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Cisco Active Network Abstraction 3.6.6 MPLS User Guide
OL-19192-01
Chapter 7 MPLS Network Faults
LDP Neighbor Down Alarm
Figure 7-1 Example of an MPLS Black Hole Scenario
In this case, Cisco ANA does the following:
• Identifies untagged label switching entries on P2 and PE3.
• Issues MPLS Black Hole Found alarms on the interfaces on both sides of the link (since the LSP is
unidirectional).
• Initiates a backward flow starting from the link on the specific untagged entries and identifies the
two LSPs traversing the link, namely:
–
LSP from PE2 to PE3
–
LSP from PE3 to PE2
• Issues Broken LSP Discovered alarms on both LSPs in PE2 and PE3, which are correlated to the
corresponding MPLS Black Hole Found alarm.
Note The clearing alarm does not activate flows to locate the LSPs that were passing through it in order to
issue a clearing alarm for Broken LSPs, but rather uses the auto-clear functionality. The gateway
periodically reviews the tickets and checks if all the alarms under each ticket are cleared or configured
as auto-cleared alarms, and whether the gateway correlation timeout has passed, in which case the
gateway closes the ticket.
After the MPLS Black Hole alarm clears, and the configured gateway correlation timeout period is
reached, the gateway can close the ticket because all the alarms correlated to MPLS Black Hole and
Broken LSP are auto-cleared.
Note If an MPLS Network Link Down event causes an IP reroute and an LDP redistribution, new LSPs might
be redirected through non-MPLS segments, which will create a black hole. In this case, Broken LSP
Discovered alarms are issued. However, the discovered broken LSPs are correlated to the Link Down
alarm and not to the MPLS Black Hole Found alarm.
LDP Neighbor Down Alarm
LDP enables neighboring P or PE routers acting as LSRs to discover peers in an MPLS network to which
they can establish LDP sessions. The sessions allow the routers to negotiate and exchange labels used
for forwarding packets.
Provider network
P2
PE2
PE3
VPN1
VPN3
VPN1
VPN3
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