Cisco Cisco IOS Software Release 12.0(24)S

AS—autonomous system. A collection of networks that share the same routing protocol and that are 
under the same system administration.

ASBR—autonomous system border router. A router that connects and exchanges information between 
two or more autonomous systems.

BGP—Border Gateway Protocol. 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. 

CE router—customer edge router. The customer router that connects to the provider edge (PE) router.

EBGP—External Border Gateway Protocol. A BGP session between routers in different autonomous 
systems (ASs). When a pair of routers in different ASs are more than one IP hop away from each other, 
an EBGP session between those two routers is called multihop EBGP. 

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

IGP—Interior Gateway Protocol. Internet protocol used to exchange routing information within an 
autonomous system. Examples of common Internet IGPs include IGRP, OSPF, and RIP. 

LDP—Label Distribution Protocol. A standard protocol between MPLS-enabled routers to negotiate the 
labels (addresses) used to forward packets. This protocol is not supported in Cisco IOS Release 12.0. 
The Cisco proprietary version of this protocol is the Tag Distribution Protocol (TDP).

LER—label edge router. The edge router that performs label imposition and disposition. 

LSR—label switch router. The role of an LSR is to forward packets in an MPLS network by looking 
only at the fixed-length label.

NLRI—Network Layer Reachability Information. 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.

P router—provider router. The core router in the service provider network that connects to provider 
edge (PE) routers. In a packet-switched star topology, a router that is part of the backbone and that serves 
as the single pipe through which all traffic from peripheral networks must pass on its way to other 
peripheral networks. 

PE router—provider edge router. The label edge router (LER) in the service provider network that 
connects to the customer edge (CE) router.

RR—route reflector. A router that advertises, or reflects, IBGP learned routes to other IBGP peers 
without requiring a full network mesh.

VPN—Virtual Private Network. A group of sites that, as a result of a set of administrative policies, can 
communicate with each other over a shared backbone. 

VPNv4 addresses—When multiple VPNs use the same address space, the VPN addresses are made 
unique by adding a route distinguisher to the front of the address. 

Refer to the

 for terms not included in this glossary.