Please refer to the network diagram below, it is a very simplified version of an actual network installation.

The Router has 3 Vlans, OSPF has been configured and all the 3 vlans have been added to Area 0 so they can communicate with each other. The Switch is a PnP Layer 2 managed switch.

With the current setup, we are facing an issue: End Device 1 cannot communicate with End Device 2. However, if we break the link between the Switch and Vlan2, the 2 End Devices are able to communicate with each other.

Can somebody please explain why this phenomenon is occuring, and how it can be solved while maintaining both the connections from the switch to the router.

enter image description here

The Router is an Extreme Summit X450a device. Refer to the sample config below:

create vlan “VLAN1”
configure vlan VLAN1 tag 10
configure vlan VLAN1 add ports 1-4 untagged
configure vlan VLAN1 ipaddress

create vlan “VLAN2”
configure vlan VLAN2 tag 20
configure vlan VLAN2 add ports 5-8 untagged
configure vlan VLAN2 ipaddress

create vlan “VLAN3”
configure vlan VLAN3 tag 30
configure vlan VLAN3 add ports 9-12 untagged
configure vlan VLAN3 ipaddress

enable ipforwarding vlan VLAN1
enable ipforwarding vlan VLAN2
enable ipforwarding vlan VLAN3

enable ipmcforwarding vlan VLAN1
enable ipmcforwarding vlan VLAN2
enable ipmcforwarding vlan VLAN3

configure ospf routerid
enable ospf
configure ospf add vlan VLAN1 area
configure ospf add vlan VLAN2 area
configure ospf add vlan VLAN3 area

configure pim add vlan VLAN1 dense
configure pim add vlan VLAN2 dense
configure pim add vlan VLAN3 dense
enable pim
  • Need configurations of the router and the switch
    – fredpbaker
    Nov 6 '13 at 4:19
  • HI fredpbaker, for the switch, we have used it as a PnP, with no configuration done. You may refer to the router config in this file db.tt/0jjqySqD
    – Muffi
    Nov 6 '13 at 6:25
  • Which ports are going to the L2 switch from the Extreme switch?
    – Daniel Dib
    Nov 6 '13 at 7:16
  • For VLAN1, i have used port 1, VLAN2 uses port 5, and VLAN3 uses port 9. So ports 5 and 9 go to the L2 switch...
    – Muffi
    Nov 6 '13 at 7:23
  • 1
    Extreme supports per vlan STP, but YLearn's explanation is plausible assuming you are not running pvst or mst. If STP is indeed the problem, you would see the vlan 3 port to the other switch blocked in STP. Assuming this is the problem, dot1q with LACP removes STP blocking issues, and provides redundancy for vlan 2 & 3 links.
    – This
    Nov 6 '13 at 9:37

The Extreme Summit X450a is actually a L3 switch, not a router, so your problem is most likley that spanning tree is putting the second (VLAN3) inter-switch link in blocking mode. When you disconnect the first (VLAN2) inter-switch link, STP transitions the second link to forwarding mode making the second station reachable.

Instead of two links carrying one VLAN each, you should be using a single tagged or trunk port between the two switches.

  • 1
    But there should be no bridging loop if he is using one VLAN per port to the switch. Looks strange to me.
    – Daniel Dib
    Nov 6 '13 at 7:12
  • I can confirm that stp is disabled on the L3 Switch, as the default configuration is STP Disabled. so i dont think STP is the case here. Yup, something is very strange.
    – Muffi
    Nov 6 '13 at 7:21
  • 1
    @DanielDib, stp is layer-2. from that perspective, a bpdu left on one port and came back in on another.
    – Ricky
    Nov 6 '13 at 7:25
  • @user79134, the strangeness is the result of a switch being plugged into a switch. I'm guessing the x450a isn't using individual MACs for each VLAN which will confuse the L2 switch. Or the L3 switch is answering ARP on the wrong interface(s). [from each end, ping each interface]
    – Ricky
    Nov 6 '13 at 7:29
  • @DanielDib, unless you are using a proprietary extension to spanning-tree, there is no VLAN distinction. As opposed to something like Cisco's per VLAN spanning-tree (PVST+) - CST, RSTP, and MST (between switches of different regions) all only have one spanning-tree topology and this is a very common configuration mistake.
    – YLearn
    Nov 6 '13 at 8:30

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