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 ?
Based on Juniper KBs: KB18291 and KB15138, I did the following:
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.
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”.
Deleted RSTP protocol statements as per Juniper documentation
- 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 ?