Dell EMC Networking, a Comparison Between Stacking and Virtual Link Trunking (VLT) A technical white paper Abstract A technical white paper comparing Stacking and Virtual Link Trunking (VLT). This whitepaper provides the essentials needed to understand and configure Stacking and VLT.
Comparison Between Stacking and VLT Revisions Date Description Author September 2019 Initial release 1.0 Umair Usmani Acknowledgements This paper was produced by the members of the Dell EMC network engineering team. The information in this publication is provided “as is.” Dell Inc. makes no representations or warranties of any kind with respect to the information in this publication, and specifically disclaims implied warranties of merchantability or fitness for a particular purpose.
Stacking and VLT Table of contents Revisions.............................................................................................................................................................................2 Acknowledgements .............................................................................................................................................................2 1 Introduction ....................................................................................................
Stacking and VLT 1 Introduction In this paper, we discuss and compare Stacking and Virtual Link Trunking (VLT) which allow us to physically connect several Dell EMC devices so that they appear as single unit to a third device. VLT connects two devices whereas stacking can connect more than two devices depending on the switch series. Both these techniques provide multipathing which allows the administrators to create redundancy by increasing bandwidth and providing active/active paths between devices.
Stacking and VLT 2 Overview of stacking In this technique, network switches are connected to operate as single unit called a stack. Such configuration can be used to quickly increase the capacity of the network. Stacking makes it easier for users to expand their network without introducing the complexity of managing multiple devices. Stackable switches can be added or removed from the stack without disturbing the overall performance of the stack.
Stacking and VLT firmware on the mismatched stack member. Once the firmware is synchronized on a member unit, the configuration on the member is updated to match the master switch. Also, when the startup configuration on the master switch is saved, it is automatically saved on the other members of the stack as well. 2.3 Election of stack master The election or re-election of stack master takes place based on the following considerations: • • • The switch is currently the stack master.
Stacking and VLT 2.5 Removal of the switch from the stack Before removing any member from the stack ensure that the other members of the stack will not become isolated from the stack due to the removal. Also, ensure that the ring topology can form a communication path around the member must be removed. Note that when removing a switch from the stack, disconnect all the links on the stack member. Also, statically re-route any traffic going through this unit.
Stacking and VLT Figure 1: Ring and cascade topology for stack In the case discussed here, we are using S4048 switch and stack groups 0 through 11 correspond to 10G stack groups with four ports each. Stack groups 12 to 17 are one 40G port each. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Stacking and VLT StandbyUnit#stack-unit 1 renumber 2 Renumbering management unit will reload the stack. Warning: Interface configuration for current unit will be lost! Proceed[confirm yes/no]:yes StandbyUnit#conf t StandbyUnit(conf)#stack-unit 2 priority 4 MasterUnit(conf)#stack-unit 1 priority 10 The stacking units can be connected while they are powered up or down.
Stacking and VLT Jumbo Capable POE Capable FIPS Mode Burned In MAC No Of MACs [Output Omitted] Dell EMC Technical White Paper : : : : : yes no disabled e4:f0:04:3f:ae:15 3
Stacking and VLT 3 Overview of Virtual Link Trunking (VLT) VLT aggregates two identical physical switches to form a single logical extended switch. This single logical entity ensures high availability and high resilience for all its connected core switches, and clients. Even though both the switches form together as a single logical unit, the control and data plan of both switches remain discrete. As a result, we can apply a switch firmware upgrade without bringing down the network.
Stacking and VLT 3.2 VLT operation Both the VLT nodes of domain always continue to forward data plane traffic in active/active mode. With the instantaneous synchronization of MAC and ARP entries, both the nodes remain active/active and continue to forward the data traffic seamlessly. 3.3 VLT functionality The same VLT domain-id should be configured on both VLT nodes. The unit-id 1 and 2 for the nodes is configured automatically.
Stacking and VLT 3.5 Link failover scenarios in VLT As shown in Figure 3, when the upstream layer-3 link 1 fails, the traffic is forced to take the alternate ECMP path to the VLT domain through link 2 to reach its destination. Figure 3: Link failure in upstream layer 3 link In this scenario, where the link 3 in the VLT port channel fails as shown in Figure 4, the traffic will then pass through the VLTi and then take the link 4 as the MAC learned on the failed VLT is now mapped to the VLTi port.
Stacking and VLT The back-up heartbeat messages are exchanged between the VLT peers through the back-up links of the management network. Therefore, if the VLTi link fails and the peers continue to exchange the heartbeat messages, the primary VLT peer knows that the secondary VLT peer is up. Since the MAC/ARP entries cannot be synchronized between the two VLT peers, the secondary VLT node closes the VLT port-channel as shown in the Figure 5.
Stacking and VLT VLTPeer1(config)# interface ethernet 1/1/38 VLTPeer1(conf-if-eth1/1/38)# no switchport VLTPeer1(conf-if-eth1/1/38)# exit VLTPeer1(config)# vlt-domain 1 VLTPeer1(conf-vlt-1)# discovery-interface ethernet 1/1/40 VLTPeer1(conf-vlt-1)# discovery-interface ethernet 1/1/38 VLTPeer1(conf-vlt-1)# vlt-mac 00:00:00:00:00:02 VLTPeer1(conf-vlt-1)# delay-restore 100 VLT configurations on Peer 2 VLTPeer2(config)# interface ethernet 1/1/40 VLTPeer2(conf-if-eth1/1/40)# no switchport VLTPeer2(conf-if-eth1/
4 Conclusion Both VLT and stacking technologies create a virtual switch where several switches are combined into one. This is not possible using standalone switches. Even though both these technologies give us similar capabilities, stacked switches act as a single switch from both data plane and control plane, so if switches need to be updated, all the switches need to be rebooted, and the network fails.
5 Appendix Campus switches Campus switches run OS6, and the maximum number of units they can support in a stack is given in the table below. Note that N1500, N2000 and N3000 cannot be stacked together. In OS 6.6, N3000 doesn’t support this branch of code. Number of switches per stack.
