Observer nTAPs : Observer nTAPs : FAQ : Understanding why Link B is active when Link A is offline
Understanding why Link B is active when Link A is offline
Link B is an active port. It is used to negotiate speeds for both Link A and Link B.
Applies to: Any copper-based nTAP.
When the main use of the nTAP is to monitor a server connection, Link A is for the server and Link B is for the router or switch. This allows the server to use a redundant link if Link B goes down, and it keeps the router or switch active if the server goes offline. Should Link A come back up, negotiations to get the link back online are enhanced because Link B already has an active link.
As already stated, Link B is an active port. It is used to negotiate speeds for both Link A and Link B. When you plug in Link A by itself, no negotiation occurs. If you plug in Link B, it negotiates a link speed with whatever device is connected to Link B. Then it negotiates with Link A at that speed. If Link A cannot use that speed, it then negotiates with the end device on Link B at a different rate until a compatible rate between the Link A device and Link B can be established.
One of the great advantages to having this capability is to use the nTAP to replicate traffic to multiple devices and not use it strictly for pass through. For example, when you use an aggregation nTAP and if you connect Link B to a SPAN, you can then pass the SPAN traffic out the two analyzer ports and have two copies of the SPAN traffic going to two different devices. You can have another device receiving the SPAN data on Link A, and if you disconnect Link A, the SPAN traffic for Link B still goes to the analyzer ports for monitoring.
An nTAP is not just for only passing bidirectional communication between Link A and Link B and copying traffic to the two analyzer ports. Take advantage of the active Link B port to daisy chain multiple TAPs together to receive multiple sets of SPAN data streams and combine the multiple SPAN sessions into a single stream. Without the ability for Link B to stay up if Link A were to go offline, you lose this capability.