1 Introducing Routing 1. Dynamic routing - information is learned from other routers, and routing protocols adjust routes automatically. 2. Static routing

1

Introducing Routing

1. Dynamic routing - information is learned from other routers, and routing protocols adjust routes automatically.

2. Static routing - network administrator configures information about remote networks manually. They are used to reduce overhead and for security.

Because of the extra administrative requirements, static routing does not have the scalability of dynamic routing.

In most networks static routes are often used in conjunction with a dynamic routing protocol.

In order to forward packets correctly, routers must learn the direction to remote networks.

Two types of routing:

www.btechebooks4u.blogspot.comwww.btechebooks4u.blogspot.com

2

Routing and Routed Protocols

Routing Protocols allow the routers to communicate with other routers to update and maintain tables.

Examples:

RIP, IGRP, EIGRP and OSPF

Routed Protocols provide enough information in their network layer address to allow packets to be forwarded from one host to another host based on the addressing scheme.

Examples:

IP, IPX, AppleTalk


3

Routing Protocols – Path DerteminationRouting consists of two basic mechanisms:

1. Path Determination

Router uses the routing table to determine the best path.

2. Switching (forwarding)

Accept a packet on one interface and forward it to a second interface

Routing protocols create and maintain routing tables:

Simplified routing table.

The Default router entry sends packets for any other destinations out S1.


4

This command sets a default route on a router:

Router(config)#

Static RoutesStatic routes between networks are manually configured by an administrator.

Static routes are added with the following command:

Router(config)# ip route 192.168.2.0 255.255.255.0 E0

ip route 0.0.0.0 0.0.0.0 192.168.1.1

Network Address Subnet Mask Gateway

Static routes out interfaces have an administrative distance of 0.

Static routes to next hop addresses have administrative distance of 1.

You can specify a non-default administrative distance for a static route:

Router(config)# ip route 0.0.0.0 0.0.0.0 192.168.1.1 130


5

Terms to Remember

default route Routing table entry that is used to direct frames for which a next hop is not

explicitly listed in the routing table.

static route A route that is explicitly configured and entered into the routing table. Static

routes take precedence over routes chosen by dynamic routing protocols.

hop Term describing the passage of a data packet between two network nodes

(for example, between two routers). See also hop count.

hop count Routing metric used to measure the distance between a source and a

destination. RIP uses hop count as its sole metric. See also hop and RIP.


6

Dynamic Routing – Distance Vector and Link-State

The success of dynamic routing depends on two basic router functions:

1. Maintenance of a routing table

2. Timely distribution of knowledge, in the form of routing updates, to other routers.

Dynamic routing relies on the routing protocol.Routing Protocols can be Distant Vector or Link-State.Hybrid protocols (like EIGRP) contain some elements of both.

Different routing protocols use different metrics to determine the best route to a network.

Administrative Distances are used to rate the trustworthiness of the various routing protocols.

7

Distance Vector Routing Protocols

RIP is a distance vector routing protocol: Uses hop count as its metric Each router the packet goes through is 1 hop

1 2

A B2 31

Router(config)#

Configuration Example:

router ripRouter(config-router)# network 172.16.0.0

The distance-vector routing algorithm passes complete routing tables to neighbor routers.

The neighbor routers combine the received routing table with their own routing tables.


8

Link State Routing ProtocolsLink-state routing algorithms (Shortest Path First algorithms), maintain a complex database of topology information.

Link-state routing uses:

Link-state routing requires more memory and processing power than distance vector, and bandwidth requirements are often higher as well.OSPF is the most commonly used Link-State Protocol.

Link-state advertisements (LSAs)

A topological database

The SPF algorithm, and the resulting SPF tree

A routing table of paths and ports to each network


9

Distance Vector or Link State?1. Also known as Bellman-Ford algorithms2. Flood routing information to all routers 3. Requests routing information from directly connected neighbors4. Complete view of the internetwork topology 5. Decisions based upon information provided by neighbors 6. Use fewer system resources7. When a network link changes state LSA are flooded through network8. Less errors, but they use more system resources9. Calculate the shortest path to all known sites on the network 10. Small update packets contain only changes 11. Slower convergence12. OSPF and IS-IS13. Do not scale well to larger systems. 14. Because they converge more quickly less prone to routing loops 15. Event-triggered updates, so convergence is fast16. Based on finding the number of hops and direction to a link 17. Passes copies of complete routing table on a periodic basis18. Each router simply inform its neighbors of its routing table19. RIP and IGRP20. more reliable, easier to debug, and less bandwidth-intensive

DVLSDVLSDVDVLSLSLSLSDVLSDVLSLS

LS

DVDVDVDV

10

Routing Protocols

Protocol Features

Distance vector, hop count metric, maximum 15 hops, broadcasts updates every 30 secs.

Cisco proprietary distance vector, bandwidth / load / delay / reliability composite metric, broadcast updates every 90 secs.

Cisco proprietary, enhanced distance vector (hybrid), load balancing, uses DUAL to calculate shortest path.

Routing updates are triggered by topology changes.

Link-state, open standard, Uses SPF algorithm. Routing updates are sent as topology changes occur.

Distance vector exterior routing protocol, used between ISPs, used to route traffic between ASs.

RIP

IGRP

EIGRP

OSPF

BGP


11

Autonomous SystemsAS is a collection of networks under a common administration and sharing a common routing strategy. ARIN, ISP, or an administrator assigns the 16 bit AS number.IGRP, EIGRP and BGP require assignment of a unique AS number.

AS 10

AS 20

Each AS has its own set of rules and policies.The AS number uniquely distinguish it from other ASs around the world.

ASs divide the global internetwork into smaller, more manageable networks.


Documents

1 Introducing Routing 1. Dynamic routing - information is learned from other routers, and routing protocols adjust routes automatically. 2. Static routing