для Cisco Cisco IOS Software Release 12.2(27)SBC

Before using the MPLS EM—MPLS LSP Multipath Tree Trace feature, you need an understanding of 
the following concepts:

As the number of MPLS deployments increases, the number of traffic types the MPLS networks carry 
could increase. In addition, load balancing on label switch routers (LSRs) in the MPLS network provides 
alternate paths for carrying MPLS traffic to a target router. The ability of service providers to monitor 
LSPs and quickly isolate MPLS forwarding problems is critical to their ability to offer services.

Prior to the release of the MPLS EM—MPLS LSP Multipath Tree Trace feature no automated way 
existed to discover all paths between provider edge (PE) routers. Troubleshooting forwarding problems 
between PEs was cumbersome.

The release of the MPLS EM—MPLS LSP Multipath Tree Trace feature provides an automated way to 
discover all paths from the ingress PE router to the egress PE router in multivendor networks that use 
IPv4 load balancing at the transit routers. Once the PE-to-PE paths are discovered, use MPLS LSP ping 
and MPLS LSP traceroute to periodically test them.

The MPLS EM—MPLS LSP Multipath Tree Trace feature requires the Cisco RFC-compliant 
implementation which is based on RFC 4379. If you do not have a Cisco IOS release that supports RFC 
4379,

 

MPLS LSP multipath tree trace does not operate to discover all PE-to-PE paths.

IPv4 load balancing at a transit router is based on the incoming label stack and the source and destination 
addresses in the IP header. The outgoing label stack and IP header source address remain constant for 
each branch being traced. 

When you execute MPLS LSP multipath tree trace on the source LSR, the router needs to find the set of 
IP header destination addresses to use all possible output paths. The source LSR starts path discovery by 
sending a transit router a bitmap in an MPLS echo request. The transit router returns information in an 
MPLS echo request that contains subsets of the bitmap in a downstream map (DS Map) in an echo reply. 
The source router can then use the information in the echo reply to interrogate the next router. The source 
router interrogates each successive router until it finds one bitmap setting that is common to all routers 
along the path. The router uses TTL expiry to interrogate the routers to find the common bits.

For example, you could start path discovery by entering the following command at the source router:

Source_LSR# trace mpls multipath ipv4 10.131.101.129/32 hashkey ipv4 bitmap 16

This command sets the IP address of the target router as 10/131.101.192 255.255.255.255 and 
configures:

The default hash key type to 8, which requests that an IPv4 address prefix and bit mask address set 
be returned in the DS Map in the echo reply.