LSPs towards loopbacks

Latest reply: Feb 25, 2014 04:19:21 1727 4 0 0

Hi dears,

 

I am simulating the attached MPLS topology. When issuing the "display mpls lsp" command on R1, I see the following output:

 

<R1>display mpls lsp
-------------------------------------------------------------------------------
                 LSP Information: LDP LSP
-------------------------------------------------------------------------------
FEC                In/Out Label  In/Out IF                      Vrf Name      
2.2.2.2/32         NULL/3        -/GE0/0/0                                    
2.2.2.2/32         1024/3        -/GE0/0/0                                    
1.1.1.1/32         3/NULL        -/-
4.4.4.4/32         NULL/1026     -/GE0/0/0                                    
4.4.4.4/32         1025/1026     -/GE0/0/0                                    
3.3.3.3/32         NULL/1025     -/GE0/0/0                                    
3.3.3.3/32         1026/1025     -/GE0/0/0                                    
5.5.5.5/32         NULL/1027     -/GE0/0/0                                    
5.5.5.5/32         1027/1027     -/GE0/0/0                                    
6.6.6.6/32         NULL/1028     -/GE0/0/0                                    
6.6.6.6/32         1028/1028     -/GE0/0/0                                    
<R1>

 

Similiar output on the other routers. Why are there two LSPs for every loopback? I would understand there would be one LSP for every loopback, but not two. And why is there in the "In interface" column always "-"?

Thank you in advance.

 

Julián

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f80029566
Created Jan 6, 2014 20:01:58 Helpful(0) Helpful(0)

Reply 2 #

Hi Nigel,

 

Now it makes sense. I have issued the same command on R2:

 

<R2>display mpls lsp

-------------------------------------------------------------------------------

LSP Information: LDP LSP

-------------------------------------------------------------------------------

FEC In/Out Label In/Out IF Vrf Name

2.2.2.2/32 3/NULL -/-

1.1.1.1/32 NULL/3 -/GE0/0/0

1.1.1.1/32 1024/3 -/GE0/0/0

4.4.4.4/32 NULL/3 -/GE0/0/2

4.4.4.4/32 1025/3 -/GE0/0/2

5.5.5.5/32 NULL/1025 -/GE0/0/2

5.5.5.5/32 1026/1025 -/GE0/0/2

3.3.3.3/32 NULL/3 -/GE0/0/1

3.3.3.3/32 1027/3 -/GE0/0/1

5.5.5.5/32 NULL/1025 -/GE0/0/1

5.5.5.5/32 1026/1025 -/GE0/0/1

6.6.6.6/32 NULL/1028 -/GE0/0/1

6.6.6.6/32 1028/1028 -/GE0/0/1

6.6.6.6/32 NULL/1028 -/GE0/0/2

6.6.6.6/32 1028/1028 -/GE0/0/2

<R2>

 

The question now is why the In Interface is blank (-). For example, for 3.3.3.3 the In Interface is blank and the Out Interface is GE0/0/1. The Out Interface is fine, but the In Interface should be either GE0/0/0 or GE0/0/2. I have issued the command on R2, because for R1 there would be only Out Interface (GE0/0/0) and none In Interface according the topology.

 

Thank for your interest,

Julián

 

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Sentinel
Created Jan 13, 2014 13:56:38 Helpful(0) Helpful(0)

Hi Julián

 

I beleive in this case the input interface is blank because the incoming interface could be "any" interface.

 

With the default mode of label assignment the router assigns a single lable per destination, this will be used by any routers sending lablled packets to this device. So basically it is saying, no matter which incoming interface the frame is received on, this is the label that will be assigned and this is the outgoing interface.

It is also possible to have different labels assigned to different interfaces, in this case you would see an incoming interface as there would be a different incoming label for each incoming interface.

 

Hope this is clear enough.

 

Regards Nigel

 

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f80029566
Created Feb 25, 2014 04:19:21 Helpful(0) Helpful(0)

Reply 4 #

Great explanation Nigel!

Thanks very much for your interest!

 

Julián

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Sentinel
Created Jan 6, 2014 09:30:18 Helpful(0) Helpful(0)

Hi Julián The reason there are two LSPs for every loopback entry is that there are two possible situations: 1. The received frame has no label (NULL). 2. The received frame has a label (specified by the local router). In both cases the output is the same, in case one a label is "pushed" into the frame, in case 2 the label is swapped. I hope this makes sense, if you look at the second column, it is showing the incoming label and the outgoing label, there is always a possibility that there will be no incoming label due to topology changes, convergence delays or simply sending routers not running MPLS. Regards Nigel
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