Summary of Rules for Routes Advertised in BGP Updates
The following list summarizes the rules dictating which routes a BGP router sends in its Update messages: ■ Send only the best route listed in the BGP table. ■ To iBGP neighbors, do not advertise paths learned from other iBGP neighbors. ■ To eBGP neighbors, do not advertise paths for which the neighbor’s AS is already in the AS_PATH PA. ■ Do not advertise suppressed or dampened routes. ■ Do not advertise routes filtered via configuration. The first two rules..
Finding an Alternative Path to a Remote Network
When the path to a network is lost, EIGRP goes to a lot of trouble to find an alternative path. The method that EIGRP uses to find alternative paths is very reliable and very fast. The following list describes the process after a neighbor goes offline: 1. The router marks the routes that were reached by sending the traffic to that neighbor. 2. The router looks in the topology table, which has every advertisement received, to determine whether there is an alterna..
The Foundation Summary provides a convenient review of many key concepts in this chapter. If you are already comfortable with the topics in this chapter, this summary might help you recall a few details. If you just read this chapter, this review should help solidify some key facts. If you are doing your final prep before the exam, the following lists and tables are a convenient way to review the day before the exam. IP addresses are • 32-bit numbers (written as four bytes)..
A variable-length subnet mask (VLSM) is designed to allow greater use of IP address space by borrowing bits from the subnet mask and allocating them to host devices. To allow a greater number of devices to connect to the Internet and intranets, the standards bodies of various routing protocols designed an IP routing algorithm to cater to IP networks with a different subnet mask than the default used in classful networks. Note Routing algorithms that support VLSM are as foll..
Special Rules Concerning Intra-area and Interarea Routes on ABRs
OSPF has a couple of rules concerning intra-area and interarea routes that take precedence over the simple comparison of the cost calculated for the various routes. The issue exists when more than one ABR connects to the same two areas. Many designs use two routers between the backbone and each nonbackbone area for redundancy, so this design occurs in many OSPF networks. The issue relates to the fact that with two or more ABRs, the ABRsthemselves, when calculating their own ..
