4
MPLS forwarding
Figure 5 MPLS forwarding
As shown in
, a packet is forwarded over the MPLS network as follows:
1.
Router B (the ingress LSR) receives a packet with no label. Then, it performs the following
operations:
a.
Identifies the FIB entry that matches the destination address of the packet.
b.
Adds the outgoing label (40, in this example) to the packet.
c.
Forwards the labeled packet out of the interface GigabitEthernet 2/0/2 to the next hop LSR
Router C.
2.
When receiving the labeled packet, Router C processes the packet as follows:
a.
Identifies the LFIB entry that has an incoming label of 40.
b.
Uses the outgoing label 50 of the entry to replace label 40 in the packet.
c.
Forwards the labeled packet out of the outgoing interface GigabitEthernet 2/0/2 to the next
hop LSR Router D.
3.
When receiving the labeled packet, Router D (the egress LSR) processes the packet as follows:
a.
Identifies the LFIB entry that has an incoming label of 50.
b.
Removes the label from the packet.
c.
Forwards the packet out of the outgoing interface GigabitEthernet 2/0/2 to the next hop LSR
Router E.
If the LFIB entry records no outgoing interface or next hop information, Router D performs the
following operations:
a.
Identifies the FIB entry by the IP header.
b.
Forwards the packet according to the FIB entry.
PHP
An egress node must perform two forwarding table lookups to forward a packet:
•
Two LFIB lookups (if the packet has more than one label).
Router A
Router B
Ingress
Router C
Router D
Egress
Router E
Router F
IP:10.1.1.1
MPLS network
IP:10.1.1.1
40
IP:10.1.1.1
50
IP:10.1.1.1
GE2/0/1
GE2/0/2
GE2/0/1
GE2/0/2
GE2/0/1
GE2/0/2
FIB table
LFIB table
LFIB table
Nexthop
Dest
Out int
Out label
10.1.0.0
GE2/0/2
40
Router C
Nexthop
Router D
Out int
GE2/0/2
Out label
50
Oper
Swap
In label
40
Nexthop
Router E
Out int
GE2/0/2
Out label
--
Oper
Pop
In label
50
Summary of Contents for HPE FlexNetwork MSR Router Series
Page 10: ...viii Index 540 ...