【正文】 (Open Shortest Path First) 110 RIP 120 External EIGRP 170 Unknown 255 (never be used) 來自 中國最大的資料庫下載 Routing Protocols Three Classes of Routing Protocols: ? Distance Vector –determines the direction (vector) and distance to any link in the interwork。 best path based on accumulated metrics from each neighbor ? Link State – use more information to recreate the exact topology of the entire interwork or at least the partition in which the router is situated , and to determine the “best route” ? One tables keeps track of directly attached neighbors ? One tables determines the topology of the entire interwork ? One tables is used for the routing table ? Hybrid – Use aspects of distance vector and link state ? Share attributes of both distance vector and link state routing ? Choose a routing path based on distance vectors ? Converge rapidly using changebased updates 來自 中國最大的資料庫下載 Comparison between Distance Vector and Link State Routing Distance Vector ? Views topology from neighbor’s perspective ? Adds distance vectors from router to router ? Frequent, periodic updates: slow convergence ? Passes copies of routing table to neighbor routers Link State ? Gets mon view of entire work topology ? Calculates the shortest path to other routers ? Eventtriggered updates: fast convergence ? Passes linkstate routing updates to others routers 來自 中國最大的資料庫下載 Distance Vector Routing Protocols Overview of Distance Vector Routing Protocols Distance Vector Routing Process Convergence Routing Loops (1) – Causes Routing Loops (2) – Possible Solutions 來自 中國最大的資料庫下載 Overview of Distance Vector Routing Protocols ? Pass periodic copies of routing table to neighbor routers and accumulate distance vector. Routers do not know the exact topology of an interwork. ? Routers discover the best path to destinations base on accumulated metrics from each neighbor ? Advantages ? Simple and easy to understand ? Easy to configure ? Disadvantages ? Possible pinhole congestion ? Do not scale well to large interworks ? High convergence time ? Examples : RIP, IGRP 來自 中國最大的資料庫下載 Distance Vector Routing Process ? Updates proceed stepbystep from router to router. ? Each router send its entire routing table to each of its adjacent neighbors. ? After a router receives an update from neighbor router, pares the update to its own routing table. ? If a better route to a work, which learned from its neighbor, the router updates its own routing table. ? Adds the cost of reaching the neighbor router. ? Problem: Routing Loops occur if the interwork’s slow convergence on a new configuration and cause inconsistent routing entries 來自 中國最大的資料庫下載 Convergence ? Convergence Definition When all of the routers on the interwork have the correct routing information in their routing tables, the interwork has converged ? Convergence Time The time required for the interwork to converge is the convergence time ? After topology changes, routers must repute routes, which disrupts routing. ? The time and process required for router reconvergence varies in routing protocols. 來自 中國最大的資料庫下載 Routing Loops (1) Causes Causes: Counting to Infinity ? Every router is not updated close to the same time!! ? Invalid updates, inappropriate way updates ? Wrong information in the routing table ? Continuously loops packets around the work Router A Router B Router D Router C Router E Network 3 Network 4 Network 5 1 1 1 2 1 3 1 4 來自 中國最大的資料庫下載 Routing Loops (2) – Possible Solutions ? Maximum Hop Count ? Specify a maximum distance vector metric number (15 for RIP) as infinity and the work which the route referred to is considered down / unreachable ? Split Horizon ? Learn a protocol’s route on an interface, do not send information about that back out that interface ? Route Poisoning ? Router keeps an entry in its routing table for the work down state (poisoned route), allowing time for other routers to repute for this topology change ? Holddowns ? Routers ignore work update information for a route for some period ? HoldDown timers work with route poisoning 來自 中國最大的資料庫下載 Link State Routing Protocols Overview of Link State Routing Protocols Link State Network Discovery Link State Topology Changes Link State Concerns Link State Problems Link State Solutions 來自 中國最大的資料庫下載 Link State Routing Protocols ? Known as Shortest Path First (SPF) Algorithm ? Maintains full knowledge of distant routers and their intercon