Cisco ASR 900 Series User Manual

or
trace mpls
{ipv4destination-address/destination-mask
| traffic-eng tunnel-interface tunnel-number
} reply mode router-alert
However, the reply with a router alert adds overhead to the process of getting a reply back to the originating
device. This method is more expensive to process than a reply without a router alert and should be used only
if there are reply failures. That is, the reply with a router alert label should only be used for MPLS LSP Ping
or MPLS LSP Traceroute when the originating (headend) device fails to receive a reply to an MPLS echo
request.
Packet Handling Along Return Path with an IP MPLS Router Alert
When an IP packet that contains an IP router alert option in its IP header or a Multiprotocol Label Switching
(MPLS) packet with a router alert label as its outermost label arrives at a device, the device punts (redirects)
the packet to the Route Processor (RP) process level for handling. This allows these packets to bypass the
forwarding failures in hardware routing tables. The table below describes how IP and MPLS packets with an
IP router alert option are handled by the device switching path processes.
Table 11: Switching Path Process Handling of IP and MPLS Router Alert Packets
Outgoing PacketProcess Switching ActionNormal Switching ActionIncoming Packet
IP packet—Router alert option
in IP header.
Forwards the packet as is.A rRouter alert option in the IP
header causes the packet to be
punted to the process switching
path.
IP packet—Router alert option
in IP header
MPLS packet— Outermost
label contains a router alert.
Adds a router alert as the
outermost label and forwards as
an MPLS packet.
A router alert option in theIP
header causes the packet to be
punted to the process switching
path.
IP packet—Router alert option
in IP header.
Removes the outermost router
alert label, adds an IP router
alert option to the IP header,
and forwards as an IP packet.
If the router alert label is the
outermost label, the packet is
punted to the process switching
path.
MPLS packet—Outermost label
contains a router alert
MPLS packet— Outermost
label contains a router alert.
Preserves the outermost router
alert label and forwards the
MPLS packet.
If the router alert label is the
outermost label, the packet is
punted to the process switching
path.
Other MPLS LSP Ping and Traceroute Command Options
The table below describes other MPLS LSP Ping and Traceroute command options that can be specified as
keywords or arguments with the ping mpls command, or with both the ping mpls and trace mpls commands.
Options available to use only on the ping mpls command are indicated as such.
MPLS Basic Configuration Guide, Cisco IOS XE Everest 16.5.1 (Cisco ASR 900 Series)
75
MPLS LSP Ping, Traceroute, and AToM VCCV
Command Options for ping mpls and trace mpls

Cisco ASR 900 Series Specifications

General

Brand	Cisco
Model	ASR 900 Series
Category	Network Router
Language	English