link advertising the interface's IP address with a
cost of 0. When flooding out a nonbroadcast interface, the LSU
or LSAck packet must be replicated to be sent to each of the
interface's neighbors. To configure point-to-multipoint,
manually override the detected OSPF network type with the
following syntax: Router(config-if)#ip ospf network
point-to-multipoint The interface should also be
configured with a frame-relay map ip command, as in the
following syntax: Router(config-if)#frame-relay map
ip address dlci [broadcast] The
broadcast keyword permits the router to send broadcasts via
the specified DLCI to the mapped neighbor or neighbors. If the
point-to-multipoint configuration is applied to the example
network, two separate frame-relay map statements would have to
be configured on the hub router, RTB.
Content 4.6
PPP and Layer 2 Considerations for Routed and Routing
Protocols 4.6.6 The keyword broadcast The
broadcast keyword provides two functions. First, it
forwards broadcasts when multicasting is not enabled. Second,
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. Protocols such as
these 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 the
selection of a designated router, OSPF treats a NBMA 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. OSPF now
automatically runs over the Frame Relay network as a broadcast
network. Note: The OSPF broadcast mechanism assumes that
IP class D addresses are never used for regular traffic over
Frame Relay. The following example maps the destination IP
address 172.16.123.1 to DLCI 100: Router(config)#interface
serial 0/0
Router(config-if)#frame-relay map IP
172.16.123.1 100 broadcast OSPF uses DLCI 100 to broadcast
updates.
Content 4.6 PPP and Layer 2
Considerations for Routed and Routing Protocols
4.6.7 Excessive fragmentation Fragmentation refers
to the reduction of a Layer 3 packet as it is placed in a Layer
2 frame. Each fragment must be less than or equal to the
original packet length. The Maximum Transmission Unit (MTU)
refers to the largest Layer 3 packet that can be forwarded out
a router or switch interface. Ethernet and serial interfaces
have default MTU values of 1500 Bytes. When an outbound
interface has a smaller MTU than the inbound interface, the
packet may have to be fragmented. Consider an Ethernet packet
of 1500 Bytes that is received on interface fastethernet 0/0,
and must be sent out interface serial 0/0 using X.25
encapsulation. X.25 supports MTU values of 576 Bytes. In this
case, the router strips the single Ethernet frame from the
Layer 3 packet and reassembles the packet into three X.25
frames. Proper fragment sequencing is maintained in the IP
header of the packet. The ID value field contains the same
value for each fragment. The Offset field value is unique, and
defines which part of the original packet is contained in this
fragment. Both sequenced and out of sequence packets are
reassembled at the destination device. To change the interface
MTU size, two commands can be used. The interface configuration
command mtu is used to change the MTU for all Layer 3
protocols. The second interface configuration command, ip
mtu is used to set the MTU for the IP protocol. While
relatively straight forward, issues can arise during MTU
configuration. If both commands are used on an interface, for
example, the ip mtu value takes precedence for IP.
However, if mtu is configured after ip mtu, then
the IP MTU value is reset to the same value as the MTU value.
Content Summary This module examined
troubleshooting techniques and common problems associated with
the following Layer 2 protocols and technologies:
- Switches and switched technologies
- PPP
- ISDN
- Frame Relay
The difficulty in
troubleshooting these technologies is the inability of common
Layer 3 troubleshooting tools, such as ping, to assist with
anything but the identification that the network is down. It is
only through a thorough understanding of the protocols and
their operation that a network technician will be able to
choose the appropriate troubleshooting methodology and IOS
commands to solve the problem in an efficient manner. In the
case of switched networks, it is quite possible to design a
functional but suboptimal network. With no apparent network
failure, it will only be the awareness of common problems and
the ability of a network technician to identify Layer 2 traffic
paths that will allow the network technician to achieve the
full performance a network is capable of. After completing this
module, the students should be confident that they can
recognize common Layer 2 problems and address them quickly and
accurately