ROUTER ON A STICK WITH 2 SWITCHES

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which topoplogy is the right one? 2 switches connected to a router or a switch connected to the router and the other switch connect to the switch?

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Posted by WebUser Charalampos Skeleton C'trattou from Cisco Support Community App

[Sandeep Choudhary · Accepted solution]

Hi,

You can connect 2 swicth to router:

When you create vlans, you are essentially creating seperate broadcast domains.  Generally when you create seperate broadcast domains, you are creating different subnets.  A layer 2 switch doesn't know how to forward traffic destined for a different broadcast domain.  You need a layer 3 device to route traffic between your vlans(broadcast domains).  I would think that ROAS is just the implementation of intervlan routing when there is no layer 3 switch available.

see the example:If you are configuring on Router

Create sub-interfaces, set 802.1Q trunking protocol and ip address on each sub-interface

Router(config)#interface f0/0

Router(config-if)#no shutdown

(Note: The main interface f0/0 doesn’t need an IP address but it must be turned on)

Router(config)#interface f0/0.0

Router(config-subif)#encapsulation dot1q 15

Router(config-subif)#ip address 192.168.15.1 255.255.255.0

Router(config-subif)#interface f0/0.1

Router(config-subif)#encapsulation dot11 20

Router(config-subif)#ip address 192.168.20.1 255.255.255.0

(Note: In the “encapsulation dot1q 15″ command, 15 is the VLAN ID this interface operates in)

Configure VLAN

Switch(config)#vlan 15

Switch(config-vlan)#name SALES

Switch(config-vlan)#vlan 20

Switch(config-vlan)#name TECH

Set ports to access mode & assign ports to VLAN

Switch(config)#interface range fa0/1

Switch(config-if)#no shutdown

Switch(config-if)# switchport mode access

Switch(config-if)# switchport access vlan 15

Switch(config-if)#interface range fa0/3

Switch(config-if)#no shutdown

Switch(config-if)#switchport mode access

Switch(config-if)# switchport access vlan 20

Hope it helps.

Regards

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[Joseph W. Doherty]

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Posting

Not enough information to say which is "right".  Either could work, much depends on capabilities of switches and router, what you're trying to do, and perhaps physical topology issues.

Everything else being equal, the router connected to both switches might allow for better performance.

[Mark3205]

Hi,

I'm hoping for a bit of advise relating to the above topic.

I have been tasked to reconfigure 2 switches( 2x2960) and 1 router (ASR1001) that are currently connected with a ROAS config with only one trunk between the ASR and SW1 and another trunk between SW1 and SW2. Each SW has multiple vlans and the connected devices need to be able to communicate with each other in different vlans.

i would like to be able to connect a second trunk from SW2 to the router and maintain the trunk between SW1&2 and implement spanning tree to avoid any L2 loops. 

The problem i am running into is the ip addressing for the sub-int of the second trunk. researching it on the cisco forums suggest that bridging would be a better option.

I have looked at creating a bridge group but the ASR will not accept the following code.

bridge irb

interface GigabitEthernet0/0.20

  bridge-group 20

interface GigabitEthernet0/1.20

  bridge-group 20

It says that the sub-int bridge-group cmd is unrecognised.

Is it the router or me.

Any suggestions welcome.

Thanks