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DES-7200 Configuration Guide Chapter 3 MPLS Reliability Configuration
3-30
application, but at least one LSR plays the active role. Therefore, the initialization phase can
involve two scenarios.
z Both ends play the active role
In case both ends play the active role, since LSP is unidirectional, there may be the following two
scenarios:
From Ingress LSR to Egress LSR: BFD for LSP; from Egress LSR to Ingress LSR:
BFD for another LSP.
The Ingress LSR will send LSP Ping echo request packet carrying my discriminator to
the Egress LSR. When Egress LSR receives the echo request packet, it can then
acquire your discriminator from the echo request packet. In this way, Egress LSR has
both my discriminator (generated by Egress LSR) and your discriminator, and then
starts to send BFD control packet to the Ingress LSR. When Ingress LSR receives the
BFD control packet, it can acquire your discriminator from the BFD control packet
received. Hence, the Ingress LSR also has my discriminator (generated by Ingress LSR)
and your discriminator. After having the discriminators of both ends, it will start to send
BFD control packet to Egress LSR, and thus the Ingress LSR and Egress LSR enter
into the initial stage of BFD session establishment.
Here we shall point out that: when Egress LSR receives the echo request packet, it
can reply with the echo reply packet (or choose not to reply). If Egress LSR does
send the echo reply packet, then such packet must carry my discriminator
(generated by Egress LSR). In this way, the Ingress LSR can acquire your
discriminator from BFD control packet or from the echo reply packet.
For Egress LSR, the working process is same as that of Ingress LSR, and we will
no give unnecessary details herein.
From Ingress LSR to Egress LSR: BFD for LSP; from Egress LSR to Ingress LSR:
BFD for IP (when there are multiple hops)
For Ingress LSR, the working process has been described before, and we will not
give unnecessary details herein. For Egress LSR, before receiving the echo
request packet sent by Ingress LSR or acquiring your discriminator, its BFD control
packets sent to Ingress LSR will be discarded.
z One active end and one passive end
The active end will send LSP Ping echo request packet carrying my discriminator to the
passive end. When the passive end receives the echo request packet, it can then acquire
your discriminator from the echo request packet. In this way, the passive end has both my
discriminator (generated by passive end) and your discriminator, and then starts to send
BFD control packet to the active end. When active end receives the BFD control packet, it