VLAN accross routers

[AJ1982]

Hi there,

I have a classroom setup of 3 routers in a triangle. Of each router I have a 2950 Switch. I would like to know what the technique is of placing the same VLAN on mutliple switches.

Eg, VLAN101 is present on switches 2 and 3, they both then get the same IP Address.

I have the example of routing multi VLANS onto one router for one SWITCH, but is there any term for multi switches and routers, sampe configs great.

Cheers AJ

(They dont teach VLAN routing on the CCNA)

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Fatman Superstar (Andrew James)

CCNA,

 

[bierhunter]

First, lay out the design of what you want to configure.

VLAN = virtual interface on the router. But...not all VLANs equal the same subnet. For example, VLAN101 for company-A maybe 10.10.0.0/24 where Company-B has VLAN101 equal to 172.17.0.0/24. So VLANs can be set to whatever subnet you want them to be. The VLAN is the router's virtual interface which then has a subnet assigned to it.

Now we get to how the routers know what VLANs are where. To a router, a VLAN is an interface with a subnet assigned to it just like a physical interface. The switch then assigns ports to the VLAN 'interface' on the router. The router worries about the subnets. The switch worries about which port belongs to which VLAN.

Think of each router/switch combo as its own network. Within that network, you can have one VLAN or multiple VLANs. Router to router, you aren't going to care about VLANs. You are going to be concerned with subnets.

For example:

RouterA is connected to SwitchA using VLAN101 and subnet 10.10.0.0/24.

RouterB is connected to SwitchB using VLAN101 and subnet 10.10.0.0/24.

In this case, traffic within each subnet is fine, however it cannot pass to the other. The router sees the subnet on its own interface, so it will not send the packet out.

If VLAN101 on RouterB/SwitchB was 10.10.1.0/24 you'd be ok.

Before I start babbling, I guess the simple answer is this.

Yes...you can have the same VLAN on each router/switch combo. However, the subnets assigned to those VLANs need to be different if you want them to talk to each other without NAT.

The routers won't see each other's VLANs. They will see each other's subnet addresses. Then you just have to think in terms of basic routing principles.

I probably made that clear as mud.

 

[1Drisnil]

Hey FatManSuperStar,

What version of CCNA did you take; v 2.1.4 cisco curriculm? Version 3.0 of Cisco's curriculm now includes 2950 switches , trunking and VLANs.

Go figure!

"...understanding "The Why" is half the battle!" - Anonymous.

 

[AJ1982]

I scrape through the version 3 exam (640-801) they didnt include OSPF and EIGRP on the original as well,


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Fatman Superstar (Andrew James)

CCNA,