Lan Switching

[Suprit Chinchodikar]

Hello,
As per the below diagram I have two switches S1 & S2. Hosts H1 & H2 are connected to the switches and are in different vlan but same subnet. 
So, by default they will not communicate as they are in two different broadcast domain.
To make them communicate either bring both the host in same vlan or add respective vlans on the interface connected between S1 and S2. But, when we do that we have a native vlan mismatch errors.
So, my questions is do we have any other mechanism in which these two hosts can communicate with each other?
Please note we cannot add vlan 20 on S1 and vlan 10 on S2 that's one condition over here.

[Richard Pidcock]

Is having the overlapping address space part of the requirement?  This is confusing to me; I'd just separate this into two distinctly different subnets as it seems the intent is to have these L2 separated anyway.

[Suprit Chinchodikar]

No, this is just a layer 2 flat network. I don't want to involve routing over here.

[KJK99]

---

@Suprit Chinchodikar wrote:

Hello,
As per the below diagram I have two switches S1 & S2. Hosts H1 & H2 are connected to the switches and are in different vlan but same subnet. 
So, by default they will not communicate as they are in two different broadcast domain.
To make them communicate either bring both the host in same vlan or add respective vlans on the interface connected between S1 and S2. But, when we do that we have a native vlan mismatch errors.
So, my questions is do we have any other mechanism in which these two hosts can communicate with each other?
Please note we cannot add vlan 20 on S1 and vlan 10 on S2 that's one condition over here.

So, my questions is do we have any other mechanism in which these two hosts can communicate with each other?

---

Apart from what has been already said, VLAN Translation/Mapping seems to be an option, but I haven't used it myself.

[Joseph W. Doherty]

Interesting.  If I understand the technology, it's basically a substitute for Q in Q, in that rather than double tagging a frame, you modify the VLAN ID value.

The expectation appears to be to use the same translation on both ends, to restore original VLAN ID, but you're thinking of doing only one translation or translate to different VLAN IDs on the other side?  i.e. V10 <> V20 on either, but just one, switch or V10 <> V99 <> V20  on both switches

[rabbdavid]

 Talking to Joseph about VLAN but on different aspects, he pointed me out this discussion, so since I a newee I tried to replicate the exercise me too. Well, if I configure the ports the link connecting the two switches in access mode, the frames pass through, and ping is working. But if I configure those ports as trunk, even setting VLAN10 on SW1 as native vlan and VLAN20 as native vlan on the other switch, hosts can't communicate and ping does not work, why? What could I do wrong?

[Torbjørn]

Which spanning-tree mode are you running? PVST and RPVST both have mechanisms to detect and prevent native-vlan mismatches, which will cause the mismatched ports to be blocked. This can be confirmed with "show spanning-tree"

[rabbdavid]

Thanks. Show spanning tree command output is: IEEE

By show running config command, output is:  spanning-tree mode pvst, on both switches

[Torbjørn]

It _should_ work if you change spanning-tree mode to MST. I don't believe it has any mechanisms to detect and block mismatches. If that doesn't work you can try to have both switches in different MST regions, such that they rely on legacy STP for handling the link. You can achieve this by setting a mst region name on one of the switches:

spanning-tree mst configuration
 name MST2

[Joseph W. Doherty]

Or for test purposes, just deactivate STP on both switches.

BTW, some time back, I did get this to work, using Packet Tracer, for both access ports and trunk ports.  Don't recall what all I had to do, beyond I ran into issues beyond CDP (which I was able to handle).

[rabbdavid]

this too is another way to get the frame pass on trunk link between switches, obviously.

[rabbdavid]

this too is another way to get the frame pass on trunk link between switches, obviously

[rabbdavid]

I do not master spanning tree subject, especially MST, however PT does not allow me to change spanning tree mode; I can chose Rapid PVST or PVST only... - Basically with native vlan mismatch, the only way I can get frames passed on the switches link, is by configuring the link ports as access ports, at least on my simulation on PT.

Thanks for your help though

[Suprit Chinchodikar]

I tried running both the versions of spanning-tree but the none of the ports connecting between S1 & S2 are getting blocked.
Also, I' am able to ping both the host after I trunk the ports and add native vlan 10 & 20 respectively.

[Richard Burts]

I am glad that our suggestions have been helpful. Thank you for marking this question as solved. This will help other participants in the community to identify discussions which have helpful information. This community is an excellent place to ask questions and to learn about networking. I hope to see you continue to be active in the community.