Add release note for BGP routing

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

Author: jab-rh

release_notes/ocp-4-18-release-notes.adoc

@@ -852,6 +852,52 @@ With this release, you can configure a multi-network policy for the following ne
 
 For more information, see xref:../networking/multiple_networks/secondary_networks/configuring-multi-network-policy.adoc[Configuring multi-network policy]
 
+[id="ocp-4-18-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 utilize multi-homing, link redundancy, and fast convergence. BGP configuration is managed with the `BGPConfiguration` custom resource (CR).
+
+When upgrading from an earlier version of {product-title} with the Metal LB Operator deployed, 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` 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 = "'"${NEW_NAMESPACE}"'"' |\
+  oc create -f -
+----
+
+. To execute the migration, run the following command:
++
+[source,terminal]
+----
+$ bash migrate.sh
+----
+
+. To confirm that the migration succeeded, run the following command:
++
+[source,terminal]
+----
+$ oc get frrconfigurations.frrk8s.metallb.io -n openshift-frr-k8s
+----
+
+For more information, see xref:../networking/bgp_routing/about-bgp-routing.adoc#about-bgp-routing[About BGP routing].
+
 [id="ocp-release-notes-networking-whitelist-blacklist-annotation-updated-allowlist-denylist_{context}"]
 ==== Updated terminology for whitelist and blacklist annotations
 The terminology for the `ip_whitelist` and `ip_blacklist` annotations have been updated to `ip_allowlist` and `ip_denylist`, respectively. Currently, {product-title} still supports the `ip_whitelist` and `ip_blacklist` annotations. However, these annotations are planned for removal in a future release.