Administrator Guide

1. Configure RSTP in the core network and on each peer switch as described in Rapid Spanning Tree Protocol (RSTP).
Disabling RSTP on one VLT peer may result in a VLT domain failure.
2. Enable RSTP on each peer switch.
PROTOCOL SPANNING TREE RSTP mode
no disable
3. Configure each peer switch with a unique bridge priority.
PROTOCOL SPANNING TREE RSTP mode
bridge-priority
Sample RSTP configuration
The following is a sample of an RSTP configuration:
Using the example shown in the Overview section as a sample VLT topology, the primary VLT switch sends BPDUs to an access device
(switch or server) with its own RSTP bridge ID. BPDUs generated by an RSTP-enabled access device are only processed by the primary
VLT switch. The secondary VLT switch tunnels the BPDUs that it receives to the primary VLT switch over the VLT interconnect. Only the
primary VLT switch determines the RSTP roles and states on VLT ports and ensures that the VLT interconnect link is never blocked.
In the case of a primary VLT switch failure, the secondary switch starts sending BPDUs with its own bridge ID and inherits all the port
states from the last synchronization with the primary switch. An access device never detects the change in primary/secondary roles and
does not see it as a topology change.
The following examples show the RSTP configuration that you must perform on each peer switch to prevent forwarding loops.
Configure RSTP on VLT peers to prevent forwarding loops—VLT peer
1 (primary)
Dell_VLTpeer1(conf)#protocol spanning-tree rstp
Dell_VLTpeer1(conf-rstp)#no disable
Dell_VLTpeer1(conf-rstp)#bridge-priority 4096
Configure RSTP on VLT peers to prevent forwarding loops—VLT peer
2 (secondary)
Dell_VLTpeer2(conf)#protocol spanning-tree rstp
Dell_VLTpeer2(conf-rstp)#no disable
Dell_VLTpeer2(conf-rstp)#bridge-priority 8192
NOTE:
When you remove the VLT configuration, RSTP is recommended as a backup solution to avoid spanning—tree
loops. In this case, you can use the following RSTP default values:
• hello–time: 2 seconds
• forward–delay: 15 seconds
• max–age: 20
There is no additional performance gain on VLT, if you configure the RSTP values other than the default values.
Configuring VLT
To configure VLT, use the following procedure.
Prerequisites:
Before you begin, make sure that both VLT peer switches are running the same Dell EMC Networking OS version and are configured for
RSTP as described in the RSTP Configuration section. For VRRP operation, ensure that you configure VRRP groups and L3 routing on
each VLT peer as described in VLT and VRRP interoperability in the Configuration Notes section. To configure VLT and create a VLT
domain in which two switches are physically connected and treated as a single port channel by access devices, configure the following
settings on each VLT peer device.
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Virtual Link Trunking (VLT)