Coloring plays very important role in Automated steering solution. Color information is exchanged by BGP extended community attribute. Egress PE node colors the service routes (VPN routes) , advertises to ingress PE with colors that signifies SRTE SLA requirement or the intent. Ingress node uses this color to match SR policy, once node identifies SR policy by matching color , it automatically steer traffic onto the SR policy. This is called Automated Steering.
There are two types of Automated steering possible,
1. Per-destination Automated steering – it automatically steers service route onto SR policy based on color and next-hop address.
2. Per-flow Automated steering – It automatically steers traffic flows based on fields like DSCP value or source address etc. (basically requires a match criteria which can identify specific flow rather than all traffic)
Lets understand the concept of Automated steering with example topology, refer to below figure 1,
In the above figure, say Node S1 is the ingress PE and S4 is the egress PE node. Requirement is to have L3VPN between S1 and S4 with SLA optimization path with metric delay.
Node S4 advertises service route x.x.x.x/y using MP-BGP with color 100 (color is exchanged using BGP extended communities) and Next-hop as S4 towards Node S1. Node 4 adds color 100 to indicate SLA requirement of low delay path and this information reaches ingress node S1 via BGP update. Now, Node S1 is configured with SR policy with color 100 and is configured with path calculation metric as delay. When Node 1 receives service routes via BGP, it tries to match route color and Next-hop with SR policy’s color and Endpoint (which are configured as SR policy).
When BGP finds matching SR policy with color 100, it installs the prefix x.x.x.x/y in the forwarding table . All traffic destined for x.x.x.x/y in Node 1, is forwarded via SR policy with color 100.
By mapping color of the BGP prefix to the color of the SR policy, traffic steering becomes automated and avoids complex configuration. This is Automated steering concept in SRTE. I hope this article is useful to understand AS concept.