Packet Handling Along Return Path With An Ip Mpls Router Alert; Other Mpls Lsp Ping And Traceroute Command Options - Cisco ASR 900 Series Manual

MPLS LSP Ping, Traceroute, and AToM VCCV
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
Incoming Packet
IP packet—Router alert option
in IP header
MPLS packet—Outermost label
contains a router alert

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.
Normal Switching Action
A rRouter alert option in the IP
header causes the packet to be
punted to the process switching
path.
A router alert option in theIP
header causes the packet to be
punted to the process switching
path.
If the router alert label is the
outermost label, the packet is
punted to the process switching
path.
If the router alert label is the
outermost label, the packet is
punted to the process switching
path.
MPLS Basic Configuration Guide, Cisco IOS XE Everest 16.5.1 (Cisco ASR 900 Series)
Command Options for ping mpls and trace mpls
Process Switching Action
Forwards the packet as is.
Adds a router alert as the
outermost label and forwards as
an MPLS packet.
Removes the outermost router
alert label, adds an IP router
alert option to the IP header,
and forwards as an IP packet.
Preserves the outermost router
alert label and forwards the
MPLS packet.
Outgoing Packet
IP packet—Router alert option
in IP header.
MPLS packet— Outermost
label contains a router alert.
IP packet—Router alert option
in IP header.
MPLS packet— Outermost
label contains a router alert.
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