Sending Back Icmp Ttl Exceeded Messages For Mpls Ttl Expired Packets; Configuring Ldp Gr - HP HSR6800 Configuration Manual

Sending back ICMP TTL exceeded messages for MPLS TTL
expired packets
After you enable an LSR to send back ICMP TTL exceeded messages for MPLS TTL expired
packets, when the LSR receives an MPLS packet that carries a label with TTL being 1, it generates
an ICMP TTL exceeded message, and send the message to the packet sender in one of the
following ways:
•
If the LSR has a route to the packet sender, it sends the ICMP TTL exceeded message to the
packet sender directly through the IP route.
•
If the LSR has no route to the packet sender, it forwards the ICMP TTL exceeded message
along the LSP to the egress, which sends the message to the packet sender.
Usually, for an MPLS packet carrying only one level of label, the first method is used. For an MPLS
packet carrying a multi-level label stack, the second method is used. However, because ASBRs,
SPEs in HoVPN applications, and carrier backbone PEs in nested VPNs might receive MPLS VPN
packets that carry only one level of labels but these devices have no IP routes to the packet senders,
the first method is not applicable. In this case, you can configure the undo ttl expiration pop
command on these devices so the devices use the second method.
For more information about HoVPN and nested VPN, see "Configuring MPLS L3VPN."
To configure the device to send back an ICMP TTL exceeded message for a received MPLS TTL
expired packet:
Step
1. Enter system view.
2. Enter MPLS view.
3. Enable the device to send
back an ICMP TTL exceeded
message when it receives an
MPLS TTL expired packet.
4. Configure the device to use
IP routes or LSPs to send
back the ICMP TTL
exceeded messages for
TTL-expired MPLS packets
that have only one level of
label.

Configuring LDP GR

MPLS has two separate planes: the forwarding plane and the control plane. Using this feature, LDP
Graceful Restart (GR) preserves the LFIB information when the signaling protocol or control plane
fails, so that LSRs can still forward packets according to LFIB, ensuring continuous data
transmission.
A device that participates in a GR process can be a GR restarter or a GR helper.
Command
system-view
mpls
ttl expiration enable
•
(Method 1) Use IP routes:
ttl expiration pop
•
(Method 1) Use LSPs:
undo ttl expiration pop
21
Remarks
N/A
N/A
Optional.
Enabled by default.
Optional.
Use either method as required.
By default, an ICMP TTL
exceeded message is sent back
along an IP route when the TTL of
an MPLS packet with a one-level
label stack expires.
This configuration does not take
effect when the MPLS packets
carry multiple levels of labels.
ICMP TTL exceeded messages
for such MPLS packets always
travel along the LSPs.