ttl expiration pop

Function

The ttl expiration pop command enables a device to use the local IP route to forward ICMP response packets after the MPLS TTL expires.

The undo ttl expiration pop command disables this function. Therefore, ICMP response packets are sent along LSPs.

By default, the LSR returns an ICMP packet using the local IP route if the received MPLS TTL-expired packet contains one label.

Format

ttl expiration pop

undo ttl expiration pop

Parameters

None

Views

MPLS view

Default Level

2: Configuration level

Usage Guidelines

Usage Scenario

On an MPLS network, when an LSR receives MPLS packets with the TTL value of 1, the LSR generates an ICMP TTL-expired message.

The LSR returns the TTL-expired message to the sender in the following ways:

If the LSR has a reachable route to the sender, it directly sends the TTL-expired message to the sender through the IP route.
If the LSR has no reachable route to the sender, it forwards the TTL-expired message along the LSP. The egress node forwards the TTL-expired message to the sender.

In most cases, the received MPLS packet contains only one label and the LSR responds to the sender with the TTL-expired message using the first method. If the MPLS packet contains multiple labels, the LSR uses the second method.

The MPLS VPN packets may contain only one label when they arrive at an autonomous system boundary router (ASBR) on the MPLS VPN. These devices have no IP routes to the sender, so they use the second method to reply to the TTL-expired messages.

Precautions

The undo mpls (system view) command deletes all configurations of the ttl expiration pop command.

Example

# Forward ICMP packets through an LSP after the MPLS TTL expires.

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