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Incorrect Route Learning Causes an AR46 to Fail to Establish an MPLS LDP Session with a Directly-connected NE40E

Publication Date:  2012-07-27 Views:  320 Downloads:  0
Issue Description

 AR46 (pos2/0/0)-155M POS-transmission- (pos1/2/1)NE40E
MPLS and LDP are enabled on interfaces directly connecting the AR46 and NE40E. The MPLS LDP session fails to be established between the AR46 and NE40E.
The two interfaces at both ends of the MPLS LDP session are configured as follows:
[ah-cz-dcn]dis cur int pos 2/0/0
interface Pos2/0/0
link-protocol ppp
ip address
ospf cost 30
mpls ldp enable
<AHHF-QGSC-N40E-DCN-P2>dis cur interface pos 1/2/1
interface Pos1/2/1
link-protocol ppp
description TO AHCUZ-XXX-AR46-DCN-PE1
undo shutdown
ip address
ospf cost 30
mpls ldp 

Alarm Information


Handling Process

Change the route and ensure that both routers use the POS interface to learn the route to the peer loopback address.
2. As the loopback address is used by default for establishing an LDP session, use the interface address in the mpls ldp transport address command for the LDP session. Note that this command is supported by only a part of versions.
3. Alternatively, run the mpls ldp remote-peer command at both ends of the LDP session. This allows a remote peer relationship to be established. 
Root Cause

 After a check, the configuration is correct but the LDP session is not established. LDP information on both interfaces is displayed as follows:
<AHHF-QGSC-N40E-DCN-P2>dis mpls ldp int pos 1/2/1
LDP Interface Information
Interface Name : Pos1/2/1
LDP ID : Transport Address :
Entity Status : Active Effective MTU : 1500
Configured Hello Timer : 15 Sec
Negotiated Hello Timer : 15 Sec
Configured Keepalive Timer : 45 Sec
Configured Delay Timer : 10 Sec
Label Advertisement Mode : Downstream Unsolicited
Hello Message Sent/Rcvd : 962/1087 (Message Count) // Number of sent and received messages ------------------------------------------------------------------------------
[ah-cz-dcn]dis mpls ldp int
Displaying information about all Ldp interface:
Interface Pos2/0/0 (address=
Label distributing enabled,bound to entity:
Generic label range configured:16 - 204799
Label Advertisement Mode: Downstream-Unsolicited
Configured KeepAlive hold time:60, Configured Hello hold time:15
Negotiated Hello hold time:15
Hello packets sent/rcv:799/0 //Number of only sent packets
The statistics show that the NE40E has sent and received packets but the AR46 only has sent packets. The route may be incorrect.
2. The routes to the loopback addresses of both routers are checked.
<AHHF-QGSC-N40E-DCN-P2>dis ip rou
Route Flags: R - relay, D - download to fib
Routing Table : Public
Summary Count : 1
Destination/Mask Proto Pre Cost Flags NextHop Interface OSPF 10 31 D Pos1/2/1 // The directly-connected POS interface on the NE40E leans the route to the peer loopback address.
[ah-cz-dcn]dis ip rou
Destination/Mask Protocol Pre Cost Nexthop Interface OSPF 10 26 GigabitEthernet0/0/2 //The GE interface on the AR46 leans learns the route to the peer loopback address.
The output shows that that the AR46 uses a different interface but not a directly-connected interface to learn the route to the peer loopback address, which causes the AR46 unable to receive replies from the NE40E. As a result, the LDP session cannot be established. 


 In one of the following situation, the LDP session cannot be established:
A link or board failure causes packet loss.
2. Configurations at both ends of an LDP session do not match.
3. A route learning error occurs.
4. A bug in the hardware or software causes a negotiation failure.