OSPF develops neighbor relationships with routers on the same link by exchanging hello messages (sometimes shortened to hellos). Upon initial exchange of hello messages, the routers add each other to their respective neighbor tables. A neighbor table is a list of connected OSPF speakers (devices running OSPF). OSPF speakers send multicast hellos with destination address 224.0.0.5 on all OSPF-enabled interfaces. OSPF sends out hello packets every 10 seconds on a broadcast link..
Enhanced IGRP can simultaneously use unequal cost paths to the same destination, which indicates that traffic can be distributed among multiple (up to four, by default) unequal cost paths to provide greater overall throughput and reliability. An alternate path is used if the variance condition is met. The variance condition depends on two factors: The local metric must be greater than the metric of the next router, so the next hop router must be closer to the destinat..
Local-Area Network Technologies
Cisco devices support multiple LAN technologies. We cover five popular technologies in this chapter: • Ethernet and IEEE 802.3 • Fast Ethernet • Gigabit Ethernet • Token Ring • Fiber Distributed Data Interface Each of these protocols operates at the data link layer of the OSI reference model, and each is used in a LAN environment to carry data from point to point at speeds from 4 Mbps to 1 Gbps. This section briefly introduces you to e..
Use the clear logging command to clear the router's internal log buffer: Router#clear logging Clear logging buffer [confirm]<enter> Router# There are times when it is useful to be able to clear your router's internal buffer log. For instance, while debugging a network problem, old debug messages can cause unnecessary confusion during stressful moments. This will clear the router log: Router#clear logging Clear logging buffer [confirm]<enter> Router>show logg..
BGP Internals and Verifying eBGP Neighbors
Similar to OSPF, the BGP neighbor relationship goes through a series of states over time. Although the Finite State Machine (FSM) for BGP neighbor states has many twists and turns, particularly for handling exceptions, retries, and failures, the overall process works as follows: Step 1. A router tries to establish a TCP connection with the IP address listed on a neighbor command, using well-known destination port 179. Step 2. When the three-way TCP connection completes, the..
