MPLS VPN Label

[jemekeren]

Hi...when connection customer to customer in vpn mpls there is a label in mpls packet that identify customer. how and the label is generated. is the mbgp process that create them? or the ldp? please correct me... is the label unidirectional too? means that from need 2 unique vpn label to support 2 way direction? tx u friend..

[Giuseppe Larosa]

Hello @nwekechampion ,

yes your understanding is correct : MPLS L3 VPN uses a stack of MPLS labels, the external one the first one is also called the IGP label and it is provided by LDP or RSVP-TE and it is the LSP to the egress PE loopback address,  and it changes at each MPLS router hop  ( P node) the PHP operation refers to the special operation performed by the last P on the path that pops the IGP label sending an MPLS label with only the VPNv4 label that travels unchanged end to end and it is allocated by the egress PE node using MP BGP af vpnv4.

Hope to help

Giuseppe

[nwekechampion]

Thanks so much!!

[reachonenetadm]

Mohammed Mahmoud,

Thanks for boiling it down so clearly. I do have a question about the resulting MTU. I show 2 labels in a PE to PE trace like so:

rtr-1#traceroute vrf CustomerA 10.16.0.1

Type escape sequence to abort.

Tracing the route to 10.16.0.1

1 192.168.0.6 [MPLS: Labels 17/20 Exp 0] 12 msec 16 msec 12 msec

Does this indicate that in order to get a PathMTU of 1500 i need an MPLS mtu of 1512 (64 bits VPN IPv4 address plus 32 bits MPLS VPN label)? TIA

[mohammedmahmoud]

Hi,

MPLS will add extra 4 bytes for each label in the label stack to the pure IP packet, which means that MPLS applications (MPLS VPN, AToM, MPLS TE, MPLS QoS) will add stacks of labels to IP packet, each is 4 bytes. To have a path MTU of 1500, you need to add the 4 bytes per label according to your MPLS applications on the PE interface connecting it to your MPLS backbone (if you face an issue with setting the interface MTU, example FE interface that doesn't support setting the interface MTU, you can rather set the MPLS MTU), and also keep in mind your interconnecting Layer 2 switches, you need to make sure that they support baby giants or further on jumbo frames.

BR,

Mohammed Mahmoud.

[reachonenetadm]

Mohammeed Mahmoud,

Thanks for your reply. I've created a new thread on this topic at http://forums.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Network%20Infrastructure&topic=WAN%2C%20Routing%20and%20Switching&topicID=.ee71a06&fromOutline=&CommCmd=MB%3Fcmd%3Ddisplay_location%26location%3D.2cc06659 . Perhaps you could take a look? Thanks again.

-Chris

[Luis_saltron]

buenas tardes

En este momento estoy observando variaciones en ciertos enlaces, dichas intermitencias son de micro segundos, asi mismo visualizo que no se esta estableciendo adhjacencia entre ellos, ya que la variaciones son en un lapso de tiempo muy corto.

Cabe destacar que no se estan presentando errores de CRC en los routers

Nota: Pienso yo que deberia un problema de comunicacion es decir un enlace de radio que deberia estar variando

Pido su ayuda para tener una idea de la situacion

Gracias y saludos

[lamav]

Luis, de que estas hablando, hombre??

Esta conversacion es sobre MPLS y VPN. Si Ud. necesita ayuda, tienes que empezar una conversacion nueva y presentar su situacion con detalles.

De todos maneras, se habla en Ingles en este message board, patron.

Gracias

Victor