LIB is a software database created in both LER and LSR. It contains mapping information of the incoming label and LSP with outgoing label. It is created during installation of the router and, subsequently, updated automatically when the new LSR and LER are added by using label distribution protocol.
When LSR is routing the packets from the incoming LSP to the outgoing LSP, it strips out the incoming label and assigns a new label to the same packet to ensure security from intruders. This process is known as label swapping or label changing.
Label switched path (LSP). Within an MPLS domain, a path is set up prior to data transmission for a given packet to travel based on forward-equivalence class (FEC). There are two types of LSPs; one is static and the other is signalled. Static LSPs are configured manually on each LSR. No signalling protocol is used in static LSP. To establish a static LSP, the operator configures ingress LER, transit LSRs and egress LER, manually specifying labels to be applied at each hop.
Contrary to this, signalled LSPs are configured only at ingress LER. When a packet is assigned to a signalled LSP, it follows a pre-established path from LSP’s ingress LER to its egress LER.
How MPLS works
Suppose an IPv4 packet arrives at the ingress router (LER) from CE with a destination address of 192.168.1.1 (Fig. 2). In MPLS domain, ingress router has a route for 192.168.1.0/20 with a next-hop of LSP. A header with a label of, say, 5, selected from its LIB, is appended to the packet and forwarded downstream. LER binds the selected label (5) according to the FEC over the IP packet and sends it through the pre-programmed LSP (2) towards LSR1.
On receipt of the labelled IP packet, LSR1 analyses the label only and ignores the IP address. It consults with its LIB for further routing. As a result, it removes the incoming label (5), winds the newly-assigned label (3) over the IP packet and sends it towards LSR2 over the assigned LSP (7).
LSR2 consults with its LIB and transmits the IP packet after swapping the incoming label (3) with the outgoing label (10) towards egress LER over the pre-assigned LSP (4). Egress LER stripes the label (10), goes through the destined IP address (192.168.1.1) and hands it over to the correct CE.
Forward-equivalence class
FEC is a representation of a group of packets that share the same requirements for their transport. All packets in such a group are provided with the same treatment en route to the destination. FEC assignment is based on:
1. Class of service requirement
2. Quality of service requirement
3. Prefixes of IP addresses
FEC based on class of service requirement. IP packets from different users are categorised on the basis of class of services these are entitled to, and these are allotted with one FEC number. For example, one FEC represents all Voice over Internet Protocol (VoIP) packets received from different users, and in MPLS domain all VoIP packets are treated equally.
FEC based on quality of service requirement. Some online services like video conferencing require constant and high-speed data transmission. If delay exceeds, there could be a loss of intelligence. Such IP packets could not be made to wait in queue. Such services deserve separate FECs.
FEC based on prefixes of IP addresses. FEC is assigned on the basis of prefixes of the IP addresses of the destination.
