[docs] cEOS: Add documentation about the forwarding agent and interface mode

docs/manual/kinds/ceos.md

@@ -317,14 +317,43 @@ To start an Arista cEOS node containerlab uses the following configuration:
 === "Startup command"
     `/sbin/init systemd.setenv=INTFTYPE=eth systemd.setenv=ETBA=1 systemd.setenv=SKIP_ZEROTOUCH_BARRIER_IN_SYSDBINIT=1 systemd.setenv=CEOS=1 systemd.setenv=EOS_PLATFORM=ceoslab systemd.setenv=container=docker systemd.setenv=MAPETH0=1 systemd.setenv=MGMT_INTF=eth0`
 === "Environment variables"
-    `CEOS:1`
-    `EOS_PLATFORM":ceoslab`
-    `container:docker`
-    `ETBA:1`
-    `SKIP_ZEROTOUCH_BARRIER_IN_SYSDBINIT:1`
-    `INTFTYPE:eth`
-    `MAPETH0:1`
-    `MGMT_INTF:eth0`
+    [//]: # (These lines have trailing spaces to force line break for proper formatting. DO NOT REMOVE!)
+    `CEOS:1`  
+    `EOS_PLATFORM:ceoslab`  
+    `container:docker`  
+    `ETBA:1`  (1)  
+    `SKIP_ZEROTOUCH_BARRIER_IN_SYSDBINIT:1`  
+    `INTFTYPE:eth`  
+    `MAPETH0:1`  
+    `MGMT_INTF:eth0`  
+    { .annotate }
+
+    1. This environment variable is related to the forwarding plane and must be set for cEOS to work properly.
+
+### Arista forwarding agent (cEOS dataplane)
+While CloudEOS has a DPDK dataplane, cEOS-lab and vEOS-lab use their own software dataplane.
+
+Two versions of the forwarding agent exist. `Arfa` (**AR** ista **F** orwarding **A** gent) is the new forwarding agent, that is supposed to be faster and more scalable. It has *nearly* reached feature parity with the older `python` forwarding agent and even has some features that do not exist in the older agent. `python` should be considered deprecated.
+
+Starting with `EOS-4.30.1F`, cEOS-lab and vEOS-lab use `Arfa` by default and the following snippet is included in the configuration by default:
+```
+platform tfa
+    personality arfa
+```
+
+The `Arfa` agent is constantly being improved and more features are being added. If you believe you've hit a limitation in `Arfa` that did not exist in `python`, contact your SE at Arista.
+
+!!!note
+    Some very advanced features such as `PTP` or `sFlow` may require additional configuration, such as `interface mode` to be set to `relay`. Please check the Arista docs for more details.
+
+### The ceos-config file
+cEOS supports an additional config file at `/mnt/flash/ceos-config` that allows some advanced configuration like system mac address (e.g. for certain CVP/CVaaS lab cases)
+
+```title="/mnt/flash/ceos-config"
+SERIALNUMBER=<Serialnumber>
+SYSTEMMACADDR=aaaa.bbbb.cccc
+```
+While it is possible to change the forwarding agent between `Arfa` and `python` via this file, it is not recommended.
 
 ### File mounts