OCPBUGS-6800: change the MTU in Local Zones deployments

Add section to change the the cluster network MTU
on the manifest stage due the limitation of AWS
Local Zones communication with the parent region.

Adding OpenShift-SDN and "important" note

Apply suggestions from code review

Co-authored-by: Kathryn Alexander <[email protected]>

Update modules/installation-localzone-generate-k8s-manifest.adoc

Apply suggestions from code review

Co-authored-by: Kathryn Alexander <[email protected]>

pr review - making correct reference

Update modules/installation-localzone-generate-k8s-manifest.adoc

Update modules/installation-localzone-generate-k8s-manifest.adoc

PR review for important notes on MTU

Author: mtulio

installing/installing_aws/installing-aws-localzone.adoc

@@ -78,6 +78,12 @@ include::modules/installation-generate-aws-user-infra-install-config.adoc[levelo
 
 include::modules/installation-localzone-generate-k8s-manifest.adoc[leveloffset=+2]
 
+[role="_additional-resources"]
+.Additional resources
+
+* xref:../../networking/changing-cluster-network-mtu.adoc#mtu-value-selection_changing-cluster-network-mtu[Changing the MTU for the cluster network]
+* xref:../../networking/changing-cluster-network-mtu.adoc#nw-ovn-ipsec-enable_configuring-ipsec-ovn[Enabling IPsec encryption]
+
 include::modules/installation-launching-installer.adoc[leveloffset=+1]
 
 include::modules/cli-installing-cli.adoc[leveloffset=+1]

modules/installation-localzone-generate-k8s-manifest.adoc

@@ -26,8 +26,53 @@ $ ./openshift-install create manifests --dir <installation_directory> <1>
 <1> For `<installation_directory>`, specify the installation directory that
 contains the `install-config.yaml` file you created.
 
-. Create the machine set manifests for the worker nodes in your Local Zone.
+. Set the default Maximum Transmission Unit (MTU) according to the network plugin:
++
+[IMPORTANT]
+====
+Generally, the Maximum Transmission Unit (MTU) between an Amazon EC2 instance in a Local Zone and an Amazon EC2 instance in the Region is 1300. See link:https://docs.aws.amazon.com/local-zones/latest/ug/how-local-zones-work.html[How Local Zones work] in the AWS documentation. 
+The cluster network MTU must be always less than the EC2 MTU to account for the overhead. The specific overhead is determined by your network plugin, for example:
+
+- OVN-Kubernetes: `100 bytes`
+- OpenShift SDN: `50 bytes`
+
+The network plugin could provide additional features, like IPsec, that also must be decreased the MTU. Check the documentation for additional information.
+
+====
+
+.. If you are using the `OVN-Kubernetes` network plugin, enter the following command:
++
+[source,terminal]
+----
+$ cat <<EOF > <installation_directory>/manifests/cluster-network-03-config.yml
+apiVersion: operator.openshift.io/v1
+kind: Network
+metadata:
+  name: cluster
+spec:
+  defaultNetwork:
+    ovnKubernetesConfig:
+      mtu: 1200
+EOF
+----
 
+.. If you are using the `OpenShift SDN` network plugin, enter the following command:
++
+[source,terminal]
+----
+$ cat <<EOF > <installation_directory>/manifests/cluster-network-03-config.yml
+apiVersion: operator.openshift.io/v1
+kind: Network
+metadata:
+  name: cluster
+spec:
+  defaultNetwork:
+    openshiftSDNConfig:
+      mtu: 1250
+EOF
+----
+
+. Create the machine set manifests for the worker nodes in your Local Zone.
 .. Export a local variable that contains the name of the Local Zone that you opted your AWS account into by running the following command:
 +
 [source,terminal]