[linux-cp]: Align lcp-auto-subint state across syncd-vpp and sonic-vpp

Both docker-syncd-vpp and docker-sonic-vpp now disable lcp-auto-subint and
carry the same rationale comment, so sairedis is the sole producer of LAG
sub-port LCP pairs in either deployment model (multi-container VM image
or single-container sonic-vpp image).

Also rewrites the existing rationale comment in docker-syncd-vpp to be
more accurate: configure_lcp_interface pairs BondEthernet<id>.<vlan> with
be<id>.<vlan> (not with PortChannel<id>.<vlan>); the bridge to
PortChannel<id>.<vlan> is a separate tc mirred step in the same SAI RIF
create. ARP replies do reach the correct LCP host tap; the kernel drops
them because the IP lives on a different netdev.

Signed-off-by: Lun Yue <17232861+lunyue-ms@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: lunyue-ms

docker-sonic-vpp/conf/startup.conf.tmpl

@@ -296,6 +296,12 @@ plugins {
 
 # Why not support sub interfaces
 linux-cp {
-   lcp-auto-subint
+   # lcp-auto-subint disabled: auto-pair "BondEthernet<id>.<vlan>" <-> "be<id>.<vlan>"
+   # punts ARP to be<id>.<vlan>, but SONiC writes the sub-port IP on
+   # "PortChannel<id>.<vlan>" (separate teamd vlan netdev), so the kernel drops
+   # the unsolicited ARP and the VPP neighbor table never gets populated.
+   # Disabled so sairedis (vpp_create_router_interface, SUB_PORT path) creates the
+   # LCP pair + tc mirred bridge between be<id>.<vlan> and PortChannel<id>.<vlan>.
+   # lcp-auto-subint
    lcp-sync
 }

docker-syncd-vpp/conf/startup.conf.tmpl

@@ -284,12 +284,12 @@ plugins {
 
 # Why not support sub interfaces
 linux-cp {
-   # lcp-auto-subint disabled: causes Bug 6 (LAG sub-port 0/8 addNeighbor).
-   # When on, VPP auto-creates host tap "be<id>.<vlan>" for BondEthernet<id>.<vlan>,
-   # but SONiC IntfMgr writes IP on "PortChannel<id>.<vlan>" (teamd vlan child),
-   # so ARP replies go to the wrong netdev. Disable so sairedis explicitly pairs
-   # BondEthernet<id>.<vlan> <-> PortChannel<id>.<vlan> via configure_lcp_interface()
-   # in SwitchVpp::vpp_create_router_interface for SAI_ROUTER_INTERFACE_TYPE_SUB_PORT.
+   # lcp-auto-subint disabled: auto-pair "BondEthernet<id>.<vlan>" <-> "be<id>.<vlan>"
+   # punts ARP to be<id>.<vlan>, but SONiC writes the sub-port IP on
+   # "PortChannel<id>.<vlan>" (separate teamd vlan netdev), so the kernel drops
+   # the unsolicited ARP and the VPP neighbor table never gets populated.
+   # Disabled so sairedis (vpp_create_router_interface, SUB_PORT path) creates the
+   # LCP pair + tc mirred bridge between be<id>.<vlan> and PortChannel<id>.<vlan>.
    # lcp-auto-subint
    lcp-sync
 }