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LDP router-id /32 - why?

Hi:

Why does the router-id (L0) have to be a /32?

If it is a /24, even though the control plane works fine, the data plane gets broken - I cannot ping end-to-end.

A show mpls ldp discovery shows no host route to the /24 loopback.

Thanks !!

3 REPLIES
Cisco Employee

Re: LDP router-id /32 - why?

The requirement for /32 loopback addresses are not an absolute necessity but rather a best practice recommendation to make sure we don't use a less specific route (aggregate route or summary route) from ingress to egress PE, which could break the LSP.

In your case, you have a /24 subnet configured on the loopback interface. By default OSPF advertises the prefix for the loopback interface as a /32 (host route) regardless of the subnet mask configured, while LDP advertises it according to the configured subnet mask.

You need to configure "ip ospf network point-to-point" under the loopback interface for ospf to advertise the prefix according to the configured network mask. If you do this then the FEC advertised by LDP and the ospf advertised prefix will match, which should solve the issue you are seeing.

I would still recommend to configure your PE loopback interfaces using a /32 subnet mask.

Hope this helps,

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: LDP router-id /32 - why?

I forgot to mention that the LSP is not setup properly because the LDP FEC (/24) and the ospf prefix (/32) don't match.

Hope this helps,

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
New Member

Re: LDP router-id /32 - why?

Thank you very much, Harold.

Very clear now.

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