Is there any source IP persistence with “Auto” link steering?
When the SD-WAN Edge is having multiple Internet WAN links, and performing local Internet breakout, there is a common question: “Is there any type of client side/source IP persistence with “Auto” link steering?”. For example, in this post, the Edge-1 is having two Internet WAN links: GE3 (98.1.2.19) and GE4 (184.1.2.27). If the Client100 (192.168.200.100) open a browser access web service such as wordpress05 (43.254.254.14), the browser can open multiple TCP sessions which result of multiple TCP flows for Edge-1 to local Internet breakout. If the flows spread across two WAN links GE3 and GE4, the web server will see the request from the same client from different source IP address, this might confuse the web server, or a load balancer in front of the web server, and cause issue for accessing the web services.
If the reader takes a look at Test 3.2 and Test 3.3 (or you still remember the result), during these two tests, the link steering setting is “Auto”. Furthermore, the traffic flows are always initiated from the same client machine Client100 (192.168.200.100). From result of Test 3.2 and Test 3.3, we can realize with “Auto” link steering, the SD-WAN Edge does not have any client side/source IP persistence consideration for selecting which WAN link to use. For example, if at time 0 the Edge-1 selected GE4 (184.1.2.27) for local Internet breakout, and then Client100 (192.168.200.100) initiate a new TCP flow at time N. At this time N, GE4 (184.1.2.27) downstream bandwidth is mostly consumed, then Edge-1 will select GE3 (98.1.2.19) for the new TCP flow. This will end up Client100(192.168.200.100) has some flows using GE4 (184.1.2.27) while some other flows using GE3 (98.1.2.19).
If source IP persistence is desired, a possible workaround is to use “Preferred” link steering instead of “Auto” link steering. Take a look of the following business polices configuration as an example:
For client IP address ends with even number, the traffic flow will match business policy “EvenIP-Prefer-GE3-UDP” or “EvenIP-Prefer-GE3-Other”, which will be preferably using GE3 (98.1.2.19). For client IP address ends with odd number, the traffic flow will match business policy “OddIP-Prefer-GE4-UDP” or “OddIP-Prefer-GE4-Other”, which will be preferably using GE4 (184.1.2.27). This sample configuration will be very similar to having client IP address persistence. The only exception is when the preferred link is down or unable to match the latency/jitter/packet loss SLA.