To reduce the amount of routing information that must be maintained and transmitted by routers, hierarchical routing architecture is used to partition the Internet into smaller-sized entities known as autonomous systems (see Figure 4-1). An autonomous system (AS) is a collection of networks under a common administration that share a common routing strategy. An AS is divided into one or more smaller entities known as routing domains. A routing domain is a set of networks that ..
Disabling Sync and Using BGP on All Routers in an AS
A second method to overcome the black-hole issue is to simply use BGP to advertise all the BGP learned prefixes to all routers in the AS. Because all routers know the prefixes, sync can be disabled safely. The downside is the introduction of BGP onto all routers, and the addition of iBGP neighbor connections between each pair of routers. (In an AS with N routers, N(N−1)/2 neighbor connections will be required.) With large autonomous systems, BGP performance and convergence ..
Any router in active state for a route must wait for a Reply to each of its Query messages. It is possible for a router to wait several minutes for all the replies, because neighboring routers might also need to go active, and then their neighbors might need to go active, and so on—each withholding its Reply message until it in turn receives all of its Reply messages. In normal operation, the process should complete; to handle exception cases, EIGRP includes a timer called ..
Forming PIM Adjacencies Using PIM Hello Messages
PIM routers form adjacencies with neighboring PIM routers for the same general reasons, and with the same general mechanisms, as many other routing protocols. PIMv2, the current version of PIM, sends Hello messages every 30 seconds (default) on every interface on which PIM is configured. By receiving Hellos on the same interface, routers discover neighbors, establish adjacency, and maintain adjacency. PIMv2 Hellos use IP protocol number 103 and reserved multicast destination ..
Adjusting for Daylight Saving Time
Some areas, such as most of North America and Europe, have consistent and common rules for when to switch between winter or Standard time and summer or Daylight Saving Time. The traditional North American rule, and the default for Cisco routers when they are configured for summer time, is to move an hour ahead at 2:00 AM on the first Sunday in April, and back an hour at 2:00 AM on the last Sunday in October. Router#configure terminal Enter configuration commands, one per li..
