Reply Mode Options For Mpls Lsp Ping And Traceroute - Cisco ASR 900 Series Manual

MPLS LSP Ping, Traceroute, and AToM VCCV
Option
router-alert
The reply with an IPv4 UDP packet implies that the device should send an IPv4 UDP packet in reply to an
MPLS echo request. If you select the ipv4 reply mode, you do not have explicit control over whether the
packet uses IP or MPLS hops to reach the originator of the MPLS echo request. This is the mode that you
would normally use to test and verify LSPs.
The reply with an IPv4 UDP packet that contains a device alert forces the packet to go back to the destination
and be processed by the Route Processor (RP) process switching at each intermediate hop. This bypasses
hardware/line card forwarding table inconsistencies. You should select this option when the originating
(headend) devices fail to receive a reply to the MPLS echo request.
You can instruct the replying device to send an echo reply with the IP router alert option by using one of the
following commands:
ping mpls
{ipv4 destination-address/destination-mask | pseudowire ipv4-address
vc-idvc-id | traffic-engtunnel-interface tunnel-number}
reply mode
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.

Reply Mode Options for MPLS LSP Ping and Traceroute

The reply mode is used to control how the responding device replies to a Multiprotocol Label Switching
(MPLS) echo request sent by an MPLS LSP Ping or MPLS LSP Traceroute command. The table below
describes the reply mode options.
router-alert
MPLS Basic Configuration Guide, Cisco IOS XE Everest 16.5.1 (Cisco ASR 900 Series)
Command Options for ping mpls and trace mpls
Description
Reply with an IPv4 UDP packet with a device alert.
This reply mode adds the router alert option to the IP
header. This forces the packet to be special handled
by the Cisco device at each intermediate hop as it
moves back to the destination.
This reply mode is more expensive, so use the
router-alert option only if you are unable to get a reply
with the ipv4 option, "Reply with an IPv4 UDP
packet."
You implement this option using the reply mode
router-alert keywords
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