Frame-relay is layer 2 packet-switching WAN technology, it is connection-oriented: a layer 2 connection must be established before a connection can be sent to a remote device. The connections used bye frame-relay are provided by virtual circuits (VCs), many VC can exist on the same physical interface. The advantage of VC that they can provide full connectivity at a much lower price, VCs are also full-duplex
Advantages of VCs
Using frame relay you can have multiple logical circuits on a same physical connection, you can use the same formula to calculate no. of VCs requirement, one nice feature of frame relay is that in all of these situations, you need only one serial interface on each router to handle the VC connections, you can easily use a lower-end router to handle a lot of VC connections, frame relay with VC is a good solution if your router has a single serial interface, but needs to connect to multiple WAN destination
There are two types of VCs
– Permanent VC (PVC)
– Switched or Semi-permanent VC (SVC)
PVC is similar to a leased line, SVC is dynamically built, and gets terminate after data has been sent, SVC is used when data is sent periodically, CCNA syllabus is focusing on configuration of PVC.
One advantage of PVC has over SVC is that SVCs must be set up when you have data to send, a fact that introduces a small amount of delay before traffic can be sent to the destination.
One advantage of SVCs is that they are temporary. Therefore, since you are using SVC only part of the time , the cost of SVC is less than that of PVC
7 most important Networking Devices
Common frame relay terms:
1) LMI 🙁 local management interface) this define how DTE (means your router) interact with DCE (router connected to frame relay switch)
LMI is a keep-alive mechanism used between DTE and DCE to ensure that both are operational and VC’s are not deleted or disable
2) DLCI: – data-link connection identifier
This value is used to uniquely identify each VC on a physical interface, using DLCI you can multiplex traffic for multiple destinations on a single physical interface. DLCI is locally significant it can change on the segment to segment basis
3) Access rate: – This is the speed of the physical connection between your router and frame relay switch
4) CIR: – Committed information rate
This is the average data rate measured over a fixed period of time, that the carrier guarantees you for particular VC
5) Bc: – Committed burst rate
It implies a smaller time period but a higher average than the CIR to allow for small burst traffic
6) Be:- Excessive bust rate
This is the fastest data rate at which provider will ever serve you the VC
7) DE: – Discard eligibility
This is used to mark frame as low priority frame
( frames exceeding Bc are marked as low priority frame)
8) Oversubscription :
when you add up all CIR of VC on an interface and if they exceed the access rate of your: you are betting that all of your VCs will not run, simultaneously, at their rates
9) FECN:
(Forward explicit congestion notification)
This value in the frame header is set by carrier switch to indicate congestion inside the carrier network to the destination device
10) BECN:
This value is set by DTE in the header of the frame relay frame to indicate congestion (from source to destination) to the source of frame relay
LMI
This is used locally between frame relay switch (DCE) and router (DTE)
As we have discussed earlier that LMI is a keep-alive mechanism
There are three different standards defined for LMI and its interaction with a frame relay DTE and DCE
-ANSI
-Q933a
-Cisco
LMI type should be matched between your router and frame relay switch
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