MPLS label size

[chopra.nitin]

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??

[Harold Ritter]

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,

[dangal.43]

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?

[Harold Ritter]

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,

[Vineeth.Annavarapu]

Hi, 

What info does the vpn label exactly consist of ? What is use of this VPN label? Already IP header contains the destination address  info with which Egress PE sends the packet to correct interface that is connect to CE. All the routing relating info is transmitted using MP  BGP updates 

[Harold Ritter]

Hi @Vineeth.Annavarapu ,

The VPN label (also referred to as service label) contains the same information as any other label.

It is used, once the packet is received on the egress PE, to forward this packet to the right VRF (CE or directly attached subnet). The IP header is not used in the core to forward the packet from ingress to egress PE, but rather the IGP label, which is normally  received via OSPF or ISIS (in the case of segment routing), LDP, RSVP or BGP-LU. In the L3VPN context, the service label is received via MP BGP (address-family vpnv4 or vpnv6). In the context of L2VPN, the service label is received via LDP (targeted LDP) or BGP.

Regards,