So what is a VLT and what does it does :
Virtual link trunking (VLT) allows physical links between two Dell switches to appear as a single virtual link to the network core or other
switches such as Edge, Access, or top-of-rack (ToR). As a result, the two physical switches appear as a single switch to the connected
Basically we are creating one logical switch out of two physical switches.
From the left we see how it looks when interconnected physically, from right how end device sees it.
Configuration steps :
1.Enable spanning tree – RSTP and PVST supported – step is optional, but nevertheless recommended.
protocol spanning-tree rstp
bridge-priority 4096 (primary VLT switch)
bridge-priority 8192 (Secondary VLT switch)
Recommended to have a root bridge on VLT master and to set STP priority to secondary VLT switch in case if the first fails no to have topology change when other third unknown device would become a root.
2. Configure ports for VLTi link :
interface range fortyGigE 0/56 , fortyGigE 0/60
interface port-channel 100
channel-member fortyGigE 0/56,60
3. Create VLT domain on both switches, don’t forget to create a backup-link
vlt domain 1
primary-priority 10 (primary VLT switch)
primary-priority 20 (Secondary VLT switch)
back-up destination 192.168.0.2 (Primary VLT switch, management interface)
back-up destination 192.168.0.3 (Secondary VLT switch management interface)
peer-link port-channel 100
Backup links are needed to have a heartbeat messages flowing between two switches.
VLT also would work without the heartbeat but then you can encounter possible split brain scenario in case of VLTi link failure.
After configuring the VLT we should get the following picture :
Now let’s attach a device to our VLT switches.
On both VLT members pick up a port for redundant connection :
interface port-channel xx
no ip address
vlt-peer-lag port-channel 110
And you are ready to go.
You can tweak the stuff like dampening – just to give some time for routing and other protocols to get online after rebooting the switch ,as ports will get up faster and devices without knowing that routing protocol is not ready yet may black hole the traffic.
You can also play with spanning-tree metrics – to have interruption after reboot as small as possible.
VLT behavior :
You can check that MACs are being synced using the command :
show mac-address-table count
Some of the of interesting points to remember (more you can find by downloading the user guide)
Some failure scenarios :
Overall VLT is a great thing for load balancing, redundancy and availability (you can upgrade the switches one by one without having a downtime) – In stack this wouldn’t be possible.
All info and images were taken from Dell User guide for S4048-ON switch, you can download it by following this link : http://downloads.dell.com/manuals/all-products/esuprt_ser_stor_net/esuprt_networking/esuprt_net_fxd_prt_swtchs/force10-s4048-on_administrator%20guide15_en-us.pdf
In user guide you can find a lot of detailed info about all the possible switch OS functions and how to use/implement/troubleshoot them.