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Connecting two Routers/switches together

Posted on 2010-04-07
Medium Priority
Last Modified: 2012-08-14
When you hook up two devices what options do you have?

L2 to L2
L3 to L3
L3 to L2

trunk dot1q or isl to another trunk
access port to access port
access port to trunk

What is recommended?

This is an area I need help to understand. A shove in the right direction would be greatly appreciated.
Question by:Dragon0x40
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Accepted Solution

Nayyar HH (CCIE RS) earned 2000 total points
ID: 30058963
I dont think you should consider connecting access to trunk as an option. Technically, it can work but across a single VLAN only so there's really no point in doing so. Access-to-Access or Trunk-to-Trunk is more appropriate.

With regarding to what options you have? Usually this is either all of the above or a subset of the above, a major factor is the devices capability.
For example

>With a L2 switch connecting to a L2 Switch you are only able to use trunk to trunk or access to access. Since there is no layer 3 support on these switches.

>With a multilayer switch connecting to a L2 switch you are only able to use L3 to access, access to access, trunk to trunk

>With a multilayer switch connectint to a L3 switch you have all optons available i.e. L3 to L3, L3 to access, trunk to trunk, access to access

Router Interfaces as you know are L3 interfaces but can also be configured to trunks. Also, switch interfaces as you know are L2 interfaces but if its a multilayer switch, interfaces can also be configured as L3 interfaces.

There is really no recommended way of connecting, but here are a few things that should aid you making your judgment.

- Device capability
- Scalability
- IGP design
- Redundancy
- Logical & Physical topology design


Author Comment

ID: 30060458
thanks nazsky,

Somehow it does not seem  right to connect an L3 port to an L2 access port. Should the L3 port have an ip address and act as a gateway for the L2 port?

It also seems strange to have trunking on an L3 port. Although I know subinterfaces in router on a stick do have dot1q enabled from what I remember.

What I am familiar with is L2 trunk ports connecting to L2 trunk ports and then an SVI as the gateway and routing between vlans. This may be the access and distribution layers but when we connect to the core and WAN it seems to be more L3 to L3.

Can you clarify this some more?
LVL 15

Assisted Solution

by:Nayyar HH (CCIE RS)
Nayyar HH (CCIE RS) earned 2000 total points
ID: 30063173
If you see the routers L3 port just as any other L3 capable device such as a PCs, you should see it appropriate. Yes, using the L3 port as a gateway for the VLAN it connects to is certainly possible.

Yes, sub-interfaces are used along with tagging/encapsulation to form trunks to layer-2 devices. Using this method can be a very cost efficient solution, it enables you to reduce the amount of physical iterfaces you would need on your router if they all can be carried on one trunked 1Gbps interface.

Yes, this is an option as well. I see what you mean, in this secnarion there are several reasons for this here are some:
> Because VLANs must exist on your access and distribution switches, you must trunk between the two device to achieve this.
> Also, there is no need for access vlans to be available on core devices hence we would only span vlans upto distrubution switches. In most cases it isnt really good to extend your Layer2 domain into your Core. The core should be able to route/switch packet as fast as possible and re-converge from single points of failures quickly. I would recommend relying on Layer3 Network protocols in the core layer as opposed to Layer2 protocols.

Remember, trunking across multiple switches introduces the potential of STP being needed. Were as connecting via L3 ports introduces the potential of IGPs being required.

Please feel free to clarify further i wasnt so clear.


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