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LDP Hello packets are generated to answer incoming Hello before forming neighbor adjacency
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Description

LDP Hello packets are generated to answer incoming Hello before forming neighbor adjacency.
According to RFC 5036, Hello packets should be generated periodically at a certain interval. By default, it is 5 sec.
But in FRR we see strange behavior. If a router has not formed an LDP neighbor adjacency yet, it answers to all received LDP Hello packets from not neighbors with new Hello packets.
That behavior is incorrect.
Example:
Network MAP:
Three routers in one broadcast domain are connected to the same switch by eth0
Configurations:
R1:

set interfaces ethernet eth0 address '192.168.139.121/24'
set interfaces loopback lo address '1.1.1.1/32'
set protocols isis interface eth0 circuit-type 'level-1'
set protocols isis interface lo
set protocols isis log-adjacency-changes
set protocols isis metric-style 'wide'
set protocols isis net '49.0001.1001.2202.1049.00'
set protocols mpls interface 'eth0'
set protocols mpls ldp discovery transport-ipv4-address '1.1.1.1'
set protocols mpls ldp interface 'eth0'
set protocols mpls ldp interface 'lo'
set protocols mpls ldp router-id '1.1.1.1'

R2:

set interfaces ethernet eth0 address '192.168.139.122/24'
set interfaces loopback lo address '1.1.1.2/32'
set protocols isis interface eth0 circuit-type 'level-1'
set protocols isis interface lo
set protocols isis log-adjacency-changes
set protocols isis metric-style 'wide'
set protocols isis net '49.0001.1001.2202.0170.00'
set protocols mpls interface 'eth0'
set protocols mpls ldp discovery transport-ipv4-address '1.1.1.2'
set protocols mpls ldp interface 'eth0'
set protocols mpls ldp interface 'lo'
set protocols mpls ldp router-id '1.1.1.2'

R3:

set interfaces ethernet eth0 address '192.168.139.123/24'
set interfaces loopback lo address '1.1.1.3/32'
set protocols isis interface eth0 circuit-type 'level-1'
set protocols isis interface lo
set protocols isis log-adjacency-changes
set protocols isis metric-style 'wide'
set protocols isis net '49.0001.1001.2202.0174.00'
set protocols mpls interface 'eth0'
set protocols mpls ldp discovery transport-ipv4-address '1.1.1.3'
set protocols mpls ldp interface 'eth0'
set protocols mpls ldp interface 'lo'
set protocols mpls ldp router-id '1.1.1.3'

When we try to connect R3 to the network, we see Hello Packets, which are generated not in a certain interval of 5s.

image.png (629×1 px, 75 KB)

This is not correct.
This issue can lead to unpredictable behaviour when there are a lot of routers in one broadcast domain.
PCAP file is attached.
LDP-hello.pcapng53 KBDownload

Details

Version
VyOS 2025.03.06-0018-rolling, VyOS 1.4.1
Is it a breaking change?
Perfectly compatible
Issue type
Bug (incorrect behavior)