DELL Force10 : VLT - Virtual Link Trunking

Do you know CISCO's Virtual port Channel? Do you want the same with DELL datacenter switches. Here we go.

General VLT overview

Virtual Link Trunking or VLT is a proprietary aggregation protocol developed by Force10 and available in their datacenter-class or enterprise-class network switches. VLT is implemented in the latest firmware releases (FTOS from 8.3.10.2) for their high-end switches like the S4810, S6000 and Z9000 10/40 Gb datacenter switches. Although VLT is a proprietary protocol from Force10, other vendors offer similar features to allow users to set up an aggregated link towards two (logical) different switches, where a standard aggregated link can only terminate on a single logical switch (thus either a single physical switch or on different members in a stacked switch setup).  For example CISCO's similar proprietary protocol is called Virtual Port Channel (aka vPC) and Juniper has another one called Multichassis LAG (MC-LAG).

VLT is a layer-2 link aggregation protocol between end-devices (servers) connected to (different) access-switches, offering these servers a redundant, load-balancing connection to the core-network in a loop-free environment, eliminating the requirement for the use of a spanning-tree protocol.[2] Where existing link aggregation protocols like (static) LAG (IEEE 802.3ad) or LACP (IEEE 802.1ax) require the different (physical) links to be connected to the same (logical) switch (such as stacked switches), the VLT, for example, allows link connectivity between a server and the network via two different switches.

Instead of using VLT between end-devices like servers it can also be used for uplinks between (access/distribution) switches and the core switches.[3]

Above VLT general description is from Wikipedia. Fore more information about VLT see http://en.wikipedia.org/wiki/Virtual_Link_Trunking

DELL published Force10 VLT Reference Architecture (PDF - link cached by google) where VLT is explained in detail so it is highly recommended to read it together with all product documentation and release notes before any real plan, design and implementation.

VLT Basic concept and terminology

The VLT peers exchange and synchronize Layer2-related tables to achieve harmonious Layer2 forwarding among the whole VLT domain, but the mechanism involved is transparent.

VLT is a trunk (as per its name) attaching remote hosts or switches.
VLTi is the interconnect link between the VLT peers. For historical reasons that is also called ICL (InterConnect Link) in the command outputs.

All the following rules apply to the VLT topologies
 2 unit per domain (as of FTOS 8.3.10.2)
 8 links per port-channel or fewer.
 Units should run the same FTOS version
 The backup should employ a different link than the VLTi, and preferably a diverse path

Simple implementation plan

Below I'll write simplified implementation plan for VLT configuration so it should be handy for any lab or proof of concept deployments.

 Implementation plan is divided in to 6 steps.

1. Check or configure spanning tree protocol
2. Check or configure LLDP
3. Check or configure out of band management leveraged for VLT backup link
4. Configure VLTi link (VLT inter connect)
5. Configure VLT domain
6. Configure VLT port-channel

Step 1 - Check or configure spanning tree protocol
Rapid Spanning-Tree should be enabled to prevent configuration and patching mistakes. STP configuration depends on customer environment and spanning tree topology preferences. Below parameters are just examples.

Switch A - configured to become RSTP root

protocol spanning-tree rstp
 no disable
 hello-time 1
 max-age 6
 forward-delay 4
 bridge-priority 4096 (if you want to have this switch as STP root)

Switch B - configured as backup root.

protocol spanning-tree rstp
 no disable
 hello-time 1
 max-age 6
 forward-delay 4
 bridge-priority 8192

Step 2 - LLDP configutration
LLDP must be enabled to advertise theirs configuration and receive configuration information form the adjacent LLDP-enabled device.

Switch A

protocol lldp
  advertise management-tlv system-description system-name
  no disable

Switch B

protocol lldp
  advertise management-tlv system-description system-name
  no disable

Step 3 - VLT backup link
VLT backup link is used to exchange heartbeat messages between the two VLT peers. The Management interface at both VLT peers to activate the backup link.

Switch A

interface management 0/0
  ip address switch-A-IP/switch-A-mask
  no shutdown

Switch B

interface management 0/0
  ip address switch-B-IP/switch-B-mask
  no shutdown

Step 4 - VLTi (interconnect) link
Now we configure the VLTi, the connection between both VLT peers. It is recommended to use a Static Port channel for redundancy reasons. Two 40GbE interfaces are enough and we bound it at the Port channel 127.  No special configuration is required at the interface or Port channel configuration level. To become a VLTi (automatically managed by the system), the port-channel should be in default mode (no switchport).

Switch A
interface port-channel 127
  description "VLTi - interconnect link"
  channel-member VLTi_INTERFACE1
  channel-member VLTi_INTERFACE2
  no ip address 
  mtu 12000
  no shutdown

Switch B
interface port-channel 127
  description "VLTi - interconnect link"
  channel-member VLTi_INTERFACE1
  channel-member VLTi_INTERFACE2
  no ip address 
  mtu 12000
  no shutdown

Note 1: Don't forget to do no shutdown for physical interfaces acting as port-channel members. Your port-channel stay down unless you put them up.
Note 2: Port-channel nor physical ports must NOT be in switchmode to be used for VLTi.
Note 3: If you are planning to use jumbo frames (bigger MTU size) then you have to use it also for VLTi links (max MTU on Force10 is 12000 so it is good idea to set it to max).

Use following configuration for all VLTi interfaces
interface VLTi_INTERFACEx
  no shutdown
  no switchmode

Verify port-channel status on both switches
show int po 127 brief

Port-channel should be up and composed from 2 ports.

Step 5 - VLT domain configuration
 We have to configure the domain number and the VLT domain options described below.

• We use the peer-link command to select which is the VLTi interface.
• We have to select the interface for the heartbeat messages exchange we use the back-up destination command with the ip address of the other VLT peer.
• We should set the primary-priority command to configure the VLT role (primary or secondary). Primary VLT node will be the switch with lower priority. 
• The system-mac mac-address command must match at both peers in the VLT domain. 
• The unit id number 0 or 1 with the unit-id command will minimize the time required for the VLT system to determine the unit ID assigned to each peer switch when one peer switch reboots.

Switch A (primary)

vlt domain 1
  peer-link port-channel 127
  back-up destination switch-B-IP
  primary-priority 1
  system-mac mac-address 02:00:00:00:00:01
  unit-id 0

Switch B (secondary)

vlt domain 1
  peer-link port-channel 127
  back-up destination switch-A-IP
  primary-priority 8192
  system-mac mac-address 02:00:00:00:00:01
  unit-id 1

For verification we can use commands below

sh vlt brief
sh vlt statistics
sh vlt backup-link

Step 5 - VLT Port Channel
It is recommended that VLTs that are facing hosts/switches should be preferably built by LACP, to benefit from the protocol negotiations. However static port-channels are also supported.

It is also recommended to configure dampening (or equivalent) on the interfaces of connected hosts/switches (access switches, not VLT peers). The reason to use dampening is that at start-up time, once the physical ports are active a newly started VLT peer takes several seconds to fully negotiate protocols and synchronize (VLT peering, RSTP, VLT backup links, LACP, VLT LAG sync, etc). The attached devices are not aware of that activity and upon activation of a physical interface, the connected device will start forwarding traffic on the restored link, despite the VLT peer unit being still unprepared. It will black-hole traffic. Dampening on connected devices (access switches) will hold an interface temporarily down after a VLT peer device reload. A reload is detected as a flap: the link goes down and then up. Dampening acts as a cold start delay, ensuring that the VLT peers are up most ready to forward before the physical interface is activated, avoiding temporary black holes. Suggested dampening time: 30 seconds to 1 minute. We use 60 seconds in our example.

So let's finally configure the port channel (dynamic LAG) that interconnect the  S4810’s (VLT Domain) to the ustream S60 what is our hypotetical L3 switch (router).

Switch A

interface port-channel 1
  description "Uplink to S60"
  no ip address
  switchport
  vlt-peer-lag port-channel 1
  no shutdown

interface tengigabit 0/PO1-INTERFACE
  port-channel-protocol lacp
    port-channel 1 mode active
  dampening 10 100 1000 60
  no shutdown

Switch B

interface port-channel 1
  description "Uplink to S60"
  no ip address
  switchport
  vlt-peer-lag port-channel 1
  no shutdown

interface tengigabit 0/PO1-INTERFACE
  port-channel-protocol lacp
    port-channel 1 mode active
  dampening 10 100 1000 60
  no shutdown

Hope it is helpful not only for me but also for someone else. Any comments are welcome.