Content 5.1 Frame Relay
Concepts 5.1.7 Stages of Inverse ARP and LMI
operation LMI status messages combined with Inverse ARP
messages allow a router to associate network layer and data
link layer addresses. When a router that is connected to a
Frame Relay network is started, it sends an LMI status inquiry
message to the network. The network replies with an LMI status
message containing details of every VC configured on the access
link. Periodically the router repeats the status inquiry, but
subsequent responses include only status changes. After a set
number of these abbreviated responses, the network will send a
full status message. If the router needs to map the VCs to
network layer addresses, it will send an Inverse ARP message on
each VC. The Inverse ARP message includes the network layer
address of the router, so the remote DTE, or router, can also
perform the mapping. The Inverse ARP reply allows the router to
make the necessary mapping entries in its address to DLCI map
table. If several network layer protocols are supported on the
link, Inverse ARP messages will be sent for each.
Content
5.2 Configuring Frame Relay
5.2.1 Configuring basic Frame Relay This section explains
how to configure a basic Frame Relay PVC. Frame Relay is
configured on a serial interface and the default encapsulation
type is the Cisco proprietary version of HDLC. To change the
encapsulation to Frame Relay use the encapsulation
frame-relay [cisco | ietf] command.
cisco Uses the Cisco proprietary Frame Relay encapsulation.
Use this option if connecting to another Cisco router. Many
non-Cisco devices also support this encapsulation type. This is
the default. ietf Sets the encapsulation method to
comply with the Internet Engineering Task Force (IETF) standard
RFC 1490. Select this if connecting to a non-Cisco router.
Cisco’s proprietary Frame Relay encapsulation uses a 4-byte
header, with 2 bytes to identify the data-link connection
identifier (DLCI) and 2 bytes to identify the packet type.Set
an IP address on the interface using the ip address
command. Set the bandwidth of the serial interface using the
bandwidth command. Bandwidth is specified in kilobits
per second (kbps). This command is used to notify the routing
protocol that bandwidth is statically configured on the link.
The bandwidth value is used by Interior Gateway Routing
Protocol (IGRP), Enhanced Interior Gateway Routing Protocol
(EIGRP), and Open Shortest Path First (OSPF) to determine the
metric of the link. This command also affects bandwidth
utilization statistics that can be found by using the show
interfaces command. The LMI connection is established and
configured by the frame-relay lmi-type [ansi | cisco
| q933a] command. This command is only needed if using
Cisco IOS Release 11.1 or earlier. With IOS Release 11.2 or
later, the LMI-type is autosensed and no configuration is
needed. The default LMI type is cisco. The LMI type is set on a
per-interface basis and is shown in the output of the show
interfaces command.These configuration steps are the same,
regardless of the network layer protocols operating across the
network. Lab Activity Lab Exercise: Configuring Frame
Relay In this lab, the student will configure a router to make
a successful connection to a local Frame Relay switch. Lab
Activity e-Lab Activity: Configuring Frame Relay In this
activity, a router will be configured to make a successful
connection to a local Frame Relay switch. Web Links
Configuring Basic Frame Relay http://www.cisco.com/en/US/tech/
tk713/tk237/technologies_configuration_
example09186a008009421c.shtml
Content
5.2 Configuring Frame Relay
5.2.2 Configuring a static Frame Relay map The local DLCI
must be statically mapped to the network layer address of the
remote router when the remote router does not support Inverse
ARP. This is also true when broadcast traffic and multicast
traffic over the PVC must be controlled. These static Frame
Relay map entries are referred to as static maps. Use the
frame-relay map protocol protocol-address dlci
[broadcast] command to statically map the remote network
layer address to the local DLCI. Lab Activity Lab
Exercise: Configuring Frame Relay PVC In this lab, the student
will configure two routers back-to-back as a Frame Relay
permanent virtual circuit (PVC). Lab Activity e-Lab
Activity: show frame-relay map In this activity, the
student will demonstrate how to use the show frame-relay map
command to display the current map entries and information
about the connections. Web Links Configuring Dynamic and
Static Mapping for Multipoint Subinterfaces
http://www.cisco.com/en/US/tech/
tk713/tk237/technologies_configuration_
example09186a008009421f.shtml
Content
5.2 Configuring Frame Relay
5.2.3 Reachability issues with routing updates in NBMA By
default, a Frame Relay network provides non-broadcast
multi-access (NBMA) connectivity between remote sites. An NBMA
environment is viewed like other multiaccess media
environments, such as Ethernet, where all the routers are on
the same subnet. However, to reduce cost, NBMA clouds are
usually built in a hub-and-spoke topology. With a hub-and-spoke
topology, the physical topology does not provide the
multi-access capabilities that Ethernet does. The physical
topology consists of multiple PVCs. A Frame Relay NBMA topology
may cause two problems: - Reachability issues regarding
routing updates
- The need to replicate broadcasts on
each PVC when a physical interface contains more than one
PVC
Split-horizon updates reduce routing loops by
not allowing a routing update received on one interface to be
forwarded out the same interface. If Router B, a spoke router,
sends a broadcast routing update to Router A, the hub router,
and Router A has multiple PVCs over a single physical
interface, then Router A cannot forward that routing update
through the same physical interface to other remote spoke
routers. If split-horizon is disabled, then the routing update
can be forwarded out the same physical interface from which it
came. Split-horizon is not a problem when there is a single PVC
on a physical interface. This would be a point-to-point Frame
Relay connection. Routers that support multiple connections
over a single physical interface have many PVCs that terminate
in a single router. This router must replicate broadcast
packets such as routing update broadcasts, on each PVC, to the
remote routers. The replicated broadcast packets can consume
bandwidth and cause significant latency to user traffic. It
might seem logical to turn off split-horizon to resolve the
reachability issues caused by split-horizon. However, not all
network layer protocols allow split-horizon to be disabled and
disabling split-horizon increases the chances of routing loops
in any network. One way to solve the split-horizon problem is
to use a fully meshed topology. However, this will increase the
cost because more PVCs are required. The preferred solution is
to use subinterfaces. Lab Activity e-Lab Activity:
Configuring Frame Relay Subinterfaces In this activity, the
student will configure three routers in a full-mesh Frame Relay
network. Web Links Problems with Running OSPF in NBMA
Mode over Frame Relay http://www.cisco.com/en/US/tech/
tk826/tk365/technologies_tech_ note09186a0080094051.shtml
Content 5.2 Configuring Frame Relay
5.2.4 Frame Relay subinterfaces To enable the forwarding of
broadcast routing updates in a hub-and-spoke Frame Relay
topology, configure the hub router with logically assigned
interfaces. These interfaces are called subinterfaces.
Subinterfaces are logical subdivisions of a physical interface.
In split-horizon routing environments, routing updates received
on one subinterface can be sent out another subinterface. In a
subinterface configuration, each virtual circuit can be