The ICMP Time Exceeded message notifies a host when a packet it sent has been discarded because it was “out of time.” Packets are not actually timed, but to prevent packets from being forwarded forever when there is a routing loop, each IP packet header includes a Time to Live (TTL) field. Routers decrement TTL by 1 every time they forward a packet; if a router happens to decrement TTL of a packet to 0, the router discards the packet and sends an ICMP Time Exceeded messag..
Defining the Router ID and Loopback Interfaces
The router needs an ID to participate in the OSPF domain. The router ID is used to identify LSAs in the OSPF database. The router ID can be defined by an administrator or left to the discretion of the router. Most people define the ID so that it is easier to track events in the network, for internal documentation, and for other system-administration purposes. The OSPF router ID can be defined by the router-id command. If the router-id command is not present, then the ..
Ethernet versus Token Passing Technologies
Token Ring, which is a token-passing LAN developed and supported by IBM, enjoyed enormous popularity during the days of IBM's Systems Network Architecture (SNA) networks, which developed in the 1970s. FDDI, a 100 Mbps optical fiber-based system, also was briefly popular due to its substantial speed advantage over the contemporary 10 Mbps Ethernet and its immunity to electrical interference. FDDI's largest drawback was the high cost of optical components. However, the lower co..
Maintaining Syslog Files on the Server
The Bourne shell script in Example 18-1 automatically rotates router logfiles to ensure that these files don't become too big and cumbersome to navigate. The script is intended to be invoked via a cron job on a daily basis, but you can also run it manually. By default, the script retains seven days worth of archived logfiles and compresses files older than two days. No arguments are required or expected. Example 18-1. rotatelog.sh #!/bin/sh # # rotatelog.sh -- a ..
Now that you have reviewed, from a generic perspective, how an IP routing protocol’s data structure interacts with a router’s IP routing table, consider the specific data structures used by EIGRP, as described in Table 6-4.
