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If you have enabled SNMP on your edge routers, the CNA system can be configured to monitor the status of your service provider links so that the CNA decision-maker process can quickly respond to a link outage. The CNA link-monitor process periodically polls the SNMP agent on your edge routers.
Without SNMP, the CNA system will have to rely solely on per-prefix outage control to determine whether a service provider access link is functioning properly. Per-prefix outage control depends on a relatively high rate of traffic measurements to function efficiently. SNMP polling will speed up the detection of outages for prefixes with low measurement rates.
For information about configuring the CNA link monitor, see Chapter 4: Polling Routers of the CNA Advanced Monitoring Guide.
In addition to enabling link monitoring, you can also add the require-bgp-snmp-poll command to your CNA configuration. Adding the this command allows the CNA system to monitor links as winner candidates where SNMP is enabled.
When BGP status polling is enabled, the link-monitor process polls the SNMP agent on each edge router for status of the BGP peering at user-determined intervals of between 30 seconds and one hour.
The require-bgp-snmp-poll command sets a flag which tells the decision-maker process to immediately stop asserting routes if BGP status is reported as unknown a user-determined number of consecutive times.
If SNMP is not running on the edge routers when the decision-maker process is started, all links will be considered to be up. If you then enable SNMP on the edge routers, the link-monitor process will function correctly (assuming it is properly configured).
If SNMP stops running on your edge routers, the decision-maker process will consider the links to be unchanged from the last reported state until SNMP resumes or the decision-maker process is restarted.
If the require-bgp-snmp-poll command is enabled but SNMP polling is not configured or there is no SNMP agent, the decision-maker process will consider all links to be up, until the SNMP polling process begins to function correctly.
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