Routers zijn machines die altijd twee of meer netwerk kaarten bevatten. Elke kaart is dan verbonden met een eigen subnet en heeft een eigen IP adres.

• Routers hebben 2 of meer netwerkkaarten• Routers 'zien' de netwerkadressen van andere verbonden netwerken• Routers kennen redundante paden en een methode om het meest optimale pad te kiezen.

Subnet A

Subnet B

Subnet C

Werking Routers

Router ontvangt een packet van een van de aangesloten subnets

IP-laag ontvangt packet(s) en stelt zonodig het packet opnieuw samen

Check de bestemming aan de hand van het IP-adres (negeer als de bestemming hetzelfde subnet heeft als de source)

Raadpleeg de routetabel voor de richting waarin het packet verstuurt moet worden.

Stuur het packet door via de network access layer naar de kaart die met het juiste subnet verbonden is.

Statische / dynamische routering

Statische routing

De beheerder voert met de hand de routetabel in en onderhoud deze ook handmatig.

Dynamische routing

Speciale protocollen (zoals RIP) verzamelen zelf routing informatie en vullen de routing tabel hiermee.

Routing

Taak van het routeren (network layer) is het afleveren van een bericht op het juiste lokale (sub)net. Om deze reden kan een routetabel volstaan met netwerk ID's ipv complete adreslijsten.

Een routetabel koppelt netwerk-id's aan het IP adres van de volgende hop (sprong)

In de routetabel wordt onderscheid gemaakt tussen direct gekoppelde netwerken en

netwerken bereikbaar via andere routers.

Routing Information Protocol (RIP)

• RIP is a distance vector routing protocol• Hop count is used as the metric for path selection• If the hop count is greater than 15, the packet will be

discarded• By default, routing updates are broadcast every 30

seconds

Routing Information Protocol (RIP)

• RIP prevents routing loops from continuing indefinitely by implementing a limit on the number of hops allowed in a path from the source to a destination.

• The maximum number of hops in a path is 15. • When a router receives a routing update that contains a

new or changed entry, the metric value is increased by 1 to account for itself as a hop in the path.

• If this causes the metric to be incremented beyond 15, it is considered to be infinity and the network destination is considered unreachable.

Routing Information Protocol (RIP)

• RIP sends routing-update messages at regular intervals. • When a router receives a routing update that includes

changes to an entry, it updates its routing table to reflect the new route.

• The received metric value for the path is increased by 1, and the source interface of the update is indicated as the next hop in the routing table.

• RIP routers maintain only the best route to a destination but can maintain multiple equal-cost paths to the destination.

Configuring RIP• The router rip command enables RIP as the routing

protocol. • The network command is then used to tell the router on

which interfaces to run RIP. • The routing process then associates specific interfaces

with the network addresses and begins sending and receiving RIP updates on these interfaces.

Configuring RIP• Router(config)#router rip

– Enables the RIP routing process • Router(config-router)#network network-number – Associates a network with the RIP routing process

Distance-vector protocolsmaken gebruik van distributed computation. Dit betekent dat elke router het beste pad naar een bestemming berekent, onafhankelijk van andere routers. Elke router deelt die berekening mee aan zijn buren. (# hops ...)

Vanaf het moment dat een router de informatie van zijn buren kent, kan hij vergelijken

met zijn eigen situatie. Mogelijk

kan hij nu een nieuw, beter pad

uitrekenen. Hij doet een update

van zijn routing tabel en verwittigt

zijn buren van de nieuwe situatie.

Dit proces loopt door tot het

routing domain gestabiliseerd is.

Distance Vector Routing Protocols

• There are several distance vector routing protocols in use today

• The most popular by a wide margin is Routing Information Protocol (RIP), followed by a Cisco proprietary protocol called Interior Gateway Routing Protocol (IGRP)

Distance Vector Routing Metric

MTU = Maximum Transmission Unit = max #bytes een interface kan verwerken

Distance Vector Routing Protocols

Convergentie• Convergentie is de snelheid en het vermogen, van een groep

toestellen in internetverbinding die een routingprotocol runnen, om in te stemmen met een veranderde topologie van het internetwerk.

• Een netwerk is geconvergeerd wanneer alle routers een consistente kennis en correcte routing tabellen.

• Routing loops kunnen zich voordoen wanneer inconsistente routing tabellen niet geupdated zijn door een trage convergentie in een veranderend netwerk

• Wanneer een netwerk nieut geconvergeerd is is het volgende mogelijk

– inconsistente routing tabellen

– inconsistente forwarding van traffiek

– routing loops

Routing Loops

• Invalid updates will continue to loop until some process stops the looping.

• This condition is called count to infinity. • Packets loop continuously around the

network in spite of the fundamental fact that the destination network is down.

• These packets loop through the network because of wrong information in the routing tables.

Routing Loops

• There are many different kinds of routing loops, but the simplest example of a routing loop occurs when one router believes the best path to a network is via a second router, and at the same time, the second router believes the best path to that network is through the first router

Routing Loops

• To reduce routing loops and counting to infinity, RIP uses the following techniques:– Count-to-infinity – Split horizon – Poison reverse – Holddown counters – Triggered updates

Routing Loops – Count to Infinity• Distance vector routing algorithms are self-correcting, but a

routing loop problem can require a count to infinity. • When a routing table update is received by a router in a

distance vector network, the hop count for the updated route is incremented by one.

• Without countermeasures to stop the count to infinity process, the distance vector metric of hop count increments each time the packet passes through another router.

• To avoid this prolonged problem, distance vector protocols define infinity as a specific maximum number.

• With this approach, the routing protocol permits the routing loop to continue until the metric exceeds its maximum allowed value. 

Count to Infinity

• By defining “infinity” as a certain number of hops away (for example, 16 for RIP), the protocol is essentially saying, “any route with a hop count of 16 is unreachable” so packets to that network will be dropped instead of forwarded

Routing Loops – Split-Horizon

• Another possible source for a routing loop occurs when incorrect information that has been sent back to a router contradicts the correct information that the router originally distributed.

• Split-horizon attempts to avoid this situation. • If a routing update about a network arrives from a router,

the router that receives the update cannot send information about that network back to the router that originally sent the update.

• Split-horizon thus reduces incorrect routing information and reduces routing overhead.

The split horizon rule says that a router should not advertise a network through the interface from which the update came.

Applying split horizon • R3 advertises the 10.4.0.0 network to R2.• R2 receives the information and updates its routing table.• R2 then advertises the 10.4.0.0 network to R1 out S0/0/0. R2 does not advertise

10.4.0.0 to R3 out S0/0/1, because the route originated from that interface.• R1 receives the information and updates its routing table.• Because of split horizon, R1 also does not advertise the information about network

10.4.0.0 back to R2.

Routing Loops – Split-Horizon

• The split horizon rule is based on the theory that it is not useful to send information about a route back in the

direction from which it came.

If router A receives an update from router D, it would not send that information back to router D.

Routing Loops – Route Poisoning• Route poisoning is used by various distance vector

protocols in order to overcome large routing loops and offer explicit information when a subnet or network is not

accessible. • Route poisoning accomplishes this by setting the hop

count to one more than the maximum.

The maximum hop count for RIP is 15.

Routing Loops – Route Poisoning

• Network 10.4.0.0 becomes unavailable due to a link failure. • R3 poisons the metric with a value of 16 and then sends out a triggered update stating that 10.4.0.0 is unavailable.• R2 processes that update. Because the metric is 16, R2 invalidates the routing entry in its routing table. • R2 then sends the poison update to R1, indicating that route is unavailable, again

by setting the metric value to 16. • R1 processes the update and invalidates the routing entry for 10.4.0.0 in its routing

table.

Route poisoning speeds up the convergence process as the information about 10.4.0.0 spreads through the network more quickly than waiting for the hop count to reach "infinity".

Routing Table Updates

• New routing tables are sent to neighboring routers on a regular basis (RIP updates occur every 30 seconds).

• However a triggered update is sent immediately in response to some change in the routing table.

• The router that detects a topology change immediately sends an update message to adjacent routers that, in turn, generate triggered updates notifying their adjacent neighbors of the change.

Holddown Timers• A count to infinity problem can be avoided by using

holddown timers• When a router receives an update from a neighbor

indicating that a previously accessible network is now inaccessible, the router marks the route as inaccessible and starts a holddown timer

Holddown Timers

• If at any time before the holddown timer expires an update is received from the same neighbor indicating that the network is again accessible, the router marks the network as accessible and removes the holddown timer.

• If an update arrives from a different neighboring router with a better metric than originally recorded for the network, the router marks the network as accessible and removes the holddown timer.

• If at any time before the holddown timer expires an update is received from a different neighboring router with a poorer metric, the update is ignored.

Holddown Timers

• Holddown timers help prevent counting to infinity but also increase convergence time.

• The default holddown for RIP is 180 seconds. • This will prevent any inferior route from being updated

but may also prevent a valid alternative route from being installed.

• The holddown timer can be decreased to speed up convergence but should be done with caution.

• The ideal setting would be to set the timer just longer that the longest possible update time for the internetwork.

Holddown Timers• The ideal setting would be to set the timer just longer

that the longest possible update time for the internetwork.

Set the holddown timer for just over 120 second

• DCEDCE Data Communications Equipment

Levert de clocking

WAN Access Provider

• DTEDTE Data Circuit-Terminal Equipment

Ontvangst de clocking

WAN gebruiker

LABO 1:

Connecting Devices with Different Media Types

Labo 2:

• Establishing a Console Session with HyperTerminal

• Configure PT Terminal to Establish a Console Session with a Cisco IOS Router.

http://www.umbc.edu/oit/sans/core/prod/telnet.html

Telnet : remote control met een plain text connection

SSH: secure shellencrypted connection

Tera Term Pro SSH = freewarehttp://www.umbc.edu/oit/sans/desktopsupport/downloads/pages/TermSSH.htm

voor software-verwijzing zie ook:http://www.umbc.edu/oit/sans/desktopsupport/downloads/

Configuratie filesConfiguratie files

operating system in NVRAM (non volatile RAM)

en

configuratieniet gesaved startup-config-file is weg !!

Router login

om veiligheidsredenen heeft de router 2 toegangsniveau's:

1. user-EXEC modehier wordt de status nagekekenhier zijn geen aanpassingen toegestaan

2. Priviliged EXEC modehier heeft de administrator full control over de router

Voorbeeld: Router> → → → User ModeRouter>enablePassword:Router# → → → Priviliged ModeRouter#disableRouter>

Examine Common IOS Show Commands

1. Examine show commands on a router

• Examine show commands in user EXEC mode show interfaces ۞ show interfaces serial 0/0/0 ۞ show version ....

• Examine show commands in privileged EXEC mode show arp ۞ show startup-config ۞ show running-config

2. Examine show commands on a switch

• Examine show commands that are common with a router

• Examine show commands unique to the switch show mac-address-table

IOS Configuration Modes

Examine configuration modes on a router

• Examine global configuration modeconfigure terminal → → → ?

• Examine other configuration modesinterface serial 0/0/0 → → → ?line console 0 → → → ?router rip → → → ?

rip: