[c-nsp] MPLS router-ID

Paul paul at gtcomm.net
Thu Aug 13 20:00:50 EDT 2015

LDP transport-address is the address of which you tell the  neighbor on 
that interface to send packets.  (such as defaults to the router id, 
like a loopback)
If the loopback, or the router id of the router you are on is not 
present in the IGP on other routers, you use the transport-address to 
tell the other router on that interface to use 'interface' or you can 
specify an IP.

example would be, remove your loopback from being advertised from ospf, 
now no other router knows how to get to your loopback, but the LDP 
router-id is still your loopback, so you override that by setting a 
per-interface IP configuration to tell the neighbor router how to 
establish a session with you

it has nothing to do with the source address of transmitted ldp hellos 
(the udp discovery packets)
It has to do with the TCP session that is created after discovery.
Router1 interface fa0/1 IP -> router 2 interface fa0/1 IP
router 1 loopback                   router2 loopback
router 1 loopback not in IGP               router2 loopback in IGP

this happens..:
router1 send LDP source -> UDP MULTICAST hello
router2 respond, -> my router id is , I will me 
master and you will be passive
router1 says OK, my router id, make a tcp connection to me
router 2 can't make a tcp connection to because it's not in the IGP!

and now...router1 interface fa0/1, add transport-address interface
<<skipping first 2 thing>>
router1 says OK, my router id(transport address) is make a tcp 
connection to me
router 2 make tcp connection to and all is ok

Obviously this is a crude representation of what happens but it should 
show you the value of the transport-address.

you can check the transport addr with show mpls ldp discovery detail

On 8/13/2015 5:30 AM, Mohammad Khalil wrote:
> I lapped it up and the source of the hello messages will be the IP assigned on the physical address , but when the session comes up , the TCP source from my side will be the transport-address
> BR,
> Mohammad
> From: eng_mssk at hotmail.com
> To: jwbensley at gmail.com; cisco-nsp at puck.nether.net
> Subject: RE: [c-nsp] MPLS router-ID
> Date: Thu, 13 Aug 2015 12:10:52 +0300
> 			What
>   is so weird is that I have configured the mpls ldp discovery
> transport-address x.x.x.x and am still seeing the ldp hello messages
> from my side sourced from the physical IP address , why?
> BR,
> Mohammad
>> From: jwbensley at gmail.com
>> Date: Tue, 11 Aug 2015 18:44:34 +0100
>> To: cisco-nsp at puck.nether.net
>> Subject: Re: [c-nsp] MPLS router-ID
>> On 11 August 2015 at 14:39, Mohammad Khalil <eng_mssk at hotmail.com> wrote:
>>> I have one of my PEs already configured with a router-id (which is my private loopback interface)
>> I assume you mean LDP router-id?
>>> Am trying to establish xconnect with my uplink provider (which uses Juniper)
>>> The SP insisted to use public IP address for LDP neighborship , I cannot change my router-id as I will lose all of my active L2VPN sessions in my network (according to what I know , I cannot have expect for one ID for my router)
>>> I configured a public IP address and advertised it into BGP to my SP and the MPLS LDP session is up , but the targeted LDP session is passive !
>> What you say configured a public IP, do you mean on the interface
>> facing the provider and then used something like "discovery
>> transport-address interface" to use the interface IP as your LDP
>> router-id?
>> Also why have you originated it into BGP? If you are using the public
>> on on the interface facing your provider they should know IP because
>> its directly connected.
>> On paper this configuration works but I'm not 100% clear of your explentaion.
>> As Nick has said, just having something like a tagged sub-interface
>> between you and your provider where the xconnect terminates would be a
>> better idea, the L3 VPNs can be in other sub-int's.
>> Cheers,
>> James.
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