Cisco Cisco IOS Software Release 12.0(22)S
MPLS VPN—Carrier Supporting Carrier—IPv4 BGP Label Distribution
Glossary
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Cisco IOS Release 12.0(22)S
Glossary
autonomous system (AS)—A collection of networks that share the same routing protocol and that are
under the same system administration.
under the same system administration.
Border Gateway Protocol (BGP)—The exterior border gateway protocol used to exchange routing
information between routers in separate autonomous systems. BGP uses Transmission Control Protocol
(TCP). Because TCP is a reliable protocol, BGP does not experience problems with dropped or
fragmented data packets.
information between routers in separate autonomous systems. BGP uses Transmission Control Protocol
(TCP). Because TCP is a reliable protocol, BGP does not experience problems with dropped or
fragmented data packets.
BGP prefixes—A route announcement using the BGP. A prefix is composed of a path of AS numbers,
indicating which networks the packet must pass through, and the IP block that is being routed. A BGP
prefix would look something like: 701 1239 42 206.24.14.0/24. (The /24 part is referred to as a CIDR
mask. The /24 indicates that there are 24 ones in the netmask for this block starting from the left hand
side. A /24 corresponds to the natural mask 255.255.255.0.
indicating which networks the packet must pass through, and the IP block that is being routed. A BGP
prefix would look something like: 701 1239 42 206.24.14.0/24. (The /24 part is referred to as a CIDR
mask. The /24 indicates that there are 24 ones in the netmask for this block starting from the left hand
side. A /24 corresponds to the natural mask 255.255.255.0.
customer edge (CE) router—The customer router that connects to the provider edge (PE) router.
External Border Gateway Protocol (EBGP)—A BGP session between routers in different autonomous
systems. When a pair of routers in different ASs are more than one IP hop away from each other, an
external BGP session between those two routers is called multihop external BGP.
systems. When a pair of routers in different ASs are more than one IP hop away from each other, an
external BGP session between those two routers is called multihop external BGP.
Internal Border Gateway Protocol (IBGP)—A BGP session between routers within the same
autonomous system.
autonomous system.
Label Distribution Protocol (LDP)—A standard protocol used by MPLS-enabled routers to assign the
labels (addresses) used to forward packets.
labels (addresses) used to forward packets.
label edge router (LER)—The edge router that performs label imposition and disposition.
label-switched path (LSP)—A sequence of hops in which a packet travels from one router to another
router by means of label switching mechanisms. A label-switched path can be established dynamically,
based on normal routing mechanisms, or through configuration.
router by means of label switching mechanisms. A label-switched path can be established dynamically,
based on normal routing mechanisms, or through configuration.
label switching router (LSR)—An LSR forwards packets in an MPLS network by looking only at the
fixed-length label.
fixed-length label.
Multiprotocol Label Switching (MPLS)—MPLS is a method for forwarding packets (frames) through
a network. It enables routers at the edge of a network to apply labels to packets (frames). ATM switches
or existing routers in the network core can switch packets according to the labels.
a network. It enables routers at the edge of a network to apply labels to packets (frames). ATM switches
or existing routers in the network core can switch packets according to the labels.
Multihop BGP—A Border Gateway Protocol between two routers in different autonomous systems that
are more than one hop away from each other.
are more than one hop away from each other.
Network Layer Reachability Information (NLRI)—BGP sends routing update messages containing
NLRI, which describes the route. In this context, an NLRI is a prefix. A BGP update message carries
one or more NLRI prefixes and the attributes of a route for the NLRI prefixes. The route attributes
include a BGP next hop gateway address, community values, and other information.
NLRI, which describes the route. In this context, an NLRI is a prefix. A BGP update message carries
one or more NLRI prefixes and the attributes of a route for the NLRI prefixes. The route attributes
include a BGP next hop gateway address, community values, and other information.
point of presence (POP)—An access point to the Internet. A POP has a unique IP address. The ISP or
online service provider (such as AOL) has one or more POPs on the Internet. ISP users dial into the POP
to connect to the Internet. A POP can reside in rented space owned by the telecommunications carrier
(such as Sprint) to which the ISP is connected. A POP usually includes routers, digital/analog call
aggregators, servers, and frequently frame relay or ATM switches.
online service provider (such as AOL) has one or more POPs on the Internet. ISP users dial into the POP
to connect to the Internet. A POP can reside in rented space owned by the telecommunications carrier
(such as Sprint) to which the ISP is connected. A POP usually includes routers, digital/analog call
aggregators, servers, and frequently frame relay or ATM switches.
provider edge (PE) router—The label edge router (LER) in the service provider network that connects
to the customer edge (CE) router.
to the customer edge (CE) router.
route reflector (RR)—A router that advertises, or reflects, IBGP learned routes to other IBGP peers
without requiring a full network mesh.
without requiring a full network mesh.