No label assigned to loopback IPs of directly connected routers

Answered Question
Mar 27th, 2009
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Hi,


I'm working on the below scenario.


R3 ---------- R1 -------------- R2

150.1.3.3 150.1.1.1 150.1.2.2 -> Lo


LDP is up between R3-R1 and R1-R2


But I do not see any label assigned to R3 & R2 lo IPs on R1 LFIB table.


R1#sh mpls ldp ne

Peer LDP Ident: 150.1.3.3:0; Local LDP Ident 150.1.1.1:0

TCP connection: 150.1.3.3.23155 - 150.1.1.1.646

State: Oper; Msgs sent/rcvd: 33/36; Downstream

Up time: 00:20:56

LDP discovery sources:

Serial2/1, Src IP addr: 150.1.13.3

Addresses bound to peer LDP Ident:

10.1.37.3 10.1.38.3 150.1.13.3 150.1.3.3

Peer LDP Ident: 150.1.2.2:0; Local LDP Ident 150.1.1.1:0

TCP connection: 150.1.2.2.11009 - 150.1.1.1.646

State: Oper; Msgs sent/rcvd: 29/28; Downstream

Up time: 00:16:39

LDP discovery sources:

FastEthernet0/0, Src IP addr: 150.1.12.2

Addresses bound to peer LDP Ident:

150.1.12.2 150.1.2.2 150.1.24.2




R1#sh mpls forward

Local Outgoing Prefix Bytes Label Outgoing Next Hop

Label Label or VC or Tunnel Id Switched interface

16 No Label 150.1.3.3/32 13243 Se2/1 point2point

17 No Label 150.1.2.2/32 0 Fa0/0 150.1.12.2

18 16 150.1.4.4/32 15063 Fa0/0 150.1.12.2

19 Pop Label 150.1.24.0/24 0 Fa0/0 150.1.12.2



R2#sh mpls ldp ne

Peer LDP Ident: 150.1.1.1:0; Local LDP Ident 150.1.2.2:0

TCP connection: 150.1.1.1.646 - 150.1.2.2.11009

State: Oper; Msgs sent/rcvd: 29/29; Downstream

Up time: 00:17:12

LDP discovery sources:

FastEthernet0/0, Src IP addr: 150.1.12.1

Addresses bound to peer LDP Ident:

150.1.12.1 150.1.1.1 150.1.13.1

Peer LDP Ident: 150.1.4.4:0; Local LDP Ident 150.1.2.2:0

TCP connection: 150.1.4.4.23266 - 150.1.2.2.646

State: Oper; Msgs sent/rcvd: 13/16; Downstream

Up time: 00:03:31

LDP discovery sources:

Serial2/0.1, Src IP addr: 150.1.24.4

Addresses bound to peer LDP Ident:

10.1.46.4 10.1.45.4 150.1.4.4 150.1.24.4

R2#sh mpls for

Local Outgoing Prefix Bytes Label Outgoing Next Hop

Label Label or VC or Tunnel Id Switched interface

16 No Label 150.1.4.4/32 13040 Se2/0.1 point2point

17 No Label 150.1.1.1/32 0 Fa0/0 150.1.12.1

18 16 150.1.3.3/32 15316 Fa0/0 150.1.12.1

19 Pop Label 150.1.13.0/24 0 Fa0/0 150.1.12.1

R2#


R3#sh mpls ldp ne

Peer LDP Ident: 150.1.1.1:0; Local LDP Ident 150.1.3.3:0

TCP connection: 150.1.1.1.646 - 150.1.3.3.23155

State: Oper; Msgs sent/rcvd: 36/34; Downstream

Up time: 00:21:36

LDP discovery sources:

Serial1/2, Src IP addr: 150.1.13.1

Addresses bound to peer LDP Ident:

150.1.12.1 150.1.1.1 150.1.13.1

R3#

R3#

R3#

R3#sh mpls fo

Local Outgoing Prefix Bytes tag Outgoing Next Hop

tag tag or VC or Tunnel Id switched interface

16 Untagged 150.1.1.1/32 0 Se1/2 point2point

17 Pop tag 150.1.12.0/24 0 Se1/2 point2point

18 17 150.1.2.2/32 0 Se1/2 point2point

19 18 150.1.4.4/32 0 Se1/2 point2point

20 19 150.1.24.0/24 0 Se1/2 point2point


If LDP neighborship is up, I thought labels will be exchanged automatically for the prefixes. Also CEF is enabled on all devices.


Any thoughts on what could be the problem or how should I troubleshoot this?


Regards

Sourabh

Correct Answer by Harold Ritter about 8 years 1 month ago

Sourabh,


The issue is that you use a /24 subnet mask on the loopback interfaces and that ospf advertises a /32 by default for a loopback interface regardless of what subnet mask is configured. Best practice is to configured a /32 on loopback interfaces. This will definitely solve the issue.


Regards

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Harold Ritter Fri, 03/27/2009 - 08:07
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Sourabh,


What platform and IOS release are you using?


Regards

souagarw Fri, 03/27/2009 - 09:32
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R1 -> 7206VXR NPE400 -> 12.2(25)S9

R2 -> 7206VXR NPE400 -> 12.2(25)S9

R3 -> C3640 -> 12.4(13b)

Harold Ritter Fri, 03/27/2009 - 11:07
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Sourabh,


Could you please post the configs.


Regards

Correct Answer
Harold Ritter Fri, 03/27/2009 - 11:52
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Sourabh,


The issue is that you use a /24 subnet mask on the loopback interfaces and that ospf advertises a /32 by default for a loopback interface regardless of what subnet mask is configured. Best practice is to configured a /32 on loopback interfaces. This will definitely solve the issue.


Regards

Giuseppe Larosa Fri, 03/27/2009 - 13:57
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Hello Harold, Sourabh


as an alternative to have OSPF to advertise loopback addresses with a prefix len less then 32 the same as LDP:


int loop m

ip ospf network point-to-point


However, in production networks /32 loopbacks are the common practice.


Hope to help

Giuseppe


shivlu jain Fri, 03/27/2009 - 22:06
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Hi Sourabh


Genrally loopbacks in ospf are advertise as stub host because of this they come as /32 as told by hritter. If you want to advertise as same subnet in that case you have to use ip ospf network point-to-point. This is help you to advertise the ospf as mentioned subnet.



regards

shivlu jain

souagarw Sat, 03/28/2009 - 02:04
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Thanks Harold for the help. Problem is resolved now.

souagarw Fri, 03/27/2009 - 09:24
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Hi Shivlu,


I understand from your blog that this issue will happen in case there is a MP-BGP session created between 2 routers using directly connected IPs.


However, in my case I have not configured MP-BGP yet on any of three devices. First I want to setup LDP between peers so that labels can be exchanged.


I understand that due to PHP, implicit null will be advertised for loopback interfaces, but in that case output on R1 should be 'POP Label' and not 'No Labels'.


Let me know if I'm missing anything.


Regards

Sourabh

Harold Ritter Fri, 03/27/2009 - 10:10
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Shivlu,


An implicit null should be seen as a "Pop Label" not as a "No Label", as the latter will cause all labels to be removed before the packet is sent out the egress interface, which will definitely be an issue if you are carrying a service label.


Regards

shivlu jain Fri, 03/27/2009 - 22:07
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Hritter


I think I were mistaken some where. Thanks for your comments.


regards

shivlu jain

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