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Community Member

MPLS label size

As per my understanding 2 labels get attached to an IP packlet, when it goes through MPLS network(BGP Vpn label/MPLS label). Each of this is 4 bytes each.

So I can say that a total of 8 bytes are attached to the packet. But what I see is that if I pump a 1500 size packet, only 4 bytes are getting attached. Can anybody tell why??

3 REPLIES
Cisco Employee

Re: MPLS label size

This would indeed be normal behavior if your ingress and egress PE are directly connected (no P router(s) in between). In such a scenario, the ingress PE does not insert the IGP label because it receives a implicit null label from the egress PE for its loopback interface address. The ingress PE therefore only insert the VPN label in the packet before it forwards it towards the egress PE.

Regerds,

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
Community Member

Re: MPLS label size

I am still in confusion why two label?, only single MPLS label should be there then which one is the other label?

what is mean by IGP label?

Cisco Employee

Re: MPLS label size

Dan,

In an MPLS VPN context, two labels are generally used. The first label, usually referred to as the IGP label, has for purpose to get the packet from the ingress to the egress PE. The second label, usually referred to as VPN label, has for purpose to send the packet to the proper egress interface or to cause the IP look up in the proper VRF on the egress PE.

The IGP label is generally learnt via LDP or RSVP. The second label is learnt via BGP VPNv4.

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
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