docs: Add dynamic routing (BGP) configuration guide.

Add a configuration and usage guide for OVN's dynamic routing
feature covering IP route exchange with external routing
daemons.

The guide is incomplete on purpose, all of these configuration
options are already described in the main ovn-nb man page and
in the architecture document.  The added value is some more
details and a few examples.

The guide includes:
- Two examples: gateway router with BGP and distributed router
  with gateway ports.
- Verification commands and best practices.

Also update the architecture document with information that
was missing:
- Add hub-spoke to the redistribution options list.
- Fix VRF table ID description to list the exact excluded
  values (0, 252, 253, 254, 255, 4294967295).
- Add Routing Protocol Redirect section.
- Add Port-to-Interface Mapping section.
- Improve connected-as-host + local-only grouping.

Also expand the dynamic-routing-v4-prefix-nexthop description
in ovn-nb.xml to explain the RFC 5549 use case.

Reported-at: https://redhat.atlassian.net/browse/FDP-3119
Assisted-by: Claude Opus 4.6, Claude Code
Acked-by: Martin Kalcok <martin.kalcok@gmail.com>
Acked-by: Mark Michelson <mmichels@redhat.com>
Signed-off-by: Dumitru Ceara <dceara@redhat.com>
Signed-off-by: Mark Michelson <mmichels@redhat.com>

Author: dceara

Documentation/automake.mk

@@ -26,6 +26,7 @@ DOC_SOURCE = \
 	Documentation/topics/test-development.rst \
 	Documentation/topics/high-availability.rst \
 	Documentation/topics/dynamic-routing/architecture.rst \
+	Documentation/topics/dynamic-routing/configuration-examples.rst \
 	Documentation/topics/dynamic-routing/index.rst \
 	Documentation/topics/incremental-processing/datapath-sync-graph.png \
 	Documentation/topics/incremental-processing/evpn-arp-graph.png \

Documentation/topics/dynamic-routing/architecture.rst

@@ -288,6 +288,9 @@ accepts a comma-separated list of the following values:
 - ``lb`` --- The VIP address of each load balancer associated with this
   router and neighboring routers.
 
+- ``hub-spoke`` --- Routes learned through OVN Interconnection (OVN-IC)
+  from other routers, enabling hub-and-spoke propagation.
+
 These options can also be set per logical router port, overriding the
 router-level setting for routes associated with that specific port.
 
@@ -322,15 +325,18 @@ When the ``tracked_port`` field is set on an ``Advertised_Route`` record,
 ``ovn-controller`` adjusts the route metric based on whether the tracked
 port is locally bound on this chassis.  Routes for locally bound ports
 receive a higher priority (lower metric value), which causes the routing
-daemon to prefer the chassis that actually hosts the workload.  This
-mechanism is particularly useful for host routes generated by the
-``connected-as-host`` redistribution mode.
+daemon to prefer the chassis that actually hosts the workload.
 
 The ``dynamic-routing-redistribute-local-only`` option further refines
 this behavior: when set to ``true``, ``ovn-controller`` only installs
 routes on the chassis where the ``tracked_port`` is locally bound,
 preventing other chassis from advertising the route at all.
 
+This is particularly useful with ``connected-as-host`` redistribution
+to ensure host routes are only announced from the chassis that owns the
+workload, providing optimal traffic forwarding and avoiding unnecessary
+traffic tromboning.
+
 IP Route Learning
 -----------------
 
@@ -391,8 +397,9 @@ The VRF routing table ID is determined by one of the following, in order
 of precedence:
 
 1. The ``dynamic-routing-vrf-id`` option on the logical router, if set to
-   a valid integer (1-4294967295, excluding reserved table IDs such as
-   ``RT_TABLE_MAIN`` and ``RT_TABLE_LOCAL``).
+   a valid integer in the range (1-4294967294), excluding the reserved
+   table IDs 252 (``RT_TABLE_COMPAT``), 253 (``RT_TABLE_DEFAULT``),
+   254 (``RT_TABLE_MAIN``), and 255 (``RT_TABLE_LOCAL``).
 
 2. The tunnel key of the logical router datapath, used as a fallback
    when ``dynamic-routing-vrf-id`` is not configured.
@@ -423,6 +430,57 @@ If ``dynamic-routing-maintain-vrf`` is ``false`` (the default), the VRF
 is expected to already exist on the chassis, managed by external tooling
 or configuration management.
 
+Routing Protocol Redirect
+-------------------------
+
+OVN can optionally redirect routing protocol control plane traffic from
+a logical router port to a logical switch port where an external routing
+daemon is listening.  This allows the routing daemon to peer using the
+router port's IP addresses through an OVN-managed logical switch port.
+
+Routing protocol redirect is entirely optional.  The routing daemon can
+establish its BGP or BFD sessions completely outside of OVN --- for
+example, on a separate physical interface, a loopback, or any other
+interface that is not managed by OVN.  Use this feature only when you
+want the routing daemon to peer using the logical router port's IP
+addresses through an OVN-managed logical switch port.
+
+The ``routing-protocols`` option on a ``Logical_Router_Port`` specifies
+which control plane protocols to redirect:
+
+- ``BGP`` --- Redirects TCP port 179 (BGP session establishment and
+  keepalives).
+
+- ``BFD`` --- Redirects UDP port 3784 (Bidirectional Forwarding
+  Detection).
+
+The ``routing-protocol-redirect`` option on the same port identifies the
+logical switch port where the routing daemon is bound.  OVN installs
+logical flows to forward matching control plane traffic between the
+router port and the specified switch port.
+
+For BGP unnumbered deployments, periodic IPv6 Router Advertisements can be
+enabled on the logical router port via the ``ipv6_ra_configs`` column.
+This provides automatic link-local address discovery, which is required
+for establishing BGP sessions over IPv6-only peering links that carry
+both IPv4 and IPv6 routes.  In order for this to function, automatic address
+discovery needs to also be supported and configured on remote BGP peer.
+
+Port-to-Interface Mapping
+-------------------------
+
+When a chassis has multiple links toward the fabric, each running BGP
+independently, the ``dynamic-routing-port-name`` option on a logical
+router port restricts route learning to a specific Linux interface.
+``ovn-controller`` resolves the port name to a Linux interface.  If the
+referenced port is bound locally, the interface name is discovered
+automatically.
+
+When automatic discovery is not available (for example, the port is not
+bound locally), the ``dynamic-routing-port-mapping`` option on the
+``Open_vSwitch`` table provides an explicit mapping from logical port
+names to Linux interface names.
+
 EVPN (Ethernet VPN) Integration
 -------------------------------