MPLS PART 1 UNDERSTAND EDGE LSR & USE OF LABELS

UNDERSTAND EDGE LSR & USE OF LABELS

The MPLS architecture describes the mechanisms to perform label switching, which combines the

benefits of packet forwarding based on Layer 2 switching with the benefits of Layer 3 routing.

Similar to Layer 2 networks (for example, Frame Relay or ATM), MPLS assigns labels to packets 

for transport across packet- or cell-based networks. The forwarding mechanism throughout the 

network is label swapping, in which units of data (for example, a packet or a cell) carry a short, 

fixed-length label that tells switching nodes along the packets path how to process and forward the data.

The significant difference between MPLS and traditional WAN technologies is the way labels 
are assigned and the capability to carry a stack of labels attached to a packet. The concept
 of a label stack enables  new applications, such as 

• Traffic Engineering
• Virtual Private Networks
• fast rerouting around link
• node failures

Every MPLS node must run one or more IP routing protocols (or rely on static routing) to

 exchange IP routing information with other MPLS nodes in the network. In this sense, every

 MPLS node  (including ATM switches) is an IP router on the control plane.

Similar to traditional routers, the IP routing protocols populate the IP routing table. 

In traditional IP routers, the IP routing table is used to build the IP forwarding cache

 (fast switching cache in Cisco IOS) or the IP forwarding table (Forwarding Information Base 

[FIB] in Cisco IOS)  used by Cisco Express Forwarding (CEF).

In an MPLS node, the IP routing table is used to determine the label binding exchange, where

 adjacent MPLS nodes exchange labels for individual subnets that

are contained within the IP routing table. The label binding exchange for unicast 

destination-based IP routing is performed using the Cisco proprietary Tag Distribution

 Protocol (TDP) or the IETF-specified Label Distribution Protocol (LDP).

The MPLS IP Routing Control process uses labels exchanged with adjacent MPLS nodes

 to build the Label Forwarding Table, which is the forwarding plane database that is

 used to forward labeled packets through the MPLS network.

As with any new technology, several new terms are introduced to describe the devices that 

make up the architecture. These new terms describe the functionality of each device and 

their roles within the MPLS domain structure. The first device to be introduced is the Label

 Switch Router (LSR). Any router or switch that implements label distribution procedures and 

can forward packets based on labels falls under this category.

Any LSR that has any non-MPLS neighbors is considered an Edge-LSR. However, if that LSR has

 any interfaces that connect through MPLS to an ATM-LSR, it also is considered to be an ATM

 edge-LSR. Edge-LSRs use a traditional IP forwarding table, augmented with labeling information,

 to label IP packets or to remove labels from labeled packets before sending them to non-MPLS 

nodes. Figure shows the architecture of an Edge-LSR.

Architecture of an Edge-LSR

Actions Performed by Various LSR Types

LSR              Type   Actions Performed by This LSR Type

• LSR              Forwards labeled packets.
• Edge-LSR    Can receive an IP packet, perform Layer 3 lookups, and impose a label stack before forwarding the packet into the LSR domain. Can receive a labeled packet, remove labels, perform Layer 3 lookups, and forward the IP packet toward its next-hop.

MPLS Label Imposition and Forwarding

MPLS Packet Forwarding and Label Switched Paths

Each packet enters an MPLS network at an ingress LSR and exits the MPLS network at
 an egress LSR.  This mechanism creates what is known as a Label Switched Path (LSP),
 which essentially describes  the set of LSRs through which a labeled packet must traverse
to reach the egress LSR for a particular FEC. This LSP is unidirectional, which means that
a different LSP is used for return traffic from a particular FEC. The creation of the LSP is
 a connection-oriented scheme because the path is set up prior to any traffic flow. 

However, this connection setup is based on topology information rather than a requirement
 for traffic flow. This means that the path is created regardless of whether any traffic actually
 is required to flow along the  path to a particular set of FECs.

Each LSR keeps two tables, which hold information that is relevant to the MPLS forwarding
 component.  The first, known in Cisco IOS as the Tag Information Base (TIB) or Label
Information Base (LIB) in standard MPLS terms, holds all labels assigned by this LSR and
the mappings of these labels to labels received from any neighbors. These label mappings
are distributed through the use of label-distribution  protocols.

Just as multiple neighbors can send labels for the same IP prefix but might not be the
actual IP next  hop currently in use in the routing table for the destination, not all the labels
within the TIB/LIB need to be used for packet forwarding. The second table, known in
Cisco IOS as the Tag Forwarding  Information Base (TFIB) or Label Forwarding Information
 Base (LFIB) in MPLS terms, is used during  the actual forwarding of packets and holds only
 labels that are in use currently by the forwarding  component of MPLS.

Edge-LSR Architecture Using Cisco IOS Terms

                            

Control Protocols used in MPLS applications:

Lab related to Labels used in MPLS:

after configuring all the routers , run routing protocol OSPF , then on the each interface enable

mpls ip , by default all the routers user LDP protocol for LIB tables .

Then for verification run # sh mpls forwarding-table & #sh mpls ldp neighbor

 on all the routers. Here i paste the output of router R1

R1#sh mpls ldp neighbor

    Peer LDP Ident: 10.1.2.1:0; Local LDP Ident 1.1.1.1:0

        TCP connection: 10.1.2.1.56975 - 1.1.1.1.646

        State: Oper; Msgs sent/rcvd: 39/39; Downstream

        Up time: 00:27:27

        LDP discovery sources:

          FastEthernet0/0, Src IP addr: 10.1.1.2

        Addresses bound to peer LDP Ident:

          10.1.1.2        10.1.2.1

R1#sh mpls forwarding-table

Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop

tag    tag or VC   or Tunnel Id      switched   interface

16     17          4.4.4.4/32        0          Fa0/0      10.1.1.2

17     18          10.1.3.0/24       0          Fa0/0      10.1.1.2

18     Pop tag     10.1.2.0/24       0          Fa0/0      10.1.1.2

R1#sh mpls ldp neighbor

    Peer LDP Ident: 10.1.2.1:0; Local LDP Ident 1.1.1.1:0

        TCP connection: 10.1.2.1.56975 - 1.1.1.1.646

        State: Oper; Msgs sent/rcvd: 39/39; Downstream

        Up time: 00:27:45

        LDP discovery sources:

          FastEthernet0/0, Src IP addr: 10.1.1.2

        Addresses bound to peer LDP Ident:

          10.1.1.2        10.1.2.1

To recap the label operations:

• Pop—The top label is removed. The packet is forwarded with the remaining label stack or as an unlabeled packet.
• Swap—The top label is removed and replaced with a new label.
• Push—The top label is replaced with a new label (swapped), and one or more labels are added (pushed) on top of the swapped label.
• Untagged/No Label—The stack is removed, and the packet is forwarded unlabeled.
• Aggregate—The label stack is removed, and an IP lookup is done on the IP packet.

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