![]() This is similar to how most protocols select an ID, by default it will select the highest IP address on a loopback interface. Each router has a unique ID called the LSR (Label Switch Router) ID.The hello packets are sent to multicast address 224.0.0.2 using source/destination UDP port 646.In the capture above you can see a couple of interesting things: Here’s what the hello packet looks like in wireshark: This IP address is then used to establish the TCP connection between the two routers. Within this hello packet, they will advertise a transport IP address. The two routers above will send multicast hello packets on their FastEthernet interfaces. Normally a loopback interface is used for the neighbor adjacency. This connection is then used for the exchange of label information. Once two routers decide to become neighbors, they build the neighbor adjacency using a TCP connection. It works a bit different than most protocols though…įirst we send UDP multicast hello packets to discover other neighbors. Like many other protocols, LDP first establishes a neighbor adjacency before it exchanges label information. Cisco created a protocol and a standard was created later. Nowadays almost everyone uses LDP instead of TDP. It’s pretty much the same story as 802.1Q/ISL or PaGP/LACP. ![]() It’s a standard, based on Cisco’s proprietary TDP (Tag Distribution Protocol). Each router will locally generate labels for its prefixes and will then advertise the label values to its neighbors. LDP is a protocol that automatically generates and exchanges labels between routers.
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