для Cisco Cisco IOS Software Releases 12.0 SP

MPLS supports per-destination and per-packet (round robin) load balancing. If per-packet load 
balancing is in effect, you should not use MPLS LSP traceroute because LSP traceroute at a transit 
router consistency checks the information supplied in the previous echo response from the directly 
connected upstream router. When round robin is employed, the path that an echo request packet 
takes cannot be controlled in a way that allows a packet to be directed to TTL expire at a given router. 
Without that ability, the consistency checking may fail during an LSP traceroute. A consistency 
check failure return code may be returned.

A platform must support LSP ping and traceroute in order to respond to an MPLS echo request 
packet.

Unless the MPLS LSP Ping/Traceroute for LDP/TE, and LSP Ping for VCCV feature is enabled 
along the entire path, you cannot get a reply if the request fails along the path at any node.

There are certain limitations when a mixture of draft versions are implemented within a network. 
The version of the draft must be compatible with Cisco’s implementation. Due to the way the LSP 
Ping draft was written, earlier versions may not be compatible with later versions because of 
changes to type, length, values (TLVs) formats without sufficient versioning information. Cisco 
attempts to compensate for this in its implementations by allowing the sending and responding 
routers to be configured to encode and decode echo packets assuming a certain version.

The network should not use TTL hiding if you want to use MPLS LSP traceroute. 

Before using the MPLS LSP Ping/Traceroute for LDP/TE, and LSP Ping for VCCV feature, you need 
an understanding of the following concepts: 

 

Internet Control Message Protocol (ICMP) ping and traceroute are often used to help diagnose the root 
cause when a forwarding failure occurs. However, they are not well suited for identifying LSP failures 
because an ICMP packet can be forwarded via IP to the destination when an LSP breakage occurs. 

The MPLS LSP Ping/Traceroute for LDP/TE, and LSP Ping for VCCV feature is well suited for 
identifying LSP breakages for the following reasons:

An MPLS echo request packet cannot be forwarded via IP because IP TTL is set to 1 and the IP 
destination address field is set to a 127/8 address.

The FEC being checked is not stored in the IP destination address field (as is the case of ICMP).

MPLS echo request and reply packets test LSPs. There are two methods by which a downstream router 
can receive packets: