Description

This KB provides information for understanding STP (Spanning Tree Protocol) standards based interoperability between Juniper EX series Switches and Cisco switches (PVST+), including the interaction between STP/RSTP and PVST+.

Solution

PVST+

On a given 802.1Q trunk, VLAN 1 BPDUs are always advertised as untagged to the well-known IEEE Spanning Tree multicast address, 01:80:c1:00:00:00. The other VLAN(s) on the trunk advertise tagged PSVT+ BPDUs to Cisco's reserved multicasts address, 01:00:0C:CC:CC:CC:CD. This allows Cisco to have a per-VLAN instance that can utilize multiple redundant trunk links by load-balancing the VLANs across them.

AS shown in figure below odd VLANs are forwarded on one of the uplink trunks and blocked on the other uplink trunk, and vice versa for the even VLANs
 
Root for Odd VLANs                               Root for Even VLANs

Switch 1 ------------------------------------ Switch 2

|                                                                       |
| FWD for ODD VLANs                                 | BLK for ODD VLANs
|                                                                       |
    ------------ Switch 3 ------------------------


One drawback to PVST+ is that it is Cisco proprietary and not embraced by any standard. PVST+ also places a large burden on switch resources such as CPU and memory, as each switch must maintain a separate STP database instance and process BPDUs on a per-port per-VLAN basis. While that might be OK with just a few VLANs, it does not scale well when 4,000 VLANs are supported.

Interoperating STP/RSTP with PVST+

By default, RSTP is enabled on all Ethernet switching enabled ports on the Juniper Networks EX series Switches. RSTP behaves the same as STP, as both only implement a single Spanning Tree instance for all VLANs over trunk.  Cisco switches, however default to their proprietary Per-VLAN spanning Tree Plus (PVST+) protocol.

Juniper Networks EX series Switches interact with the various types of BPDUs the Cisco switches send out. Since IEEE STP BPDU is standards- based and advertises to the well known IEEE reserved multicasts address. EX-series switches will process the IEEE STP BPDU. However, since PVST+ BPDU is a proprietary protocol and advertises to Cisco's reserved multicasts address, the EX series switches will treat it as any other multicasts packet and flood it on all forwarding ports within the VLAN in which the respective BPDUs are received.

There is one additional caveat to PVST+, is by default, the path cost assigned by PVST+ is based on Cisco's STP cost convention, not the IEEE STP cost. Therefore, the path cost needs to be changed so that it complies with IEEE STP path cost and all similar links (like FastE, GbE) have the same link-cost value. On the Cisco devices, the command "spanning-tree pathcost method long" must be entered to make it compliant with IEEE STP path cost. this command will be applied to all Cisco switch ports.

Since RSTP only has single Spanning Tree instance for all VLANs across a trunk and there is no VLAN awareness, all traffic will be forwarded or blocked based on the VLAN 1 topology.

Lastly, if Cisco is peering with Juniper and is running PVST, then Juniper links need to have VSTP enabled.

Modification History

5/8/2024- added note - 
'Lastly, if Cisco is peering with Juniper and is running PVST, then Juniper links need to have VSTP enabled.'

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