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MPLS Overview

Multiprotocol Label Switching (MPLS) is a method for engineering traffic patterns by assigning short labels to network packets that describe how to forward them through the network. MPLS is independent of routing tables or any routing protocol and can be used for unicast packets.

The MPLS framework supports traffic engineering and the creation of virtual private networks (VPNs). Traffic is engineered (controlled) primarily by the use of signaling protocols to establish label-switched paths (LSPs). VPN support includes Layer 2 and Layer 3 VPNs and Layer 2 circuits.

When you enable your device to allow MPLS traffic, the device performs packet-based processing and functions as a standard Junos router.

Caution: When packet forwarding mode is changed to MPLS, all flow-based security features are deactivated, and the device performs packet-based processing only. Flow-based services such as security policies, zones, NAT, ALGs, chassis clustering, screens, firewall authentication, and IPsec VPNs are unavailable on the device. However, MPLS can be enabled in flow-based packet forwarding mode for selected traffic using firewall filters. For more information, please see the following application note: https://www.juniper.net/us/en/local/pdf/app-notes/3500192-en.pdf

This overview contains the following topics:

Label Switching

In a traditional IP network, packets are transmitted with an IP header that includes a source and destination address. When a router receives such a packet, it examines its forwarding tables for the next-hop address associated with the packet's destination address and forwards the packet to the next-hop location.

In an MPLS network, each packet is encapsulated with an MPLS header. When a router receives the packet, it copies the header as an index into a separate MPLS forwarding table. The MPLS forwarding table consists of pairs of inbound interfaces and path information. Each pair includes forwarding information that the router uses to forward the traffic and modify, when necessary, the MPLS header.

Because the MPLS forwarding table has far fewer entries than the more general forwarding table, the lookup consumes less processing time and processing power. The resultant savings in time and processing are a significant benefit for traffic that uses the network to transit between outside destinations only.

Label-Switched Paths

Label-switched paths (LSPs) are unidirectional routes through a network or autonomous system (AS). In normal IP routing, the packet has no predetermined path. Instead, each router forwards a packet to the next-hop address stored in its forwarding table, based only on the packet's destination address. Each subsequent router then forwards the packet using its own forwarding table.

In contrast, MPLS routers within an AS determine paths through a network through the exchange of MPLS traffic engineering information. Using these paths, the routers direct traffic through the network along an established route. Rather than selecting the next hop along the path as in IP routing, each router is responsible for forwarding the packet to a predetermined next-hop address.

Figure 1 shows a typical LSP topology.

Figure 1: Typical LSP Topology

In the topology shown in Figure 1, traffic is forwarded from Host C1 to the transit network with standard IP forwarding. When the traffic enters the transit network, it is switched across a preestablished LSP through the network. In this example, an LSP might switch the traffic from Router R4 to Router R2 to Router R1. When the traffic exits the network, it is forwarded to its destination by IP routing protocols.

Label-Switching Routers

Routers that are part of the LSP are label-switching routers (LSRs). Each LSR must be configured with MPLS so that it can interpret MPLS headers and perform the MPLS operations required to pass traffic through the network. An LSP can include four types of LSRs:

• Inbound router—The only entry point for traffic into MPLS. Native IPv4 packets are encapsulated into the MPLS protocol by the inbound router. Each LSP can have only one inbound router. Inbound routers are also known as ingress routers.
• Transit router—Any router in the middle of an LSP. An individual LSP can contain between 0 and 253 transit routers. Transit routers forward MPLS traffic along the LSP, using only the MPLS header to determine how the packet is routed.
• Penultimate router—The second-to-last router in the LSP. The penultimate router in an LSP is responsible for stripping the MPLS header from the packet before forwarding it to the outbound router.
• Outbound router—The endpoint for the LSP. The outbound router receives MPLS packets from the penultimate router and performs an IP route lookup. The router then forwards the packet to the next hop of the route. Each LSP can have only one outbound router. Outbound routers are also known as egress routers.

Labels

To forward traffic through an MPLS network, MPLS routers encapsulate packets and assign and manage headers known as labels. A label is a 20–bit unsigned integer in the range 0 through 1,048,575. The routers use the labels to index the MPLS forwarding tables that determine how packets are routed through the network.

When a network's inbound router receives traffic, it inserts an MPLS label between the IP packet and the appropriate Layer 2 header for the physical link. The label contains an index value that identifies a next-hop address for the particular LSP. When the next-hop transit router receives the packet, it uses the index in the MPLS label to determine the next-hop address for the packet and forwards the packet to the next router in the LSP.

As each packet travels through the transit network, every router along the way performs a lookup on the MPLS label and forwards the packet accordingly. When the outbound router receives a packet, it examines the header to determine that it is the final router in the LSP. The outbound router then removes the MPLS header, performs a regular IP route lookup, and forwards the packet with its IP header to the next-hop address.

Label Operations

Each LSR along an LSP is responsible for examining the MPLS label, determining the LSP next hop, and performing the required label operations. LSRs can perform five label operations:

• Push—Adds a new label to the top of the packet. For IPv4 packets arriving at the inbound router, the new label is the first label in the label stack. For MPLS packets with an existing label, this operation adds a label to the stack and sets the stacking bit to 0, indicating that more MPLS labels follow the first.

  When it receives the packet, the inbound router performs an IP route lookup on the packet. Because the route lookup yields an LSP next hop, the inbound router performs a label push on the packet, and then forwards the packet to the LSP next hop.

• Swap—Replaces the label at the top of the label stack with a new label.

  When a transit router receives the packet, it performs an MPLS forwarding table lookup. The lookup yields the LSP next hop and the path index of the link between the transit router and the next router in the LSP.

• Pop—Removes the label from the top of the label stack. For IPv4 packets arriving at the penultimate router, the entire MPLS label is removed from the label stack. For MPLS packets with an existing label, this operation removes the top label from the label stack and modifies the stacking bit as necessary—sets it to 1, for example, if only a single label remains in the stack.

  If multiple LSPs terminate at the same outbound router, the router performs MPLS label operations for all outbound traffic on the LSPs. To share the operations among multiple routers, most LSPs use penultimate hop popping (PHP).

• Multiple push—Adds multiple labels to the top of the label stack. This action is equivalent to performing multiple push operations.

  The multiple push operation is used with label stacking, which is beyond the scope of this topic.

• Swap and push—Replaces the top label with a new label and then pushes a new label to the top of the stack.

  The swap and push operation is used with label stacking, which is beyond the scope of this topic.

Penultimate Hop Popping

Multiple LSPs terminating at a single outbound router load the router with MPLS label operations for all their outbound traffic. Penultimate hop popping (PHP) transfers the operation from the outbound router to penultimate routers.

With PHP, the penultimate router is responsible for popping the MPLS label and forwarding the traffic to the outbound router. The outbound router then performs an IP route lookup and forwards the traffic. For example, if four LSPs terminate at the same outbound router and each has a different penultimate router, label operations are shared across four routers.

LSP Establishment

An MPLS LSP is established by one of two methods: static LSPs and dynamic LSPs.

Static LSPs

Like a static route, a static LSP requires each router along the path to be configured explicitly. You must manually configure the path and its associated label values. Static LSPs require less processing by the LSRs because no signaling protocol is used. However, because paths are statically configured, they cannot adapt to network conditions. Topology changes and network outages can create black holes in the LSP that exist until you manually reconfigure the LSP.

Dynamic LSPs

Dynamic LSPs use signaling protocols to establish themselves and propagate LSP information to other LSRs in the network. You configure the inbound router with LSP information that is transmitted throughout the network when you enable the signaling protocols across the LSRs. Because the LSRs must exchange and process signaling packets and instructions, dynamic LSPs consume more resources than static LSPs. However, dynamic LSPs can avoid the network black holes of static LSPs by detecting topology changes and outages and propagating them throughout the network.