Underlay/overlay AS and auto-derived route targets

[jbemmel]

The current configs (e.g. https://github.com/srl-labs/srl-rt5-l3evpn-basics-lab/blob/main/startup_configs/leaf1.conf#L52) use a unique AS for the underlay at each leaf, and then manually override the EVPN route target (e.g. https://github.com/srl-labs/srl-rt5-l3evpn-basics-lab/blob/main/startup_configs/leaf1.conf#L140) such that the services on both leaves are interconnected through a common RT

Alternatively, the underlay AS on all leaves could be set to the overlay AS (65535, see https://github.com/srl-labs/srl-rt5-l3evpn-basics-lab/blob/main/startup_configs/leaf1.conf#L90) with a local-as override to a unique value for each leaf. That way, there is no need to override the auto-derived RT for each IP-VRF tenant service.

The latter option is easier to automate, simplifying incremental changes when adding new services / tenants

[hellt]

I didn't go with the overlay AS in the default BGP config because it would break the story line. But operationally it makes sense