Refer to the exhibit. When implementing an LDP protocol, an engineer experienced an issue between two directly connected routers and noticed that no LDP neighbor exists for 1.1.1.1. Which factor should be the reason for this situation?
A.
LDP needs to be enabled on the R2 loopback interface.
B.
LDP needs to be enabled on the R2 physical interface.
The answer is B.. you have to have a transport IP which is the interface that is used to actually send the hellos. You are configured with a targeted session but MPLS is not passing on the interface, that's why it will show you sending hellos but it's not FORWARDING them.
Router(config-if)#do sh mpls ldp disc det
Local LDP Identifier:
2.2.2.2:0
Discovery Sources:
Targeted Hellos:
2.2.2.2 -> 1.1.1.1 (ldp): active/passive, xmit/recv
Hello interval: 10000 ms; Transport IP addr: 2.2.2.2
LDP Id: 1.1.1.1:0; no host route to transport addr
Src IP addr: 1.1.1.1; Transport IP addr: 1.1.1.1
Hold time: 90 sec; Proposed local/peer: 90/15 sec
Reachable via 0.0.0.0/0
Password: not required, none, in use
Before session protection
Router(config)#do sh mpls ldp disc det
Local LDP Identifier:
2.2.2.2:0
Discovery Sources:
Interfaces:
GigabitEthernet0/0 (ldp): xmit/recv
Enabled: Interface config
Hello interval: 5000 ms; Transport IP addr: 2.2.2.2
LDP Id: 1.1.1.1:0; no host route to transport addr
Src IP addr: 172.16.0.1; Transport IP addr: 1.1.1.1
Hold time: 15 sec; Proposed local/peer: 15/15 sec
Reachable via 0.0.0.0/0
Password: not required, none, in use
Clients: IPv4
Targeted Hellos:
2.2.2.2 -> 1.1.1.1 (ldp): active/passive, xmit/recv
Hello interval: 10000 ms; Transport IP addr: 2.2.2.2
LDP Id: 1.1.1.1:0; no host route to transport addr
Src IP addr: 1.1.1.1; Transport IP addr: 1.1.1.1
Hold time: 90 sec; Proposed local/peer: 90/90 sec
Reachable via 0.0.0.0/0
Password: not required, none, in use
After session Protection
Router(config-if)#do sh mpls ldp disc det
Local LDP Identifier:
2.2.2.2:0
Discovery Sources:
Targeted Hellos:
2.2.2.2 -> 1.1.1.1 (ldp): active/passive, xmit/recv
Hello interval: 10000 ms; Transport IP addr: 2.2.2.2
LDP Id: 1.1.1.1:0; no host route to transport addr
Src IP addr: 1.1.1.1; Transport IP addr: 1.1.1.1
Hold time: 90 sec; Proposed local/peer: 90/15 sec
Reachable via 0.0.0.0/0
Password: not required, none, in use
Sure R2 is not receiving hellos, so C sounds good. But WHY is R2 not receiving hellos, because LDP is not enabled on any physical interface. You can see in the output the discovery sources are blank. So B is a better answer than C.
Just to bring more confusion...
The output shows that R2 does not have MPLS enabled on any interface, or MPLS enabled interface is shutdown (tested in the lab), so not 100% B.
We can see that R2 have a configured targeted LDP to R1. The basic reason for LDP session not being up on R2 is because it didn't receive any hello from R1. Targeted LDP just needs both ends to be configured and pure IP connectivity between ends.
Any comments ?
Correct answer is c, R2 sending hellos but not receiving any.
upvoted 3 times
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