We have Juniper EX 4200 as core switch at two sites connected Cisco 2960s and Cisco 3560s (access layer switches). For even-numbered VLANs, one Juniper switch is root bridge and for odd-numbered VLANs, other Juniper switch is the root bridge.

We have Cox and Verizon Metro-E links connecting core switches (Juniper EX 4200 at both sites).

I want to do VLAN load sharing using VSTP but somehow it is not working as expected. I want to pass some VLANs through COX and some through Verizon. When there is any issue with Cox, all VLAN traffic pass through Verizon and vice-versa. RSTP is also enabled on both Juniper switches.

I see MAC flapping in log messages on all Cisco access layer switches when I bring up both Metro-E links together. When only Cox is connected, everything works fine. When only Verizon is connected, everything works fine. But when BOTH COX and Verizon are connected, network gets disrupt and I see MAC flapping on all Cisco switches. All cisco switches are running PVST.

Anybody knows what is happening and why VSTP is not working when both COX and VERIZON Metro-E links are active ?

Update (09-Dec-2013):=====

Based on Juniper KBs: KB18291 and KB15138, I did the following:

  1. I enabled a common native vlan 50 (and shutdown vlan 1) on all Juniper and Cisco switches and configure the trunk ports where Cisco switches connect to Juniper for native vlan. (This is because Spanning-tree BPDUs are exchanged via Native VLAN between Cisco and Juniper). By default, Cisco native vlan is vlan 1 and there is no native vlan on Juniper. So Juniper does not understand the BPDUs and treats them as broadcast traffic flooding them to the corresponding VLAN. Because of this STP between Cisco and Juniper does not converge.

  2. Changed Cisco Spanning tree mode from PVST to Rapid-PVST (Juniper recommends changing Cisco spanning-tree mode from default - PVST to Rapid-PVST). Rapid-PVST converges well with Juniper spanning tree protocol “VSTP”.

  3. Deleted RSTP protocol statements as per Juniper documentation

  4. Entered vstp interface priority command for VLANs and Native VLAN on Juniper switches

Now when Cox and Verizon links are up at the same time, I see that some Cisco switches that hang-off to juniper core switches at both sites go down. I also see in Juniper (using command "show ethernet-switching interfaces") that some interfaces where Cisco switches are connected are blocked by STP.

Can someone figure out what is happening ?

  • Could you add a diagram that details your network (detailing devices and all interconnects with their relevant details)? I often find that diagrams really helping in diagnosing spanning tree issues.
    – YLearn
    Commented Dec 9, 2013 at 19:20
  • Again, why are you clinging to the duct tape and super glue path? Take the time to setup MST properly, and this will never be a problem again.
    – Ricky
    Commented Dec 9, 2013 at 21:16
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could post and accept your own answer.
    – Ron Maupin
    Commented Jan 3, 2021 at 19:37

2 Answers 2


Rule #1: in a mixed vendor environment, one avoids use of vendor proprietary protocols

There are (apparently) several complications attempting to use VSTP (a Juniper protocol) with PVST (RPVST actually, a Cisco protocol) -- while they both run a per-vlan RSTP instance, they don't do it exactly the same (something about tagged/untagged native vlans, etc.)

Your best bet would be to use an open, documented standard with rules everybody obeys. That would be MST (802.1s, now part of 802.1q.) Of course, MST is a great deal more complicated to setup. (been there... complex hundreds of vlans across 4 vendors)

All this assumes the carriers aren't screwing with the traffic. If I'm following your description, the metro-e links are between juniper switches and the cisco's hang off the juniper's at each site. If that's true, there's only one path from a cisco to the other side, so mac flaps shouldn't be possible -- unless there's a pair of junipers at each end round-robin'ing traffic between sites. (or there are etherchannels that aren't setup/running correctly)

  • Yes...Metro-E links are between Juniper switches and cisco's hang off the junipers at each site. But still MAC flaps on all cisco switches when both COX and VERIZON are up at the same time.
    – starcity
    Commented Nov 26, 2013 at 17:40

Are the carriers passing your L2 control frames? Quite often you need to specifically ask them to do so.

You can verify by checking your BPDUs sent and received on both switches when they are both up

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