Warn user about IBGP sessions without IGP (fixes #1048)

docs/module/bgp.md

@@ -265,6 +265,10 @@ The BGP transformation module builds a list of BGP neighbors for every node. Tha
 
 See the [IBGP Data Center Fabric](bgp_example/ibgp.md) example for more details.
 
+```{tip}
+_netlab_ generates a warning for routers that have IBGP sessions without an underlying IGP. To turn off that warning (for example, when using IBGP-over-EBGP EVPN design), set **defaults.bgp.warnings.missing_igp** to _False_.
+```
+
 (bgp-ebgp-sessions)=
 **EBGP sessions**
 * Whenever multiple nodes connected to the same link use different AS numbers, you'll get a full mesh of EBGP sessions between them.

netsim/defaults/hints.yml

@@ -10,6 +10,9 @@ evpn:
     You could use 'bgp.as' parameter to specify the global AS. Otherwise, specify
     the global AS used by EVPN in 'vrf.as' parameter if you use VRFs, or in
     'evpn.as' parameter if you use EVPN in bridging-only scenarios.
+bgp:
+  igp: >
+    Add a supported IGP (ospf, isis, eigrp) to the list of modules.
 report:
   source: >
     A report can be specified in a file with .j2 suffix within 'reports' subdirectory in

netsim/modules/__init__.py

@@ -18,9 +18,9 @@
 # List of attributes we don't want propagated from defaults to global/node
 #
 no_propagate_list = [
-  "attributes","extra_attributes","features",
-  "requires","supported_on","no_propagate",
-  "config_after","transform_after"]
+  "attributes", "extra_attributes", "features",
+  "requires", "supported_on", "no_propagate",
+  "config_after", "transform_after", "warnings" ]
 
 """
 Return the authoritative list of all modules.

netsim/modules/bgp.py

@@ -150,6 +150,7 @@ def get_remote_ibgp_endpoint(n: Box) -> Box:
 """
 def build_ibgp_sessions(node: Box, sessions: Box, topology: Box) -> None:
   rrlist = find_bgp_rr(node.bgp.get("as"),topology)
+  has_ibgp     = False                            # Assume we have no IBGP sessions (yet)
 
   # If we don't have route reflectors, or if the current node is a route
   # reflector, we need BGP sessions to all other nodes in the same AS
@@ -161,6 +162,7 @@ def build_ibgp_sessions(node: Box, sessions: Box, topology: Box) -> None:
           neighbor_data = bgp_neighbor(n,n_intf,'ibgp',sessions,get_neighbor_rr(n))
           if not neighbor_data is None:
             node.bgp.neighbors.append(neighbor_data)
+            has_ibgp = True
 
   #
   # The node is not a route reflector, and we have a non-empty RR list
@@ -172,6 +174,30 @@ def build_ibgp_sessions(node: Box, sessions: Box, topology: Box) -> None:
         neighbor_data = bgp_neighbor(n,n_intf,'ibgp',sessions,get_neighbor_rr(n))
         if not neighbor_data is None:
           node.bgp.neighbors.append(neighbor_data)
+          has_ibgp = True
+
+  if not has_ibgp:
+    return
+
+  # Do we have to warn the user that IBGP sessions work better with IGP?
+  # The warning could be disabled in the settings, and we assume it doesn't make
+  # sense complaining if the node has no loopback interfaces (routing on hosts)
+  #
+  ibgp_warning = topology.defaults.bgp.warnings.missing_igp and 'loopback' in node
+  igp_list     = topology.defaults.bgp.warnings.igp_list
+
+  if ibgp_warning:                                # Does the user want to get the warning?
+    for igp in igp_list:                          # If so, check the viable IGPs
+      if igp in node.module:                      # ... and if any one of them is in the list of node modules
+        ibgp_warning = False                      # ... life is good, turn off the warning
+        break
+
+  if ibgp_warning:
+    log.error(
+      f'Node {node.name} has IBGP sessions but no IGP',
+      category=Warning,
+      module='bgp',
+      hint='igp')
 
 """
 build_ebgp_sessions: create EBGP session data structure

netsim/modules/bgp.yml

@@ -83,3 +83,6 @@ features:
   ipv6_lla: Can run EBGP sessions over IPv6 link-local addresses
   rfc8950: Can run IPv4 AF over IPv6 LLA EBGP session
   community: Granular BGP community propagation
+warnings:
+  missing_igp: True
+  igp_list: [ ospf, eigrp, isis ]