I guess the main reason is that TCP session between two LDP neighbors does not necessarily need to be established on the same link that Hello messages are send on.
Refer to the topology on the picture I attached. Let's say we are establishing LDP between R1 and R3's e 0/0, but loopback interfaces are advertised via OSPF not on e 0/0 interfaces, but rather via R1's e 0/1 and R3's e 0/2. So Hello packets are send via one interfaces and TCP keepalives via another. (Can't really think of case where you would need configuration like this, but anyway)
If only Hello packets are used for checking if our neighbor is down, we won't be able to determine if there's problem with TCP session (for example, if R2 goes down), so that's why you need both type of messages.
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I have two switches connected with two trunk links.
On the first switch spanning tree is switched off.
On the second switch, MST is turned on.
Ports configuration is as follows:
interface Ethernet0/2 switchport trunk allowed vlan 2-4 switchport trunk encapsulation dot1q switchport trunk native vlan 4 switchport mode trunk ! interface Ethernet0/3 switchport trunk allowed vlan 2-4 switchport trunk encapsulation dot1q switchport trunk native vlan 4 switchport mode trunk
(the configuration is the same on both switches)
I expect second switch to block one of the trunk ports: however, both of them are in FORWARDING.
I dumped traffic between two switches and I saw no BPDUs are send back to switch 2. Switch 1 receives them but instead of sending them back to switch 2 does nothing.
When I change native valn to 1 (and allow it on trunk link), everything works correctly and one of the port is being blocked.
Why it happens?
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