Frame Relay 101
Last updated
Last updated
Frame Relay is a Layer 2 WAN technology that uses the concept of virtual circuits to transport data. The Frame Relay network is packet switched, but packets towards a destination share a common label, so they follow a strict path according to the label, giving the impression of an end-to-end circuit to the upper layer protocols.
Usually, devices in a Frame Relay network have 2 main roles: The customer Frame Relay Router and the provider Frame Relay Switch. I like to call them like this because it makes the difference more clear between the 2 roles. One difference between a switch and a router is that, typically, a router makes forwarding decisions based on the Layer 3 address, and a switch based on the Layer 2 address. In Frame Relay, the customer router will receive or create a Layer 3 packet. It will look at its destination address and based on the routing table it will have to forward it over a Frame Relay interface. To do this, it will have to encapsulate the Layer 3 packet into a Layer 2 “frame” before sending it out the Frame Relay interface. The next device on the link is the provider Frame Relay Switch, which will forward the frame based only on the Layer 2 information, without looking at Layer 3. This is why the devices in the provider’s Frame Relay cloud are called Frame Relay Switches.
It is important to understand what device you are configuring because the configuration differs on a Frame Relay Router from the one on a Frame Relay Switch.
Most of the time you will see that the Frame Relay Router is called the DTE device, and the Frame Relay Switch is called the DCE device.
In Telecommunications, DTE is a Data Terminal Equipment and is the last device in a communication line, while DCE is the Data Communication Equipment and is the device that the DTE connects to, in order to access the communication path. This is why the interface on the Frame Relay Router is defined as DTE interface, while the interface on the Frame Relay Switch is defined as DCE interface. What about interfaces between Frame Relay Switches? They are called NNI – Network to Network Interface.
Let me point out that this denomination is different from the DTE and DCE ends of a serial connection, because even though the Frame Relay DCE device is usually the DCE end of the link and the DTE device is usually the DTE link, the connection can still work if the Frame Relay DTE device is the DCE end of the link and the Frame Relay DCE device is the DTE end of the link
As I said earlier, the Frame Relay network offers a virtual circuit to the upper layer protocols, by encapsulating the Layer 3 information in the Layer 2 frames, and by marking each frame with its appropriate Data Link Connection Identifier. The name says it all. The DLCI will identify the virtual circuit that each frame belongs to. One thing that must be remembered here is that the DLCI identifies the circuit on a link by link basis. This means that the the same virtual circuit will be identified by different DLCI values on each link from the source to the destination.
In the example below, R1 uses 2 different circuits to reach R2 and R3 respectively. R1 will encapsulate traffic for R2 in frames with DLCI set to 102 and will send them towards FRS1. FRS1 knows it must take frames with DLCI 102 and send them towards FRS0 with DLCI set to 912. When FRS0 receives frames with DLCI 912 will switch them over to FRS2 and will change the DLCI to 452. As the last device in the Frame Relay Cloud, FRS2 will forward the frames to R2 but it will also replace the DLCI value with 201. The same process happens for traffic from R1 to R3, but this time the DLCIs used are 103, 913, 743 and 301
To complicate thinks a bit, there are different type of encapsulations that can be used. Cisco routers default to cisco encapsulation, but there is also an ietf encapsulation available, that should be used when connecting to non-Cisco devices. Luckily, many other devices support cisco encapsulation, but also, Cisco devices work even with a mismatched encapsulation. They will send packets with the defined encapsulation but will accept packets with either encapsulation. So in real life there is no need to worry about this, but it is recommended to match the encapsulation even for the often underrated reason of readability.
Even if it is not very clear, we should understand that hitting enter will set the encapsulation to cisco, while the other options are ietf and MFR.
To see the encapsulation type used on an interface use:
Another important topic here is that the frame-relay encapsulation must match end-to-end, that is between the DTE devices, and not between the DTE and the DCE. There is something else that should match between the DTE and the DCE and that is the LMI Type.
You will see later that encapsulation can be different on each virtual circuit, but by default, each virtual circuit will inherit the settings on the interface. To see how encapsulation actually looks like, you should read this article about Frame Relay Encapsulation
Frame Relay started in the mid 1980’s when both ITU-T (known at the time as CCITT) and ANSI where trying to standardize the technology, but it took off at the beginning of the 1990’s when “The Gang of Four” – Cisco, StrataCom, Northern Telecom and DEC – created a consortium that would focus on accelerating the introduction and interoperability of Frame Relay products. The most important development they came up with, was the LMI – Local Management Interface, which is a set of extensions complementary to the existing Frame Relay standards that add several new features to it, like Virtual Circuit Status Messages, Multicasting or Global Addressing. Support for Multicasting and Global Addressing extensions is optional, but VC Status Messages is expected to be implemented by most vendors.
VC Status Messages offers a very useful service for Frame Relay, that gives an end-to-end status of the virtual circuit from one DTE to another. The status of a virtual circuit can be ACTIVE, INACTIVE or DELETED.
For a quick view of the status of each virtual circuit, I use:
An ACTIVE status means everything is OK and the virtual circuit can be used to send data from one DTE to another.
An INACTIVE status means the virtual circuit is not completed, but the problem is not on the local link between the DTE and the DCE, but beyond the first DCE, in the provider network or at the other end of the connection.
A DELETED status means the DTE and the DCE do not have the same information regarding the virtual circuit. Most often, the virtual circuit on the DTE is configured with a different DLCI than on the DCE.
To set the LMI type, you use the command below to select either Cisco, ANSI Annex D or ITU Q933-A (Annex A)
but probably the best option would be to just let the default LMI auto-sense to work. This feature, available on Cisco routers, will discover the LMI type used by the DCE device and will use it. To enable auto-sense, if an lmi-type has been already set, use:
To see the LMI type that is used:
You can also see what DLCI is used for LMI, with:
| LMI Type | Listens on | Available DLCIs |
|---|---|---|
Cisco | 1023 | 16-1007 |
ANSI | 0 | 16-991 |
ITU | 0 | 16-991 |
LMI Status inquires are sent every 10 sec by default (show as type1 in debug frame LMI)
Full Status updates are sent every 6th inquiry (show as type0 in debug frame LMI)
DTE will report the status of each configured DLCI. The MTU size limits the number of DLCIs on a link. When using an MTU of 1500 Bytes a maximum only 296 DLCIs can be included in one status message. If the DCE device doesn’t receives 3 LMI Status messages it considers the link down.
The Back-to-Back Frame Relay configuration is one where, God knows why, you would want to connect to routers on a serial link using Frame Relay encapsulation. First, we will have to enable frame-relay encapsulation and set a Layer 3 address
As we saw at the beginning, a Frame Relay router expects to be the DTE end of the link so it must connect to a DCE device. But this time, both devices are DTE devices. In order to make the link operational, one device must become the serial DCE device and this is done by setting clock rate on the interface. Use “?” to see all available clock rates.
* In newer version of the IOS, this is not needed anymore as all serial interfaces have a default clock rate, but only the DCE uses it. Each router still thinks it is a Frame Relay DTE, so by default it expects to communicate over LMI with the DCE device. Unless LMI is disabled, the link will not come up! So next thing to do is to disable LMI on both ends using:
One more thing must be done to achive connectivity. The routers must agree on the DLCI to use. This can be done by assigning a DLCI to the interface and using Inverse ARP, or by setting up a static mapping between IP and DLCI, but we’ll see how this works in the next episode.
End-to-end Keepalives are a different kind of keepalive, that test the end-to-end connectivity over Frame Relay, that is from one DTE to another. This way, the status of the PVC can be better monitored. To enable EEK, first you must define a map-class:
The EEK works by sending a Request on one side, and replying with a response to it on the other side. Each side can don both or any of these actions.
Bidirectional: This end will both send requests and reply to requests. The other end must be bidirectional also in order to work. Used when the upstream path is different from the downstream path
Request: This end will only send and wait for replies. The other end must be set to reply or passive-reply in order to work
Reply: This end will only respond to requests. The other end must be set to Request in order to work
Passive-reply: This end will reply but will not track the requests received. The other end must be set to Request in order to work
The keepalive configuration can be done with the following commands:
Finally, apply the map-class on the frame-relay interface, using:
To monitor, use:
