Fix: Mark phantom links used to create multihop EBGP sessions

The ebgp.multihop plugin creates bogus ('phantom' sounds so much better)
links to persuade the BGP module to establish BGP sessions between
endpoints of multihop EBGP sessions.

Unfortunately, other modules (in particular 'routing.static') find
those links and want to use them.

With this change, ebgp.multihop plugin marks the interfaces attached to
those links with the '_phantom_link' flag, and the nexthop resolution
for static routes avoids those interfaces.

At the moment, the bogus static route next hops seem to be the only
undesired side effect of those phantom links, but of course something
else is bound to pop up eventually :(

Fixes #2414

Author: ipspace

netsim/extra/ebgp.multihop/plugin.py

@@ -83,6 +83,7 @@ def pre_link_transform(topology: Box) -> None:
       if 'vrf' in intf:
         intf.bgp._vrf = intf.vrf
       intf._bgp_session = True
+      intf._phantom_link = True
       intf.ifname = f'_ebgp_multihop_{s.linkindex}'
 
   topology.links.extend(sessions)

netsim/modules/routing.py

@@ -829,6 +829,8 @@ def resolve_node_nexthop(sr_data: Box, node: Box, topology: Box) -> Box:
 
   node_found = False
   for intf in node.interfaces:
+    if intf.get('_phantom_link',False):
+      continue
     for ngb in intf.neighbors:
       if ngb.node != sr_data.nexthop.node:
         continue

tests/topology/expected/ebgp.utils.yml

@@ -13,6 +13,7 @@ bgp:
         password: Funny
       interfaces:
       - _bgp_session: true
+        _phantom_link: true
         bgp:
           local_as: 123
           multihop: 255
@@ -21,6 +22,7 @@ bgp:
         ipv4: 10.0.0.2/32
         node: r2
       - _bgp_session: true
+        _phantom_link: true
         bgp:
           local_as: 456
           multihop: 255