Print a warning if a LAG link uses a Linux bridge (#2038)

Closes #2036

Author: ipspace

docs/module/lag.md

@@ -66,6 +66,7 @@ links:
      ifindex: 50
 ```
 
+(lag-multi-provider)=
 ### Caveat: Multi-provider Labs
 
 _netlab_ implements links between containers and virtual machines with Linux bridges that block LACP packets. Package virtual machines into [vrnetlab](clab-vrnetlab) containers to connect them to pure containers.

netsim/modules/lag.py

@@ -355,6 +355,37 @@ def populate_mlag_peer(node: Box, intf: Box, topology: Box) -> None:
 
   intf.pop('vlan',None)                                                     # Remove any VLANs provisioned on peerlinks
 
+"""
+Sanity check: virtual LAG links that would have to use a Linux bridge would not work
+
+This function depends on the exact sequence of events during the transformation process:
+
+* LAG module creates extra "virtual lag" links to generate the bond interfaces
+* Multi-provider code marks those links as multi-provider links and adds a "bridge" attribute
+* The sanity check is run before the virtual LAG links are removed
+"""
+def check_bridge_links(topology: Box) -> None:
+  err_cache: dict = {}
+  for link in topology.links:
+    if '_virtual_lag' not in link:                          # Not a virtual LAG link
+      continue
+
+    if 'bridge' not in link:                                # Not a multi-provider link, we're OK
+      continue
+
+    nodes = sorted([ intf.node for intf in link.interfaces ])
+    n_text = " and ".join(nodes)                            # Find the nodes attached to the link
+    if n_text in err_cache:                                 # Do not create more than one warning
+      continue                                              # ... for a pair of nodes
+
+    log.warning(
+      text=f'The LAG link between {n_text} is using a Linux bridge. LACP will not work',
+      more_hints = [ 'See https://netlab.tools/module/lag/#lag-multi-provider for more details' ],
+      module='lag',
+      flag='lag.bridge')
+    
+    err_cache[n_text] = True                                # Remember we already warned the user
+
 class LAG(_Module):
 
   """
@@ -402,6 +433,7 @@ def link_pre_link_transform(self, link: Box, topology: Box) -> None:
   def module_post_transform(self, topology: Box) -> None:
     if log.debug_active('lag'):
       print(f'LAG module_post_transform: Cleanup "virtual_lag" links')
+    check_bridge_links(topology)
     topology.links = [ link for link in topology.links if '_virtual_lag' not in link ]
 
   """