As per my previous post regarding how MPLS traverse in Data Plane Operation Frame-Mode, here i would like to write down about the MPLS Label Stack Header.
As per the previous studies we all know that the Layer 2 Frame Header are as below:
Above is the Layer 2 Frame Header with Unlabeled IP Packets, for Various Reason and Switching Performance being the first MPLS Label is inserted in between the Layer 3 IP Packets and Layer 2 Header as shown below:
Due to the way MPLS Label is inserted in between the Layer 3 Contents of Layer 2 Frame and Layer 2 Header, the MPLS Label header is also called SHIM Header.
MPLS Label consists of 20 Bits, the COS information bit which is also called Experimental bits consists of 3 Bits and 8 Bit of TTL (TIME-to-LIVE) ( which has the identical functions in loop detection as the IP TTL field ) and 1 bit of Bottom of Stack bit, which are all shown as below:
Above Frame are consists of 32 Bit Header in total and each are separated for different uses, the Bottom of Stack bit implements an MPLS Label Stack, where it defines the combination of two or more label header attached to Single Packets, although MPLS don’t have label stack on Simple Unicast IP routing, but for other MPLS application which are MPLS-Based VPN and MPLS Traffic Engineering do rely heavily on the Stack.
When the MPLS Header is inserted in between the Layer 3 IP Packet Header and Layer 2 Header, the sending router must have some sort of information for the receiving routers to indicates that the transmitted packet is not pure IP Datagram and it has some sort of label packets attached to it, for this below Protocols are defined above layer 2:
1. In LAN Environment labeled packets carrying unicast and multicast layer 3 packets uses ethertype value of 8847 and 8848 Hex, which are used directly on Ethernet, FastEthernet, GigEthernet as well as part of SNAP Header on other LAN Media.
2. In PPP where the points are using PPP encapsulation a new Network Control Protocol introduced which is called MPLS Control Protocol (MPLSCP) and labels are market with the PPP protocol field value of 8281 Hex.
3. In Frame-relay DLCI Peer Between Routers MPLS Packets are marked with Frame-Relay SNAP Network Layer Protocol ID (NLPID) followed by SNAP Header with the ethertype value of 8847 Hex.
4. In ATM Forum Virtual Circuit MPLS Packets are marked with a SNAP Header that uses the Same ethertype which are used by LAN (8847 and 8848 Hex)
Below Table summarize the Above points:
Packet over SONET/SDH:
Frame-Relay PVCs Similar:
Label Over ATM PVCs: