Configuring sub-interface IP Address across multiple routers

[Mezta1988]

Hi,

I'm currently studying CCNA and I'd like to clarify something regarding the LAB which I'm working on. Going straight to the point, I want to establish connections for Vlan 10 and 20 across the the network. however I have no idea on how properly setup the IP Address on the Sub-Interface. As far as I know each sub interface in each of every router will serve as the

default gateway

of for Vlan's in Network R1,2 & 3. 

I would really appreciate if someone can advise me if this topology is possible or not.

Vlan 10: 172.16.0.0/27
Vlan 20: 172.16.0.32/27
R1 Se 0/0/0 172.16.0.65/30 ----> R2 Se 0/0/0 172.16.0.66 /30
R2 Se 0/0/1 172.16.0.7130 -----> R3 Se 0/0/1 172.16.0.72 /30
R3 Se 0/0/0 172.16.0.75/30 ----> R1 Se 0/0/1 172.16.0.76 /30

Routing protocol used: RIPv2

[Joseph W. Doherty]

By default, you would have three VLANs 10 and three VLANs 20.  Each VLAN set might be supported via subinterfaces on each router, but you would also need unique address blocks for each.

Assuming you only want one VLAN 10 and one VLAN 20 you would need to transport L2 across your routers.  Which may be possible using a 2911, but I wonder whether that's the intent of this lab.

[David Ruess · Accepted solution]

Hello,

Did you make this lab yourself or following a lab guide? This lab will not function as is.

1. You cant have VLANs separated by a L3 network like you do. (at least at the CCNA level)

2. The CCNA (or any Cisco exam as far as I know) does not use RIP as a routing protocol. You should either use OSPF or static routing as that's what's needed for the CCNA.

The reason it wont work is simple. Imagine you're R1. You need to send a packet to VLAN 10. It doesn't know about VLANs but it knows about VLAN 10s network of

172.16.0.0/27

How does it know which VLAN 10 to send it to. Well in the scenario it will try to send traffic on its locally connected VLAN 10 network. If the host is on R2 then traffic is dropped since it doesn't have that host on its VLAN 10 network, its on another device.

Hope this helps

-David

[Richard Burts]

There have been 2 good responses to the question. I suggest a slightly different approach.

The original post suggests that there is a single vlan 10 in 3 locations. In reality there are 3 separate vlans which happen to have the same name. (And remember that the name of the vlan is locally significant, not globally significant.) But these vlans are entirely separate. If we remember that a vlan is a broadcast domain, then it should be clear that the broadcast domain connected to router 1 is separate and independent from the broadcast domain on router 2 which is separate and independent from the broadcast domain on router 3.

[David Ruess]

That was also my initial thought but looking back over the OP message the VLAN 10s and the VLAN 20s have the same subnet for all 3 domains which overlaps and wouldn't be feasible. The the names are locally significant the networks are not. VLAN 10 has a network of 

172.16.0.0/27

on all 3 routers (I assumed from the information given). 

[Joseph W. Doherty]

"I suggest a slightly different approach."

Sorry Rick, but what is the slight difference you're suggesting?

Perhaps just a further explanation what each router's VLANs provide (i.e. broadcast domains)?

I had noted there would be three VLAN 10s (also ditto for VLAN 20), by default (default perhaps being unclear, by which I'm assuming only L3 between the router without any L2 overlay).

[Richard Burts]

Joseph

Did any of the prior posts make the point that several vlans can exist and use the same name? That was what I thought was a slightly different approach.

[Richard Burts]

Having thought about it a bit more I have this to add: pretty clearly the original poster was assuming that if several vlans in different locations had the same name then they were the same vlan (vlan 10 is the same vlan in 3 locations). I was trying to make the point that the vlan name is a descriptor and does not have any significance about whether it is all one vlan or is several different vlans using the same name. I thought that was slightly different from the points that you and David were making.

[Joseph W. Doherty]

Same (VLAN) name?

Well, that is a different approach as I saw "Vlan 10" as VLAN (ID) 10, not some unknown numbered (ID) VLAN named "Vlan 10", which can, of course, be done.

As to whether my prior post made the point there can be multiple VLANs with the same ID, I thought mine did, as I explicitly described (for the OP topology) there are three VLANs for both 10 and 20.  But, as you didn't "see" that, very possible other readers would not either.  So, thank you for joining the discussion and expounding that point (again).

(Also thank you for answering my question, as I was scratching my head wondering what the "different approach" was.  I would now say your initial reply was a "different explanation" or even a "better explanation".)

[Richard Burts]

Joseph

You and I recognize vlan10 as the vlan ID (and that is the technically correct most accurate identification). But not so easy for someone in the early stages of learning networking. I was trying to suggest something easier to understand (and admittedly less precise).

You and I know that a vlan is a local broadcast domain and that while it is possible to have several vlans with the same vlan ID they are not the same vlan. I think it was not so obvious to the original poster. I am not claiming that my explanation was better, I was just aiming for different. The OP can go with the one that he understands the best.

[Mezta1988]

Yes sir, I made this lab my self out of curiosity, however, it's clear to me now that this type of approach is simply not applicable.

[M02@rt37 · Accepted solution]

Hello @Mezta1988

You can not achieve this, in this particular way.

You can not have same subnet/IPs/networks separated by Routers. Routers route networks... how Router1, as example, would route subnet dedicated to VLAN 10 because this VLAN is on the LAN-side behind this router. To achieve this, PC vlan 10 behind Switch2 talk to PC15 behind Switch1, you could use L2VPN VPWS or other things, but not with your topology.

To make it easier for your topology:

- VLAN 10 (only him) on Switch1

- VLAN 20 (only him) on Switch2

- VLAN 10 (only him) on Switch3

Router1 interface facing Switch 1 is a sub-interface, gatewau of VLAN 10.

Router2 interface facing Switch 2 is a sub-interface, gatewau of VLAN 20.

Router3 interface facing Switch 1 is a sub-interface, gatewau of VLAN 30.

You could use RIP for Routing protocol, but as @David Ruess suggested to you, this protocol is no more in CCNA exam. Start with static routing, and if it is OK, go to OSPF or EIGRP.

[Mezta1988]

Than you sir for providing your direct solution.

[M02@rt37]

You're so welcome @Mezta1988 

[Mezta1988]

Thank you everyone for your comments and suggestions, I am doing my self-pace study for CCNA. All your comments really helped me to understand some of the basic questions that I have in my head for the past few days.