Routing between VLANs

By edepa ·

I am researching this topic before drafting a design and would like some guidance.

Reduced to its simplest, the problem is this. I have two Cisco Catalyst L2 switches (switch SA and switch SB) and one Cisco 1811 router (router RA). All three devices support VLANs with 802.1Q encapsulation.

The VLANs will be port-based on both switches. The number of VLANs is irrelevant but it should be taken that there are more than two to make this scenario sensible. It will be necessary to route IP between VLANs on SA and VLANs on SB, using RA.

In so far as connections between switches and routers are concerned, in order to achieve this routing, I understand that the switches should be connected to the router using trunk ports. I understand that the advantage of this method over that of connecting the router to the switches using access ports on the switches is that the former scales better to handling several VLANs.


This conversation is currently closed to new comments.

Thread display: Collapse - | Expand +

All Answers

Collapse -

This might help with your problem for "Routing between VLANs"

Collapse -

Rouring between VLANs

by edepa In reply to This might help with your ...

Thank-you for replying. I had read that article. While it provides some insights, I understood that article to apply to the problem of physically setting up routing between two L3 devices (such as the Catalyst 3000 devices) that implement VLANs. Individually, these devices are themselves capable of routing between the VLANs configured and the ISL/802.10 protocols are used to achieve inter - L3 device routing of the VLANs.

What I had a problem with, but now believe to have solved, is the the physical connection between an 802.1q-compliant router and 802.1q - L2 devices. Since then, I found that the router should be connected to the trunk ports on the switches and subinterfaces defined on each router interface that is connected in this way. I found these two articles, particularly the wikipedia one, very helpful:



Collapse -


by Jellimonsta In reply to Rouring between VLANs

Sounds like you got it.
You would trunk the port to the router from the switch.
Then define the subinterfaces on the router.

int fa 0/1
desc Trunk to Router-1
switchport trunk encaps dot1q
switchport mode trunk
switchport trunk native vlan <primary vlan>


int fa0/0.1
ip add

int fa0/0.2
ip add

int fa0/0.3
ip add

Don't forget to make the IP of the main router sub interface (main VLAN) your default gateway on the switch.
Good luck.

Collapse -

Thanks a million!

by edepa In reply to Yup...

Grand Jellimonsta, thanks for your detailed reply.

It was the wikipedia article that pinned down the picture.

I hope to implement by the end of August (still have to purchase the devices).


Collapse -

No problem.

by Jellimonsta In reply to Thanks a million!

Good luck with the config! :)

Related Discussions

Related Forums