InterVlan Routing: Which is better?

Unanswered Question
Oct 1st, 2007

We have something like this:


Router

|

|

L3-Switch

|

|

L2-Switch


I came up with following two ways to configure router and switches to achieve InterVLAN routing. Please tell me which way is better and why? Please also let me know if you know better way.


Thank much,


Method 1:

Router's ethernet port connected to L3-Switch is confgiured with 172.16.1.1/24


L3-Switch is connected to Router via L2 port. VLANs were configured as follow:

int vlan 1

ip address 172.16.1.2 255.255.255.0

int vlan 100

ip address 172.16.2.1 255.255.255.0

int vlan 200

ip address 172.16.3.1 255.255.255.0

***eigrp and default route was configured appropriately


L2-Switch is connected to L3-Switch via a trunk port. Vlan 1 (just for management) was configured:

int vlan 1

ip address 172.16.1.2 255.255.255.0


Method 2:

Router's ethernet port connected to L3-Switch is confgiured with 172.16.1.1/24


L3-Switch is connected to Router via L3 port and address is 172.16.1.2. VLANs were configured as follow:

int vlan 100

ip address 172.16.2.1 255.255.255.0

int vlan 200

ip address 172.16.3.1 255.255.255.0

***eigrp and default route was configured appropriately


L2-Switch is connected to L3-Switch via a trunk port.

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Overall Rating: 4 (2 ratings)
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Edison Ortiz Mon, 10/01/2007 - 20:03

If you are planning to use Vlan1 for other hosts, option 1 is the correct choice.


In option 2, you are wasting a whole class C on a point-to-point connection.


BTW, on option 1, the IP address on L2-Switch must be different - try 172.16.1.3



Jon Marshall Mon, 10/01/2007 - 22:55

Hi


As Edison says if you use vlan 1 for any clients then you need to look at option 1.


I would not use vlan 1 for management, use any other vlan that is not in use for any clients/servers.


It's difficult to go with either option without a full description of topology but i would tend to do all inter-vlan routing on the switch. This means that the link between the router and the L3 switch would be a L3 point-to-point link. As Edison says, this would waste a whole class C in your addressing scheme but


1) it needn't if you subnet it down to a /30

2) It is private addressing anyway so hopefully you should have enough spare addressing.


I don't think their is a right/wrong way to do this to be honest.


Jon

smallrain_2 Tue, 10/02/2007 - 04:38

Hi Jon, Edison;


Thank much for your comments.


PS. L2-Switch's IP in options 1 was a typo

Joseph W. Doherty Tue, 10/02/2007 - 04:38

In general, when doing LAN routing, try to do it on L3 switches since many are often considerably faster than "pure" routers.


You should not need to trunk the VLANs to the router unless you wanted to do another variation for redunancy. A redunancy design would have both your L3 switch and router as HSRP gateways on all VLANs, the L3 switch being primary. If the L3 switch fails, the router would take over inter VLAN routing. (This only works as long as router isn't connected to the L3 switch. Also, depending on the physical topology, a trunk alone from the router may be insufficient.)

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