routing and routing protocols ccna 2 – chapter 6

Routing and Routing Protocols

CCNA 2 – Chapter 6

Routing Types

Static Route Operation

Hoboken#show ip route

Codes: C - connected, S - static,

S 172.16.1.0/24 [1/0] is directly connected, Serial0

C 192.168.2.0/24 is directly connected, Ethernet0

ip route command

RTR(config)# ip route prefix mask {address | interface} [distance]

prefix IP route prefix for the destination.

mask Prefix mask for the destination.

address IP address of the “next hop” that can be used to reach that network.

interfaceNetwork interface to use (exit-interface)

distance (Optional) An administrative distance.

Static Route Operation

• If the exit interface (gateway) is “down” the static route will not be put in the routing table.

Administrative Distance and Metric

[ administrative distance / routing metric (or cost) ]

The cost for all static routes is “0”

The default administrative distance for static routes is “1”

Hoboken#show ip route

Codes: C - connected, S - static,

S 172.16.1.0/24 [1/0] is directly connected, Serial0

C 192.168.2.0/24 is directly connected, Ethernet0

Administrative Distance

Administrative Distance is the “trustworthiness” of the routing information.Lower the administrative distance the more trustworthy the information.If the router hears about a route to the same network from more than one source it

will use the administrative distance to decide which route to put in the routing table.

Examples

Examples

• The network 0.0.0.0 and mask 0.0.0.0 are known as a “default route”

• Can be written 0.0.0.0/0

• Known as a “quad zero” route”

Static Routing

Configuring static routes

Routers do not need to configure static routes for their own directly connected networks.

We need to configure static routes for networks this router needs to reach.

We will need to configure static routes for the other routers as well, as “routing information about a path from one network to another does not provide routing information about the reverse, or return path.”

Convergence – When all the routers in the network (AS) have accurate and consistent information, so that proper routing and packet forwarding can take place.

Convergence will not happen until all the routers have complete and accurate routing information, meaning we must configure static routes on all the routers before packets will be correctly delivered.

Recursive LookupThe router knows it can get to 192.168.1.0/24 network by forwarding the packets

to the router at the ip address of 172.16.0.2

How does the router know how to get to the ip address 172.16.0.2?

RTA(config)#ip route 192.168.1.0 255.255.255.0 172.16.0.2

RTA#show ip route

Codes: C - connected, S - static,

C 172.16.0.0/16 is directly connected, Serial0

S 192.168.1.0/24 [1/0] via 172.16.0.2

C 192.168.2.0/24 is directly connected, Ethernet0

172.16.0.0/16 192.168.1.0/24

.1 .1.2 .2

RTA RTB RTC192.168.2.0/24

.1 .1

10.1.0.0/16

s0 s0 s1s1e0 e0

1

2

Static Routing

Static routes in the real-world

In the example above, there is only one route, link, between ABC’s network and the ISP.

When there is only a single route to a network, this is known as a stub network.

It is very common for the ISP to have a static route pointing to it’s customers’ networks, in this case ABC.

ABC

ISP

10.1.1.2/24

10.1.1.1/24

172.16.0.0/16

ip route 172.16.0.0 255.255.0.0 10.1.1.2

Common uses for Static Routes

Static routes in the real-world (continued)What about ABC and sending packets to the ISP – packets going to the Internet?It is also common for customer networks to use a special kind of static route, known as a default

static route.Of course we will examine this later throughout the rest of this course, but for now we specify the

network and mask as “0.0.0.0 0.0.0.0” (pronounced “quad-zero”).This tells the router to forward all packets to this next-hop address (or exit interface) that do not

have an explicit route in the routing table.

ABC

ISP

10.1.1.2/24

10.1.1.1/24

172.16.0.0/16

ip route 172.16.0.0 255.255.0.0 10.1.1.2

ip route 0.0.0.0 0.0.0.0 10.1.1.1

Default

Common uses for Static Routes

Any packets not matching the routes 172.16.0.0/16 or 10.1.1.0/24 are sent to the router 10.1.1.1 – where it is now their “problem.”

ABC

ISP

10.1.1.2/24

10.1.1.1/24

172.16.0.0/16

ip route 172.16.0.0 255.255.0.0 10.1.1.2

ip route 0.0.0.0 0.0.0.0 10.1.1.1

RTB#show ip route

Gateway of last resort is 10.1.1.1 to network 0.0.0.0

C 172.16.0.0/16 is directly connected, Ethernet0

10.0.0.0/24 is subnetted, 1 subnets

C 10.1.1.0 is directly connected, Serial1

S* 0.0.0.0/0 [1/0] via 10.1.1.1

Default

Common uses for Static Routes

Summarizing static routes

There are many times when a single static route can replace several static routes.

In other words, summarizing several static routes into a single static route.

Host 1

Host 2 Host 3

Hub

Hub Hub

S0 S0

S0 S1

E0 E0

E0

172.16.2.0/24

172.16.3.0/24

172.16.1.0/24

192.168.1.0/24

192.168.2.0/24

SanJose2

SanJose1

Baypointe

.1

.1

.1

.1 .1

.2.2

172.16.0.0/24

Baypointe

The three static routes can be summarized into a single route:

Baypointe(config)# ip route 172.16.1.0 255.255.255.0 192.168.1.2

Baypointe(config)# ip route 172.16.2.0 255.255.255.0 192.168.1.2

   Baypointe(config)# ip route 172.16.3.0 255.255.255.0 192.168.1.2

Summarized route:

Baypointe(config)# ip route 172.16.0.0 255.255.0.0 192.168.1.2

The summarized route will now include all three subnets!

Be sure to use the proper mask – 255.255.0.0!

Using a 255.255.255.0 mask will only route for 172.16.0.0/24 subnet and not 172.16.1.0/24, 172.16.2.0/24 or 172.16.3.0/24.

Summarizing static routes

BaypointeSummarized route:

  Baypointe(config)# ip route 172.16.0.0 255.255.0.0 192.168.1.2

Advantages:

Fewer routes in the routing table – faster routing table lookup.

Subnets can be added and deleted on 172.16.0.0 network without having to change static route on Baypointe router.

Host 1

Host 2 Host 3

Hub

Hub Hub

S0 S0

S0 S1

E0 E0

E0

172.16.2.0/24

172.16.3.0/24

172.16.1.0/24

192.168.1.0/24

192.168.2.0/24

SanJose2

SanJose1

Baypointe

.1

.1

.1

.1 .1

.2.2

172.16.0.0/24

Summarizing static routes

Verify static routesCopy running-config startup-config

Dynamic Routing Protocols

Routed Protocols vs. Routing Protocols

Autonomous Systems

• An autonomous system (AS) is a collection of networks under a common administration sharing a common routing strategy.

• To the outside world, an AS is viewed as a single entity. The AS may be run by one or more operators while presenting a consistent view of routing to the external world.

• The American Registry of Internet Numbers (ARIN), a service provider, or an administrator assigns an identifying 16-bit number to each AS.

Routing Protocols

• The goal of a routing protocol is to build and maintain the routing table.

• This table contains the learned networks and associated ports for those networks.

• Routers use routing protocols to manage information received from other routers, information learned from the configuration of its own interfaces, along with manually configured routes.

Types of Routing Protocols

• Distance Vector: RIP, IGRP, EIGRP

• Link State: OSPF, IS-IS

• Path Vector: BGP

• Note: IGRP and EIGRP are Cisco Proprietary

Distance Vector Routing Protocols

• “Routing by rumor”

• Each router receives a routing table from its directly connected neighbor routers.

Router B receives information from Router A.

Router B adds a distance vector number (such as a number of hops), which increases the distance vector.

Then Router B passes this new routing table to its other neighbor, Router C.

This same step-by-step process occurs in all directions between neighbor routers.

Distance Vector Routing Protocols

Distance Vector Routing Protocols

• Routing table updates occur when the topology changes. As with the network discovery process, topology change updates proceed step-by-step from router to router.

• With some routing protocols routing tables updates happen on a periodic basis.

Link State Routing Protocol Operations

• Link-state advertisements (LSAs) – A link-state advertisement (LSA) is a small packet of routing information that is sent between routers.

• Topological database – A topological database is a collection of information gathered from LSAs.

• SPF algorithm – The shortest path first (SPF) algorithm is a calculation performed on the database resulting in the SPF tree.

• Routing tables – A list of the known paths and interfaces.

Link State Routing Protocol Operations

Path Determination

A router determines the path of a packet from one data link to another, using two basic functions:

• A path determination function

• A switching function

Path Determination

• The switching function is the internal process used by a router to accept a packet on one interface and forward it to a second interface on the same router.

• A key responsibility of the switching function of the router is to encapsulate packets in the appropriate frame type for the next data link.

Configuring Dynamic Routing

Configuring Dynamic Routing

GAD(config)#router rip

GAD(config-router)#network 172.16.0.0

Network command two things:Tells the router which interfaces that will participate in this dynamic routing

protocol, which interfaces it will send and receive routing updates on.Tells other routers the networks in its routing updates that it is directly

connect to.

• The network command is used on only directly connected networks.

• With RIP and IGRP, only need to use the classful address (no subnets).

Router(config)#router rip

Router(config-router)#network 172.16.0.0

Router(config-router)#network 160.89.0.0

Distance Vector vs. Link State

Requests routing information from directly connected neighbors

Slower Convergence Decisions based upon

information provided by neighbors  

Flood routing information to all routers

Event-triggered updates, so convergence is fast

Complete view of the internetwork topology