Chapter 5 Part 1 Dynamic Interior Routing Information Mechanisms

Designing Routing and Switching Architectures Howard C. Berkowitz  Chapter 4 reviews the internals of routing protocols to the extent relevant to network design.  Neighbors-share a common transmission medium.

Designing Routing and Switching Architectures Howard C. Berkowitz Dynamic Routing Principles  Dynamic routing is the most common source of routing information.  In network design you want to balance static and dynamic routing.

Designing Routing and Switching Architectures Howard C. Berkowitz Dynamic Routing Principles  Logical steps in path determination: Address discovery Neighbor discovery Specialization Forwarding Routing information exchange Dead detection

Designing Routing and Switching Architectures Howard C. Berkowitz Dynamic Routing Principles  Router’s Own Identity Early protocols IP address of interface sending the packet sufficient Modern Protocols Router identifier that is not associated with a physical interface. Uniquely identifies the router Determining the router’s identity is the first step of initialization. Router builds internal data structures Starts to load its tables with information about directly connected media.

Designing Routing and Switching Architectures Howard C. Berkowitz Dynamic Routing Principles Generates a preliminary set of routes and submits them to the RIB installation process. After local routes are installed, or in parallel to their installation, the router attempts to find neighbors and exchange information with them.

Designing Routing and Switching Architectures Howard C. Berkowitz

Designing Routing and Switching Architectures Howard C. Berkowitz Dynamic Routing Principles  Information to be exchanged Dynamic routing protocol The existence of neighboring routers The existence of reachable destinations. The desirability of paths to reachable destinations(might)

Designing Routing and Switching Architectures Howard C. Berkowitz

Designing Routing and Switching Architectures Howard C. Berkowitz Dynamic Routing Principles  Path Calculation and Routing Information Flooding. Two algorithms Distance Vector  RIP, IGRP(Interior Gateway Routing Protocol), EIGRP(Enhanced)-Diffusing update algorithm(DUAL) Link-state  OSPF(Open Shortest Path First), IS-IS(Intermediate System- to-Intermediate System.

Designing Routing and Switching Architectures Howard C. Berkowitz Dynamic Routing Principles Area Collection of routers and routes. Boundary defines functional area. Distance vector Each network entity generates a map of its paths to all connected nodes in the network.  Create cost tables that contain the router’s perception of the cost to all destinations. Only sends this map to adjacent neighbors Link-State Mirror image of distance vector. Create cost maps only of the paths to their adjacent neighbors Flood entire routing domain(or area) with these maps. More resource intensive.

Designing Routing and Switching Architectures Howard C. Berkowitz Dynamic Routing Principles Distance vector RIP, IGRP  No idea which router generated a route  No idea when route was generated.  Old data to build tables that generate loops. EIGRP  Measures to prevent loop formation.

Designing Routing and Switching Architectures Howard C. Berkowitz Dynamic Routing Principles Link-State OSPF  Large networks  Updates contain originator and age of originator.  Persistent topological database  Copy in each router in an area. IS-IS  Appropriate for some specialized applications  Primarily used in large internet service providers  Telecom operational networks(SONET)

Designing Routing and Switching Architectures Howard C. Berkowitz Dynamic Routing Principles RIPv1 and IGRP Small networks Converge much more slowly Cannot handle classless addressing. RIPv2 Can handle classless routes No scaling advantages over RIPv2 No real idea of route summarization.

Designing Routing and Switching Architectures Howard C. Berkowitz Dynamic Routing Principles  Convergence Exists when the router has a valid path to all reachable destinations. Factors involved in convergence Time needed to load an initial set of routes. Failure detection time after which reconvergence can begin. Time for announcement of a new route to propagate from the most distant router to the local router.

Designing Routing and Switching Architectures Howard C. Berkowitz Dynamic Routing Principles Factors involved in convergence(cont.) Rules that prevent route announcements from being accepted for a period of time. Delays before recomputing routing tables.

Designing Routing and Switching Architectures Howard C. Berkowitz

Designing Routing and Switching Architectures Howard C. Berkowitz

Designing Routing and Switching Architectures Howard C. Berkowitz Dynamic Routing Principles  Problem of overloading Protocol Mechanisms Some mechanism that started with a single function has additional functions assigned to it. First generation (RIP)-mechanism of tracking hop count two purposes Loop detection through count-to-infinity Route metric

Designing Routing and Switching Architectures Howard C. Berkowitz Dynamic Routing Principles Second-generation of distance vector (IGRP)  Deleted hop count Link-state  Do not use hop count EIGRP  Does not use hop count.

Designing Routing and Switching Architectures Howard C. Berkowitz Dynamic Routing Principles Hop Count and Loop detection Infinite loops Count-to-infinity Hop count steadily increments as the routing loop continues. Count reaches an infinite value.  RIP value is 16

Designing Routing and Switching Architectures Howard C. Berkowitz Dynamic Routing Principles Split Horizon  Mechanism to prevent routing loops  Two types  Simple split horizon-prevents re-advertising to router where it first learned.  Poisoned reverse-sends an update back to the originator, but with an infinite metric to indicate the sending router believes the route to be down. Holddown timers  Reduces loops-takes longer to recover from failures.  After router hears that a given route is down, it does not hear any updates for the given holdown time.

Designing Routing and Switching Architectures Howard C. Berkowitz

Designing Routing and Switching Architectures Howard C. Berkowitz Dynamic Routing Principles Metrics in Modern Routing Protocols. IGRP and EIGRP  Primarily base their metrics on bandwidth OSPF  Primarily base their metrics on bandwidth in most implications.

Designing Routing and Switching Architectures Howard C. Berkowitz Dynamic Routing Principles Periodic vs. Triggered Updates:Dead Routers Periodic announcements like that are a source of overhead. Updates when something changes. RIP and IGRP  If a router does not send out updates, it can be assumed to be down. OSPF, IS-IS, EIGRP  Low overhead function called hello subprotocol

Designing Routing and Switching Architectures Howard C. Berkowitz Dynamic Routing Principles  Neighbor and Adjacency Relationships Newer protocols try to be selective to optimize the use of bandwidth. Newer protocols send updates only when they represent a significant change. They send updates only to the routers affected by the changes. OSPF sends changes only to designated routers (DRs) DRs only send the most recent significant updates to non- designated routers. EIGRP routers do not propagate updates that do not change the routing table of the router that received the update.

Designing Routing and Switching Architectures Howard C. Berkowitz Dynamic Routing Principles Peering and Workload EIGRP talks to all neighbors, but its diffusing update mechanism limits propagation of information that is not useful. DR mechanisms in OSPF and IS-IS are a good example of traffic reduction optimization in broadcast networks.

Designing Routing and Switching Architectures Howard C. Berkowitz Dynamic Routing Principles Media Issues 4 Basic medium types:  Broadcast  Nonbroadcast multiple access (NBMA)  Packet switching  Point-to-point  Demand  dialups

Designing Routing and Switching Architectures Howard C. Berkowitz Dynamic Routing Principles Stub Media Some routers serve stub networks(networks that do not have routers) Load sharing Do not need to propagate routing updates because there are no routers to process it. Default routes and local routing information is all that is needed.

Designing Routing and Switching Architectures Howard C. Berkowitz

Designing Routing and Switching Architectures Howard C. Berkowitz Dynamic Routing Principles  Sharing Information Among Routing Sources. Acceptance General process of receiving routing information Acceptance policy Rules used to accept routing information Must insure there is no feedback between routers RFC 1812 Router should be relatively suspicious in accepting data from other routing systems. Routers must be paranoid about accepting routing data from anyone and must be especially careful when they distribute routing information provided by another party.

Designing Routing and Switching Architectures Howard C. Berkowitz Dynamic Routing Principles Redistribution(term invented by Cisco) General acceptance of routing information from sources other than the current source. Importing and exporting One route learns from another when it imports/exports routes. Inject Inject foreign routes into a routing process. Must import and export for actual transfer to take place.

Designing Routing and Switching Architectures Howard C. Berkowitz Dynamic Routing Principles Importing is logically just before installation in RIB Should design the redistribution to minimize the full mutual exchange of routing information and make it heirarchical whenever possible to simplify loop prevention.

Designing Routing and Switching Architectures Howard C. Berkowitz