Cisco Systems Networking AcademyS2 C 11

Routing Basics

Routing Tables

• An IP routing table consists of destination network addresses and next hop pairs

• At each stop, the next destination is calculated • The network layer provides best-effort end-to-end

packet delivery across interconnected networks• After the path is selected, the router forwards the

packet

How Routers Route

• As frames are received, the data link layer header is removed and discarded and the network layer frame is sent to the appropriate network layer process

• Network protocol header is examined to determine destination of packet

• Packet is then passed back to data link layer where it is encapsulated in a new frame and queued for delivery to appropriate interface

How Routers Route - 2

• Each line between the routers has a number that routers use as a network address

• Consistency of Layer 3 addresses across internetwork improves use of bandwidth by preventing unnecessary broadcasts

• Consistent end-to-end addressing enables network layer to find a path to destination without burdening devices or links with broadcasts

Network and Host Addressing

• Network Address – path part used by router

• Host Address – specific port or device

• Destination router Ands subnet mask to network part of address to determine subnet that contains the host address

• Most network protocol addressing schemes use some form of a host or node address.

Path Selection and Packet Switching

• A router generally relays a packet from one data link to another, using two basic functions:– a path determination function – a switching function

• The switching function allows a router to accept a packet on one interface and forward it through a second interface.

• The path determination functions selects best interface to use to send out the packet

Routed and Routing Protocol

• Routed protocol used between routers to direct user traffic – examples IP, IPX

• Routing protocol used between routers to maintain routing tables – examples RIP, IGRP, EIGRP, OSPF

Network Layer Protocol Operations

• Layer 2 addresses may be changing constantly as packets work their way through network but layer 3 addresses are constant

• Each router provides its services to support upper-layer functions (CCNA says 3 levels can be supported)

• End system addresses frame using MAC address of intermediate system

Multiple Routing Protocols

• Routers can support multiple independent routing protocols– This capability allows router to deliver packets

from several routed protocols over the same data links

Static Dynamic Routes

• Static Route– Uses programmed route the network

administrator physically enters into router

• Dynamic Route– Uses route that routing protocol adjusts

automatically for topology or traffic changes

Static Routes

• Allow you to hide parts of network– Dynamic routing reveals everything known

about a network– Static routing allows you to specify information

to reveal

• Stub network (only one possible path) – conserves resources

Default Route

• Default route used when next hop is not specified in routing table– Assumes and trusts next router will have a best

path to destination or contain another default route

Why Dynamic Routing?

• An alternate route can substitute for a failed route

• Dynamic routing protocols can also direct traffic from the same session over different paths in a network for better performance– Known as load sharing

Dynamic Routing Operations

• Success depends on:– Maintenance of routing tables

– Timely distribution of knowledge in form of routing updates

• Routing Protocol describes– How to send updates

– What knowledge is contained in updates

– When to send updates

– How to locate recipients of the updates

Routing Metric ComponentsThe smaller the metric the better

• Hop Count

• Ticks

• Cost

• Bandwidth

• Delay

• Load

• Reliability

Three Classes of Routing Protocols

• Distance Vector– Determines direction and distance (hop count)

• Link State a.k.a. Shortest Path First– Recreates exact topology of entire network

• Hybrid – combination of distance vector and link state– Combines aspects of distance vector & link

state

Time to Convergence

• The time it takes all routers to share the same information about the network

• When topology changes routers must recompute routes (disrupts routing)

• Time to reconvergence varies with routing protocols

Distance Vector

• Routers pass period copies of routing tables communicating topology changes

• Each routeer receives routing tables from directly connected routers

• Accumulates network distances• Does not allow router to know exact topology of

entire network• Each router sends entire routing table

– Contains total path cost and logical address of first router on path

Routing Loops

• Occur when slow convergence causes inconsistent routing entries

• New updates contain paths to failed routes– Information is propagated to other routers– Invalid updates will continue to loop until some process

stops the looping• Condition called “COUNT TO INFINITY”

– Avoid by defining infinity (number of loops)– Hold-down timers (route marked inaccessible and hold-

down timer started) – no conflicting poorer information accepted from other routers until time expires

Split Horizon

• Incorrect information sent to a router contradicts correct information it just sent

• Split Horizon solves problem– if a routing update about Network 1 arrives

from Router A, Router B or Router D cannot send information about Network 1 back to Router A.

– Thus is reduces incorrect routing information and routing overhead

Link State Basics• Shortest Path First

– Complex database of topology information– Table maintains full knowledge of distant

routers

• Uses– 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

Link State Routing 2

• Algorithms rely on using same link-state updates– Whenever topology changes, routers share information

• Convergence achieved because each router– keeps track of its neighbors: each neighbor's name,

whether the neighbor is up or down, and the cost of the link to the neighbor.

– constructs an LSA packet that lists its neighbor router names and link costs, including new neighbors, changes in link costs, and links to neighbors that have gone down.

–

Achieving Convergence Continued

– sends out this LSA packet so that all other routers receive it.

– when it receives an LSA packet, records the LSA packet in its database so that it updates the most recently generated LSA packet from each router.

– completes a map of the internetwork by using accumulated LSA packet data and then computes routes to all other networks by using the SPF algorithm.

• Each time LSA packet caused change in link-state database, SPF (link-state algorithm) recalculates best paths & updates routing table

Link State Concerns

• Processing Requirements– Use Dijkstra’s algorithm to compute the SPF (requires

processing task proportional to number of links in network multiplied by number of routers)

• Memory Requirements• Bandwidth requirements

– During initial discovery process, all routers send LSA packets to each other – floods network and temporarily reduce bandwidth available for routed traffic

Link State Continued

• Most important aspect to to make certain all routers get necessary LSA packets

• Need to synchronize large networks to keep updates correct

• Order of router startup alters topologies learned• If LSA distribution is not done correctly, result is

invalid routes• Scaling up on large networks can expand the

problem

Comparison

• Distance Vector– Views topology from

neighbor’s view– Adds distance vectors

from router to router– Frequent, periodic

updates; slow convergence

– Copies of routing tables passed to neighbors

• Link State– Common view of

entire network topology

– Shortest path calculated to routers

– Event-triggered updates; faster convergence

– Link-state routing updates passed

Hybrid

• Balanced-hybrid routing– Uses distance vectors with more accurate

metrics– Use topology changes to trigger routing

database updates– Converges rapidly– Uses fewer resources (bandwidth & memory)

• Example is OSI’s IS-IS and Cisco EIGRP

LAN to WAN Routing

• Routers enable LAN-to-WAN packet flow by keeping the end-to-end source and destination addresses constant while encapsulating the packet in data link frames, as appropriate, for the next hop along the path.

Routers

• Devices that implement network services• Provide interfaces for wide range of links and

subnetworks at wide range of speeds• Active and intelligent network nodes that help

manage the network– Provide dynamic control over resources– Support tasks and goals for connectivity, reliable

performance, mgm control, & flexibility– Route and switch but also sequence based on priority

and filtering