72
Resetting LDP sessions
If you change LDP session parameters when some LDP sessions are up, the LDP sessions cannot function
normally. In this case, reset LDP sessions so the LDP peers will renegotiate parameters and establish new
sessions.
Use the following command to reset LDP sessions:
Task Command
Remarks
Reset LDP sessions.
reset mpls ldp
[
all
| [
vpn-instance
vpn-instance-name
] [
fec
mask
|
peer
peer-id
] ]
Available in user view
Managing and optimizing MPLS forwarding
Configuring TTL processing mode at ingress
At the ingress of an LSP, a label stack encapsulated with a TTL field is added to each packet. Whether the
label TTL takes the IP TTL or not depends on whether IP TTL propagation is enabled:
•
With IP TTL propagation enabled: When the ingress labels a packet, it copies the TTL value of the
original IP packet to the TTL field of the label. When an LSR forwards the labeled packet, it
decrements the TTL value of the label at the stack top by 1. When an LSR pops a label, it copies the
TTL value of the label at the stack top back to the TTL field of the IP packet. In this case, the TTL value
of a packet is decreased hop by hop when forwarded along the LSP. Therefore, the result of tracert
will reflect the real path along which the packet has traveled.
Figure 21
Label TTL processing when IP TTL propagation is enabled
•
With IP TTL propagation disabled: When the ingress labels a packet, it does not copy the TTL value
of the original IP packet to the TTL field of the label, and the label TTL is set to 255. When an LSR
forwards the labeled packet, it decrements the TTL value of the label at the stack top by 1. When an
LSR pops a label, it compares the IP TTL and the label TTL and uses the smaller value as the TTL of
the IP packet. In this case, the result of tracert does not show the hops within the MPLS backbone,
as if the ingress and egress were connected directly.
Ingress
Egress
LSR D
TTL
TTL
IP TTL
Label TTL
TTL 253
TTL 252
TTL 252
TTL 249
TTL 248
TTL 252
TTL 250
Copy TTL value
TTL 251
TTL 252
LSR A
LSR B
LSR C
LSR E
TTL 249
TTL 249
The penultimate hop copies
the label TTL to the IP packet
and pops the label.
