The Ethernet Bandwidth Notification (ETH-BN) function is used by a server MEP to signal changes in link bandwidth to a client MEP.
This functionality is for point-to-point microwave radios to modify the downstream traffic rate toward the microwave radio to match its microwave link rate. When a microwave radio uses adaptive modulation, the capacity of the radio can change based on the condition experienced by the microwave link. For example, in adverse weather conditions that cause link degradation, the radio can change its modulation scheme to a more robust one (which reduces the link bandwidth) to continue transmitting. This change in bandwidth is communicated from the server MEP on the radio, using ETH-BN Message (ETH-BNM), to the client MEP on the connected router. The server MEP transmits periodic messages with ETH-BN information including the interval, the nominal, and currently available bandwidth. A port MEP with the ETH-BN feature enabled processes the information in the CFM PDU. The operational port egress rate can be modified to adjust the rate of traffic sent to the radio.
A port MEP supports the client side reception and processing of the ETH-BNM sent by the server MEP. By default, processing is disabled. A port that can process an ETH-BNM is a configuration specific to that port, even when the port is a LAG member port. The ETH-BN configuration on the LAG member ports does not have to be the same. However, mismatches in the configuration on these member ports could lead to significant differences in operational egress rates within the same LAG. Different operational rates on the LAG member ports as a result of ETH-BN updates are not considered when hashing packets to the LAG member ports.
The no form of the configure port ethernet eth-cfm mep eth-bn receive CLI command sets the ETH-BN processing state on the port MEP. A port MEP supports untagged packet processing of ETH-CFM PDUs at domain levels 0 and 1 only. The port client MEP sends the ETH-BN rate information received to be applied to the port egress rate in a QoS update. A pacing mechanism limits the number of QoS updates sent. The configure port ethernet eth-cfm mep eth-bn rx-update pacing CLI command allows the updates to be paced using a configurable range of 1 to 600 seconds (the default is 5 seconds). The pacing timer begins to countdown following the most recent QoS update sent to the system for processing. When the timer expires, the most recent update that arrived from the server MEP is compared to the most recent value sent for system processing. If the value of the current bandwidth is different from the previously processed value, the update is sent and the process begins again. Updates with a different current bandwidth that arrive when the pacing timer has already expired are not subject to a timer delay. See the 7450 ESS, 7750 SR, 7950 XRS, and VSR Interface Configuration Guide for more information about these commands.
A complementary QoS configuration is required to allow the system to process nominal bandwidth updates from the CFM engine. The configure port ethernet no eth-bn-egress-rate changes CLI command is required to enable the QoS function to update the port egress rates based on the current available bandwidth updates from the CFM engine. By default, the function is disabled.
Both the CFM and the QoS functions must be enabled for the changes in current bandwidth to dynamically update the egress rate.
When the MEP enters a state that prevents it from receiving the ETH-BNM, the current bandwidth last sent for processing is cleared and the egress rate reverts to the configured rate. Under these conditions, the last update cannot be guaranteed as current. Explicit notification is required to dynamically update the port egress rate. The following types of conditions lead to ambiguity:
administrative MEP shutdown
port admin down
port link down
eth-bn no receive transitioning the ETH-BN function to disable
If the configure port ethernet eth-bn-egress-rate-changes command is disabled using the no option, CFM continues to send updates, but the updates are held without affecting the port egress rate.
The ports supporting ETH-BN MEPs can be configured for network, access, or hybrid modes. When ETH-BN is enabled on a port MEP and the config>port>ethernet>eth-cfm>mep>eth-bn>receive and the QoS config>port>ethernet>eth-bn-egress-rate-changes contexts are configured, the egress rate is dynamically changed based on the current available bandwidth indicated by the ETH-BN server.
The port egress rate is capped by the minimum of the configured egress rate and the maximum port rate. The minimum egress rate is one kbyte. If a current bandwidth of zero is received, it does not affect the egress port rate and the previously processed current bandwidth continues to be used.
The client MEP requires explicit notification of changes to update the port egress rate. The system does not timeout any previously-processed current bandwidth rates using a timeout condition. The specification does allow a timeout of the current bandwidth if a message has not been received in 3.5 times the ETH-BN interval. However, the implicit approach can lead to misrepresented conditions and has not been implemented.
When starting or restarting the system, the configured egress rate is used until an ETH-BNM arrives on the port with a new bandwidth request from the ETH-BN server MEP.
An event log is generated each time the egress rate is changed based on reception of an ETH-BNM. If an ETH-BNM is received that does not result in a bandwidth change, no event log is generated.
The destination MAC address can be a Class 1 multicast MAC address (that is, 01-80-C2-00-0x) or the MAC address of the port MEP configured. Standard CFM validation and identification must be successful to process any CFM PDU.
For information about the eth-bn-egress-rate-changes command, see the 7450 ESS, 7750 SR, 7950 XRS, and VSR Interface Configuration Guide.
The PDU used for ETH-BN information is called the Bandwidth Notification Message (BNM). It is identified by a sub-OpCode within the Ethernet Generic Notification Message (ETH-GNM).
Table: BNM PDU format fields shows the BNM PDU format fields.
| Label | Description |
|---|---|
MEG Level |
Carries the MEG level of the client MEP (0 to 7). This field must be set to either 0 or 1 to be recognized as a port MEP. |
Version |
The current version is 0. |
OpCode |
The value for this PDU type is GNM (32). |
Flags |
Contains one information element: Period (3 bits) to indicate how often ETH-BNM messages are transmitted by the server MEP. Valid values are:
|
TLV Offset |
This value is set to 13. |
Sub-OpCode |
The value for this PDU type is BNM (1). |
Nominal Bandwidth |
The nominal full bandwidth of the link, in Mbytes/s. This information is reported in the display but not used to influence QoS egress rates. |
Current Bandwidth |
The current bandwidth of the link in Mbytes/s. The value is used to influence the egress rate. |
Port ID |
A non-zero unique identifier for the port associated with the ETH-BN information, or zero if not used. This information is reported in the display, but is not used to influence QoS egress rates. |
End TLV |
An all zeros octet value. |
The show eth-cfm mep eth-bandwidth-notification display output includes the ETH-BN values received and extracted from the PDU, including the last reported value and the pacing timer. If n/a appears in the field, it means that field has not been processed.
The base show eth-cfm mep output is expanded to include the disposition of the ETH-BN receive function and the configured pacing timer.
The show port port-id detail is expanded to include an ETH-BNM section. This section includes the egress rate disposition and the current egress BN rate being used.