Cabletron Systems HSIM-W6 Guía de instalación descargar pdf (Pagina 19)

HSIM-W84 Firmware Support

HSIM-W84 Installation Guide 2-7

Switched Virtual Circuits, or SVCs, are available on a call-by-call basis using the SVC signaling

protocol (Q.933). The network must quickly establish the connection, and allocate bandwidth

based on the user’s request.

In a Frame Relay frame, user data packets are not changed in any way. A two byte header is

appended to the frame. Contained in this header is a 10-bit number called the Data Link

Connection Identiﬁer (DLCI). The DLCI is the “virtual circuit” number which corresponds to a

particular destination. The DLCI allows data coming into a Frame Relay switch to be sent across

the network using a three-step process: Check the integrity of the frame and discard it if it is in

error, look up the DLCI in a table and if not intended for this link, discard the frame. If the frame

passes the previous tests, relay the frame toward its destination out the speciﬁc port speciﬁed in the

table.

The ANSI standard deﬁnes a mechanism for the network to signal the existence of congestion,

called Explicit Congestion Notiﬁcation (ECN) bits. Frame Relay uses FECN (Forward ECN) and

BECN (Backward ECN) bits to notify end user devices about network congestion. Although the

Frame Relay protocol does not respond to congestion, some higher layer protocols for end user

devices may respond to ECNs by recognizing that delays have increased, or that frames have been

dropped.

The IP-OSPF Routing Protocol

Open Shortest Path First (OSPF) is a link state routing protocol developed for Internet Protocol

(IP) networks. OSPF distributes routing information between routers belonging to a single

autonomous system. In an autonomous system, routers exchange routing information through a

common routing protocol.

OSPF was designed primarily for the Internet environment and supports variable length subnet

masks, Type of Service (TOS) based routing, packet authentication, and the tagging of externally

derived routing information.

OSPF, based upon link-state technology, was developed by the Internet Engineering Task Force

(IETF). The IETF developed OSPF based upon the shortest path ﬁrst algorithm to serve large,

heterogeneous networks.

A key feature of OSPF is the speed in which it responds to topological changes, commonly

referred to as “convergence time.” OSPF generates a minimal amount of routing protocol trafﬁc

compared to most distance vector protocols.

Cabletron’s implementation of OSPF is based on RFC 1247 — OSPF Version 2 and RFC 1253 —

OSPF Version 2 Management Information Base.

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Cabletron Systems HSIM-W6 Guía de instalación Pagina 19