BCMSN Lab - Building the BCMSN lab

Step 1 - Building the BCMSN lab in GNS3

Note that if you try connecting two switches in GNS3, you are instantly prompted with an error stating 'can't connect these devices'.

You are receiving this error because GNS3 cannot emulate a Cisco switch. *Gasp* *Horror* *Shock*! Heh, it's true. GNS3 uses the dynamips/dynagen engine as mentioned earlier and therefore the same functions won't work here. A workaround to this is to use an NM-16ESW network module in a 3640 router to emulate a switching environment.

Basically the NM-16ESW module is a Cisco EtherSwitch Module designed for Integrated Services Routers. So what exactly is an EtherSwitch model you ask ehy... ? Well "Cisco EtherSwitch modules for Cisco Integrated Services Routers offer branch office customers the option to integrate switching and routing in one platform. Combining robust Layer 3 flexible WAN routing with low-density line-rate Layer 2 switching, the EtherSwitch modules provide straightforward configuration, easy deployment and integrated management in a single platform." quotes the Cisco site. Sounds something awfully close to a layer 3 switch, except the latter is designed towards inter-vlan routing at wire speed (not WAN) . Also a Layer 3 switch is a switch with a router inside whereas this effectively adds an Etherswitch module to a router.

Anyhow, if you are interested in knowing more about the NM-16ESW module, please visit the following link: http://www.cisco.com/en/US/prod/collateral/routers/ps259/product_data_sheet09186a00801aca3e.html

Having said that let's now move on to step two which is knowing how exactly do we do this in our virtual lab?

Step 2 - Adding the NM-16 EtherSwitch Module to a Cisco router

-Let's add three c3600 routers that shall eventually be converted to a 'switch' for the purpose of setting up the BCMSN lab in GNS3.
Note you can use any router you want. The c3600 is designed for small-to-medium scale businesses and therefore is one that isn't too strong on the RAM.

-After adding them in GNS3, Right-click on the router and change the hostname from 'R1' to 'S1' in-order to differentiate the switches from the routers in your lab. Do the same for R2 and R3.

-Before connecting all the 3 'switches' using crossover links, right-click S1 and select 'configure', select the specific switch 'S1' on the left column and then choose the 'slots' tab to add the 'NM-16ESW' module to the 'slot0:' adapter. Click OK. You'll receive a prompt stating: You mist use manual mode when adding a link in NM-16ESW modules. Meaning, upon connecting to other switches/routers you want to manually choose the 0/0 interface as we added the module to the same slot. (Just to be on the safe side it's best to right-click the switch and do so using the 'manual' option whilst connecting links. It's possible that GNS3 recognises the aforementioned slot automatically, but that's a chance we don't wanna take).

-Repeat the same on S2 and S3 except that S2 will have NM-16ESW modules on two slots (one for getting connected to S1 and the other to connect to S3). I think each slot-number co-relates to it's interface, e.g: slot0 = f0/0, slot6 = f0/6.

-Now connect the three switches manually using the crossover cable (would have to be 'CAT 5' or 'CAT 5e' IRL) to the appropriate interface on each switch (i.e: fa0/0).
(Do not connect the 3rd switch back to the 1st switch as we don't want to mess around STP just yet, even though Cisco switches might be programmed to enable STP by default upon receiving a BPDU. I believe it is only the low-end Alcatel switches and so forth that do not have it on by default. Enterprise switching eqiuipment such as that like Cisco probably don't have that problem.)

-Right-click each switch and choose 'start'. Green light pops up on each switch confirming POST successful and connectivity established.


In the next chapter, we start the basic switching configuration, including configuring VLAN's and setting up VTP. A lot of it, if not all will be revision from CCNA.