[lldp] Replace per-port portidsubtype loop with global ifname directive

[ZhaohuiS]

Description of PR

Summary:
Cherry-pick of sonic-net/sonic-buildimage#26873 for the 202412 branch.

Replace the per-port portidsubtype configuration loop in lldpd.conf.j2 with a single global configure lldp portidsubtype ifname directive.

The per-port loop was added by #26144 to prevent MAC-based Port IDs on the first LLDP frame, but it causes LLDP neighbor churns on high-scale systems — startup delays proportional to port count (e.g., 20+ seconds on 512-port systems) lead to neighbor table instability.

#26873 fixes this by using the global configure lldp portidsubtype ifname directive instead. The trade-off is that the Port ID in the first LLDP packet uses the kernel interface name (e.g., Ethernet64) rather than the platform alias (e.g., etp9a). After lldpmgrd configures the per-port alias, subsequent packets use the correct alias. The key improvement is that the Port ID is no longer a MAC address, which was the original problem #26144 aimed to solve.

Observed behavior:

# First LLDP packet — Port ID is kernel interface name (not MAC, not alias)
ARISTA01T1#show lldp nei
Port          Neighbor Device ID       Neighbor Port ID    TTL
---------- ------------------------ ---------------------- ---
Et1           str5-sn5640-6            Ethernet64          120

# After lldpmgrd sets alias — Port ID becomes platform alias
ARISTA01T1#show lldp nei
Port          Neighbor Device ID       Neighbor Port ID    TTL
---------- ------------------------ ---------------------- ---
Et1           str5-sn5640-6            etp9a               120

Note: The original PR had conflicts on 202412 because the lldpd-ipv4-iface-with-ports.conf test file does not exist on this branch, and the lldpd-ipv6-iface.conf has a different baseline. These conflicts are resolved in this PR.

Type of change

• Bug fix

Back port request

• 202412

Approach

What is the motivation for this PR?

#26144 introduced per-port portidsubtype configuration to prevent MAC-based Port IDs. However, on high-scale systems this causes LLDP neighbor churns due to slow startup. #26873 fixes it with a global directive that eliminates the startup delay while still avoiding MAC-based Port IDs.

How did you do it?

Applied the same change as sonic-net/sonic-buildimage#26873 with conflict resolution:

1. lldpd.conf.j2: Replaced the per-port Jinja2 loop with global configure lldp portidsubtype ifname
2. lldpd-ipv4-iface.conf: Added configure lldp portidsubtype ifname before pause
3. lldpd-ipv6-iface.conf: Added configure lldp portidsubtype ifname before pause
4. lldpd-ipv4-iface-with-ports.conf: Does not exist on 202412, skipped

How did you verify/test it?

Verified the generated config matches expected output format. The original PR was tested on sonic-net master. Observed LLDP behavior on str5-sn5640-6 confirms Port ID transitions from kernel ifname to alias after lldpmgrd configures it.

Any platform specific information?

N/A

Supported testbed topology if it's a new test case?

N/A

Documentation

N/A

[ZhaohuiS]

@r12f could you pls help review ?