This frame is very similar to the HDLC frame except for the missing control field here. DLCI field:. The first part of DLCI is of 6 bits and the second part is of 4 bits. They together form a 10 bit data link connection identifier. It is not used by the frame relay protocol. Extended Address EA :. Discard Eligibility DE :. In the overload situations a frame may have to be discarded. The frame relay address has been extended from the original 2 byte address to 3 or 4 byte address, in order to increase the range of DLCIs.
Its long form is Voice Over Frame Relay. Voice is transferred in the digital form with the help of PCM. But the quality of sound is not very good. A mechanism to check if data is flowing. A multicast mechanism. A mechanism to check the status of a switch. Congestion control and quality of service:. One of the advantages of frame relay is that it provides congestion control and quality of service. Advantages of frame relay:. Some of the advantages of frame relay are:. Streamlined communication process.
The number of functions of a protocol at the user-network interface is reduced. Lower delay. Higher throughput. Frame relay can be used at access speeds upto 2 Mbps. Frame Relay is cost- effective, partly due to the fact that the network buffering requirements are carefully optimized.
Compared to X. Frame Relay is also much faster than X. Disadvantages of frame relay:. Frames are delivered unreliably. Packets may not be delivered in the same sequence as that at the sending end. Packets having errors are simply discarded. Frame relay does not provide flow control. In business scenarios, where there is unpredictable and high-volume traffic, Frame Relay is one of the best choices.
It also remains a great choice for medium- to large-sized networks, which makes use of star or mesh connectivity. In business scenarios, where there is a slow connection or continuous traffic flow due to applications like multimedia, Frame Relay is not a recommended choice. By: Brad Rudisail Contributor. By: Kaushik Pal Contributor.
By: Leah Zitter Contributor. Dictionary Dictionary Term of the Day. Techopedia Terms. Connect with us. Sign up. Term of the Day. Best of Techopedia weekly. Rate enforcement on a per-VC basis: You can configure a peak rate to limit outbound traffic to either the CIR or some other defined value such as the excess information rate EIR.
This allows for finer granularity in the prioritisation and queuing of traffic, giving you more control over the traffic flow on an individual VC. If incoming traffic from Ethernet is larger than the token bucket filter, the traffic is buffered up in the frame-relay traffic queue. To view a flow chart showing packet flow when you implement Frame Relay traffic shaping, please see Frame Relay Traffic Shaping Flowchart.
This command shows the status of the permanent virtual circuit PVC , packets in and out, dropped packets if there is congestion on the line via forward explicit congestion notification FECN and backward explicit congestion notification BECN , and so on.
For a detailed description of the fields used with the show frame-relay pvc command, click here. If you have the output of a show frame-relay pvc command from your Cisco device, you can use Output Interpreter registered customers only to display potential issues and fixes.
You can use Output Interpreter registered customers only to display potential issues and fixes. This may be because either there is no mapping or incorrect mapping for the local DLCI in the frame-relay cloud or the remote end of the PVC is Deleted. This command is not enabled for point-to-point subinterfaces.
It is useful for multipoint interfaces and subinterfaces only. Sample output is shown below:. For a detailed description of the fields used with the show frame-relay map command, please see Documentation on frame relay Commands.
If you have the output of a show frame-relay map command from your Cisco device, you can use Output Interpreter registered customers only to display potential issues and fixes.
Configuration messages called bridge protocol data units BPDUs are used in the spanning-tree protocols supported in Cisco bridges and routers. These flow at regular intervals between bridges and constitute a significant amount of traffic because of their frequent occurrence.
There are two types of spanning-tree protocols in transparent bridging. For example, a Cisco using two Frame Relay interfaces, each with four subinterfaces, with a total of eight DLCIs, and associated maps needs the following:. This section contains portions of possible show interface command output you may encounter while troubleshooting.
Explanations of the output are provided as well. You need to troubleshoot the problem with a loopback test. To do a loopback test, follow the steps below:. Set the serial line encapsulation to HDLC and keepalive to 10 seconds. To do so, issue the commands encapsulation hdlc and keepalive 10 under the serial interface. There should not be any misses. An extended ping of 0x is helpful in resolving line problems since a T1 or E1 derives clock from data and requires a transition every 8 bits.
B8ZS ensures that. A heavy zero data pattern helps to determine if the transitions are appropriately forced on the trunk. A heavy ones pattern is used to appropriately simulate a high zero load in case there is a pair of data inverters in the path. The alternating pattern 0x represents a "typical" data pattern. If your pings fail or if you get cyclic redundancy check CRC errors, a bit error rate tester BERT with an appropriate analyzer from the telco is needed.
Check to make sure the Cisco router is transmitting data. You will most likely need to check the line integrity using loop tests at various locations beginning with the local CSU and working your way out until you get to the provider's Frame Relay switch. See the previous section for how to perform a loopback test. If you did not turn keepalives off, this line of output means that the router is talking with the Frame Relay provider's switch. You should be seeing a successful exchange of two-way traffic on the serial interface with no CRC errors.
Keepalives are necessary in Frame Relay because they are the mechanism that the router uses to "learn" which data-link connection identifiers DLCIs the provider has provisioned. To watch the exchange, you can safely use debug frame-relay lmi in almost all situations. The debug frame-relay lmi command generates very few messages and can provide answers to questions such as:. Notice the status of "DLCI " in the output above. The possible values of the status field are explained below:.
You can start sending it traffic with this DLCI in the header. This means that the switch - or a particular queue on the switch - for this PVC is backed up, and you stop transmitting in case frames are spilled. But it was programmed at some point in the past. Configuring a DLCI that the switch doesn't have will show up as a 0x4. IP split horizon checking is disabled by default for Frame Relay encapsulation so routing updates will come in and out the same interface.
Additionally, certain protocols such as AppleTalk, transparent bridging, and IPX cannot be supported on partially meshed networks because they require "split horizon," in which a packet received on an interface cannot be transmitted out the same interface, even if the packet is received and transmitted on different virtual circuits. You are not able to ping your own IP address on a multipoint Frame Relay interface.
Furthermore, you are not able to ping from one spoke to another spoke in a hub and spoke configuration. But if you configure a static map using the frame-relay map command for your own IP address or one for the remote spoke to use the local DLCI, you can then ping your devices.
The broadcast keyword provides two functions: it forwards broadcasts when multicasting is not enabled, and it simplifies the configuration of Open Shortest Path First OSPF for non-broadcast networks that use Frame Relay.
The broadcast keyword might also be required for some routing protocols -- for example, AppleTalk -- that depend on regular routing table updates, especially when the router at the remote end is waiting for a routing update packet to arrive before adding the route.
By requiring selection of a designated router, OSPF treats a non-broadcast, multi-access network such as Frame Relay in much the same way as it treats a broadcast network.
In previous releases, this required manual assignment in the OSPF configuration using the neighbor interface router command. When the frame-relay map command is included in the configuration with the broadcast keyword, and the ip ospf network command with the broadcast keyword is configured, there is no need to configure any neighbors manually. See the ip ospf network interface command for more detail. Once you create a specific type of subinterface, you cannot change it without a reload.
For example, you cannot create a multipoint subinterface serial0. To change it, you need to either reload the router or create another subinterface.
Approximately DLCIs can be configured on a single physical link, given a bit address. Because certain DLCIs are reserved vendor-implementation-dependent , the maximum is about The range for a Cisco LMI is These are the DLCIs carrying user-data. The total number of interfaces and subinterfaces per system is limited by the number of interface descriptor blocks IDBs that your version of Cisco IOS supports.
An IDB is a portion of memory that holds information about the interface such as counters, status of the interface, and so on. Higher speed interfaces require more memory than lower speed interfaces. If the full status update message is larger than the interface MTU, the packet is dropped, and the interface giant counter is incremented.
When changing the MTU, ensure the same value is configured at the remote router and intervening network devices. Please note that these numbers vary slightly, depending on the LMI type. The maximum DLCIs per router not interface platform guideline, based on extrapolation from empirical data established on a Cisco router platform, are listed below:.
Note: These numbers are guidelines only, and assume that all traffic is fast-switched. Dynamic routing protocols, and other protocols like IPX SAP that exchange database tables, send hellos and forwarding information messages which must be seen and processed by the CPU.
As a general rule, using static routes will allow you to configure a larger number of VCs on a single Frame Relay interface. Assign DLCIs to their subinterfaces before you enable the main interface to ensure that frame-relay inverse-arp works properly. In case it does malfunction, follow the steps below:. Each RIP packet can contain up to 25 route entries, for a total of bytes; 36 bytes of this total are header information, and each route entry is 20 bytes. Therefore, if you advertise routes over a Frame Relay link configured for 50 DLCIs, the result is 1 MB of routing update data every 30 seconds, or kbps of bandwidth consumed.
On a T1 link, this bandwidth represents This amount of overhead is considerable, and it is borderline acceptable, but the committed information rate CIR would have to be in the region of the access speed. Obviously, anything less than a T1 would incur too much overhead. For example:. Each IGRP packet can contain route entries, for a total of bytes, 38 of which are header information, and each route entry is 14 bytes.
If you advertise routes over a Frame Relay link configured with 50 DLCIs, the request is approximately KB of routing update data every 90 seconds, or 64 kbps of bandwidth consumed. On a T1 link, this bandwidth would represent 4. This overhead is an acceptable amount:. Each RTMP packet can contain up to 94 extended route entries, for a total of bytes, 23 bytes of header information, and each route entry is 6 bytes.
To remain within an acceptable level of overhead 15 percent or less , a T1 rate is required. Each IPX RIP packet can contain up to 50 route entries for a total of bytes, 38 bytes of header information, and each route entry is 8 bytes. To remain within an acceptable level of overhead 15 percent or less , a rate of kbps is required. Each IPX SAP packet can contain up to seven advertisement entries for a total of bytes, 38 bytes of header information, and each advertisement entry is 64 bytes.
To remain within an acceptable level of overhead 15 percent or less , a rate of greater than 2 Mbps is required. Obviously, SAP filtering is required in this scenario. In some cases, the keepalive on the Cisco device needs to be set slightly shorter about 8 seconds than the keepalive on the switch. You'll see the need for this if the interface keeps coming up and down. Serial interfaces, which are by default multipoint, are non-broadcast media, while point-to-point subinterfaces are broadcast.
If you are using static routes, you can point to either the next hop or the serial subinterface. For multipoint, you need to point to the next hop. The router needs to know that this is a broadcast interface for OSPF to work. OSPF and multipoint can be very troublesome.
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