Clarifies some aspects of the kubeadm install (#5883)

docs/setup/independent/create-cluster-kubeadm.md

@@ -62,8 +62,9 @@ cloud providers is difficult.
 {% capture prerequisites %}
 
 1. One or more machines running Ubuntu 16.04+, CentOS 7 or HypriotOS v1.0.1+
-1. 1GB or more of RAM per machine (any less will leave little room for your
+1. 2 GB or more of RAM per machine (any less will leave little room for your
    apps)
+1. 2 CPUs or more on the master
 1. Full network connectivity between all machines in the cluster (public or
    private network is fine)
 {% endcapture %}
@@ -232,7 +233,7 @@ Please select one of the tabs to see installation instructions for the respectiv
 
 {% capture calico %}
 
-The official Calico guide is [here](http://docs.projectcalico.org/latest/getting-started/kubernetes/installation/hosted/kubeadm/).
+Refer to the Calico documentation for a [kubeadm quickstart](https://docs.projectcalico.org/latest/getting-started/kubernetes/), a [kubeadm installation guide](http://docs.projectcalico.org/latest/getting-started/kubernetes/installation/hosted/kubeadm/), and other resources.
 
 **Note:**
 
@@ -266,6 +267,9 @@ kubectl apply -f https://raw.githubusercontent.com/projectcalico/canal/master/k8
  - For flannel to work correctly, `--pod-network-cidr=10.244.0.0/16` has to be passed to `kubeadm init`.
  - flannel works on `amd64`, `arm`, `arm64` and `ppc64le`, but for it to work on an other platform than
 `amd64` you have to manually download the manifest and replace `amd64` occurences with your chosen platform.
+ - Set `/proc/sys/net/bridge/bridge-nf-call-iptables` to `1` by running `sysctl net.bridge.bridge-nf-call-iptables=1`
+to pass bridged IPv4 traffic to iptables' chains. This is a requirement for some CNI plugins to work, for more information
+please see [here](https://kubernetes.io/docs/concepts/cluster-administration/network-plugins/#network-plugin-requirements).
 
 ```shell
 kubectl apply -f https://raw.githubusercontent.com/coreos/flannel/v0.9.0/Documentation/kube-flannel.yml
@@ -274,6 +278,10 @@ kubectl apply -f https://raw.githubusercontent.com/coreos/flannel/v0.9.0/Documen
 
 {% capture kube-router %}
 
+Set `/proc/sys/net/bridge/bridge-nf-call-iptables` to `1` by running `sysctl net.bridge.bridge-nf-call-iptables=1`
+to pass bridged IPv4 traffic to iptables' chains. This is a requirement for some CNI plugins to work, for more information
+please see [here](https://kubernetes.io/docs/concepts/cluster-administration/network-plugins/#network-plugin-requirements).
+
 Kube-router relies on kube-controll-manager to allocate pod CIDR for the nodes. Therefore, use `kubeadm init` with the `--pod-network-cidr` flag.
 
 Kube-router provides pod networking, network policy, and high-performing IP Virtual Server(IPVS)/Linux Virtual Server(LVS) based service proxy.
@@ -284,6 +292,10 @@ For information on setting up Kubernetes cluster with Kube-router using kubeadm
 
 {% capture romana %}
 
+Set `/proc/sys/net/bridge/bridge-nf-call-iptables` to `1` by running `sysctl net.bridge.bridge-nf-call-iptables=1`
+to pass bridged IPv4 traffic to iptables' chains. This is a requirement for some CNI plugins to work, for more information
+please see [here](https://kubernetes.io/docs/concepts/cluster-administration/network-plugins/#network-plugin-requirements).
+
 The official Romana set-up guide is [here](https://github.com/romana/romana/tree/master/containerize#using-kubeadm).
 
 **Note:** Romana works on `amd64` only.
@@ -295,6 +307,10 @@ kubectl apply -f https://raw.githubusercontent.com/romana/romana/master/containe
 
 {% capture weave_net %}
 
+Set `/proc/sys/net/bridge/bridge-nf-call-iptables` to `1` by running `sysctl net.bridge.bridge-nf-call-iptables=1`
+to pass bridged IPv4 traffic to iptables' chains. This is a requirement for some CNI plugins to work, for more information
+please see [here](https://kubernetes.io/docs/concepts/cluster-administration/network-plugins/#network-plugin-requirements).
+
 The official Weave Net set-up guide is [here](https://www.weave.works/docs/net/latest/kube-addon/).
 
 **Note:** Weave Net works on `amd64`, `arm` and `arm64` without any extra action required.