Cisco Cisco IOS Software Release 12.2(27)SBC

MPLS-Aware NetFlow captures this label if you indicate 2 (second from the top) as a label of 
interest. 

The 16 represents the third label in the MPLS label stack.

MPLS-Aware NetFlow captures this label if you indicate 3 (third from the top) as a label of interest. 

Lb4–Lb6 represents the fourth to sixth labels in the MPLS stack. LSRs in an MPLS cloud add up to 
six labels to the MPLS label stack.

MPLS-Aware NetFlow captures these labels if you indicate 4, 5, or 6 as a label of interest.

The B represents miscellaneous bits. These include the following: 

Exp—Three bits reserved for experimental use

S—End-of-stack bits, set to 1 for the last entry in the stack and to 0 for every other entry

Time To Live (TTL)—Eight bits used to encode a hop count (or time to live) value 

 shows a sample CSC topology and the incoming MPLS label stack on multiple LSRs as the 

packet travels through the network. This is what the stack might look like at a provider backbone LSR. 

In the example in 

, a hierarchical Virtual Private Network (VPN) is set up between two customer 

edge (CE) routers: 

Traffic flows from the CE router to a provider edge (PE) router, possibly one belonging to an Internet 
service provider (ISP). Here, a VPN label (16) is imposed on the inbound IP packet. 

The ISP network eventually connects to an Internet backbone provider where a CSC label (42) is 
imposed on the label stack. 

As packets traverse the backbone network, an LDP label (33) is imposed on the label stack.

At the inbound interface shown in 

, MPLS-Aware NetFlow captures the MPLS label stack 

and reports that the top label is 33, the second label is 42, and the third label is 16. 

88307

16

25

33

42

IP

IP

Carrier's carrier

MPLS

Backbone MPLS

Carrier's carrier

MPLS

16

VPN

Label

stack

IP

25

Lx

16

VPN

IP

42

CSC

16

VPN

IP

CSC VPN IP

VPN IP

33

LDP

Label Label type Location

in stack

33

LDP

1 (top)

42

CSC

2

16

VPN

3