The router has three tasks when supporting virtual inline mode:
- It must forward both incoming and outgoing WAN traffic to the SD-WAN appliance.
- It must forward traffic to its destination (WAN or LAN).
- It must monitor the health of the SD-WAN appliance so that the appliance can be bypassed if it fails.
In virtual inline mode, the packet forwarding methods can create routing loops if the routing rules do not distinguish between a packet that has been forwarded by the appliance and one that has not. You can use any method that makes that distinction.
A typical method involves dedicating one of the router’s Ethernet ports to the appliance and creating routing rules that are based on the Ethernet port on which packets arrive. Packets that arrive on the interface dedicated to the appliance are never forwarded back to the appliance, but packets arriving on any other interface can be.
The basic routing algorithm is:
- Do not forward packets from the appliance back to the appliance.
- If the packet arrives from the WAN, forward it to the appliance.
- If packet is destined for the WAN, forward to the appliance.
- Do not forward LAN-to-LAN traffic to the appliance.
- Traffic shaping is not effective unless all WAN traffic passes through the appliance.
Note: When considering routing options, keep in mind that returning data, not just outgoing data, must flow through the appliance. For example, placing the appliance on the local subnet and designating it as the default router for local systems does not work in a virtual inline deployment. Outgoing data would flow through the appliance, but incoming data would bypass it. To force data through the appliance without router reconfiguration, use inline mode.
If the appliance fails, data should not be routed to it. By default, Cisco policy based routing does no health monitoring. To enable health monitoring, define a rule to monitor the appliance’s availability, and specify the “verify-availability” option for the “set ip next-hop” command. With this configuration, if the appliance is not available, the route is not applied, and the appliance is bypassed.
Important: Citrix recommends virtual inline mode only when used with health monitoring. Many routers that support policy-based routing do not support health-checking. The health-monitoring feature is relatively new. It became available in Cisco IOS release 12.3(4)T.
Following is an example of a rule for monitoring the availability of the appliance:
!- Use a ping (ICMP echo) to see if appliance is connected track 123 rtr 1 reachabilit y ! rtr 1 type echo protocol IpIcmpecho 192.168.1.200 schedule 1 life forever start-time now
This rule pings the appliance at 192.168.1.200 periodically. You can test against 123 to see if the unit is up.