Bridging two vlans together with a crossover cable

[wabob]

Hello.

We are in process of migrating IP connectivity for network devices in one subnet from VLAN 1 to another VLAN. We are planning to connect a crossover cable on the core switch which will bridge an access port in VLAN 1 to an access port in the new VLAN. There are no other access ports anywhere in the new VLAN yet. We are hoping this will work so we can do an incremental migration instead of all-or-nothing.

Are there any precautions I should take on the bridging access ports with regards to spanning-tree, such as bpdufilter or bpduguard enable?

Thanks!

 

[unclerico]

are you physically recabling these devices?? if not, why don't you make the interface connecting the core switch to the other switch(es) a trunk port and then for each port change the vlan?? you will be able to migrate one port at a time.

The other way I am interpreting this is that you want to create a loop on your core switch by plugging a crossover cable from one port to another on the same switch.

I hate all Uppercase... I don't want my groups to seem angry at me all the time! =)
- ColdFlame (vbscript forum)

 

[wabob]

The IP gateway is still on VLAN 1. Once the devices are all migrated, we will take the SVI off VLAN 1 and put the same IP gateway address on the new VLAN, which does not have an SVI now.

There are about two hundred devices on this IP subnet spread over about thirty or more downstream switches (most with redundancy). We are trying to migrate the devices one at a time from VLAN 1 to the other VLAN with minimal disruption of connectivity.

 

[cisconooblet]

Question: Why are you migrating vlan1 to vlanX with the same ip addressing scheme? Just curious?


Why not create a new ip subnet for the new vlan, sub interface on the router, and then migrate each port over one at the time to the new vlan?

 

[wabob]

The IPs of the devices to be migrated cannot be changed (long story). That is why I want to bridge vlan 1 and vlan X together until migration is complete.

 

[unclerico]

you can use fallback bridging to do this. the steps are a) create a bridge group and b) assign the SVI's to the bridge group.

I hate all Uppercase... I don't want my groups to seem angry at me all the time! =)
- ColdFlame (vbscript forum)

 

[wabob]

VlanX does not currently have an SVI.

I have the two access ports shut down which are to be bridged together. Bpduguard and bpdufilter are enabled on both ports.

 

[VinceWhirlwind]

Your original question "bridge an access port in VLAN 1 to an access port in the new VLAN. " will work on 3750s (for example) but not 3500s (for example).

The 3750s will issue regular complaints about "mismatched VLAN" (or something) but will pass traffic.
3500s disable the interface and pass nothing.

Your question about spanning-tree is best answered as follows: don't use spanning-tree, it's a sign of poor design.

 

[burtsbees]

Vince---I think you can disable err-disable to prevent that in 3500's...could be wrong...

Burt

 

[VinceWhirlwind]

I vaguely remember trying to force it to work on the 3500s without luck.
I have very little exposure to 3500s these days, except for throwing them away.

I suspect 2960s & 3560s will behave like 3750s and allow you to bridge two VLANs.