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Slide 1:

Dynamic Routing and OSPF (part 1)

IP routing:

IP routing Each router or host makes its own routing decisions Sending machine does not have to determine the entire path to the destination Sending machine just determines the next-hop along the path. This process is repeated until the destination is reached Forwarding table consulted to determine the next-hop

IP routing:

IP routing Classless routing route entries include destination next-hop mask (prefix-length) indicating size of address space described by the entry Longest match for a given destination, find longest prefix match in the routing table example: destination is routing table entries are and

IP routing:

IP routing Default route where to send packets if don’t have an entry for the destination in the routing table most machines have a single default route often referred to as a default gateway

Static routing:

Static routing each router manually configured with a list of destinations and the next hop to reach those destinations ideal for small number of destinations or “stub” networks stub network - network with only one or two paths to the rest of the network

Dynamic Routing:

Dynamic Routing routers compute routing tables dynamically based on information provided by other routers in the network routers communicate topology to each other via different protocols routers then compute one or more next hops for each destination - trying to calculate the most optimal path

Static and Dynamic Routing:

Static and Dynamic Routing Static routing is a simplistic approach Shortcomings: Cumbersome to configure Cannot adapt to link/node failures, addition of new nodes and links Doesn't scale to large networks Solution: Dynamic Routing

Desirable Characteristics:

Desirable Characteristics Automatically detect and adapt to network topology changes Optimal routing Scalability Robustness Simplicity Speed of convergence Some control of routing choices (e.g. which links we prefer to use)

Convergence - Why do I care?:

Convergence - Why do I care? Convergence is when all the routers have the same routing information When a network is not converged, there is network downtime Packets don't get to where they are supposed to be going: routing loops, black holes Occurs when there is a change in the status of a router or link

Dynamic Protocols:

Dynamic Protocols Metrics can be calculated based on a single characteristic of a path or by combining multiple characteristics Metrics commonly used: Bandwidth Hop count Cost administratively defined metrics

OSPF magic exercise:

OSPF magic exercise delete your static routes config t no ip route x.x.x.x y.y.y.y z.z.z.z enter the following: router ospf 1 network x.x.x.x area 0 x.x.x.x = ip address of your backbone interface redistribute connected subnets

OSPF magic exercise:

OSPF magic exercise Verify connectivity to all PCs in the network Do not save your config

Dynamic Routing Protocols and OSPF (part 2):

Dynamic Routing Protocols and OSPF (part 2)

Types of Routing Protocols:

Types of Routing Protocols EGP Exterior Gateway Protocol Example: BGP IGP Interior Gateway Protocol Example: OSPF, RIP

Types of Routing Protocols :

Types of Routing Protocols Link-state Distance-vector


IGP Used within a single Autonomous System (AS) Within a single network

Other Interior Gateway Protocols (IGPs):

Other Interior Gateway Protocols (IGPs) RIP Lots of scaling problems RIPv1 is classful and officially obsolete RIPv2 is classless EIGRP Proprietry (Cisco only) IS/IS The forerunner of OSPF Multiprotocol (OSPF is IP only)

Distance Vector Protocols:

Distance Vector Protocols Listen to neighboring routes Install all routes in a table Advertise all routes in table Very simple Very Stupid example: RIP


RIP routing information protocol distance-vector algorithm cost is hop count broadcast information to all neighbors every 30 seconds


RIP A B D E C ROUTING TABLE for A A - B 1 C 2 D 3 E 2

Why not use RIP?:

Why not use RIP? Distance Vector algorithm Broadcasts everything (not scalable) Metric is hop-count only Infinity of 16 (not large enough) Slow convergence (routing loops) Poor robustness


OSPF Open Shortest Path First Dynamic IGP (Interior Gateway Protocol) Use within your own network Link state algorithm

Shortest Path First:

Shortest Path First A B C D 15 3 4 4 7 Metric: Link Cost

Link State Algorithm:

Link State Algorithm Each router maintains a database containing map of the whole topology Links State (including cost) All routers have the same information All routers calculate the best path to every destination

Link State Algorithm (con):

Link State Algorithm (con) Any link state changes are flooded across the network "Global spread of local knowledge”

Link State vs. Distance vector:

Link State vs. Distance vector Distance Vector views net topology from neighbor’s perspective adds distance vectors from route to router frequent, periodic updates; slow convergence passes copies of routing table to neighbor routers

Link State vs. Distance vector:

Link State vs. Distance vector Link-State gets common view of entire network topology calculates the shortest path to other routers event-triggered updates; faster convergence passes link-state routing updates to other routers

Distance Vector and Link State Protocols:

Distance Vector and Link State Protocols Distance vector routers compute the best path from information passed to them from neighbors Link State routers each have a copy of the entire network map Link State routers compute best routes from this local map

Note: Routing is not the same as Forwarding:

Note: Routing is not the same as Forwarding Forwarding: passing packets along to the next hop There is only one forwarding table Just has prefix and next-hop info Routing: populating the forwarding table You might have multiple routing databases - e.g. both OSPF and BGP Routing databases have more information

Routing and Forwarding:

Routing and Forwarding OSPF BGP Static Forwarding Table On Cisco, if the same prefix is received from multiple protocols, the "administrative distance" is used to choose between them


OSPF open shortest path first dynamic IGP not distance vector Link-State algorithm

OSPF: How it works (1):

OSPF: How it works (1) "Hello" packets sent periodically on all OSPF-enabled interfaces become "neighbors" establishes that link can carry data used to determine if neighbor is up Adjacencies (virtual point-to-point links) formed between some neighbors

How it works (2):

How it works (2) Once an adjacency is established, trade information with your neighbor Topology information is packaged in a "link state announcement" Announcements are sent ONCE, and only updated if there's a change (or every 30 minutes)

How it works (3):

How it works (3) Each router sends Link State Announcements (LSAs) over all adjacencies LSAs describe router's links, interfaces and state Each router receives LSAs, adds them into its database, and passes the information along to its neighbors

How it works (4):

How it works (4) Each router builds identical link-state database Runs SPF algorithm on the database to build SPF tree Forwarding table built from SPF tree

How it works (5):

How it works (5) When change occurs: Broadcast change All routers run SPF algorithm Install output into forwarding table


HELLO Broadcast* HELLO on network segment Receive ACK Establishes 2-way communication Repeat periodically Default: HELLO sent every 10 seconds Default: if no HELLO heard for 40 seconds, link is assumed to be dead Now establish adjacencies * Actually uses Multicast addresses (, so that non-OSPF devices can ignore the packets

The HELLO packet:

The HELLO packet Router priority Hello interval Router dead interval Network mask List of neighbors These must match HELLO HELLO HELLO


Neighbors Bi-directional communication Result of OSPF hello packets Need not exchange routing information

Who is adjacent?:

Who is adjacent? "Adjacent" neighbors exchange routing information Not all neighbors are adjacent On a point-to-point link everyone On broadcast medium not everyone why?

Broadcast neighbors:

Broadcast neighbors A B C D Order of N^2 adjacencies

Broadcast medium:

Broadcast medium Select a neighbor: Designated Router (DR) All routers become adjacent to DR Exchange routing information with the DR DR updates all the other neighbors Scales Adjacencies reduced from N^2 to 2N Backup Designated Router (BDR)

LSAs propagate along adjacencies:

LSAs propagate along adjacencies DR BDR

Other nice features of OSPF:

Other nice features of OSPF Authentication (optional) Equal-cost multipath more than one "best" path - share traffic Proper classless support (CIDR) Multiple areas For very large networks (>150 routers) Aggregate routes across area boundaries Keep route flaps within an area Proper use of areas reduce bandwidth and CPU utilisation Backbone is Area 0

Cisco OSPF commands and configuration:

Cisco OSPF commands and configuration show ip route show ip ospf neighbor show ip ospf database

Configuring OSPF:

Configuring OSPF router ospf <process-id> network x.x.x.x m.m.m.m area <area-id> m.m.m.m = wildcard mask 0 = don’t care bit 1 = check bit mask for exact match network area 0 network area 0

Classroom Layout:

A C B F E I G D H J Router PC HUB Router PC HUB Router PC HUB Router PC HUB Router PC HUB Router PC HUB Router PC HUB Router PC HUB Router PC HUB Router PC HUB SWITCH Classroom Layout

Serial Links for exercise:

A C B F E I G D H J Serial Links for exercise

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