3com S7906E 설치 설명서

 

1-6 

Structure of an LSR 

 

 

As shown in 

, an LSR consists of two planes: 

z

 

Control plane: Implements label distribution and routing, establishes the LFIB, and builds and tears 
LSPs. 

z

 

Forwarding plane: Forwards packets according to the LFIB. 

An LER forwards both labeled packets and IP packets on the forwarding plane and therefore uses both 
the LFIB and the FIB. An ordinary LSR only needs to forward labeled packets and therefore uses only 
the LFIB. 

When establishing an LSP hop by hop, LDP uses the information in the routing tables of the LSRs along 
the path to determine the next hop. The information in the routing tables is provided by routing protocols 
such as IGPs and BGP. LDP only uses the routing information indirectly; it has no direct relationship 
with routing protocols. 

On the other hand, existing protocols such as BGP and RSVP can be extended to support label 
distribution. 

In MPLS applications, it may be necessary to extend some routing protocols. For example, 
MPLS-based VPN applications requires that BGP be extended to propagate VPN routing information, 
and MPLS-based Traffic Engineering (TE) requires that OSPF or IS-IS be extended to carry link state 
information. 

By integrating both Layer 2 fast switching and Layer 3 routing technologies, MPLS features improved 
route lookup speed. However, with the development of the application specific integrated circuit (ASIC) 
technology, route lookup speed is no longer the bottleneck hindering network development. This makes 
MPLS not so outstanding in improving forwarding speed.