Bfd Operation - Alcatel-Lucent 7450 Manual

BFD Operation

2. If the router interface is up and only one of the LDP IPv4 or LDP IPv6 interfaces goes
3. When the router interface comes up from a down state, and one of either the LDP IPv4 or
Given the above behavior, it is recommended that the user configures the sync timer to a value
which allows enough time for both the LDP IPv4 and LDP IPv6 sessions to come up.
BFD Operation
The operation of BFD over a LDP interface tracks the next-hop of prefix IPv4 and prefix IPv6 in
addition to tracking of the LDP peer address of the Hello adjacency over that link. This tracking is
required as LDP can now resolve both IPv4 and IPv6 prefix FECs over a single IPv4 or IPv6 LDP
session and, as such, the next-hop of a prefix will not necessarily match the LDP peer source
address of the Hello adjacency. The failure of either or both of the BFD session tracking the FEC
next-hop and the one tracking the Hello adjacency will cause the LFA backup NHLFE for the FEC
to be activated, or the FEC to be re-resolved if there is no FRR backup.
The following CLI command allows the user to decide if they want to track only with an IPv4
BFD session, only with an IPv6 BFD session, or both:
configure>router>ldp>interface-parameters>interface>bfd-enable [ipv4] [ipv6]
This command provides the flexibility required in case the user does not need to track both Hello
adjacency and next-hops of FECs. For example, if the user configures bfd-enable ipv6 only to
save on the number of BFD sessions, then LDP will track the IPv6 Hello adjacency and the next-
hops of IPv6 prefix FECs. LDP will not track next-hops of IPv4 prefix FECs resolved over the
same LDP IPv6 adjacency. If the IPv4 data plane encounters errors and the IPv6 Hello adjacency
is not affected and remains up, traffic for the IPv4 prefix FECs resolved over that IPv6 adjacency
will be black-holed. If the BFD tracking the IPv6 Hello adjacency times out, then all IPv4 and
IPv6 prefix FECs will be updated.
The tracking of a mLDP FEC has the following behavior:
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down, no action is taken.
LDP IPv6 sessions comes up, IGP starts the sync timer at the expiry of which the interface
metric is restored to its configured value. All static routes with the ldp-sync option
enabled are also activated at the expiry of the timer.
IPv4 and IPv6 mLDP FECs will only be tracked with the Hello adjacency because they do
not have the concept of downstream next-hop.
The upstream LSR peer for an mLDP FEC supports the multicast upstream FRR
procedures, and the upstream peer will be tracked using the Hello adjacency on each link
or the IPv6 transport address if there is a T-LDP session.
The tracking of a targeted LDP peer with BFD does not change with the support of IPv6
peers. BFD tracks the transport address conveyed by the Hello adjacency which
bootstrapped the LDP IPv6 session.
7450 ESS MPLS Guide