ttl expiration pop

Function

The ttl expiration pop command enables a device to reply to a transmit end with ICMP reply messages over IP routes after MPLS TTLs in the packets expire.

The undo ttl expiration pop command disables IP routing and enables ICMP reply messages to pass through LSPs.

By default, if a device receives a packet with a single label and an expired MPLS TTL, the device replies to the transmit end with an ICMP reply message over an IP route.

Format

ttl expiration pop

undo ttl expiration pop

Parameters

None

Views

MPLS view

Default Level

2: Configuration level

Task Name and Operations

Task Name	Operations
ip-stack	write

Usage Guidelines

Usage Scenario

By default, if the device receives packets with expired MPLS TTLs, the VRP replies to the source with ICMP reply messages. If the IP routes are unreachable, the VRP discards the packet with the expired MPLS TTL. As a result, the tracert operation cannot provide forwarding path information carried in the ICMP message for the transmit end, which causes difficulties in analyzing faults. To prevent ICMP reply message loss, run the this command to enable ICMP reply messages to pass along an LSP.

The function of this command applies to the inter-AS VPN Option B. This command can be run on ASBRs in the inter-AS VPN Option B.

Prerequisites

The MPLS view has been displayed using the mpls command.

Precautions

The undo mpls command deletes all configurations related to this command.

Example

# Enable a device to forward ICMP reply messages along an LSP if the device receives packets with expired TTLs.

<HUAWEI> system-view
[~HUAWEI] mpls
[*HUAWEI-mpls] undo ttl expiration pop