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Icnd23 s03l02
 

Icnd23 s03l02

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    Icnd23 s03l02 Icnd23 s03l02 Presentation Transcript

    • Determining IP Routes Introducing Distance Vector Routing © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-1
    • Outline • Overview • Distance Vector Route Selection • Routing Information Maintenance • Routing Inconsistencies with Distance Vector Routing Protocols • Count to Infinity Prevention • Techniques to Eliminate Routing Loops • Implementation of Techniques to Eliminate Routing Loops • Summary © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-2
    • Distance Vector Routing Protocols • Routers pass periodic copies of their routing table to neighboring routers and accumulate distance vectors. © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-3
    • Sources of Information and Discovering Routes • Routers discover the best path to destinations from each neighbor. © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-4
    • Selecting the Best Route with Metrics © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-5
    • Maintaining Routing Information • Updates proceed step by step from router to router. © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-6
    • Inconsistent Routing Entries • Each node maintains the distance from itself to each possible destination network. © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-7
    • Inconsistent Routing Entries (Cont.) • Slow convergence produces inconsistent routing. © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-8
    • Inconsistent Routing Entries (Cont.) • Router C concludes that the best path to network 10.4.0.0 is through Router B. © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-9
    • Inconsistent Routing Entries (Cont.) • Router A updates its table to reflect the new but erroneous hop count. © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-10
    • Count to Infinity • The hop count for network 10.4.0.0 counts to infinity. © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-11
    • Defining a Maximum • A limit is set on the number of hops to prevent infinite loops. © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-12
    • Routing Loops • Packets for network 10.4.0.0 bounce (loop) between Routers B and C. © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-13
    • Split Horizon • It is never useful to send information about a route back in the direction from which the original information came. © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-14
    • Route Poisoning • Routers advertise the distance of routes that have gone down to infinity. © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-15
    • Poison Reverse • Poison reverse overrides split horizon. © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-16
    • Holddown Timers • The router keeps an entry for the “possibly down state” in the network, allowing time for other routers to recompute for this topology change. © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-17
    • Triggered Updates • The router sends updates when a change in its routing table occurs. © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-18
    • Eliminating Routing Loops © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-19
    • Eliminating Routing Loops (Cont.) © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-20
    • Eliminating Routing Loops (Cont.) © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-21
    • Eliminating Routing Loops (Cont.) © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-22
    • Eliminating Routing Loops (Cont.) © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-23
    • Eliminating Routing Loops (Cont.) © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-24
    • Summary • Distance vector routing protocols generate periodic routing updates addressed to directly connected routing devices. Routers running a distance vector routing protocol send periodic updates even if there are no changes in the network. • When a router receives an update from a neighbor’s router, the router compares the update with its own routing table. The router adds the cost of reaching the neighbor’s router to the path cost reported by the neighbor to establish a new metric. • Routing inconsistencies occur if slow internetwork convergence or a new configuration causes incorrect routing entries. © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-25
    • Summary (Cont.) • Distance vector protocols define infinity as some maximum number. The routing protocol then permits the routing table update loop until the metric exceeds its maximum allowed value. • There are five techniques for eliminating routing loops on distance vector routing networks: split horizon, route poisoning, poison reverse, holddown timers, and triggered updates. • All five techniques can be used together to eliminate routing loops in area networks. © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-26
    • © 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-27