Cisco Cisco IOS Software Release 12.0(13)S7

BGP—Border Gateway Protocol. An interdomain routing protocol that replaces External Gateway 
Protocol (EGP). BGP systems exchange reachability information with other BGP systems. BGP is 
defined by RFC 1163. 

EGP—Exterior Gateway Protocol. An internet protocol for exchanging routing information between 
autonomous systems. EGP is documented in RFC 904. EGP is not to be confused with the general term 
exterior gateway protocol. EGP is an obsolete protocol that was replaced by Border Gateway Protocol 
(BGP). 

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

CSC—Carrier Supporting Carrier. A situation where one service provider allows another service 
provider to use a segment of its backbone network. The service provider that provides the segment of the 
backbone network to the other provider is called the backbone carrier. The service provider that uses the 
segment of the backbone network is called the customer carrier.

LDP—Label Distribution Protocol. A standard protocol between Multiprotocol Label Switching 
(MPLS)-enabled routers that is uses in the negotiation of the labels used to forward packets. The Cisco 
proprietary version of this protocol is the Tag Distribution Protocol (TDP). 

LDP peer—A label switch router (LSR) that is the receiver of label space information from another 
LSR. If an LSR has a label space to advertise to another LSR, or to multiple LSRs, one Label 
Distribution Protocol (LDP) session exists for each LSR (LDP peer) receiving the label space 
information. 

MD5—Message Digest 5. A one-way hashing algorithm that produces a 128-bit hash. Both MD5 and 
Secure Hash Algorithm (SHA) are variations on MD4 and are designed to strengthen the security of the 
MD4 hashing algorithm. Cisco uses hashes for authentication within the IPSec framework. SNMP v.2 
uses MD5 for message authentication, to verify the integrity of the communication, to authenticate the 
message origin, and to check its timeliness. 

MPLS—Multiprotocol Label Switching. A switching method that forwards IP traffic through use of 
labels. Each label instructs the routers and the switches in the network where to forward a packet based 
on preestablished IP routing information. 

PE router—provider edge router. A router that is part of a service provider’s network connected to a 
customer edge (CE) router. All Multiprotocol Label Switching (MPLS) Virtual Private Network (VPN) 
processing occurs in the PE router. 

VPN—Virtual Private Network. Enables IP traffic to travel securely over a public TCP/IP network by 
encrypting all traffic forwarded from one network to another. A VPN uses tunneling to encrypt all 
information at the IP level. 

VRF—A VPN routing and forwarding instance. A VRF consists of an IP routing table, a derived 
forwarding table, a set of interfaces that use the forwarding table, and a set of rules and routing protocols 
that determine what goes into the forwarding table. In general, a VRF includes the routing information 
that defines a customer VPN site that is attached to a PE router. 

See the 

 for terms not included in this glossary.