PLANET XGS3-24042 - Manual (Page 649)

79-3
LSP are separately called the upstream and downstream LSR, along the direction of data transmission. In the
next figure, R2 is the downstream LSR of R1,while R1 is the upstream LSR of R2.
Figure 79-2 Label Switched Path LSP
The function of LSP, the same as the virtual circuit of ATM and Frame Relay, is a unidirectional path form the
ingress of a MPLS network to its egress. Each router along the LSP is a LSR.
When downstream LSRs advertise labels to their upstream LSRs, all labels as a series and the LSR
sequence compose a LSP. LSP will map the IP layer route information to a link layer switched path. LSP is a
unidirectional packet forwarding path, along which, packets are always forwarded form an upstream LSR to a
downstream one. To forward packets in the opposite direction, creating an entirely new and independent LSP
is necessary. LSP always relates FEC with LSP. This relationship between FEC and LSP is called mapping
packets to LSP.
1. The rules of mapping packets to LSP:
（1） If there is only one LSP, which includes a host-address FEC unit with the same destination address
as the packet, map the packet to it;
（2） If there is more than one LSP satisfying condition 1, map the packet to any one of them.
（3） If there is only one LSP, whose address-prefix FEC unit can match the packet, map the packet to it.
（4） If there is more than one LSP satisfying condition 3, choose a LSP based on the Longest Prefix
Match principle;
（5） If a packet will definitely pass through a specific egress LSR, and there is a LSP, the prefix FEC unit
bounded to which is the address of that egress LSR, map the packet to this LSP.
2. Additional Rules:
（1） If the destination address of the packet matches no LSP, the packet will be sent along the LSP with
the same address as its Egress Router, as long as the LSP has an Address-prefix FEC unit.
（2） If a packet matches two LSPs, one of which includes a host-address FEC unit, and the other an
address-prefix FEC unit, always map the packet to the first one.
（3） If the packet matches no LSP with a host-address FEC unit, it should not be sent along a LSP even
if whose host-address FEC unit is the same as the packet’s egress router address.
（4） The creation of LSP is based on connections, which are the result of topology information rather
than the demand of data flow. That is to say, no matter data forwarded by this router exist or not, the
LSP will always be created.
Label Merging
R21 R22 R21 R22
R1 R2 R3 R4 R1 R2 R3 R4 R2 R3 R4