can we use a using a routed port as a gateway instead of SVI on a l3 switch ?

[SJ K]

Hi guys,

If I have only a L3 switch and a router.

The common setup would be to have the devices connected to the L3 switch.

Setup a SVI for the VLAN and have the devices's gateway pointing to the SVI.

Then create a routed ported that is physically connected to the router as below

q1) Is there anyway that I can create/use a new routed port as the gateway instead of SVI without adding additional hardware ?

Reason for asking this because I believe in order to use a routed port I would have to add in a L2 switch between the devices and the L3 switch. Have the devices hook up to the L2 switch and connect the L2 switch to the routed port on the L3 switch. Then set the gateway for the devices as the routed port's IP.

q2) In that case, does the routed port need to belong to the same VLAN (100) - as I see in the cisco website that routed port does not belongs to any VLAN.

Please advise.

Regards,
Noob

[Reza Sharifi]

Hi,

In this case, you can use the layer-3 switch as layer-2 and terminate the gateway on the router directly, but you still need to have an interface on the router that has an IP address in the same segment as vlan 100 )192.168.1.1).  If you use a switch as your gateway, you need an SVI and than a layer-3 routed port to the router just like your diagram.

HTH

[SJ K]

Hi Reza,

Thanks for replying.

q1) Assuming i use the l3 switch as a L2 switch and terminate the gateway on the router directly, do we need to configure any VLAN setting on the router interface ?  Do we also need to configure the switchport on the L2 that is connected to the Router to be in the required VLAN ?

q2) Assuming i am going to use my L3 switch as a gateway as well as for InterVLAN routing - does it mean that the only usage of the routed port is for connecting to the router.

What else can routed port on L3 switch be use for ; beside connecting to a router ?

Can I connect a L3 routed port to a L2 switch and have all the devices connecting to the same L2 switch have their gateway as the L3 routed port IP ?

Regards,
Noob

[Reza Sharifi]

Hi,

q1) Assuming i use the l3 switch as a L2 switch and terminate the gateway on the router directly, do we need to configure any VLAN setting on the router interface ?  Do we also need to configure the switchport on the L2 that is connected to the Router to be in the required VLAN ?

No vlan is needed on the router.  If you are connecting only one vlan the switch side will be configured as access port and on the router all you need is one ip.  If you have multiple vlans traversing the same physical interface than you need to trunk the interface on the switch side and use Sub-interfaces on the router with dot1q trunking  This design is called "router on a stick"

http://www.cisco.com/c/en/us/support/docs/lan-switching/inter-vlan-routing/14976-50.html

q2) Assuming i am going to use my L3 switch as a gateway as well as for InterVLAN routing - does it mean that the only usage of the routed port is for connecting to the router.

What else can routed port on L3 switch be use for ; beside connecting to a router ?

Correct, the link between the router and the switch is a transit link.

Can I connect a L3 routed port to a L2 switch and have all the devices connecting to the same L2 switch have their gateway as the L3 routed port IP ?

If that would means the gateway is the router and switch is layer-2 than yes.

HTH

[SJ K]

Hi Reza,

Thanks for coming back.

What i meant for Q2 is

q2) Assuming i am going to use my L3 switch as a gateway as well as for InterVLAN routing - does it mean that the only usage of the routed port is for connecting to the router.
What else can routed port on L3 switch be use for ; beside connecting to a router ?

Can I connect a L3 routed port to a L2 switch and have all the devices connecting to the same L2 switch have their gateway as the L3 routed port IP ?

That I will added in a L2 switch, the L3 switch's routed port will act as the gateway.

Device --> L2 switch --> (routed ported) on L3 switch --> (another routed port) on L3 switch ---> Router.

Can it works that way ?

Regards,
Noob

[Jon Marshall · Accepted solution]

Yes it will work but only if you have just one L2 switch.

If you had multiple switches with the same vlan connecting to the same L3 switch then it obviously wouldn't work for two reasons -

1) traffic within the same vlan could not go past the L3 port ie. the vlan/IP subnet terminates on that port

2) more obviously you cannot have multiple L2 switches connecting to different ports all sharing the same physical L3 port as their default gateway.

Jon

[SJ K]

Hi Jon,

Thanks for your reply !

1) traffic within the same vlan could not go past the L3 port ie. the vlan/IP subnet terminates on that port

Not sure if i understood clearly - what if I have only 1 L2 switch connecting to the L3 switch's routed port and the rest of the L2 switches are daisy chain up or stack together.  Does it solve the problem ?

Regards,
Noob

[Reza Sharifi]

Referencing your diagram, if the L2 switch has only one vlan (we call it vlan 10)  you can make the connection between the L2 switch and L3 switch an access port and the SVI on the L3 switch. 

If the L2 switch has multiple vlans, (10, 20) you need to make the connection between the L2 switch and L3 switch trunk and all SVIs will be on the L3 switch (common design). If you want to keep the port as an access port than you need a physical connection per vlan.  This is not a common design as it does not scale.

If you add a second or 3rd switch, it follows the same logic.  If the 2nd or 3rd switch have other vlans (30, 40) you also need a trunk port from each switch to the L3 switch and SVIs on L3 switch.

Is that clear?

HTH
 

[SJ K]

Hi Reza,

Cleared.  However, for my diagram, it is using a routed port on the L3 switch (192.168.1.1)

Referencing your diagram, if the L2 switch has only one vlan (we call it vlan 10)  you can make the connection between the L2 switch and L3 switch an access port and the SVI on the L3 switch

Do you mean -> an access port on the L2 and a routed port on the L3 ?

Regards,
Noob

[Reza Sharifi]

Do you mean -> an access port on the L2 and a routed port on the L3 ?

correct, the L2 switch is purely layer 2 with no IPs (just vlan) and the L3 switch has both the vlan and the SVI (IP).

HTH

[SJ K]

Hi Reza,

Sorry i am confused, i am not going to use any SVI on the L3 switch (except for the management vlan). 

As per my diagram, there will be 2 routed port, 1 acting as the gateway for the L2 switch and the end-devices and another 1 connecting to the router as shown in my diagram above.

Did i get anything wrong ?

Regards,
Noob

[Reza Sharifi · Accepted solution]

Hi,

lets break it down to 2 questions:

As per my diagram, there will be 2 routed port, 1 acting as the gateway for the L2 switch

Correct, the gateway for your PC is 192.168.1.1 witch will be on the L3 switch using an SVI and SVI has an IP address of 192.168.1.1. Since this is a vlan gateway you have to have to an SVI and can't be a point-to-point routed interface.

The 10.10.10.1 and 2 addresses that connect the L3 switch to the router is a routed Point-to-point interface, meaning no need for SVI.

HTH

[SJ K]

Hi Reza,
 

Thanks for replying.

Why can't we use a routed port on the L3 switch as the gateway for the end-devices ? Why must it be an SVI ?

Regards,
Noob

[Jon Marshall]

You can use a routed port if you want, it's just not that scalable.

If there is only one L2 switch or there are multiple switches daisy chained with only one vlan then it will work.

If you connect multiple L2 switches to different ports on the L3 switch then you need an SVI if the same vlan is on multiple switches.

By far the commonest setup is to use SVIs on L3 switches, I have never yet used a L3 port oin a L3 switch as a gateway for clients in a vlan but if you only had one vlan and you wanted to do it this way you could.

Jon

[SJ K]

Hi Jon , Reza,

Thanks and duly noted. It is just that i am thinking of what else can a routed port be put to use beside being a point to point transit link for the actual router.

Thanks again for all the advices and validation.!

Regards,
Noob