Before jumping into MPLS concepts, it is a good idea to familiarize yourself with the terminology and lingo used in MPLS.
Table 2-1. MPLS Terminology
| Upstream
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A router that is closer to the source of a packet, relative to another router.
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| Downstream
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A router that is farther from the source of a packet, relative to another router. As a packet traverses a network, it is switched from an upstream router to its downstream neighbor.
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| Control plane
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Where control information such as routing and label information is exchanged.
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| Data plane/forwarding plane
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Where actual forwarding is performed. This can be done only after the control plane is established.
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| Cisco Express Forwarding (CEF)
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The latest switching method used in Cisco IOS. It utilizes an mtrie-based organization and retrieval structure. CEF is the default forwarding method in all versions of Cisco IOS Software Release 12.0 and later.
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| Label
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A fixed-length tag that MPLS forwarding is based on. The term label can be used in two contexts. One term refers to 20-bit labels. The other term refers to the label header, which is 32 bits in length. For more details on labels, see the later section "What Is a Label?".
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| Label binding
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An association of an FEC (prefix) to a label. A label distributed by itself has no context and, therefore, is not very useful. The receiver knows to apply a certain label to an incoming data packet because of this association to an FEC.
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| Label imposition
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The process of adding a label to a data packet in an MPLS network. This is also referred to as "pushing" a label onto a packet.
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| Label disposition
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The process of removing a label from a data packet. This is also referred to as "popping" a label off a packet.
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| Label swapping
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Changing the value of the label in the MPLS header during MPLS forwarding.
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| Label Switch Router (LSR)
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Any device that switches packets based on the MPLS label.
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| Label Edge Router (LER)
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An LSR that accepts unlabeled packets (IP packets) and imposes labels on them at the ingress side. An LER also removes labels at the edge of the network and sends unlabeled packets to the IP network on the egress side.
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| Forwarding Equivalence Class (FEC)
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Any set of properties that map incoming packets to the same outgoing label. Generally, an FEC is equivalent to a route (all packets destined for anything inside 10.0.0.0/8 match the same FEC), but the definition of FEC can change when packets are routed using criteria other than just the destination IP address (for example, DSCP bits in the packet header).
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| Label-Switched Path (LSP)
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The path that a labeled packet traverses through a network, from label imposition to disposition.
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| Label stack
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Apart from the label exchanged between LSRs and their neighbors, for applications such as MPLS-VPN, an end-to-end label is exchanged. As a result, a label stack is used instead of a single MPLS label. An important concept to keep in mind is that the forwarding in the core is based just on the top-level label. In the context of MPLS TE, label stacking is required when a labeled packet enters an MPLS TE tunnel.
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| Forwarding Information Base (FIB)
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The table that is created by enabling CEF on the Cisco routers.
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| Label Information Base (LIB)
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The table where the various label bindings that an LSR receives over the LDP protocol are stored. It forms the basis of populating the FIB and LFIB tables.
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| Tag Information Base (TIB)
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The older, "tag-switching" name for the LIB.
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| Explicit null
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The opposite of implicit null. In the control plane, the last hop sends a label value of 0 (for IPv4) to the penultimate hop. The label value is never used for lookup. Explicit null provides some advantages that implicit null doesn't. It is used in network devices that don't support implicit null, or to carry EXP bits all the way to the tunnel tail.
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| Implicit null
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The concept of not using a label on the last hop of an LSP in the forwarding plane. Implicit null has some performance advantages. In the control plane, the last hop of the LSP advertises a label value of 3 to indicate implicit null.
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| Penultimate Hop Popping (PHP)
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After receiving the egress router, a labeled packet pops off the label and does an IP lookup in the CEF table. This means that the egress router must do two lookups for every packet exiting the network. To reduce this burden placed on the egress router, PHP allows the penultimate hop router to remove the top-level label, which allows the LER to forward the packet based on a single lookup. The router that is immediately upstream of the tail of an MPLS TE tunnel also performs PHP.
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| P/PE and C/CE
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P and PE routers are LSRs and LERs in the context of MPLS-VPN. The term P comes from routers being in the provider network. C routers are routers found in the customer network. CE routers are the routers on the customer edge facing the provider. PE routers are provider edge routers, which connect to the CE routers. CE routers normally run plain IP (not required to be MPLS-aware).
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| Label Distribution Protocol (LDP)
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One of the many protocols in place to distribute the label bindings between an LSR and its neighbor. Other mechanisms include RSVP, used in MPLS TE, and MP-BGP, used in MPLS-VPN.
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| Tag Distribution Protocol (TDP)
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The predecessor of LDP, TDP is a Cisco-proprietary protocol that acts much like LDP. You can use TDP if interoperability between Cisco and non-Cisco devices is not important.
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| Resource Reservation Protocol (RSVP)
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This protocol was originally intended as a signaling protocol for the Integrated Services (IntServ) quality of service (QoS) model, wherein a host requests a specific QoS from the network for a particular flow. This reservation could be within an enterprise network or over the Internet. RSVP with a few extensions has been adapted by MPLS to be the signalling protocol that supports MPLS TE within the core. RSVP theory is standardized in RFC 2205 and RFC 3209. It is covered in greater detail in Chapter 4, "Path Calculation and Setup."
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| Constrained Routing LDP (CR-LDP)
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This is an alternative approach to RSVP that acts as a signalling protocol to achieve MPLS TE. Cisco routers support RSVP rather than CR-LDP for traffic engineering LSP setup. CR-LDP is not covered in this book.
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