Delta Electronics Sonet OC-48/SDH User Manual download pdf (Page 13)

Chapter 2

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Interfaces

Point-to-point interfaces When you’re using an encapsulation type of PPP, Cisco-HDLC,

ATM, or Frame Relay, the default MTU for the inet and iso protocols is 4470 bytes. The mpls

protocol family uses a value of 4458 bytes.

Broadcast interfaces Both a Gigabit Ethernet and a Fast Ethernet interface share the same

properties for protocol MTU sizes. The inet family uses 1500 bytes, the iso family uses 1497

bytes, and the mpls family uses 1488 bytes.

The difference between the protocol MTU and the interface MTU discussed in

the “Physical Properties” section earlier in this chapter is quite important. The

interface MTU is the largest size packet able to be sent on the physical media.

This value includes all Layer 2 overhead information, such as the destination

MAC address on Ethernet, or the labels in an MPLS environment. The Cyclic

Redundancy Check (CRC) information is not included in this value, however.

Each encapsulation type has a payload field where higher-layer information is

stored. This payload field is the size of the protocol MTU. This is the largest

amount of logical protocol data, including the protocol header, able to be sent

on a particular interface.

Virtual Circuit Addressing

An interface configured for use on an ATM, Frame Relay, or Ethernet VLAN network requires

the addition of a Layer 2 virtual circuit address. We examine these options next.

ATM VPI and VCI

An Asynchronous Transfer Mode (ATM) network uses the concept of a virtual path and a virtual

circuit to connect two devices. The path is represented by a virtual path identifier (VPI), which can

be thought of as a logical conduit between the devices. Each VPI in a network may contain mul-

tiple logical circuits represented by a virtual circuit identifier (VCI), which is the actual connection

between the devices.

Each logical unit in the router is assigned a VPI/VCI Layer 2 address. The path values range

from 0 to 255, while the circuits on that path can be between 0 and 4089. These values are

locally significant so that the two connected devices agree on their usage. The specific pair can

then be used elsewhere in the ATM network, allowing for greater overall scalability.

As an example, assume that the Merlot router is connected to Riesling through an ATM

interface. Ten logical units are created on the interface, each with a unique IPv4 address and

VCI value. When data packets are passed between the two routers, the VCI address at Layer 2

helps determine which logical unit should receive and process the packet.

Frame Relay DLCI

In a manner similar to ATM, a Frame Relay network uses data link connection identifiers (DLCIs)

to address packets at Layer 2. The DLCI value is the logical circuit between the two devices, which

is also locally significant. Each logical interface unit assigned a DLCI becomes a Frame Relay per-

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Delta Electronics Sonet OC-48/SDH User Manual Page 13