For this scenario, two sets of tests were performed by using the following configuration:
- Cisco switches act as the root bridge for the respective VLANs
- Dell EMC S4128-SW1 acts as the root bridge
Cisco Nexus as the root
The image below depicts the physical and logical network topology respectively. The logical spanning tree network topology shows how port 9 is forwarding and port 8 is being blocked on S4128-SW1. The black arrow describes the traffic flow for VLANs 10 and 20. This is the normal behavior of having a single spanning tree instance for all VLANs where the Dell EMC switch runs RSTP.
For Cisco N5Ks running RPVST+, two different spanning tree instances are created. From the N5K1 perspective, the VLAN 20 root bridge is N5K2, and so it creates a separate instance pointing to N5K2 as the root switch. From the N5K2 perspective, the VLAN 10 root bridge is N5K1, and it creates a separate instance pointing to N5K1 as the root switch for that VLAN as shown in the N5K1 VLAN 20 spanning tree instance and N5K2 VLAN 10 spanning tree instance sections below.
Test steps
- Create two tagged streams of VLAN ID 20 with source port 3, and destination ports 1 and 2.
- Shut down ethernet1/1/9 on S4128-SW1 to simulate a fail-over scenario and check for any traffic disruption.
- Recover ethernet1/1/9 on S4128-SW1 and check for any traffic disruption.
S4128-SW1# show spanning-tree active
Spanning tree enabled protocol rstp with force-version stp
Executing IEEE compatible Spanning Tree Protocol
Root ID Priority 24577, 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 50
Flush Indication threshold 65535
Interface
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 2 32769 002a.6a0d.a17c 128.156
ethernet1/1/9 128.548 128 2000 FWD 0 24577 547f.eeac.13c1 128.158
Interface
VLAN0020
Spanning tree enabled protocol rstp
Root ID Priority 32788
Address 002a.6a0d.a17c >> N5K2 MAC address
Cost 2
Port 159 (Ethernet1/31)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32788 (priority 32768 sys-id-ext 20)
Address 547f.eeac.13c1
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role Sts Cost Prio.Nbr Type
---------------------------------------
Eth1/3 Desg FWD 2 128.131 P2p
Eth1/30 Desg FWD 2 128.158 P2p
Eth1/31 Root FWD 2 128.159 P2p
VLAN0010
Spanning tree enabled protocol rstp
Root ID Priority 24586
Address 547f.eeac.13c1 >> N5K1 MAC address
Cost 2
Port 159 (Ethernet1/31)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)
Address 002a.6a0d.a17c
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Interface Role Sts Cost Prio.Nbr Type
-------------------------------------------
Eth1/3 Desg FWD 2 128.131 P2p
Eth1/28 Desg FWD 2 128.156 P2p
Eth1/31 Root FWD 2 128.159 P2p >> pointing to N5K1 as the root switch
Results
As expected in this test, there are two different spanning tree modes. One mode is a purely vendor proprietary implementation (RPVST+), and the other is the standard-based (RSTP).
When e1/1/9 is disabled, the alternate root e1/1/8 on S4128-SW1 moves to the forwarding state immediately. The behavior is expected in RSTP. Since this is the alternate root port, a BPDU exchange does not take place within the network which allows for fast switch over and forwarding times.
With RSTP, a direct message exchange takes place between point-to-point links. This exchange consists of an RSTP BPDU proposal message and an agreement message. Proposal and agreement BPDUs are used to negotiate fast-convergence parameters should a failure occur. For 802.1Q tagged VLANs (all VLANs besides VLAN 1), Cisco switches send their BPDUs only to the reserved Cisco multicast address of 01-00-0C-CC-CC-CD. Unless the Dell EMC switch is also listening to this multicast address, it will only have visibility and an understanding of the logical topology of the CST. For a Cisco bridge running PVST+ to be able to converge with a third-party bridge running 802.1w RSTP, VLAN 1 must be allowed on all 802.1Q trunks that interconnect them.
The output below shows that for VLAN 20, BPDUs are sent only to the Cisco reserved address that the Dell EMC switch floods on all its other ports. That means that the rapid failover and convergence that RSTP offers is only evident on VLAN 1.
N5K1# debug spanning-tree all
N5K1# 2019 Jan 7 16:32:40.844091 stp: MTS: dropping MTS_OPC_DEBUG_WRAP_MSG msg id (48856751) on q 7
2019 Jan 7 16:32:41.022537 stp: handling batch flush timer
2019 Jan 716:32:41.272546 stp: inserting instance 1 hello timer in C queue
2019 Jan 7 16:32:41.272565 stp: Malloc in fu_cq_node_alloc@../utils/fsmutils/cqueue.c[922]-
2019 Jan 7 16:32:41.360178 stp: BPDU Rx: Received BPDU on vb 1 VLAN 20 port Ethernet1/31 pkt_len 64
bpdu_len 42 netstack flags 0x00ed enc_type sstp
2019 Jan 7 16:32:41.360207 stp: BPDU RX: vb 1 VLAN 20 port Ethernet1/31 len 64 flags 0xed: Ethernet
Hdr 01000ccccccd<-002a6a0da146 type/len 0032: SNAP aa aa 03 00000c 010b SSTP CFG P:0000 V:02 T:02 F:3c R:60:14:00:2a:6a:0d:a1:7c 00000000 B:60:14:00:2a:6a:0d:a1:7c 809f A:0000 M:0014
Dell EMC PowerSwitch S4128F-ON as the root
The following scenario has the S4128-SW1 as the root. The spanning tree network topology in the figure below shows how port 9 and port 8 on the S4128-SW1 are forwarding, and how port 31 on N5K2 is blocked.With Dell EMC acting as root bridge, the Cisco N5Ks converge only for default instances (VLAN 1). Port 31 on N5K2 is placed in blocking state as shown in the figure below.The following output shows how BPDUs for VLAN 10 are sent only to the Cisco reserved multicast address, 01-00-0C-CC-CC-CD. For non-default VLANs 10 and 20, the Dell EMC S4128 switch acts as a hub and floods the RPVST BPDUs on all other ports, which the Cisco N5K2 receives on port 28. The Cisco N5K2 then places one of its ports in blocking mode (ethernet1/31) to avoid a loop in that VLAN.N5K2# debug spanning-tree all
N5K2#2019 Jan 28 15:46:33.577122 stp: MTS: dropping MTS_OPC_DEBUG_WRAP_MSG msg id (244602708) on q 8
2019 Jan 28 15:46:33.782530 stp: handling batch flush timer
2019 Jan 28 15:46:34.032568 stp: handling batch flush timer
2019 Jan 28 15:46:35.232858 stp: BPDU Rx: Dropping redundant SSTP packet received on port Ethernet1/31 VLAN VLAN0001
2019 Jan 28 15:46:35.232871 stp: BPDU RX: vb 1 VLAN 10, ifi 0x1a01b000 (Ethernet1/28
2019 Jan 28 15:46:35.232883 stp: BPDU Rx: Received BPDU on vb 1 VLAN 10 port Ethernet1/28 pkt_len 64 bpdu_len 42 netstack flags 0x00ed enc_type sstp
2019 Jan 28 15:46:35.232908 stp: BPDU RX: vb 1 VLAN 10 port Ethernet1/28 len 64 flags 0xed: Ethernet Hdr 01000ccccccd<-547feeac13a5 type/len 0032: SNAP aa aa 03 00000c 0 10b SSTP CFG P:0000 V:02 T:02 F:3c R:60:0a:54:7f:ee:ac:13:c1 00000000 B:60:0a:54:7f:ee:ac:13:c1 809e A:0000 M:0014 H:0002 F:000f T:0000
Test steps
- Create two tagged streams of VLAN ID 20 with source port 3, and destination ports 1 and 2.
- Shut down ethernet1/1/9 on S4128-SW1 to simulate a fail-over scenario and check for any traffic disruption.
- Recover ethernet1/1/9 on S4128-SW1 and check for any traffic disruption.
Results
The blocked link, ethernet1/31 on N5K2 starts forwarding when e1/1/9 is shut down.