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, 108 insertions, 0 deletions

diff --git a/src/migration-scripts/bgp/6-to-7 b/src/migration-scripts/bgp/6-to-7
new file mode 100644
index 000000000..7a11bbf3a
--- /dev/null
+++ b/src/migration-scripts/bgp/6-to-7
@@ -0,0 +1,108 @@
+#!/usr/bin/env python3
+#
+# Copyright VyOS maintainers and contributors <maintainers@vyos.io>
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License version 2 or later as
+# published by the Free Software Foundation.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# T7760: Remove per VRF setting for system-as option and replace it with
+#        local-as if required.
+
+from vyos.configtree import ConfigTree
+
+def migrate(config: ConfigTree) -> None:
+    vrf_base = ['vrf', 'name']
+    bgp_base = ['protocols', 'bgp']
+
+    if not config.exists(vrf_base):
+        return
+
+    global_asn = None
+    if config.exists(bgp_base + ['system-as']):
+        global_asn = config.return_value(bgp_base + ['system-as'])
+
+    for vrf in config.list_nodes(vrf_base):
+        # bail out early if there is no per VRF BGP instance defined
+        vrf_bgp_base = vrf_base + [vrf] + bgp_base
+        if not config.exists(vrf_bgp_base):
+            continue
+
+        # This is a mandatory node in the old design but keep it optional
+        # if one want's to load a weird config
+        if config.exists(vrf_bgp_base + ['system-as']):
+            system_as = config.return_value(vrf_bgp_base + ['system-as'])
+            config.delete(vrf_bgp_base + ['system-as'])
+
+            # If there is no existing global BGP instance - start one
+            if not config.exists(bgp_base):
+                config.set(bgp_base + ['system-as'], value=system_as)
+                global_asn = system_as
+
+            vrf_neighbor_base = vrf_bgp_base + ['neighbor']
+            vrf_peer_group_base = vrf_bgp_base + ['peer-group']
+            if config.exists(vrf_neighbor_base):
+                for neighbor in config.list_nodes(vrf_neighbor_base):
+                    # Neighbor already has local-as option set, do not touch it
+                    if config.exists(vrf_neighbor_base + [neighbor, 'local-as']):
+                        #print(f'VRF {vrf} BGP neighbor {neighbor} has local-as set - do not migrate!')
+                        continue
+
+                    # Check if the neighbor uses a peer-group which has the local-as
+                    # option set, do not touch it either
+                    peer_group = None
+                    if config.exists(vrf_neighbor_base + [neighbor, 'peer-group']):
+                        peer_group = config.return_value(vrf_neighbor_base + [neighbor, 'peer-group'])
+                        # Check if the peer-group has a local-as option set
+                        if config.exists(vrf_peer_group_base + [peer_group, 'local-as']):
+                            #print(f'VRF {vrf} BGP neighbor {neighbor} uses peer-group {peer_group} which has local-as set - do not migrate!')
+                            continue
+
+                    # BGP local-as option is only allowed for eBGP speakers
+                    if global_asn == system_as:
+                        continue
+
+                    config.set(vrf_neighbor_base + [neighbor, 'local-as', system_as, 'no-prepend', 'replace-as'])
+                    config.set_tag(vrf_neighbor_base + [neighbor, 'local-as'])
+
+                    # We do also need to take care about BGP internas. When using local-as routes with our own AS
+                    # previously in the path will get rejected:
+                    # bgpd: x.x.x.x(Unknown) rcvd UPDATE about 192.0.2.0/24 IPv4 unicast -- DENIED due to: as-path contains our own AS;
+                    # Set allowas-in option
+                    allowas_in_numer = ['allowas-in', 'number']
+                    for afi in ['ipv4-labeled-unicast', 'ipv4-multicast', 'ipv4-unicast', 'ipv4-vpn',
+                                'ipv6-labeled-unicast', 'ipv6-multicast', 'ipv6-unicast', 'ipv6-vpn']:
+                        afi_neighbor_base = vrf_neighbor_base + [neighbor, 'address-family', afi]
+                        afi_peer_group_base = vrf_peer_group_base + [peer_group, 'address-family', afi]
+
+                        # No need to change anything on an AFI not in use
+                        if not config.exists(afi_neighbor_base) and not config.exists(afi_peer_group_base):
+                            continue
+
+                        allowas_in_value = 0
+                        print(neighbor, afi)
+                        # Check if there is any allowas-in definition for a peer-group
+                        if peer_group and config.exists(afi_peer_group_base + allowas_in_numer):
+                            allowas_in_value = int(config.return_value(afi_peer_group_base + allowas_in_numer))
+                            #print(f'peer-group {peer_group} allowas-in for {afi} is {allowas_in_value}')
+
+                        # Per neighbor "allowas-in" definition takes higher precendence
+                        if config.exists(afi_neighbor_base + allowas_in_numer):
+                            allowas_in_value = int(config.return_value(afi_neighbor_base + allowas_in_numer))
+                            #print(f'neighbor {neighbor} allowas-in for {afi} is {allowas_in_value}')
+
+                        # Increment allowas-in by 1 as we now have one more entry
+                        allowas_in_value += 1
+                        # Clip allowas-in to 10 - max supported by FRR platform
+                        if allowas_in_value > 10: allowas_in_value = 10
+
+                        # Set per neighbor allowas-in which always takes precedence over the peer-group definition
+                        config.set(afi_neighbor_base + allowas_in_numer, value=allowas_in_value, replace=True)