How to Configure Frame Relay On Cisco Router
You learn about How to Configure Frame Relay On Cisco with serial interface of a router. You also learn how to configure point-to-point sub-interfaces.Basic Frame Relay Configuration Commands
Frame Relay is configured on a Cisco router from the Cisco IOS command line interface (CLI) . The following table shows the mandatory and optional steps to configure Frame Relay .| Type of Task | Description |
|---|---|
| Required Tasks |
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| Optional tasks |
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Below is the topology of three routers that is used in this section, although the initial focus is on the Frame Relay link between R1 and R2, the 10.1.1.0/24 network.
STEP 1. SET THE IP ADDRESS IN THE INTERFACE
On a Cisco router, Frame Relay is generally supported on synchronous serial interfaces. Use the ip address command to set the IPv4 address of the interface.In the link between R1 and R2, address 10.1.1.1/24 was assigned to S0 / 0/1 of R1 and IPv4 address 10.1.1.2/24 to S0 / 0/1 of R2.
With the ipv6 address command , routers R1 and R2 were also configured with the following IPv6 addresses:
- R1 was configured with the global unicast IPv6 address 2001: DB8: CAFE: 1 :: 1/64 and the static link-local address FE80 :: 1.
- The R2 was configured with the global unicast IPv6 address 2001: DB8: CAFE: 1 :: 2/64 and the static link-local address FE80 :: 2.
Note : By default, the Cisco IOS uses EUI-64 to automatically generate the link-local IPv6 address on an interface.
STEP 2. CONFIGURE ENCAPSULATION
The encapsulation frame-relay interface configuration command [ cisco | ietf ] enables Frame Relay encapsulation and allows Frame Relay processing on the supported interface.- The cisco encapsulation type is the default Frame Relay encapsulation enabled on the supported interfaces. Use this option if you connect to another Cisco router.
Many non-Cisco devices also support this type of encapsulation. It uses a 4-byte header, with 2 bytes to identify the DLCI and 2 bytes to identify the type of packet.
- The ietf encapsulation type complies with RFC 1490 and RFC 2427. Use this option if you connect to a non-Cisco router.
STEP 3. SET THE BANDWIDTH
Use the bandwidth command to set the bandwidth of the serial interface. Specify the bandwidth in kb / s. This command notifies the routing protocol that the bandwidth was statically configured on the link. The EIGRP and OSPF routing protocols use the bandwidth value to calculate and determine the link metric.STEP 4. SET THE TYPE OF LMI (OPTIONAL)
Manual configuration of the LMI type is optional, since Cisco routers automatically detect the type of LMI by default. Remember that Cisco supports three types of LMI: Cisco, ANSI Annex D and Q933-A Annex A. The default LMI type for Cisco routers is Cisco.R1 Configuration Example:
R1 (config) # interface Serial0 / 0/1
R1 (config-if) # bandwidth 64
R1 (config-if) # ip address 10.1.1.1 255.255.255.0
R1 (config-if) # ipv6 address 2001: db8: cafe: 1 :: 1/64
R1 (config-if) # ipv6 address fe80 :: 1 link-local
R1 (config-if) # encapsulation frame-relay
The show interfaces serial command verifies the configuration, including Frame Relay layer 2 encapsulation and the default LMI type cisco.
Note that this command shows the IPv4 address, but does not include any of the IPv6 addresses. Use the show ipv6 interface command or the show ipv6 interface brief command to verify IPv6.
Note : The no encapsulation frame-relay command eliminates Frame Relay encapsulation on the interface and returns the interface to the default HDLC encapsulation.
2. SETTING UP A STATIC FRAME RELAY MAP
Cisco routers support all network layer protocols using Frame Relay, such as IPv4, IPv6, IPX and AppleTalk. Address assignment to DLCI is achieved by dynamic or static address assignment.
The dynamic assignment is performed by the inverse ARP feature . Because reverse ARP is enabled by default, no additional commands are required.
The static assignment is manually configured on a router (following the topology of Image 1). The establishment of static allocation depends on the needs of the network.
To assign between a next hop protocol address and a DLCI destination address, use the command:
frame-relay map protocol protocol-address dlci [ broadcast ]
| Parameter | Description |
|---|---|
| protocol | Define the supported protocol, bridge or logical link control: ip (IPv4), ipv6, AppleTalk, decnet, dlsw, ipx, llc2, rsrb, vines and xns. |
| protocol-address | Defines the network layer address of the destination router interface. |
| dlci | Defines the local DLCI that is used to connect to the remote protocol address. |
| broadcast | (Optional) Allows broadcast and multicast transmissions through the virtual circuit. This allows the use of dynamic routing protocols in the VC. |
THE BROADCAST PARAMETER
Frame Relay, ATM and X.25 are multiaccess networks without broadcast (NBMA). NBMA networks only allow the transfer of data from one computer to another through a VC or a switching device. This requires that you play the packages manually to all destinations.The use of the broadcast keyword is a simplified way to resend routing updates. It allows IPv4 broadcasts and broadcasts to spread to all nodes. It also allows IPv6 multicasting through PVC. When the keyword is enabled, the router converts broadcast and multicast traffic to unicast traffic.
Image shows how to use keywords when configuring static address assignments. Note that the first assignment of Frame Relay IPv6 to a global unicast address does not include the broadcast keyword.
However, the broadcast keyword is used in the assignment to the link-local address. IPv6 routing protocols use link-local addresses for multicast routing updates. Therefore, only the link-local address map requires the broadcast keyword to resend multicast packets.
VERIFICATION OF A STATIC FRAME RELAY MAP
To verify the Frame Relay assignment, use the show frame-relay map command .
Note that there are three Frame Relay assignments. There is an assignment for IPv4 and two for IPv6, one for each of the IPv6 addresses.
PROBLEMS AND CONNECTION SOLUTION
By default, most Frame Relay networks provide NBMA connectivity between remote sites using a hub-and-spoke topology.In a NBMA Frame Relay topology, when a single multipoint interface must be used to interconnect multiple sites, connection problems may arise from routing updates. With distance vector routing protocols, split horizon connection problems may arise, as well as multicast and broadcast reproduction. With link state routing protocols, problems with choosing the DR / BDR can cause connection problems.
SPLIT HORIZON
The split horizon rule is a loop prevention mechanism for distance vector routing protocols such as EIGRP and RIP. This rule reduces routing loops by preventing a routing update that is received on an interface from being forwarded from the same interface.
- In the Frame Relay hub-and-spoke topology, the remote router R2 (a spoke router) sends an update to the central office router R1 (the router hub).
- The R1 connects several PVCs through a single physical interface.
- R1 receives multicast on its physical interface; however, the split horizon cannot forward that routing update through the same interface to other remote routers (spoke).
Note : The split horizon is not a problem if only one PVC (a single remote connection) was configured on a physical interface. This type of connection is point to point.
MULTICAST AND BROADCAST PLAYBACK
As shown in the figure, due to the split horizon, when a router supports multipoint connections through a single interface, the router must reproduce the broadcast and multicast packets.
In the case of routing updates, the updates must be reproduced and sent on each PVC to the remote routers. These reproduced packets consume bandwidth and cause significant latency variations in user traffic.
NEIGHBOR DISCOVERY: DR AND BDR
Link state routing protocols, such as OSPF, do not use the split horizon rule to avoid loops. However, connection problems with the DR / BDR may arise.In NBMA networks, OSPF operates in network mode without broadcast by default, and neighbors are not discovered automatically. Neighbors can be statically configured. However, make sure that the router hub becomes a DR.
Remember that an NBMA network behaves like Ethernet, and in Ethernet a DR is needed to exchange routing information between all routers in a segment. Therefore, only the router hub can function as a DR, because it is the only router that has PVC with the rest of the routers.
TROUBLESHOOTING CONNECTION ISSUES
- There are several ways to solve the routing connection problem:
- Disable the split horizon : a method to solve the connection problems caused by the split horizon can be to deactivate the split horizon. However, disabling the split horizon increases the chances of routing loops forming in the network. In addition, only IP allows you to disable the split horizon; IPX and AppleTalk do not allow it .
- Full mesh topology: This topology increases costs.
- Subinterfaces : In a Hub-and-spoke Topology of Frame Relay, the router hub can be configured with logically assigned interfaces called "subinterfaces".
FRAME RELAY SUBINTERFACES
Frame Relay can divide a physical interface into several virtual interfaces called "subinterfaces."
A subinterface is simply a logical interface that is directly associated with a physical interface. Therefore, a Frame Relay subinterface can be configured for each of the PVCs entering a physical serial interface.
To enable the forwarding of broadcast routing updates on a Frame Relay network, you can configure the router with logically assigned subinterfaces.
- When using a subinterface configuration, each VC can be configured as a point-to-point connection.
- A partial mesh network can be divided into several smaller, full-mesh, point-to-point networks.
- A unique network address can be assigned to each point-to-point subnet. This allows each subinterface to work similarly to a leased line.
- Using a point-to-point Frame Relay subinterface, each pair of point-to-point routers is in its own subnet. This allows packets received in a subinterface to be sent through another subinterface, even if the packets are forwarded by the same physical interface.
FRAME RELAY SUBINTERFACES CONFIGURATION
Frame Relay subinterfaces can be configured in mode:- Point-to-point : A single point-to-point subinterface establishes a PVC connection to another physical interface or subinterface in a remote router. In this case, each pair of point-to-point routers is in its own subnet, and each point-to-point subinterface has a single DLCI. In a point-to-point environment, each subinterface functions as a point-to-point interface. For each point-to-point VC, there is a different subnet. Therefore, routing update traffic is not subject to the split horizon rule.
- Multipoint : A single multipoint subinterface establishes several PVC connections to several physical interfaces or subinterfaces on remote routers. All the interfaces that participate are in the same subnet. The subinterface functions as an NBMA Frame Relay interface, so routing update traffic is subject to the split horizon rule. All multipoint VCs belong to the same subnet.
When configuring subinterfaces, the encapsulation frame-relay command is assigned to the physical interface. All other configuration items, such as the network layer address and the DLCIs, are assigned to the subinterface.
SUBINTERFACE CONFIGURATION
Subinterfaces address the limitations of Frame Relay networks by providing a way to subdivide a partial mesh Frame Relay network into a number of smaller full-mesh or point-to-point subnets. Each subnet is assigned its own network number and appears before the protocols as if it could be reached through a different interface.POINT-TO-POINT SUBINTERFACES CONFIGURATION
To create a subinterface, use the command:router (config-if) # interface serial number.subinterface-number { multipoint | point-to-point }
To simplify troubleshooting, use the DLCI as a subinterface number. You must also specify whether the interface is point-to-multipoint or point-to-point with the multipoint or point-to-point keyword, since there is no default value. These keywords are defined in the table:
| Parameter | Description |
|---|---|
| subinterface-number | The subinterface number must be in the range of 1 to 4294967293. The interface number that precedes the point (.) Must match the physical interface number to which this subinterface belongs. |
| multipoint | Select this option if all routers are in the same subnet. |
| point-to-point | Select this option so that each pair of point-to-point routers have their own subnet. Point-to-point links typically use a 255.255.255.252 subnet mask |
R1 (config-if) # interface serial 0/0 / 0.103 point-to-point
DLCI CONFIGURATION
If the subinterface is configured as point-to-point, the local DLCI of the subinterface must also be configured to distinguish it from the physical interface. DLCI is also required for multipoint subinterfaces with reverse ARP enabled for IPv4. Not required for multipoint subinterfaces configured with static route maps.The Frame Relay service provider assigns the DLCI numbers. These numbers range from 16 to 992 and, in general, only have local importance. The interval varies depending on the LMI used.
The frame-relay interface-dlci command configures the local DLCI in the subinterface, as shown in Figure 2:
router (config-subif) # frame-relay interface-dlci dlci-number
| Parameter | Description |
|---|---|
| dlci-number | Defines the local DLCI number that links to the subinterface. This is the only way to link a DLCI derived from LMI with a subinterface, since LMI does not know the subinterfaces. Use the frame-relay interface-dlci command only in subinterfaces. |
EXAMPLE: POINT-TO-POINT SUBINTERFACE CONFIGURATION
The previous topology is shown, but with point-to-point subinterfaces. Each PVC is a different subnet. The physical interfaces of the router are divided into subinterfaces, with each subinterface in a different subnet.
To configure subinterfaces in a physical interface, the following steps are required:
- Step 1 . Remove any network layer address assigned to the physical interface. If the physical interface has an address, local subinterfaces do not receive frames.
- Step 2 . Configure Frame Relay encapsulation on the physical interface using the encapsulation frame-relay command .
- Step 3 . Create a logical subinterface for each of the defined PVCs. Specify the port number, followed by a period (.) And the subinterface number. To simplify problem solving, it is suggested that the subinterface number match the DLCI number.
- Step 4 . Configure an IP address for the interface and set the bandwidth.
- Step 5 . Configure the local DLCI in the subinterface using the frame-relay interface-dlci command. Remember that the Frame Relay service provider assigns the DLCI numbers.
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