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C

HAPTER

 47: MPLS A

RCHITECTURE

n 

5 Label retention mode

There are two label-retention modes: liberal label retention mode and conservative 
label retention mode.

Suppose there are two LSRs: Ru and Rd. For a specific FEC, if LSR Ru has received 
the label binding from LSR Rd, in case Rd is not the next hop of Ru and Ru saves 
this binding, then it is the liberal label retention. And if Ru discards this binding, 
then it is the conservative label retention mode.

In case it is required that LSR is capable of adapting route variation rapidly, you can 
use the liberal label retention mode. In case it is required that a few labels are 
saved in LSR, you can use the conservative label retention mode.

n 

Label distribution protocol (LDP) is the signaling control protocol in MPLS, which 
controls binding labels and FECs between LSRs and coordinates a series of 
procedures between LSRs.

The basic composing unit of MPLS network is LSR (Label Switching Router). It runs 
MPLS control protocol and L3 routing protocol, exchanges routing messages with 
other LSRs and create the routing table, maps FECs with IP packet headers, binds 
FECs with labels, distributes label binding messages, establishes and maintains 
label forwarding table.

The network consisting of LSRs is called MPLS domain. The LSR that is located at 
the edge of the domain is called edge LSR (LER, Labeled Edge Router). It connects 
an MPLS domain with a non-MPLS domain or with another MPLS domain, 
classifies packets, distributes labels (as ingress LER) and distracts labels (as egress 
LER). The ingress LER is termed as ingress and egress LER as egress.

The LSR that is located inside the domain is called core LSR, which provides 
functions such as label swapping and label distribution. The labeled packets are 
transmitted along the LSP (Label Switched Path) composed of a series of LSRs.