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VLAN confusion. Am I just dense.

Flyberius
Occasional Visitor

VLAN confusion. Am I just dense.

Hi,

 

I have a number of HP V1910-48G switches.  All currently sitting on VLAN1.

 

We have a core switch and off of that secondary and tertiary switches.  On the secondary switch I configured some of the ports to be on VLAN2 and configured a Cisco router to route between VLAN1 (192.168.2.0/24) and VLAN2 (192.168.4.0/24).  The routing aspect works beautifully. 

 

What I then did was to configure some ports on the tertiary switch to also be on VLAN2.  I then connected a VLAN2 port from Secondary switch to a VLAN2 port on the tertiary switch, expecting any device on VLAN2 of one switch to be able to communicate with VLAN2 of the other.  This does not appear to be the case.  If I am plugged into a VLAN2 port of the secondary switch I cannot reach any device plugged into the VLAN2 port of the tertiary switch.  And vice versa.

 

I am new to vlans and whilst I thought I understood the basic concept I am clearly doing something very wrong.

 

I have attached a picture to help people understand what I have set up.

 

 

P.S. This thread has been moved from Switches, Hubs, Modems (Legacy ITRC forum) to Web and Unmanaged. - Hp Forum Moderator

6 REPLIES
Richard Brodie_1
Honored Contributor

Re: VLAN confusion. Am I just dense.

It could be that spanning tree is blocking the link between your secondary and tertiary switches.

 

"show span" should tell you.

Flyberius
Occasional Visitor

Re: VLAN confusion. Am I just dense.

I have telneted to the switches in question but the Show Span command does not appear to work.

 

Further more I do not seem to have any of the commands listed in the Command Reference which is rather odd.

 

Is it possible to view the spanning tree from the GUI?

 

 

Does the connection between the two switches need to be a trunk?  I tried all manner of combinations of Access Hybrid and Trunk but none of them seem to allow traffic through.

Flyberius
Occasional Visitor

Re: VLAN confusion. Am I just dense.

Ignore me, I just saw this:  _cmdline-mode on

 

Although the show span command doesn't appear to do anything...

 

Richard Brodie_1
Honored Contributor

Re: VLAN confusion. Am I just dense.

Oops, V1910, sorry. 'display stp' then

Flyberius
Occasional Visitor

Re: VLAN confusion. Am I just dense.

This is the port on Switch 2 that is connecting to the port on switch 3.

 

 

----[Port48(GigabitEthernet1/0/48)][FORWARDING]----
 Port Protocol       :enabled
 Port Role           :CIST Designated Port
 Port Priority       :128
 Port Cost(Legacy)   :Config=auto / Active=20
 Desg. Bridge/Port   :32768.20fd-f1ca-fc0d / 128.48
 Port Edged          :Config=enabled / Active=enabled
 Point-to-point      :Config=auto / Active=true
 Transmit Limit      :10 packets/hello-time
 Protection Type     :None
 MST BPDU Format     :Config=auto / Active=legacy
 Port Config-
 Digest-Snooping     :disabled
 Rapid transition    :true
 Num of Vlans Mapped :1
 PortTimes           :Hello 2s MaxAge 20s FwDly 15s MsgAge 2s RemHop 20
 BPDU Sent           :30695
          TCN: 0, Config: 30695, RST: 0, MST: 0
 BPDU Received       :0
          TCN: 0, Config: 0, RST: 0, MST: 0

 

 

 

And this is the port on Switch 3 connecting to switch 2.  Interestingly it is show as Discarding, whatever that means.  Doesn't sound good.

 

----[Port18(GigabitEthernet1/0/18)][DISCARDING]----
 Port Protocol       :enabled
 Port Role           :CIST Alternate Port
 Port Priority       :128
 Port Cost(Legacy)   :Config=auto / Active=20
 Desg. Bridge/Port   :32768.20fd-f1ca-fc0d / 128.48
 Port Edged          :Config=enabled / Active=disabled
 Point-to-point      :Config=auto / Active=true
 Transmit Limit      :10 packets/hello-time
 Protection Type     :None
 MST BPDU Format     :Config=auto / Active=legacy
 Port Config-
 Digest-Snooping     :disabled
 Num of Vlans Mapped :1
 PortTimes           :Hello 2s MaxAge 20s FwDly 15s MsgAge 2s RemHop 68
 BPDU Sent           :1
          TCN: 0, Config: 1, RST: 0, MST: 0
 BPDU Received       :30779
          TCN: 0, Config: 30779, RST: 0, MST: 0

 

 

Richard Brodie_1
Honored Contributor

Re: VLAN confusion. Am I just dense.

What you have here is a spanning tree problem; this confuses a lot of people when multiple VLANs are involved.

Switch 1 appears to be the root switch: ignoring the router, the spanning tree consists of the two red uplinks.
The green link between switches 2 and 3 is a redundant link, and spanning tree will block the link (at one end as you observed). Sometimes this is called the East-West problem: traffic goes up and down the spanning tree but not sideways.

What confuses people is that standard spanning tree doesn't care about VLANs, it's essentially colour blind. Cisco's proprietary Per VLAN Spanning Tree family of protocols, on the other hand isn't.
You can achieve similar results to PVST with MSTP but only with a lot of fiddly configuration.

There's a lot of interest in both proprietary and standardised ways of replacing STP and having multiple links active at once. Cisco has FabricPath, HP has IRF then there is SPB and TRILL from the IEEE and IETF respectively.

For now, the simplest thing to do is probably to have all the VLANs on all 3 interswitch links, even if you have no ports on VLAN 2 on Switch 1.