Flex-algo and On-demand Next-hop/Automated Steering together

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Flex-algo and ODN/Automated Steering | In this topic, we will bring all SR-TE ingredients together like Flex-algo, On-demand Next-hop and Automated Steering.


Till now in my earlier articles , we have discussed Flex-algo , ODN and Automated Steering and their usage  in SR traffic engineering . Please refer to below links for those detailed articles,

On-demand Next-hop
SR Policy

Now, we will see how ODN and Automated steering leverages Flex-algo. As a reminder, ODN enables automatic instantiation of SR Policy path when service routes are received from egress node via BGP (VPN routes). Automated Steering steers service routes into the SR policy . SR policy is identified by color of the service route and Next-hop. There is no change in their functionalities  while leveraging Flex-algo to encode SR-TE path.

The way it is done , Flex-algo SID algorithm is mapped to color which expresses the SLA constraint in the ODN template. For example, low-delay SLA constraint which is identified by color 100 and Flex-algo 130 is defined to provide low-delay path. So, SR Policy color is mapped to Flex-algo in the ODN template configured on Headend node. Below is the sample config for that, (using Cisco CLI commands for illustration purpose only).



Mapping Color with Flex-algo in ODN template example

Now, will use sample topology as given below for further illustration,

Figure 1

Suppose there is requirement of L3VPN from Node S1 to Node S4 with SR-TE ODN and SLA optimization of low-delay path.When VPN routes are received on headend node S1 with BGP nexthop as S4 with color 100 , ODN functionality instantiates SR Policy 1 to Node S4  based on the ODN template for color 100.ODN template restricts SR Policy to use Flex-algo SID as described earlier on how SR policy color is mapped with Flex-algo . SID list consists of just single SID , that is, Flex-algo prefix-SID 16304 correspond to Flex-algo 130 of Node S4 as shown .

BGP on Node S1 uses Automated Steering to steer the VPN routes into ODN based SR Policy which is identified by color 100. This is how ODN, Automated steering in SR-TE leverages Flex-algo to fulfil SLA optimization requirements.

Flex-algo is integral part of SR-TE architecture , Flex-algo SIDs are considered for SR-TE path calculation and can be included while creating SR policy.

I hope you will like this articles, please don’t hesitate to write comment below for any clarification or question you may have.

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