Concept Guide

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.
1. Configure the VLT interconnect for the VLT domain. The primary and secondary switch roles in the VLT domain are
automatically assigned after you configure both sides of the VLTi.
NOTE:
If you use a third-party ToR unit, to avoid potential problems if you reboot the VLT peers, Dell EMC recommends
using static LAGs on the VLTi between VLT peers.
2. Enable VLT and create a VLT domain ID. VLT automatically selects a system MAC address.
3. Configure a backup link for the VLT domain.
4. (Optional) Manually reconfigure the default VLT settings, such as the MAC address and VLT primary/ secondary roles.
5. Connect the peer switches in a VLT domain to an attached access device (switch or server).
920
Virtual Link Trunking (VLT)