Virtual Inter-Switch Trunk (vIST)

Split MultiLink Trunking provides subsecond failover when a switch fails. Virtual Inter-Switch Trunk (vIST) improves upon that Layer 2 and Layer 3 resiliency by using a virtualized IST channel through the SPBM cloud. The vIST channel carries the vIST control traffic and data traffic during an SMLT failover. This feature dramatically improves resiliency over other methods.

Because vIST uses a virtual channel and because IS-IS runs over it, vIST eliminates the potential single point of failure with a dedicated MLT. The vIST channel is always up as long as there is SPBM connectivity between the vIST peers.

vIST interoperates between any two devices that support vIST, and the devices do not have to be of the same type.

vIST creates a virtualized channel through the SPBM cloud, and this channel connects two SMLT devices to form a virtualized Switch Cluster. Note that the SPBM cloud can consist of as few as two nodes.

Important

Do not change the system MTU to less than the default value of 1950 bytes. The system MTU must be 1950 or jumbo because of the header size increase when transmitting packets over the SPBM cloud.

Note

• Users may observe a momentary increase in activity when a MAC delete message is received from a peer. This is due to vIST engaging in MAC learning activities. This is a normal operational procedure.

• For proper traffic flow, if a Layer 2 VSN is created on one vIST peer, it must also be created on the other vIST peer. For more information on Layer 2 VSN, see Layer 2 VSN Configuration.

vIST configuration note

If you need to update the vIST VLAN IP address on vIST peers by deleting and recreating the vIST vlan IP address (for example, as part of maintenance), ensure that you update one vIST BEB at a time.

Caution

Always perform vIST configuration updates under no traffic. Otherwise, it results in traffic loss.

Before you begin updating a device, as a first step, isolate the device by shutting down all the links and failing over the traffic to its vIST peer. Then, delete and recreate the vIST VLAN IP on the device and save your configuration. When bringing the device back into operation, first unshut those network-to-network interface (NNI) ports that bring up vIST, followed by the SMLT configured ports, and then all the remaining ports, to prevent network loops or duplicate traffic.

For information on vIST configuration, see Create a Virtual IST or Create a Virtual IST using EDM.

vIST operational note

When you enable IST and boot the chassis, the SMLT enabled trunk ports (SMLT ports) are automatically locked. A timeout mechanism automatically unlocks the SMLT ports when the IST control channel fails to establish within a reasonable amount of time. The timeout mechanism prevents the SMLT ports from being locked forever. Initially 240 seconds are allowed for the switch to determine the IST VLAN status.

The IST VLAN is considered up if at least one port is forwarding traffic and an ARP entry is populated for the IP address of the IST peers. Once the IST VLAN is up, the timeout value is reset to 60 seconds. The IST control channel must be up within the timeout period. If the timeout period is exceeded, then the SMLT ports are automatically unlocked and a message is logged stating that the SMLT ports are unlocked due to a timeout.

Note

If the IST filter is enabled before the timeout, then the IST filter is unaffected and remains enabled.

If a virtual BMAC mismatch occurs between two vIST peers, incorrect information can show in CLI regarding the ports that the client information is learned on, such as MAC addresses or IPv6 neighbor addresses. This can occur before the mismatch is detected, because the two peers receive traffic from each other and learn the information on the local ports. Each vIST peer synchronizes the information as learned locally, even when learned from vIST peer. You can resolve the mismatch by changing the virtual BMAC on one of the vIST peers, but the trafffic might remain affected. After confirming the mismatch is resolved from a log report, you can resolve the traffic issue by disabling and enabling the router ISIS on both vIST peers.