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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- 3 LMI protocol options are available in <strong>Cisco</strong> IOS – cisco (default), q933a (ITU-T), and ansi.<br />
LMI is locally significant, hence the routers at both ends do not need to use the same LMI type.<br />
LMI must be configured to match the LMI configuration on the local Frame Relay switch.<br />
- Frame Relay uses virtual circuit (VC) – the logical connection created between 2 DTE devices.<br />
Thousands of devices are connected to a Frame Relay network, and VC is used to represent a<br />
link between 2 routers. A VC can be either a PVC or SVC.<br />
RT1<br />
DLCI 100<br />
DLCI 200<br />
Virtual Circuit<br />
Frame Relay<br />
Figure 23-2: Frame Relay Virtual Circuits and DLCIs<br />
- Permanent Virtual Circuits (PVCs) are permanent established connections that allow<br />
communication without call setup. PVCs are being setup and preconfigured by service provider<br />
(the leased line concept). PVCs are the most common type in use today.<br />
- Switched Virtual Circuits (SVCs) are temporary established connections that are dynamically<br />
established when needed. A VC is established before data transmission and terminated after data<br />
transmission (the dial-up or phone call concept). SVCs are very rarely being used today.<br />
- Data Link Connection Identifier (DLCI) is the Frame Relay address in a Frame Relay header<br />
(10-bit number) that used to identify a VC between 2 DTEs. It allows a Frame Relay interface to<br />
distinguish between different VCs as many VCs can be terminated on a same interface.<br />
Ex: 3 branch offices are connected to the HQ. Without Frame Relay, the HQ router would need 3<br />
serial interfaces for point-to-point connections (leased lines) to all branches (a scalability issue).<br />
With Frame Relay, the HQ router just needs a single physical access link to a service provider’s<br />
Frame Relay network and use DLCI to identify all the PVCs to branch offices. Frame Relay is<br />
less expensive than point-to-point links (leased lines) when connecting multiple sites together.<br />
- The DLCI addresses in Frame Relay perform the same functions as MAC addresses on Ethernet.<br />
Frame Relay is a L2 protocol. Whenever a Frame Relay switch forwards an IP datagram<br />
encapsulated in a Frame Relay frame, it would read the destination DLCI address in the frame<br />
and forward to the corresponding VC accordingly.<br />
- In Figure 23-2, RT1 has 2 VCs but only one access link (multiple VCs can share the same<br />
access link). A DLCI of 100 identifies the VC that connects to RT2 and a DLCI of 300 identifies<br />
the VC that connects to RT3. Whenever RT1 would like to forward a packet to RT2, it<br />
encapsulates the Layer 3 packet with a Frame Relay header with a DLCI of 100 which represents<br />
the VC to RT2, and a trailer and send out the frame (it becomes a L2 frame). The Frame Relay<br />
switches will then correctly forward the frame to RT2. On RT2, a different DLCI number can be<br />
used to identify the same VC.<br />
158<br />
Virtual Circuit<br />
DLCI 300<br />
DLCI 400<br />
RT2<br />
RT3<br />
Copyright © 2008 Yap Chin Hoong<br />
yapchinhoong@hotmail.com
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