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: 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