net/frr: Improve docs

[Monviech]

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net/frr could use some more docs, as discussed. Mostly focused on simple BGP, OSPF related routing. Explaining some of the available options.

Not going to dive too deep into advanced options since the crowd who uses these options know what they are doing, and I am not that deep. :)

[Monviech]

Current document:

https://docs.opnsense.org/manual/dynamic_routing.html

• Explain some of the menus and options better. Focus on OSPF, BGP and BFD. Since iBGP can do IPv4 and IPv6 I do not want to get into OSPFv3 too much.

• Remove pictures and replace with text and tables to improve future maintainability.

• Some general how tos:

  • OSPF with IPsec VTI to create failover VPN routing between two OPNsense firewalls with multiple WAN connections
  • BGP to dynamically change default route to the internet based on path costs
    • Considerations with NAT
    • Using Route Maps and Prefix Lists to prevent unwanted routes being received or advertised (especially important when peering with public AS)
  • BFD to increase the responsiveness of BGP neighbors

• Potential Drawbacks:

  • Issues with states when using CARP and BGP at the same time
  • Asymmetric routing
  • When peering with multiple eBGP AS from OPNsense Firewalls in HA, how it could be necessary to use Unicast CARP on the WAN interfaces