Glossary

448 Glossary

A

ABR See area border router.

address-family A sub-configuration command allowing for MPLSspecific configuration in BGP and RIPv2.

area border router (ABR) An OSPF router that has interfaces configured for two or more areas.

ASBR See autonomous system boundary router.

ATM label switch router (ATM-LSR) An ATM switch that is capable of forwarding packets based on labels.

ATM-LSR See ATM label switch router.

ATM-LSR domain A series of ATM-LSRs connected together with

LC-ATM interfaces.

autonomous system boundary router (ASBR) An OSPF router that has at least one interface in the OSPF domain and one interface connecting to an external network.

B

backbone are The OSPF Area 0.

backbone router An OSPF router that has at least one interface in Area 0.

C

CEF See Cisco Express Forwarding.

Central Services MPLS VPN topology A Central Services MPLS VPN topology is a network where there is some central service, such as data storage facilities or media content, that is being accessed by different sites. These different sites have no access to each other, only to the central service site.

Glossary 449

Cisco Express Forwarding (CEF) CEF creates an optimized, “cached” version of the routing table. CEF is a requirement for MPLS and tag switching.

control plane A component of the MPLS architecture that is responsible for binding a label to network routes and distributing those bindings among other MPLS-enabled routers.

D

data plane A component of the MPLS architecture where information that is created and maintained from the control plane is actually used. Also known as the forwarding plane.

dedicated router In a peer-to-peer VPN, a dedicated router is a service provider router that is connected to a single customer.

down bit The down bit is used to prevent routing loops between customer routes and the service provider OSPF super-backbone. When a route is redistributed from MP-IBGP into OSPF, the down bit is set in the Options field of the OSPF LSA header. Another PE router, receiving an LSA with the down bit set, does not redistribute the route into MP-IBGP.

downstream A term used when discussing MPLS label binding. Downstream refers to an originating or neighboring router that advertises a route.

downstream-on-demand Downstream-on-demand occurs when an upstream LSR, using the Label Request message, requests a label from its downstream neighbor.

E

edge label switch router (edge-LSR) An edge router that is also an LSR.

edge-LSR See edge label switch router.

egress router An edge router where packets leave the network.

450 Glossary

export route target An export route target is applied by a PE router when VRF routes are redistributed into MP-BGP.

extranet A VPN category where sites from different companies are connected together.

F

FEC See forwarding equivalence class.

FIB See forwarding information base.

forwarding equivalence class (FEC) An FEC is a grouping of IP packets that are all treated the same way. For unicast-based routing, an IP prefix is the equivalent of an FEC.

forwarding information base (FIB) A FIB is essentially a cached version of the IP routing table that eliminates the need for a route-cache.

forwarding plane A component of the MPLS architecture where information that is created and maintained from the control plane is actually used. Also known as the data plane.

full-mesh topology A VPN topology where each site has a connection to every other site.

H

hub-and-spoke topology A VPN topology where spoke sites are all connected to a central hub site but not to each other.

I

import route target Used by a PE router to select, in conjunction with redistribution, which routes from MP-BGP to redistribute into a particular VRF.

independent control Independent control means to immediately bind a label to an FEC.

Glossary 451

ingress router An edge router where packets enter the network.

internal router An OSPF router that has all configured interfaces in the same OSPF area.

intranet A class of VPNs where all connected sites are associated with the same company.

L

Label Distribution Protocol (LDP) The Label Distribution Protocol (LDP) is the IETF version of Cisco’s TDP. LDP is used to bind labels to network routes.

label forwarding information base (LFIB) The LIB is built in the control plane, and only those labels in use reside in the LFIB. The LFIB is a subset of the LIB.

label imposition The point in the transit of a packet through a service provider network where the label is applied by a router and used by subsequent devices to label-switch the packet.

label information base (LIB) A mapping of incoming labels to outbound labels, along with outbound interface and link information.

label stacking An MPLS feature where more than one label can be carried. Label stacking is useful for applications such as traffic engineering and VPNs.

label switch controller (LSC) An external controller that, when added to an ATM switch, enables the switch to exchange routes and labels with its neighbors.

label-switched path (LSP) A unidirectional set of LSRs that the labeled packet must flow through to get to a particular destination.

label switch router (LSR) A router that is capable of forwarding packets based on MPLS labels.

LC-ATM A label-switching-controlled ATM interface where the VPI/VCI is assigned through MPLS or tag switching (LDP or TDP).

452 Glossary

LDP See Label Distribution Protocol.

leased lines A dedicated private link through a service provider network. See also point-to-point connection.

LFIB See label forwarding information base.

LIB See label information base.

link state advertisement (LSA) Advertisements used by OSPF to exchange routing information with other OSPF-enabled routers.

LSA See link state advertisement.

LSC See label switch controller.

LSP See label-switched path.

LSR See label switch router.

M

MP-BGP See Multi-Protocol BGP.

MP-BGP backbone When Multi-Protocol BGP is deployed through a service provider network, the service provider backbone is referred to as the MP-BGP backbone.

MPLS See Multi-Protocol Label Switching.

MPLS label stack Another way of referring to the MPLS label or to a stack of MPLS labels.

MPLS VPN A VPN made possible with MPLS.

Multi-Protocol BGP (MP-BGP) An expanded BGP that has extensions used to carry MPLS-specific attributes through a network.

Multi-Protocol Label Switching (MPLS) A technology where labels are used to switch packets instead of route them. MPLS, as a technology, evolved from early attempts to glue the IP world and ATM world together. What we know as MPLS today is, for the most part, a standardized version of Cisco’s proprietary tag switching.

Glossary 453

N

Network Layer Reachability Information (NLRI) A way of referring to an IP prefix in BGP.

NLRI See Network Layer Reachability Information.

O

optimal routing Optimal routing is the process of a router selecting the best path for sending traffic.

ordered control Ordered control occurs when an upstream LSR must wait on a label to be received from its downstream LSR. Ordered control takes longer to set up a label-switched path (LSP) and is used by MPLSenabled ATM label switch routers (ATM-LSRs).

OSPF domain A set of OSPF routers belonging to the same autonomous system.

OSPF super-backbone When running OSPF in an MPLS VPN, the service provider MP-BGP backbone is the OSPF super-backbone. The OSPF superbackbone eliminates the OSPF Area 0 requirement.

overlay A type of connection that uses an underlying technology to facilitate connections.

overlay MPLS VPN topology An MPLS VPN topology where a site participates in more than one MPLS VPN.

P

partial-mesh topology A VPN topology where some sites are connected to all other sites and some sites do not connect to every other site.

peer-to-peer VPNs A type of VPN where customer routers connect to service provider routers instead of having virtual circuits between sites.

penultimate hop popping A process by which the next-to-last router in an LSP removes a label and forwards it as unlabeled IP.

454 Glossary

point-to-point connections A dedicated private link through a service provider network. See also leased lines.

popping The process of removing the MPLS label.

pushing The process of applying the MPLS label.

R

RD See route distinguisher.

redistribution The process of importing routes from another routing protocol or process.

redundant hub-and-spoke topology A VPN topology where spoke sites have more than one connection to a central site. The redundant hub-and- spoke topology usually uses more than one service provider for spoke site connectivity.

route distinguisher (RD) A 64-bit value that is used to keep possibly overlapping address from actually doing so in MP-IBGP.

routing bit A Cisco IOS mechanism used to ensure proper path selection. When a route is received by a PE with the down bit set, the routing bit is cleared. With the routing bit cleared, the route never shows up in the routing table of the PE, even if it is the best route as determined by OSPF.

routing context A VRF-specific set of configurations for a routing protocol.

S

shared router In a peer-to-peer VPN, a shared router is a service provider router that is connected to by different customers.

shim header Another way of referring to the MPLS label.

simple MPLS VPN topology An MPLS VPN where all sites are part of the same single MPLS VPN.

Glossary 455

static routes Manually configured non-dynamic routes. In MPLS, static routes must always have the outbound interface specified. The next hop IP address is optional.

T

Tag Distribution Protocol (TDP) Cisco’s proprietary protocol that is used to bind tags (which are the same as MPLS labels) to network routes in the routing table.

tag field A field, not used by the OSPF protocol, that the service provider AS number is mapped into. The mapping of the AS number into the tag field is used to prevent routing loops.

tag switching router (TSR) A router capable of switching packets based on tags instead of routing them.

TDP See Tag Distribution Protocol.

TLV See Type-Length-Value.

traffic engineering A process by which traffic is optimized to follow certain paths based on specified requirements.

TSR See tag switching router.

Type-Length-Value (TLV) The Type-Length-Value (TLV) is the LDP hop-count object. When an ATM-LSR receives a Label Request message with the TLV, it increments the hop-count value by 1.

U

unsolicited downstream A condition where a downstream router advertises a new binding without waiting for the neighbors to send a request.

upstream A term used when discussing MPLS label binding. Upstream refers to a route that was learned from the originating or neighboring router.

456 Glossary

V

VCI See virtual circuit identifier.

VC merge See virtual circuit merge.

virtual circuit identifier (VCI) The address contained in the ATM cell header that is used to designate the virtual channel within the virtual path on the physical ATM link.

virtual circuit merge (VC merge) Used to solve cell-interleaving problems and allows the ATM-LSR to preserve label space. VC merge is enabled by default on an ATM-LSR.

virtual path identifier (VPI) An identifier in the ATM cell header that is used to designate the virtual path on the ATM physical link.

virtual private network (VPN) A virtual private network is a network that overlays public network infrastructure and that provides its own routing, security, and quality of service configuration.

virtual router A condition where a single router appears to be many routers to customers. Customer routing tables are kept separate even though they all connect to the same router.

virtual routing and forwarding (VRF) table A dedicated routing table, with routing table mechanisms, for a particular customer on a PE router.

VPI See virtual path identifier.

VPN See virtual private network.

VPNv4 routes A term for MP-BGP routes where the route distinguisher is prepended to the NLRI.

VRF See virtual routing and forwarding tables.