CCNA Complete Guide 2nd Edition.pdf - Cisco Learning Home
CCNA Complete Guide 2nd Edition.pdf - Cisco Learning Home
CCNA Complete Guide 2nd Edition.pdf - Cisco Learning Home
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- Frame Relay DLCIs are locally significant, which means that addresses only need to be unique<br />
on a particular local access link – there can be only one 123, Jalan ABC, 58000, Kuala Lumpur,<br />
but there can be a 123, Jalan ABC in every city.<br />
RT1<br />
DLCI 100<br />
DLCI 200<br />
DLCI 100<br />
DLCI 100<br />
Figure 23-3: Frame Relay Local Addressing and Global Addressing<br />
- Local addressing DLCIs must be unique on an access link to represent different VCs, but the<br />
same DLCI number can be used on multiple access links in the Frame Relay network.<br />
- Global addressing is a way of choosing DLCI numbers when planning a Frame Relay network<br />
by making DLCI addressing look like LAN addressing. But a Frame Relay header only has a<br />
single DLCI field, not both Source and Destination DLCI fields as expected in data link headers.<br />
Figure 23-3B shows a frame sent by RT1 to RT2 with DLCI 200 in the header, the Frame Relay<br />
switch which RT2 connected to will change the field to DLCI 100 before it forwards to RT2.<br />
As a result, the sender treats the DLCI field as a destination address while the receiver<br />
treats the DLCI field as a source address.<br />
- Link Access Procedure Frame Bearer Services (LAPF) specification defines the Frame Relay<br />
frame header and trailer used to encapsulate L3 packets. Always remember that the header only<br />
has a single DLCI field, not both Source and Destination DLCI fields as in other L2 headers.<br />
Figure 23-4: LAPF Header and Trailer<br />
- The LAPF specification does not have a Protocol Type field which is important to identify the<br />
encapsulated Layer 3 protocols, eg: IP, IPX, AppleTalk. The 2 implementations are the default<br />
<strong>Cisco</strong>-proprietary encapsulation and RFC 1490 / 2427 IETF encapsulation (enabled with the<br />
encapsulation frame-relay ietf interface subcommand) when connecting to non-<strong>Cisco</strong> router.<br />
- Frame Relay switches don’t care and ignore the Frame Relay encapsulation type, but the DTEs at<br />
both ends must be configured to use the same encapsulation type. Additionally, the encapsulation<br />
type can be different for each VC.<br />
159<br />
Global<br />
DLCI 100<br />
DLCI 200<br />
FR Frame<br />
DLCI 100<br />
DLCI 100<br />
Global<br />
DLCI 200<br />
Global<br />
DLCI 300<br />
Figure 23-3A: Frame Relay Local Addressing Figure 23-3B: Frame Relay Global Addressing<br />
LAPF Header<br />
DLCI (10 bits)<br />
FECN, BECN, DE (1 bit each)<br />
RT2<br />
RT3<br />
RT1<br />
DLCI 300<br />
Encapsulate Layer 3<br />
Packet<br />
LAPF Trailer<br />
FCS<br />
RT2<br />
RT3<br />
Copyright © 2008 Yap Chin Hoong<br />
yapchinhoong@hotmail.com
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