bgp: T7760: deprecate per bgp vrf instance system-as node

Originating from the bug in T7665. To avoid potential issues down the line - and
given that there's no compelling technical reason to retain the system-as CLI
node under per-VRF BGP configuration, which cannot be achieved through
alternative means - the maintainers have collectively decided to deprecate the
following command:

set vrf name <name> protocols bgp system-as <asn>

Starting with VyOS 1.4.4, this CLI command will be considered deprecated. While
it will still be accepted, it will no longer have any operational effect. A
deprecation warning will be displayed at commit time, indicating that the BGP
ASN from the global BGP configuration is now used instead.

A migration script will handle the transition and perform the following actions:
* Ensure a global BGP configuration exists; if not, initialize one.
* Iterate over all configured VRFs to determine whether a BGP instance exists
* For any insance, update the configuration to use the global system-as
  and apply the local-as ASN no-prepend replace-as option on all affected
  neighbors to preserve existing behavior.
* If a neighbor is already configured with a local-as directive, that neighbor
  will be excluded from the migration process, as it already follows a custom
  configuration.
* Add allowas-in per neighbor option. Required to not deny prefix received
  updates due to as-path contains our own global ASN.

1 files changed, 24 insertions, 10 deletions

diff --git a/src/conf_mode/protocols_bgp.py b/src/conf_mode/protocols_bgp.py
index fe5740a18..ab0e7e44a 100755
--- a/src/conf_mode/protocols_bgp.py
+++ b/src/conf_mode/protocols_bgp.py
@@ -18,6 +18,7 @@ from sys import exit
 from sys import argv
 
 from vyos.base import Warning
+from vyos.base import DeprecationWarning
 from vyos.config import Config
 from vyos.configverify import has_frr_protocol_in_dict
 from vyos.configverify import verify_prefix_list
@@ -210,8 +211,27 @@ def verify(config_dict):
 
         return None
 
-    if 'system_as' not in bgp:
-        raise ConfigError('BGP system-as number must be defined!')
+    ERR_MSG_GLOBAL_VRF_AS_MISSING = 'BGP "system-as" number must be defined! Use "set protocols ' \
+                                    'bgp system-as <asn>" to define a global BGP instance AS number.'
+    system_as = None
+    if vrf:
+        system_as = dict_search('dependent_vrfs.default.protocols.bgp.system_as', bgp)
+        if not system_as:
+            raise ConfigError(ERR_MSG_GLOBAL_VRF_AS_MISSING)
+
+        if 'system_as' in bgp:
+            tmp_as = bgp['system_as']
+            DeprecationWarning(f'CLI command "vrf name {vrf} protocols bgp system-as ' \
+                               f'{tmp_as}" is ignored and will be removed in VyOS 1.5! ' \
+                               f'\n\nGlobal "protocols bgp system-as {system_as}" option ' \
+                                'applies, use per neighbor "local-as" option to override.')
+
+    elif 'system_as' not in bgp:
+        raise ConfigError(ERR_MSG_GLOBAL_VRF_AS_MISSING)
+
+    # Cache global defined system AS number used in further checks
+    if not system_as:
+        system_as = bgp['system_as']
 
     # Verify BMP
     if 'bmp' in bgp:
@@ -257,7 +277,7 @@ def verify(config_dict):
                 if 'remote_as' in peer_config:
                     is_ibgp = True
                     if peer_config['remote_as'] != 'internal' and \
-                            peer_config['remote_as'] != bgp['system_as']:
+                            peer_config['remote_as'] != system_as:
                         is_ibgp = False
 
                     if peer_group not in peer_groups_context:
@@ -278,7 +298,7 @@ def verify(config_dict):
                 # Neighbor local-as override can not be the same as the local-as
                 # we use for this BGP instane!
                 asn = list(peer_config['local_as'].keys())[0]
-                if asn == bgp['system_as']:
+                if asn == system_as:
                     raise ConfigError('Cannot have local-as same as system-as number')
 
                 # Neighbor AS specified for local-as and remote-as can not be the same
@@ -369,12 +389,6 @@ def verify(config_dict):
                     if 'source_interface' in peer_config['interface']:
                         raise ConfigError(f'"source-interface" option not allowed for neighbor "{peer}"')
 
-            # Local-AS allowed only for EBGP peers
-            if 'local_as' in peer_config:
-                remote_as = verify_remote_as(peer_config, bgp)
-                if remote_as == bgp['system_as']:
-                    raise ConfigError(f'local-as configured for "{peer}", allowed only for eBGP peers!')
-
             for afi in ['ipv4_unicast', 'ipv4_multicast', 'ipv4_labeled_unicast', 'ipv4_flowspec',
                         'ipv6_unicast', 'ipv6_multicast', 'ipv6_labeled_unicast', 'ipv6_flowspec',
                         'l2vpn_evpn']: