Week Eleven Agenda Attendance Announcements Mimic Simulator Lab Assignment 4-1-2, Basic Routing and LAN Switching Configuration Review Week Ten Information Current Week Information Upcoming Assignments
Week Eleven Topics Review Week Ten Information • Interior Versus Exterior Routing Protocols • What is convergence? • Autonomous Systems • Definitions • Loop Free Path Current Week Information
Interior Versus Exterior Routing Protocols • Routing protocols designed to work inside an autonomous system are categorized as interior gateway protocols (IGPs). • Protocols that work between autonomous systems are classified as exterior gateway protocols (EGPs). • Protocols can be further categorized as either distance vector or link-state routing protocols, depending on their method of operation.
Interior Versus Exterior Routing Protocols An interior gateway protocol (IGP) is a routing protocol that is used within an autonomous system (AS). Two types of IGP. Distance-vector routing protocols each router does not possess information about the full network topology.It advertises its distances to other routers and receives similar advertisements from other routers. Using these routing advertisements each router populates its routing table. In the next advertisement cycle, a router advertises updated information from its routing table. This process continues until the routing tables of each router converge to stable values.
Interior Versus Exterior Routing Protocols Distance-vector routing protocols make routing decisions based on hop-by-hop. A distance vector router’s understanding of the network is based on its neighbors definition of the topology, which could be referred to as routing by RUMOR. Route flapping is caused by pathological conditions, hardware errors, software errors, configuration errors, intermittent errors in communications links, unreliable connections within the network which cause certain reach ability information to be repeatedly advertised and withdrawn.
Interior Versus Exterior Routing Protocols In Cisco networks, with distance vector routing protocols flapping routes can trigger routing updates with every state change. Cisco trigger updates are sent when these state changes occur. Traditionally, distance vector protocols do not send triggered updates.
Interior Versus Exterior Routing Protocols Link-state routing protocols, each node possesses information about the complete network topology. Each node then independently calculates the best next hop from it for every possible destination in the network using local information of the topology. The collection of best next hops forms the routing table for the node. This contrasts with distance-vector routing protocols, which work by having each node share its routing table with its neighbors. In a link-state protocol, the only information passed between the nodes is information used to construct the connectivity maps.
Routing Protocols • Interior routing protocols are designed for use in a network that is controlled by a single organization • RIPv1RIPv2, EIGRP, OSPF and IS-IS are all Interior Gateway Protocols
Link State Analogy • Each router has a map of the network • Each router looks at itself as the center of the topology • Compare this to a “you are here” map at the mall • The map is the same, but the perspective depends on where you are at the time
Link State Routing Protocol • The link-state algorithm is also known as Dijkstra's algorithm or as the shortest path first (SPF) algorithm • The link-state routing algorithm maintains a complex database of topology information • The link-state routing algorithm maintains full knowledge of distant routers and how they interconnect. They have a complete picture of the network
Exterior Gateway Routing Protocol An exterior routing protocol is designed for use between different networks that are under the control of different organizations • An exterior routing routes traffic between autonomous systems • These are typically used between ISPs or between a company and an ISP • BGPv4is the Exterior Gateway Protocol used by all ISPs on the Internet
What is Convergence • Routers share information with each other, but must individually recalculate their own routing tables • For individual routing tables to be accurate, all routers must have a common view of the network topology • When all routers in a network agree on the topology they are considered to have converged
Why is Quick Convergence Important? • When routers are in the process of convergence, the network is susceptible to routing problems because some routers learn that a link is down while others incorrectly believe that the link is still up • It is virtually impossible for all routers in a network to simultaneously detect a topology change.
Convergence Issues Factors affecting the convergence time include the following: • Routing protocol used • Distance of the router, or the number of hops from the point of change • Number of routers in the network that use dynamic routing protocols • Bandwidth and traffic load on communications links • Load on the router • Traffic patterns in relation to the topology change
What are Autonomous Systems? • An Autonomous System (AS) is a group of routers that share similar routing policies and operate within a single administrative domain. • An AS can be a collection of routers running a single IGP, or it can be a collection of routers running different protocols all belonging to one organization. • In either case, the outside world views the entire Autonomous System as a single entity.
Autonomous System AS Numbers • Each AS has an identifying number that is assigned by an Internet registry or a service provider. • This number is between 1 and 65,535. • AS numbers within the range of 64,512 through 65,535are reserved for private use. • This is similar to RFC 1918 IP addresses. • Because of the finite number of available AS numbers, an organization must present justification of its need before it will be assigned an AS number. • An organization will usually be a part of the AS of their ISP
Autonomous System • Each AS has its own set of rules and policies. • The AS number uniquely distinguish it from other ASs around the world.
Definitions Metricis a numeric value used by routing protocols to help determine the best path to a destination. RIP uses the metric hop count number . The lower the numeric value, the closer the destination. OSPF uses the metric bandwidth. EIGRP uses bandwidth
Definitions • Flat routing protocol is when all routing information is spread through the entire network. • Hierarchical routing protocol are typically classless link-state protocols. This means that classless means that routing updates include subnet masks in their routing updates. • Administrative distance is the measure used by Cisco routers to select the best path when there are two or more different routes to the same destination from two different routing protocols. Administrative distance defines the reliability of a routing protocol. Each routing protocol is prioritized in order of most to least reliable (believable) using an administrative distance value. A lower numerical value is preferred.
EIGRP Characteristics EIGRP is an advanced distance vector protocol that employs the best features of link-state routing.
OSPF Characteristics OSPF is the standardized protocol for routing IPv4. Since it’s initial development, OSPF has been revised to be implemented with the latest router protocols. • Developed for large networks (50 routers or more) • Must be a backbone area • Routers that operate on boundaries between the backbone and non-backbone are called, Area Border Routers (ABR) • OSPF is a link state protocol
OSPF Characteristics When the OSPF topology table is fully populated, the SPF algorithm calculates the shortest path to the destination. Triggered updates and metric calculation based on the cost of a specific link ensure quick selection of the shortest path to the destination.
OSPF Characteristics OSPF is link-state routing protocol RIP and EIGRP are distance-vector (routing by rumor) routing protocols, susceptible to routing loops, split-horizon, and other issues. OSPF has fast convergence RIP hold-down timers can cause slow convergence. OSPF supports VLSM and CIDR RIPv1 does not
OSPF Characteristics • Cisco’s OSPF metric is based on bandwidth • RIP is based on hop count • OSPF only sends out changes when they occur. • RIP sends entire routing table every 30 seconds, IGRP every 90 seconds • OSPF also uses the concept of areas to implement hierarchical routing • A large internetwork can be broken up into multiple areas for management and route summarization
OSPFCharacteristics • Two open-standard routing protocols to choose from: RIP, simple but very limited, or OSPF, robust but more sophisticated to implement. EIGRP is Cisco proprietary
OSPF Characteristics When all routers are configured into a single area, the convention is to use area 0(zero) If OSPF has more than one area, it must have an area 0 Multi-area OSPF becomes more complicated to configure and understand OSPF Routing Domain • Single Area OSPF uses only one area, usually Area 0
OSPF Characteristics 1. Flooding of link-state information The first thing that happens is that each node, router, on the network announces its own piece of link-state information to all other routers on the network. This includes who their neighboring routers are and the cost of the link between them. Example: “Hi, I’m Router A, and I can reach Router B via a T1 link and I can reach Router C via an Ethernet link.” Each router sends these announcements to all of the routers in the network.
OSPF Characteristics 4. Shortest Path First Tree This algorithm creates an SPF tree, with the router making itself the root of the tree and the other routers and links to those routers, the various branches. 5. Routing Table Using this information, the router creates a routing table.
Large OSPF Networks Large link-state table Each router maintains a LSDB for all links in the area The LSDB requires the use of memory Frequent SPF calculations A topology change in an area causes each router to re-run SPF to rebuild the SPF tree and the routing table. A flapping link will affect an entire area. SPF re-calculations are done only for changes within that area.
Issues with large OSPFNetworks Large routing table Typically, the larger the area the larger the routing table. A larger routing table requires more memory and takes more time to perform the route look-ups. Solution: Divide the network into multiple areas
OSPF Uses “Areas” Hierarchical routing enables you to separate large internetworks (autonomous systems) into smaller internetworks that are called areas. With this technique, routing still occurs between the areas (called inter-area routing), but many of the smaller internal routing operations, such as recalculating the database –re-running the SPF algorithm, are restricted within an area
OSPF Uses “Areas” Changes in one area are generally not propagated (spread) to another Route summarization is extensively used in multi-area OSPF
OSPF Router Types Internal: Routers with all their interfaces within the same area Backbone: Routers with at least one interface connected to area 0 ASBR:(Autonomous System Boundary Router): Routers that have at least one interface connected to an external internetwork (another autonomous system) ABR: (Area Border Router): Routers with interfaces attached to multiple areas.
IS - IS Characteristics • IS-IS is an Open System Interconnection (OSI) routing protocol originally specified by International Organization for Standardization (ISO) • IS-IS is a dynamic, link-state, intra-domain, interior gateway protocol (IGP) • IS-IS was designed to operate in an OSI Connectionless Network Service (CLNS) environment • It was not originally designed to work with the IP protocol
IS - IS Characteristics • Extensions were added so that IS-IS can route IP packets • IS-IS operates at Layer 3 (Network) of the OSI model • IS-IS selects routes based upon a cost metric assigned to links in the IS-IS network • A two-level hierarchy is used to support large routing domains • A large domain can be administratively divided into areas
OSPF and IS – IS Similarities • Classless • Link-state databases an Dijkstra’s algorithm • Hello packets to form and maintain adjacencies • Use areas to form hierarchical topologies • Support address summarization between areas • Link-state representation, aging, and metrics • Update, decision, and flooding processes • Convergence capabilities • Deployed on ISP backbones
IS – IS and the OSI Protocol Suite • The OSI suite of protocols were never widely implemented at the Layers 3-7 because the TCP/IP Protocols at these layers became the de-facto standard. • Layers 1 and 2 Protocols are widely used: IEEE 802.3, FDDI, IEEE 802.5, etc.
OSI Terminology • End system (ES) is any non-routing network node (host) • Intermediate system (IS) is a router • An area is a logical entity formed by a set of contiguous routers, hosts, and the data links that connect them • Domain is a collection of connected areas under a common administrative authority(think AS) • The areas are connected to form a backbone
IS – IS is Designed to be Hierarchical An OSI network is a hierarchy of these entities: • Domain -any portion of an OSI network under a common administration • Area –a part of a domain, broken up for easier management • Backbone –areas connect to other areas through the backbone
IS – IS is Hierarchical There are four levels of routing: • Level 0, routing between an ES and IS • Level 1, routing between ISs in the same area • Level 2, routing between different areas in the same domain • Level 3, routing between separate domains