Add release note for BGP routing

- https://issues.redhat.com/browse/OSDOCS-10810

Author: jab-rh

release_notes/ocp-4-19-release-notes.adoc

@@ -460,6 +460,52 @@ For more information, see xref:../updating/preparing_for_updates/updating-cluste
 
 With this release, {product-title} routes can be configured with third-party certificate management solutions, utilising the `.spec.tls.externalCertificate` field in the route API. This allows you to reference externally managed TLS certificates through secrets, streamlining the process by eliminating manual certificate management. By using externally managed certificates, you reduce errors, ensure a smoother certificate update process, and enable the OpenShift router to promptly serve renewed certificates. For more information, see xref:../networking/routes/secured-routes.adoc#nw-ingress-route-secret-load-external-cert_secured-routes[Creating a route with externally managed certificate].
 
+[id="ocp-4-19-support-for-bgp-routing-protocol_{context}"]
+==== Support for the BGP routing protocol
+
+The Cluster Network Operator (CNO) now supports enabling Border Gateway Protocol (BGP) routing. With BGP, you can import and export routes to the underlying provider network and use multi-homing, link redundancy, and fast convergence. BGP configuration is managed with the `FRRConfiguration` custom resource (CR).
+
+When upgrading from an earlier version of {product-title} in which you installed the MetalLB Operator, you must manually migrate your custom frr-k8s configurations from the `metallb-system` namespace to the `openshift-frr-k8s` namespace. To move these CRs, enter the following commands:
+
+. To create the `openshift-frr-k8s` namespace, enter the following command:
++
+[source,terminal]
+----
+$ oc create namespace openshift-frr-k8s
+----
+
+. To automate the migration, create a `migrate.sh` file with the following content:
++
+[source,bash]
+----
+#!/bin/bash
+OLD_NAMESPACE="metallb-system"
+NEW_NAMESPACE="openshift-frr-k8s"
+FILTER_OUT="metallb-"
+oc get frrconfigurations.frrk8s.metallb.io -n "${OLD_NAMESPACE}" -o json |\
+  jq -r '.items[] | select(.metadata.name | test("'"${FILTER_OUT}"'") | not)' |\
+  jq -r '.metadata.namespace = "'"${NEW_NAMESPACE}"'"' |\
+  oc create -f -
+----
+
+. To run the migration script, enter the following command:
++
+[source,terminal]
+----
+$ bash migrate.sh
+----
+
+. To verify that the migration succeeded, enter the following command:
++
+[source,terminal]
+----
+$ oc get frrconfigurations.frrk8s.metallb.io -n openshift-frr-k8s
+----
+
+After the migration is complete, you can remove the `FRR-K8s` custom resources from the `metallb-system` namespace.
+
+For more information, see x!ref:../networking/bgp_routing/about-bgp-routing.adoc#about-bgp-routing[About BGP routing].
+
 [id="ocp-4-19-networking-gateway-api-controller_{context}"]
 ==== Support for using the Gateway API to configure cluster Ingress traffic
 We will have details here when {product-title} {product-version} is released.