How is a sub-second failure of a transport link detected in a Cisco SD-WAN network?
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A. B. C. D.B.
In a Cisco SD-WAN network, there are different mechanisms that can be used to detect a sub-second failure of a transport link.
Option A: Hellos are sent between the WAN Edge routers and the vSmart controller. This option is partially correct. Hellos are sent between the WAN Edge routers and the vSmart controller to maintain connectivity and exchange information about the network topology. However, Hellos are not used to detect link failures but rather to detect the loss of connectivity between the routers and the controller.
Option B: BFD (Bidirectional Forwarding Detection) runs on the IPsec tunnels between WAN Edge routers. This option is the most accurate answer. BFD is a protocol that can be used to quickly detect link failures in a network. It works by periodically sending lightweight detection packets over a link and waiting for a response from the other end. If no response is received within a certain time period, the link is considered to have failed. In a Cisco SD-WAN network, BFD can be used on the IPsec tunnels between WAN Edge routers to detect sub-second link failures.
Option C: BGP (Border Gateway Protocol) is used between WAN Edge routers and the vSmart controller. This option is also partially correct. BGP is used in a Cisco SD-WAN network to exchange routing information between the WAN Edge routers and the vSmart controller. However, BGP is not specifically designed to detect link failures but rather to route traffic around them.
Option D: Link state change messages are sent between vSmart controllers. This option is not correct. Link state change messages are not used in a Cisco SD-WAN network to detect link failures. These messages are used in other types of networks, such as OSPF (Open Shortest Path First), to inform routers of changes in the network topology.
In summary, the most accurate answer to the question is B. BFD runs on the IPsec tunnels between WAN Edge routers to quickly detect sub-second link failures in a Cisco SD-WAN network.