Cisco Cisco IOS Software Release 12.0(22)S

AS—autonomous system. A collection of networks under a common administration sharing a common 
routing strategy. 

ASBR—autonomous system boundary router. An edge router located between an Open Shortest Path 
First (OSPF) autonomous system and a non-OSPF network. ASBRs run both OSPF and another routing 
protocol, such as RIP. 

BGP—Border Gateway Protocol. A routing protocol that exchanges network reachability information 
with other BGP systems, which may be within the same autonomous system or between multiple 
autonomous systems.

CE router—customer edge router. A router that is part of a customer network and that interfaces to a 
provider edge (PE) router. 

EBGP—External Border Gateway Protocol. A Border Gateway Protocol between routers located in 
different autonomous systems. 

IBGP—Internal Border Gateway Protocol. A Border Gateway Protocol between routers within the same 
autonomous system. 

LDP—label distribution protocol. A standard protocol used by MPLS-enabled routers to assign the 
labels (addresses) used to forward packets.

LSP—label-switched path. 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.

LSR—label switching router. An LSR forwards packets in an MPLS network by looking only at the 
fixed-length label.

MPLS—Multiprotocol Label Switching. 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. 

Multihop BGP—A Border Gateway Protocol between two routers in different autonomous systems that 
are more than one hop away from each other.

NLRI—Network Layer Reachability Information. BGP routers exchange network layer reachability 
information, which includes the full route (BGP AS numbers) to reach the destination network. 

PE router—provider edge router. A router that is part of a service provider's network connected to a 
customer edge (CE) router. All MPLS VPN processing occurs in the PE router.

POP—point of presence. 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. 

RD—route distinguisher. An 8-byte value that is concatenated with an IPv4 prefix to create a unique 
VPN-IPv4 prefix.

RIP—Routing Information Protocol. An internal gateway protocol used to exchange routing 
information within an autonomous system. RIP uses hop count as a routing metric.

VPN—Virtual Private Network. VPNs connect branch offices and remote users through a shared or 
public network, such as the Internet, and provide the same security and availability as a private network. 
Because VPNs use an existing shared WAN infrastructure, costs are lower and deployment is faster than