This is called a label stack. Each entry in the label stack contains four fields:.
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In the case of Ethernet frames this is done through the use of EtherType values 0x and 0x, for unicast and multicast connections respectively. This is a type of router located in the middle of an MPLS network. It is responsible for switching the labels used to route packets. When an LSR receives a packet, it uses the label included in the packet header as an index to determine the next hop on the label-switched path LSP and a corresponding label for the packet from a lookup table.
The old label is then removed from the header and replaced with the new label before the packet is routed forward. Alternatively, under penultimate hop popping this function may instead be performed by the LSR directly connected to the LER.
Devices that function only as transit routers are similarly called P Provider routers. Label-switched paths LSPs are established by the network operator for a variety of purposes, such as to create network-based IP virtual private networks or to route traffic along specified paths through the network. When an unlabeled packet enters the ingress router and needs to be passed on to an MPLS tunnel, the router first determines the forwarding equivalence class FEC for the packet and then inserts one or more labels in the packet's newly created MPLS header.
The packet is then passed on to the next hop router for this tunnel.
rick gallahers mpls training guide building multi protocol label switching networks Manual
When a labeled packet is received by an MPLS router, the topmost label is examined. Based on the contents of the label a swap , push impose or pop dispose operation is performed on the packet's label stack. Routers can have prebuilt lookup tables that tell them which kind of operation to do based on the topmost label of the incoming packet so they can process the packet very quickly.
Indeed, transit routers typically need only to examine the topmost label on the stack. The forwarding of the packet is done based on the contents of the labels, which allows "protocol-independent packet forwarding" that does not need to look at a protocol-dependent routing table and avoids the expensive IP longest prefix match at each hop. At the egress router, when the last label has been popped, only the payload remains.
This can be an IP packet, or any of a number of other kinds of payload packet. The egress router must therefore have routing information for the packet's payload, since it must forward it without the help of label lookup tables. An MPLS transit router has no such requirement. In some special cases, the last label can also be popped off at the penultimate hop the hop before the egress router.
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This is called penultimate hop popping PHP. This may be interesting in cases where the egress router has lots of packets leaving MPLS tunnels, and thus spends inordinate amounts of CPU time on this. By using PHP, transit routers connected directly to this egress router effectively offload it, by popping the last label themselves. The path is set up based on criteria in the FEC. The path begins at a label edge router LER , which makes a decision on which label to prefix to a packet, based on the appropriate FEC. It then forwards the packet along to the next router in the path, which swaps the packet's outer label for another label, and forwards it to the next router.
Multiprotocol Label Switching
The last router in the path removes the label from the packet and forwards the packet based on the header of its next layer, for example IPv4. The router which first prefixes the MPLS header to a packet is called an ingress router. The last router in an LSP, which pops the label from the packet, is called an egress router. Routers in between, which need only swap labels, are called transit routers or label switch routers LSRs. Since bidirectional communication is typically desired, the aforementioned dynamic signaling protocols can set up an LSP in the other direction to compensate for this.
When protection is considered, LSPs could be categorized as primary working , secondary backup and tertiary LSP of last resort. These are mainly used for multicasting purposes. If one wants to carry two different types of traffic between the same two routers, with different treatment by the core routers for each type, one has to establish a separate MPLS path for each type of traffic. Multicast was for the most part an after-thought in MPLS design. The paths an LSR knows can be defined using explicit hop-by-hop configuration, or are dynamically routed by the constrained shortest path first CSPF algorithm, or are configured as a loose route that avoids a particular IP address or that is partly explicit and partly dynamic.
In a pure IP network, the shortest path to a destination is chosen even when the path becomes congested. In addition to the constraint of RSVP bandwidth, users can also define their own constraints by specifying link attributes and special requirements for tunnels to route or not to route over links with certain attributes. For end-users the use of MPLS is not visible directly, but can be assumed when doing a traceroute : only nodes that do full ip routing are shown as hops in the path, thus not the MPLS nodes used in between, therefore when you see that a packet hops between two very distant nodes and hardly any other 'hop' is seen in that providers network or AS it is very likely that network uses MPLS.
In the event of a network element failure when recovery mechanisms are employed at the IP layer, restoration may take several seconds which may be unacceptable for real-time applications such as VoIP. Frame Relay aimed to make more efficient use of existing physical resources, which allow for the underprovisioning of data services by telecommunications companies telcos to their customers, as clients were unlikely to be utilizing a data service percent of the time.
In more recent years, Frame Relay has acquired a bad reputation in some markets because of excessive bandwidth overbooking by these telcos. Telcos often sell Frame Relay to businesses looking for a cheaper alternative to dedicated lines ; its use in different geographic areas depended greatly on governmental and telecommunication companies' policies. Many customers are likely to migrate from Frame Relay to MPLS over IP or Ethernet within the next two years, which in many cases will reduce costs and improve manageability and performance of their wide area networks.
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While the underlying protocols and technologies are different, both MPLS and ATM provide a connection-oriented service for transporting data across computer networks. In both technologies, connections are signaled between endpoints, connection state is maintained at each node in the path, and encapsulation techniques are used to carry data across the connection. The most significant difference is in the transport and encapsulation methods. Packets must be segmented, transported and re-assembled over an ATM network using an adaptation layer, which adds significant complexity and overhead to the data stream.
MPLS, on the other hand, simply adds a label to the head of each packet and transmits it on the network. Differences exist, as well, in the nature of the connections. Establishing two-way communications between endpoints requires a pair of LSPs to be established. Because 2 LSPs are required for connectivity, data flowing in the forward direction may use a different path from data flowing in the reverse direction. MPLS can stack multiple labels to form tunnels within tunnels. Modern routers are able to support both MPLS and IP natively across a common interface allowing network operators great flexibility in network design and operation.
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