Add release note for BGP routing

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

Author: jab-rh

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

@@ -439,6 +439,52 @@ Starting in {product-title} 4.14, Extended Update Support (EUS) is extended to t
 [id="ocp-release-notes-networking_{context}"]
 === Networking
 
+[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.