Need more specific routes for underlay/bootstrap egress in switch zone

[rcgoodfellow]

Currently the switch zone relies on a default route to reach the underlay and boostrap networks. If a techport attached dhcpv6 server advertises a default router, then we will have intermittent comms as route selection will pick the techport from time to time where we really want the oxControlService0 or the oxBootstrap0 interfaces.

To protect against this, we should have more specific routes in the switch zone for the underlay and bootstrap network prefixes.

For the underlay network, we should rely on a route to AZ /48 out oxControlService0 and for the bootstrap we should rely on a static route out oxBootstrap0 for fdb0:a840:25::/40

[rcgoodfellow]

Added corresponding documentation issue on DHCPv6 limitations here

• oxidecomputer/docs#710