Hello,
Let me talk about a few concepts first:
1- LSR (Label Switching Router)
Label Switching Router (LSR) refers to network equipment that can perform MPLS label switching and packet forwarding, and is also called MPLS node. LSR is the basic element in MPLS network, and all LSRs support MPLS protocol.
2- LER (edge LSR)
The LSR located at the edge of the MPLS domain is called LER (Label Edge Router). If an LSR has an adjacent node that does not run MPLS, then the LSR is the LER.
MPLS forwards based on labels. When IP packets enter the MPLS network, the LER of the MPLS entry analyzes the contents of the IP packets and adds appropriate labels to these IP packets. All nodes in the MPLS network forward data based on the labels. When the IP packet leaves the MPLS network, the label is deleted by the egress edge router.
LSP (Label Switched Path), the path that IP packets traverse in the MPLS network is called Label Switched Path (LSP). LSP is a unidirectional path, which is consistent with the direction of data flow.
The starting node of the LSP is called an ingress (Ingress); the node in the middle of the LSP is called an intermediate node (Transit); the end node of the LSP is called an egress node (Egress). An LSP can have 0, 1 or more intermediate nodes, but there is only one ingress node and egress node.
Ingress
The starting node of an LSP, an LSP can only have one Ingress.
The main function of Ingress is to put a new label on the packet and encapsulate it into an MPLS packet for forwarding.
Intermediate node (Transit)
An intermediate node of an LSP, an LSP may have multiple transits.
The main function of Transit is to search the label forwarding information table and complete the forwarding of MPLS packets through label switching.
Egress
At the end of an LSP, an LSP can only have one egress.
The main function of Egress is to pop up the label and restore the original message to forward accordingly.
I hope this will clarify you!
Thanks
