What is label stack in MPLS?

Label stacking is the encapsulation of an MPLS packet inside another MPLS packet – that is, adding an MPLS header “on top of” (hence stacking) an existing MPLS header. The result of stacking is the ability to tunnel one MPLS LSP inside another LSP.

What is the format of MPLS labels?

MPLS Label Format | MPLS Frame format As shown a MPLS label is inserted between layer 2 header and layer 3 header of any packet. It is four bytes in size and consists of following fields. TC: 3 bit in size. It defines class of service.

What are two types of MPLS labels?

MPLS Label Distribution – LDP Part2

  • MPLS Reserved Labels Range. Labels 0 through 15 are reserved labels.
  • Implicit NULL Label (Label3) The implicit NULL label is the label that has a value of 3.
  • Explicit NULL Label (Label 0 V4/ 2 V6)
  • Router Alert Label (Label 1)

What is bottom of stack in MPLS?

MPLS supports multiple labels, which are stacked. If the BoS field of a label is set to 1, the label is at the bottom of the label stack. TTL: an 8-bit field indicating a time to live (TTL) value. This field implements the same function as the TTL field in IP packets.

What is inner label and outer label in MPLS?

The outer label identifies the segments of the LSP between the ingress PE and the egress PE, but it does not identify how the egress PE should forward the packet. The inner label identifies the egress PE’s forwarding details, in particular the outgoing interface for the unlabeled packet.

What is the bottom label in a label stack called?

How many bits is an MPLS label?

20 bits
The MPLS layer lies between layer 2 and 3 of the model ie the Data Link and the Network Layer. That’s why it is also known as 2.5 layer protocol or “shim” protocol. Label: The label field is of 20 bits, hence the label could take values from 0 to 2^20–1, or 1,048,575.

How are MPLS labels assigned?

One locally unique label is assigned to each IP destination found in the main routing table and stored in the LIB table. Propagation of local labels to adjacent routers where these labels might be used as next-hop labels (stored in the FIB and LFIB tables to enable label switching).

What is LSR in MPLS?

A label switch router (LSR) is a router that supports MPLS. It is capable of understanding MPLS labels and of receiving and transmitting a labeled packet on a data link.

How are MPLS labels distributed?

LDP is a protocol that automatically generates and exchanges labels between routers. Each router will locally generate labels for its prefixes and will then advertise the label values to its neighbors. It’s a standard, based on Cisco’s proprietary TDP (Tag Distribution Protocol).

Are MPLS labels unique?

A LSP is not characterised by a globally unique MPLS label; instead, labels have only local significance at a specific router interface (similar to the way VLAN-IDs are used at layer 2), and MPLS routers are therefore responsible for assigning the correct label to each packet for the next hop.