OS10 Enterprise Edition User Guide Release 10.4.1.0

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VLT backup

VLT backup link is an additional link to discovery interfaces that is used to check the availability of the adjacent node in the VLT setup.

When VLTi interface goes down, the system differentiates the link failure and node failure. If the VLTi link fails, all the VLT nodes are communicated about node liveliness through the backup link.

Based on the node liveliness information, the VLT LAG/port is in up state in the primary VLT peer and in down state in the secondary VLT peer.

Configure the VLT backup link using the backup destination { ip-address | ipv6 ipv6–address} [vrf management] [interval interval-time]

Example configuration:
OS10(config)# vlt-domain 1
                                 OS10(conf-vlt-1)# backup destination 10.16.151.110 vrf management interval 30
                                 
                                 OS10(config)# vlt-domain 1
                                 OS10(conf-vlt-1)# backup destination ipv6 1::1 vrf management interval 30
                                 
                              

The following examples describe different cases where VLT backup link can be used:

MAC and ARP not synchronized

When VLTi fails, MAC address learnt after the failure is not synchronized with VLT peers. This leads to continuous flooding of traffic instead of unicast.

Due to wrong hashing, ARP learning might fail leading to traffic being dropped.

In the following illustration, after VLTi is down VLT peer 1 learns MAC address of Host 2.

As VLTi link fails, the VLT peer1 is not synched up with the MAC address of Host2 . Due to this, if the traffic from Host1 is hashed to VLT peer2, then the VLT peer2 floods the traffic. Yet, the traffic would not reach Host2 as the VLT port between VLT peer 2 and Swicth 3 is down.

VLT backup link disabled

When VLT backup link is enabled, the secondary VLT peer 2 identifies the node liveliness through the backup link. If the primary is up, the secondary peer brings down the VLT LAG ports. Now the traffic from Host1 reaches VLT peer 1 and then reaches the destination, that is Host2. In this case the traffic is unicasted instead of flooding, as shown in the following illustration.

VLT backup link enabled

STP failure:

When VLTi is down, STP may fail to detect any loops in the system, which creates data loop in an L2 network.

In the following illustration, STP is running in all the three switches. In the steady state, VLT peer 1 is elected as the root bridge.

When VLTi is down, both the VLT nodes become primary. In this state, VLT peer 2 sends STP BPDU to TOR assuming that TOR sends BPDU to VLT peer 1. Due to this, VLT peer 2 does not receive BPDU on the VLT port, but receives TOR BPDU from orphan port. The STP in VLT peer 2 assumes that there is no loop in the system and opens up both the VLT and the orphan ports. This creates a data loop in the system which brings down the system.

VLT backup link disabled

When VLT backup link is enabled, the secondary VLT peer identifies the node liveliness of primary through the backup link. If the primary VLT peer is alive, the secondary VLT peer brings down the VLT LAG ports. In this scenario, the STP opens up the orphan port and there is no loop in the system as shown in the following illustration.

VLT backup link enabled

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