- Gives the service provider the ability to make a “cloud of bandwidth”
- Service providers realize that no one uses 100% of their bandwidth 100% of the time. Because of this they can over commit the bandwidth.
- Make it cheaper to get connections over point-2-point leased lines.
Point-2-Point Looks like this:
Frame Relay kind of looks like this:
Frame Relay Terminology you will need to know
Committed Information Rate (CIR)
- Logical speed of the line.
- Min Bandwidth the service provider will give you
Local Access Rate (LAR)
- Speed of the Physical cable. It will most likely be > than the CIR
- Physical speed of the of the cable you connect your equipment to the provider with.
Local Managment Interface (LMI)
- The Language you speak between your router and service provider (3 types… Cisco, ANSI and one other)
- Signaling Protocol. Provides statistics from the service provider on the line
- Status – Quality and dropping of packets
- LMI can send DLCI info.
- NOTE – See diagram above for LMI
Data Link Connection Identifier (DLCI)
- Ethernet uses MAC addresses, Frame Relay uses DLCI
- DLCIs are LOCALLY SIGNIFICANT ONLY!!!!!!!!
Permanent Virtual Circuit (PVC)
- It is the logical circuit between routers in frame relay
- This is something the service provider sets up.
- Every PVC has a CIR
- The Greater # of PVCs, the Grater the Cost ($)
How DLCIs work
DLCIs can have a number from 16 to 1024.
I watched a presentation that explained DLCs and I have to say it was the best description I have ever heard it went something like this.
I head to the New York Airport because I need to take a flight to Toronto. When I get to the airport I look at my ticket and see that in order to get to Toronto I need to go to “New York Gate 102” to get on the plane. I then get on a plane and trust that the pilot will get me safely to Toronto. When the plane lands safely in Toronto we get off the plan and come through “Toronto Gate 201”. I then leave the airport and call a taxi to where I need to go.
The Gates are like DLCI’s. Unlike in Ethernet where the MAC address is the source address on your local computer that eventually talks to a remote MAC address on another, the DLCI closest to where you leave is actually the Destination Address and the DLCI where the packet leaves the frame relay cloud is actually the address of where you came from if you took the trip backwards!
So while DLCIs are like MAC in that they are addresses for Frame relay they are interpreted much, much differently. Basically in reverse!
So DLCI 607 is actually the destination address of Router 7. DLCI 706 is the destination address of R6 FROM R7. The two DLCI’s on Router R6 can NEVER be the same because it is LOCALLY SIGNIFICANT. That being said R7 and R5 could both be changed to 607 and R6 wouldn’t care as long as the Frame Relay cloud was configured properly by the service provider.
Let’s look at it one more time! 605 is the destination of R5 on R6. On R5 506 is the destination address of Router R6! It’s important to understand DLCIs!!!!!
One more thing you should note is that the red lines represent the PVC between the routers!
Frame Relay PVC Design
There are 3 ways you can design the PVCs. They are
Hub & Spoke
- Cheapest to deploy
- Single Point of failure
- Delays can become an issue
- Exponentially Expensive! Each time you add a new location, you then need to create redundant PVCs to all other routers as well.
- Because of this it doesn’t scale well.
- Best compromise on performance and price
- There is redundancy on the most important parts of the network only where needed.
Frame Relay Point Design
Besides PVCs you can also design how each point is designed.
- Each router is in the same subnet.
- Multiple DLCIs are mapped to an interface
- Causes problems with Split Horizon. ie. Typically a router will not allow routes it learned to leave on the same interface. This prevents routing loops.
- All Routers are on different subnets
- Point-2-Point sub-interface created for EACH peer
- Split Horizon isn’t an issue for P-2-P design.
I hope this helps you with the basics of frame relay!