The following figure describes the physical and logical spanning tree view of the network under a common/single spanning tree configuration. Two sets of tests were performed using the following configuration:
- Default common/single spanning tree instance that is created by MST and RSTP
- Two spanning tree instances on the Cisco switches with VLANs 10 and 20 assigned to each instance respectively
Common spanning tree instance
The following command examples show how Cisco switches share the same spanning tree region, same instances, and RSTP running under-the-hood. It is reasonable to expect great convergence times.
N5K1# show spanning-tree mst
MST0 VLANs mapped: 1-9,11-19,21-4094
Bridge address 547f.eeac.13c1 priority 24576 (24576 sysid 0)
Root this switch for the CIST
Regional Root this switch
Operational hello time 2 , forward delay 15, max age 20, txholdcount 6
Configured hello time 2 , forward delay 15, max age 20, max hops 20
Interface Role Sts Cost Prio.Nbr Type
-------------------------------------------------
Eth1/3 Desg FWD 2000 128.131 Edge P2p
Eth1/30 Desg FWD 2000 128.158 P2p Bound (STP)
Eth1/31 Desg FWD 2000 128.159 P2p
N5K2# show spanning-tree mst
##### MST0 VLANs mapped: 1-9,11-19,21-4094
Bridge address 002a.6a0d.a17c priority 32768 (32768 sysid 0)
Root address 547f.eeac.13c1 priority 24576 (24576 sysid 0)
port Eth1/31 path cost 0
Regional Root address 547f.eeac.13c1 priority 24576 (24576 sysid 0)
internal cost 2000 rem hops 19
Operational hello time 2 , forward delay 15, max age 20, txholdcount 6
Configured hello time 2 , forward delay 15, max age 20, max hops 20
Interface Role Sts Cost Prio.Nbr Type
-------------------------------------------------
Eth1/3 Desg FWD 2000 128.131 P2p
Eth1/28 Desg FWD 2000 128.156 P2p Bound (STP)
Eth1/31 Root FWD 2000 128.159 P2p
S4128-SW1# show spanning-tree active
Spanning tree enabled protocol rstp with force-version stp
Executing IEEE compatible Spanning Tree Protocol
Root ID Priority 24576, Address 547f.eeac.13c1
Root Bridge hello time 2, max age 20, forward delay 15
Bridge ID Priority 32768, Address f48e.385f.3dca
Configured hello time 2, max age 20, forward delay 15
Flush Interval 200 centi-sec, Flush Invocations 49
Flush Indication threshold 65535
Interface Designated
Name PortID Prio Cost Sts Cost BridgeID PortID
------------------------------------------------------------------------
ethernet1/1/3 128.524 128 2000 FWD 2000 32768 f48e.385f.3dca 128.524
ethernet1/1/8 128.544 128 2000 BLK 0 32769 002a.6a0d.a17c 128.156
ethernet1/1/9 128.548 128 2000 FWD 0 24576 547f.eeac.13c1 128.158
Interface
Name Role PortID Prio CostSts Cost Link-type Edge
------------------------------------------------------------------------
ethernet1/1/3 Desg 128.524 128 2000 FWD 2000 AUTO No
ethernet1/1/8 Altr 128.544 128 2000 BLK 0 AUTO No
ethernet1/1/9 Root 128.548 128 2000 FWD 0 AUTO No
Test steps
- Create two tagged streams of VLAN ID 10 and 20 with source port 3, and destination ports 1 and 2.
- Shut down ethernet1/1/9 on S4128-SW1 switch and measure traffic loss and duration of traffic loss.
- Recover ethernet1/1/9 on S4128-SW1 and measure traffic loss and duration of traffic loss.
Results
RSTP instance of OS10 converged with mst0 in Cisco. Ideally, traffic disruption should be negligible because all switches are under the same spanning tree region. Upon shutting down e1/1/9, the blocked link (port 8) moves into the forwarding state immediately.
When ethernet1/1/9 is restored, ethernet1/1/8 goes into a blocking status.
When Dell EMC switch converged initially, it recognized port 8 as the ALTR or alternate port, ready to take over if (root) port 9 were to fail. When this transition takes place, none of the BPDUs are transmitted since there is no interaction with another switch; therefore, the only events that take place are related to the link operational status (on e1/1/8 and e1/1/9) and the RSTP timers when transitioning from "Discarding" to "Forwarding". In this case, when using RSTP, transitioning from DISCARDING
> LEARNING
> FORWARDING
is almost instant.
Multiple spanning tree instance
For this test, multiple spanning tree instances are configured on the Cisco switches.
VLAN 10 was assigned to instance 1 and N5K1 switch is configured as the root switch for this instance. VLAN 20 was assigned to instance 2 and N5K2 switch is configured as the root switch for this instance. Although multiple spanning tree instances have been configured on the Cisco switches, we expect the results to be identical such as when having a single spanning tree instance. The following command examples depict the spanning tree instances on the Cisco switches:
N5K1# show spanning-tree mst 1
##### MST1 VLANs mapped: 10
Bridge address 547f.eeac.13c1 priority 24577 (24576 sysid 1)
Root this switch for MST1
Interface Role Sts Cost Prio.Nbr Type
-------------------------------------------------
Eth1/3 Desg FWD 2000 128.131 P2p
Eth1/30 Desg FWD 2000 128.158 P2p
Eth1/31 Desg FWD 2000 128.159 P2p
N5K2# show spanning-tree mst 1
##### MST1 VLANs mapped: 10
Bridge address 002a.6a0d.a17c priority 28673 (28672 sysid 1)
Root address 547f.eeac.13c1 priority 24577 (24576 sysid 1)
port Eth1/31 cost 2000 rem hops 19
Interface Role Sts Cost Prio.Nbr Type
-------------------------------------------------
Eth1/3 Desg FWD 2000 128.131 P2p
Eth1/28 Desg FWD 2000 128.156 P2p
Eth1/31 Root FWD 2000 128.159 P2p
N5K1# show spanning-tree mst 2
##### MST2 VLANs mapped: 20
Bridge address 547f.eeac.13c1 priority 28674 (28672 sysid 2)
Root address 002a.6a0d.a17c priority 24578 (24576 sysid 2)
port Eth1/31 cost 2000 rem hops 19
Interface Role Sts Cost Prio.Nbr Type
-------------------------------------------------
Eth1/3 Desg FWD 2000 128.131 P2p
Eth1/30 Desg FWD 2000 128.158 P2p
Eth1/31 Root FWD 2000 128.159 P2p
N5K2# show spanning-tree mst 2
##### MST2 VLANs mapped: 20
Bridge address 002a.6a0d.a17c priority24578 (24576 sysid 2)
Root this switch for MST2
Interface Role Sts Cost Prio.Nbr Type
-------------------------------------------------
Eth1/3 Desg FWD 2000 128.131 P2p
Eth1/28 Desg FWD 2000 128.156 P2p
Eth1/31 Desg FWD 2000 128.159 P2p
Test steps
- Create two tagged streams of VLAN ID 10 and 20 with source port 3, and destination ports 1 and 2.
- Shut down port 8 on the S4128-SW1 switch, measure the traffic loss, and duration of the traffic loss.
- Recover port 8 on S4128-SW1 switch, measure the traffic loss, and the duration of the traffic loss.
Results
The results were identical as having a single spanning tree instance. MST uses RSTP timers, so the convergence times should be identical.