Microwave links are often used in Ethernet access ring topologies and the bandwidth provided by the microwave link depends
on environmental factors like fog, rain, and snow, which can drastically affect the bandwidth.
This feature relies on the Ethernet CFM to assess the environmental conditions on either end of the microwave link and automatically
change the modulation to provide optimal bandwidth. The Ethernet CFM monitors the microwave link bandwidth, and when a link
degradation is detected, notifies the router to take action on the degraded microwave link.
In IP/MPLS, the nodes are unaware of any changes to the bandwidth on the microwave link and the Gigabit Ethernet connection
to the nodes remain constant. To ensure optimal routing and traffic transport across the access network, a mechanism has been
implemented to notify the IP/MPLS access nodes of any ACM events on the microwave links. This enables microwave radio transceivers,
which support ACM, to report link bandwidth information to an upstream Ethernet switch.
The vendor-specific message (VSM) in Y.1731 is used to notify Cisco routers of ACM events, and the bandwidth available on
the microwave link. Acting on this information, the node can change the Hierarchical Quality of Service (H-QoS), adjust the
Interior Gateway Protocol (IGP) metric of the link to the new capacity or remove the degraded link.
H-QoS Policy Adjustment
H-QoS policy adjustment is the process of adjusting the egress H-QoS policy parameters on the IP/MPLS access node connected
to the microwave link. This modifies the parent shaper rate to match the current bandwidth of the microwave link. It also
adjusts the child class parameters to ensure correct priority and bandwidth-guaranteed traffic.
If the available bandwidth is less than the total bandwidth required by Expedited Forwarding (EF) and Assured Forwarding (AF)
classes, the operator can choose to drop AF class traffic or remove the link from the service.
IGP Metric Adjustment
The IP/MPLS access node can adjust the IGP metric on the microwave link to align it with the available bandwidth. This will
trigger an IGP SPF recalculation, allowing the IGP to get the correct bandwidth for routing traffic.
Link removal is the process of removing the microwave link from the IGP. This occurs when the bandwidth loss breaches the
threshold set by the operator. It sets off the resiliency mechanisms in the network, and the degraded link is bypassed, resulting
in minimal traffic loss. The degraded link is not brought administratively down. When it is up, the microwave equipment can
signal to the access node about its status and usability.