ces into one virtual link is formally known as a Link Aggrega- tion Group (LAG) and specified in IEEE 802.1AX-2020 (formerly IEEE 802.3ad). When a PTP message is queued to a LAG interface, the driver must select one physical link from the group down which to forward that message. That decision may be made based on some algorithm, such as one based on hashing several fields from the IP header. Each end of the link makes that decision somewhat autonomously and therefore the link chosen by each side of the link can differ. This difference in link selection in opposite directions can also cause asymmetry. When two PTP BCs are connected to each other, then it is possible to ensure that the PTP imple- mentation can be enhanced to enable the engineer to configure the LAG to use a specific physical link to transmit PTP messages. The LAG standard also includes the Link Aggregation Control Protocol (LACP) that is used to con- trol a LAG link. It is used to communicate and negotiate parameters between the two ends of the link. When two T-BCs are connected to each other, the PTP implementation can be enhanced to use LACP to negotiate the link selection. On the other hand, with PTP-unaware nodes, the NE at each end of the link has no idea how to treat transit PTP messages, and SO it just treats them according to its configured link selection algorithm. Figure 11-9 illustrates this problem. Node 1 puts PTP in link 2 Node 2 puts PTP in link 3 Link 1 Link 2 Link 3 PTP-unaware PTP-unaware node 1 node 2 Figure 11-9 Asymmetry in LAG Bundles in PTP-Unaware Nodes Note that there are other forms of asymmetry that could arise from factors external to the network elements and instead arise in the links between them. This can be as simple as a mismatch in length between the transmit and receive fibers, where somebody might have patched one direction of the path with a 1-m patch cable and the other with a 10-m cable. It can also arise where a fiber may have been cut and the repairer added a length (loop) of spare fiber when splicing it ba