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Cisco Employee

No label assigned to loopback IPs of directly connected routers

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

1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

Re: No label assigned to loopback IPs of directly connected rout

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

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
12 REPLIES
Cisco Employee

Re: No label assigned to loopback IPs of directly connected rout

Sourabh,

What platform and IOS release are you using?

Regards

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
Cisco Employee

Re: No label assigned to loopback IPs of directly connected rout

R1 -> 7206VXR NPE400 -> 12.2(25)S9

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

R3 -> C3640 -> 12.4(13b)

Cisco Employee

Re: No label assigned to loopback IPs of directly connected rout

Sourabh,

Could you please post the configs.

Regards

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
Cisco Employee

Re: No label assigned to loopback IPs of directly connected rout

Enclosed herewith configs.

Cisco Employee

Re: No label assigned to loopback IPs of directly connected rout

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

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
Hall of Fame Super Silver

Re: No label assigned to loopback IPs of directly connected rout

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

Silver

Re: No label assigned to loopback IPs of directly connected rout

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

Cisco Employee

Re: No label assigned to loopback IPs of directly connected rout

Thanks Harold for the help. Problem is resolved now.

Silver

Re: No label assigned to loopback IPs of directly connected rout

Hi Sourabh

I have sen the output you are getting no labels on R1 becasue R3 and R2 will advertise its directly conncted interace to R1 as implicit null.

You can test it with loopbacks also.

http://shivlu.blogspot.com/2008/11/what-will-happen-if-loopbacks-are-not.html

regards

shivlu jain

Cisco Employee

Re: No label assigned to loopback IPs of directly connected rout

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

Cisco Employee

Re: No label assigned to loopback IPs of directly connected rout

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

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
Silver

Re: No label assigned to loopback IPs of directly connected rout

Hritter

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

regards

shivlu jain

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