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The CNA system requires a BGP session and at least one network advertisement by a managed router. Without a BGP announcement, the CNA decision-maker process will never populate its database.
If you are using some protocol other than BGP to route your traffic to your WAN edge router and VPN device, you will need to add some BGP statements to their configurations so that at least one BGP prefix is announced to the CNA system. Since the only purpose of the announcement is to trigger the database construction, this can be any prefix, such as the router’s loopback address.
The following example shows how the statements would look in a Cisco router configuration:
router bgp 65002 no synchronization network 1.0.0.0 neighbor 172.16.6.14 remote-as 65002
The AS number in the BGP statement can be your network’s AS number or a private number, such as the 65002 AS number shown in the example. If the same number is used in both the router bgp and neighbor remote-as statements, the peering will be an iBGP session.
The network 1.0.0.0 statement forces the router to announce the loopback address (1.0.0.0) to the CNA system.
The no synchronization command guarantees that BGP will route to your remote network. Without the command, BGP will consider any path invalid unless it can find the address in its interior routing table, and the router will never install the route in its routing table. (If your interior routing is already working correctly, the no synchronization command may not be necessary, but it won’t hurt anything if it is present and is good insurance.)
The 172.16.6.14 address used in the neighbor remote-as command is the FastEthernet address of the CNA system’s engine module.
You will also need to use LSRR or GRE tunnels and PBR as described above in the section Measuring Performance to Remote Sites across both your VPN tunnels and the WAN links. These mechanisms will be used by the CNA system to send active probes to the other sites. These probes provide the traffic measurements used by the CNA decision-maker process to decide which of the two paths should be used at any given moment.
You will also need to make the Administrative Distance of whatever routing protocol you are using higher than 200 (the Administrative Distance associated with iBGP), so that the routes asserted by the CNA system will win over the routes selected by your routing protocol.
If your routers implement the OSPF (Open Shortest Path First) protocol, for example, you could set the Administrative Distance to 201 or higher by adding the Cisco IOS distance statement to the router ospf configuration block on each router:
router ospf 100 ... distance 201 ...
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