H3C Technologies H3C S7500E Series Switches - Manual (Page 50)

2-8
MPLS data forwarding
Figure 2-7 MPLS forwarding process diagram
Router A
Router B
Ingress
Router C Router D
Egress
Router E
IP:4
0.1
.1.
2
MPLS domain
40 IP:40.1.1.2 50 IP:40.1.1.2
IP
:4
0.
1.1.
2
NHLFE
ILM FIB
Vlan-int4
Out int Oper Out label
Swap 50
Token
0
Next hop
30.1.1.2 Vlan-int3
Out int Oper Out label
Push 40
Token
0
Next hop
20.1.1.2
50
In label Token In int
Vlan-int4 40
In label Token
0
In int
Vlan-int3
Vlan-int3
20.1.1.1/24 Vlan-int3
20.1.1.2/24
Vlan-int4
30.1.1.2/24
Vlan-int4
30.1.1.1/24
Vlan-int5
40.1.1.2/24
Vlan-int5
40.1.1.1/24 Vlan-int2
10.1.1.2/24
Vlan-int2
10.1.1.1/24
Dest Token
40.1.1.0/24 0
As shown in Figure 2-7 , in an MPLS domain, a packet is forwarded in the following procedure:
The ingress (Router B) receives a packet carrying no label. Router B determines the FEC of the
packet according to the destination address, and searches the FIB table for the Token value. As
the Token value is not Invalid, Router B looks for the corresponding NHLFE entry of the Token
value. According to the NHLFE entry, Router B pushes label 40 to the packet, and then forwards
the labeled packet to the next hop LSR (Router C) through the outgoing interface (Vlan-interface
3).
Upon receiving the labeled packet, Router C looks for the ILM entry according to the label (40) to get
the Token value. As the Token value is not null, Router C looks for the corresponding NHLFE
entry of the Token value. According to the NHLFE entry, Router C swaps the original label with
label 50, and then forwards the labeled packet to the next hop LSR (Router D) through the
outgoing interface (Vlan-interface 4).
Upon receiving the labeled packet, Router D (the egress) looks for the ILM entry according to the label
(50) to get the Token value. As the Token is null, Router D removes the label from the packet. If
the ILM entry records the outgoing interface, Router D forwards the packet through the outgoing
interface; if no outgoing interface is recorded, router D forwards the packet according to the IP
header of the packet.
PHP
In an MPLS network, when an egress node receives a labeled packet, it looks up the LFIB, pops the
label of the packet, and then performs the next level label forwarding or performs IP forwarding. That
is, an egress node needs to do forwarding table lookup twice to forward a packet: looks up the LFIB
twice, or looks up the LFIB once and the FIB once.