router as a default gateway). This way all internal traffic is L3 switched which is much better than making a single FA link on the router do all the internal routing. An odd situation can happen when you have multiple subnets, and the router exists on one of the subnets that other devices, aside from the switch, reside. If any of those devices were to use the router as a default gateway while other devices were using the switch, you end up with split routing and users will not get consistent performance. Using the separately routed interface/vlan ensures that split routing can't happen.

In the end, I try to use the separately routed interface/vlan whenever possible even if only one vlan exists. It requires a relatively small amount of effort, and it lays the ground work for when additional vlans are needed.
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by:Sandeep Gupta
Sandeep Gupta earned 664 total points
ID: 38809672
second scenario is straight forwad ..you connect two boxes directly and give the IP on interface.

In first scenario..both the interfaces are directly conncted and it is a property of vlan you applied ..vlan can be used as L2 as well as L3. here you uses vlan as L3 by giving it ip.

you applied vlan 1 as access so that port and link property do not change..

if you apply vlan 1 on port as trunk ..it will not work.
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