chore(doc): add information about policies and rephrase

docs/advance/with-bgp.en.md

@@ -1,16 +1,18 @@
 # BGP Support
 
-Kube-OVN supports broadcasting the IP address of the Pod or Subnet to the outside world via BGP protocol,
-so that the outside world can access the Pod directly through the Pod IP.
+Kube-OVN supports broadcasting the IP address of Pods/Subnets to the outside world via the BGP protocol,
+so that the outside world can access the Pods directly through their Pod IP.
 To use this feature, you need to install `kube-ovn-speaker` on specific nodes and
 add the corresponding annotation to the Pod or Subnet that needs to be exposed to the outside world.
 
-## Install kube-ovn-speaker
+Kube-OVN also supports broadcasting the IP address of services of type `ClusterIP` via the same annotation.
 
-`kube-ovn-speaker` use [GoBGP](https://osrg.github.io/gobgp/) to publish routing information to the outside world and
-set the next-hop route to itself.
+## Installing `kube-ovn-speaker`
 
-Since the node where `kube-ovn-speaker` is deployed needs to carry return traffic, specific labeled nodes need to be selected for deployment:
+`kube-ovn-speaker` uses [GoBGP](https://osrg.github.io/gobgp/) to publish routing information to the outside world and to
+set the `next-hop` route to itself.
+
+Since the nodes where `kube-ovn-speaker` is deployed need to carry return traffic, specific labeled nodes need to be selected for deployment:
 
 ```bash
 kubectl label nodes speaker-node-1 ovn.kubernetes.io/bgp=true
@@ -38,13 +40,13 @@ Modify the corresponding configuration in yaml:
 - `neighbor-as`: The AS number of the BGP Peer.
 - `cluster-as`: The AS number of the container network.
 
-Deploy yaml:
+Apply the YAML:
 
 ```bash
 kubectl apply -f speaker.yaml
 ```
 
-## Publish Pod/Subnet Routes
+## Publishing Pod/Subnet Routes
 
 To use BGP for external routing, first set `natOutgoing` to `false` for the corresponding Subnet to allow the Pod IP to enter the underlying network directly.
 
@@ -58,15 +60,56 @@ kubectl annotate subnet ovn-default ovn.kubernetes.io/bgp=true
 Delete annotation to disable the publishing:
 
 ```bash
-kubectl annotate pod perf-ovn-xzvd4 ovn.kubernetes.io/bgp-
+kubectl annotate pod sample ovn.kubernetes.io/bgp-
 kubectl annotate subnet ovn-default ovn.kubernetes.io/bgp-
 ```
 
-## BGP Advance Options
+## Publishing Services of type `ClusterIP`
+
+To announce the ClusterIP of services to the outside world, the `kube-ovn-speaker` option `announce-cluster-ip` needs to be set to `true`.
+See the advanced options for more details.
+
+Set the annotation to enable publishing:
+
+```bash
+kubectl annotate service sample ovn.kubernetes.io/bgp=true
+```
+
+Delete annotation to disable the publishing:
+
+```bash
+kubectl annotate service sample ovn.kubernetes.io/bgp-
+```
+
+## Announcement policies
+
+There are 2 policies used by `kube-ovn-speaker` to announce the routes:
+
+- **Cluster**: this policy makes the Pod IPs/Subnet CIDRs be announced from every speaker, whether there's Pods
+that have that specific IP or that are part of the Subnet CIDR on that node. In other words, traffic may enter from
+any node hosting a speaker, and then be internally routed in the cluster to the actual Pod. In this configuration
+extra hops might be used. This is the default policy to Pods and Subnets.
+- **Local**: this policy makes the Pod IPs be announced only from speakers on nodes that are actively hosting
+them. In other words, traffic will only enter from the node hosting the Pod marked as needing BGP advertisement,
+or from the node hosting a Pod with an IP belonging to a Subnet marked as needing BGP advertisement.
+This makes the network path shorter as external traffic arrives directly to the physical host of the Pod.
+
+NOTE: You'll probably need to run `kube-ovn-speaker` on every node for the **Local** policy to work.
+If a Pod you're trying to announce lands on a node with no speaker on it, its IP will simply not be announced.
+
+The default policy used is **Cluster**. Policies can be overriden for each Pod/Subnet using the `ovn.kubernetes.io/bgp` annotation:
+
+- `ovn.kubernetes.io/bgp=cluster` or the default `ovn.kubernetes.io/bgp=yes` will use policy **Cluster**
+- `ovn.kubernetes.io/bgp=local` will use policy **Local**
+
+NOTE: Announcement of Services of type `ClusterIP` doesn't support any policy other than **Cluster** as routing to the actual pod
+is handled by a daemon such as `kube-proxy`. The annotation for Services only supports value `yes` and not `cluster`.
+
+## BGP Advanced Options
 
 `kube-ovn-speaker` supports more BGP parameters for advanced configuration, which can be adjusted by users according to their network environment:
 
-- `announce-cluster-ip`: Whether to publish Service routes to the public, default is `false`.
+- `announce-cluster-ip`: Whether to publish routes for Services of type `ClusterIP` to the public, default is `false`.
 - `auth-password`: The access password for the BGP peer.
 - `holdtime`: The heartbeat detection time between BGP neighbors. Neighbors with no messages after the change time will be removed, the default is 90 seconds.
 - `graceful-restart`: Whether to enable BGP Graceful Restart.