When a labeled packet is sent from LSR A to the neighboring LSR B, the label value carried by the IP packet
is the label value that LSR B assigned to represent the forwarding equivalence class of the packet. Thus, the
label value changes as the IP packet traverses the network.
MFI Control-Plane Services
The MFI control-plane provides services to MPLS applications, such as Label Distribution Protocol (LDP)
and Traffic Engineering (TE), that include enabling and disabling MPLS on an interface, local label allocation,
MPLS rewrite setup (including backup links), management of MPLS label tables, and the interaction with
other forwarding paths (IP Version 4 [IPv4] for example) to set up imposition and disposition.
MFI Data-Plane Services
The MFI data-plane provides a software implementation of MPLS forwarding in all of these forms:
•
Imposition
•
Disposition
•
Label swapping
Time-to-Live Propagation in Hierarchical MPLS
Cisco IOS XR software provides the flexibility to enable or disable the time-to-live (TTL) propagation for
locally generated packets that are independent of packets forwarded form a customer edge (CE) device.
The IP header contains a field of 8 bits that signifies the time that a packet still has before its life ends and is
dropped. When an IP packet is sent, its TTL is usually 255 and is then decremented by 1 at each hop. When
the TTL field is decremented down to zero, the datagram is discarded. In such a case, the router that dropped
the IP packet for which the TTL reached 0 sends an Internet Control Message Protocol (ICMP) message type
11 and code 0 (time exceeded) to the originator of the IP packet.
Related Topics
Configuring the Time-to-Live Propagation in Hierarchical MPLS, on page 159
MPLS Maximum Transmission Unit
MPLS maximum transmission unit (MTU) indicates that the maximum size of the IP packet can still be sent
on a data link, without fragmenting the packet. In addition, data links in MPLS networks have a specific MTU,
but for labeled packets. All IPv4 packets have one or more labels. This does imply that the labeled packets
are slightly bigger than the IP packets, because for every label, four bytes are added to the packet. So, if n is
the number of labels, n * 4 bytes are added to the size of the packet when the packet is labeled. The MPLS
MTU parameter pertains to labeled packets.
How to Implement MPLS Forwarding
These topics explain how to configure a router for MPLS forwarding.
Cisco IOS XR MPLS Configuration Guide for the Cisco CRS Router, Release 5.1.x
158
Implementing MPLS Forwarding
MFI Control-Plane Services
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