24
Step Command
Remarks
4.
Configure the router 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.
•
(Approach 1) Use IP routes:
ttl expiration pop
•
(Approach 2) Use LSP routes:
undo ttl expiration pop
Optional.
Use either approach 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.
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 router that participates in a GR process plays either of two roles:
•
GR restarter, the router that gracefully restarts due to a manually configured command or a fault. It
must be GR-capable.
•
GR helper, neighbor of the GR restarter. A GR helper maintains the neighbor relationship with the
GR restarter and helps the GR restarter restore its LFIB information. A GR helper must be
GR-capable.
Figure 12
LDP GR
As shown in
, two LDP peers perform GR negotiation when establishing an LDP session. The LDP
session established is GR capable only when both peers support LDP GR.
The working procedure of LDP GR is as follows:
GR helper
GR helper
GR helper
GR restarter
LDP session with GR capability
