© university of new hampshire interoperability laboratory ipv4 routing summer training 2004 kari...

© UNIVERSITY of NEW HAMPSHIRE INTEROPERABILITY LABORATORY

IPv4 RoutingIPv4 Routing

Summer training 2004Kari Revier

Barbara Hill

UNIVERSITY of NEW HAMPSHIRE

INTEROPERABILITY LABORATORY IPv4 Routing

Presentation Goals:Presentation Goals:Presentation Goals:Presentation Goals:

•Understand what routing is and how it is used

•Compare Distance Vector and Link State protocols.

•Learn basics of OSPF and RIP

•Learn how a Routing Table works

•Understand what routing is and how it is used

•Compare Distance Vector and Link State protocols.

•Learn basics of OSPF and RIP

•Learn how a Routing Table works



What is Routing?What is Routing?What is Routing?What is Routing?

• In internetworking,[Routing is] the process of moving a packet of data from source to destination. Routing is usually performed by a dedicated device called a router. Routing is a key feature of the Internet because it enables messages to pass from one computer to another and eventually reach the target machine. Each intermediary computer performs routing by passing along the message to the next computer. Part of this process involves analyzing a routing table to determine the best path. (http://www.webopedia.com/TERM/R/routing.html)

• In internetworking,[Routing is] the process of moving a packet of data from source to destination. Routing is usually performed by a dedicated device called a router. Routing is a key feature of the Internet because it enables messages to pass from one computer to another and eventually reach the target machine. Each intermediary computer performs routing by passing along the message to the next computer. Part of this process involves analyzing a routing table to determine the best path. (http://www.webopedia.com/TERM/R/routing.html)



Routing vs. BridgingRouting vs. BridgingRouting vs. BridgingRouting vs. Bridging

What is the difference between Routing and Bridging?

What is the difference between Routing and Bridging?



Routing vs. Bridging cont.Routing vs. Bridging cont.Routing vs. Bridging cont.Routing vs. Bridging cont.

• Bridging is involved with the first two layers of the OSI model (Physical, Data link).

“A bridge is a device that connects two local-area networks (LANs), or two segments of the same LAN that use the same protocol”. (http://www.webopedia.com/TERM/B/bridge.html)

• Though Bridges (aka Switches) are faster and less expensive than routers, Bridges don’t are not aware of IP addresses and the networking layer, and thus cannot make very well informed decisions as to the best path traffic should take.

• Bridging is involved with the first two layers of the OSI model (Physical, Data link).

“A bridge is a device that connects two local-area networks (LANs), or two segments of the same LAN that use the same protocol”. (http://www.webopedia.com/TERM/B/bridge.html)

• Though Bridges (aka Switches) are faster and less expensive than routers, Bridges don’t are not aware of IP addresses and the networking layer, and thus cannot make very well informed decisions as to the best path traffic should take.



Routing vs. Bridging cont.Routing vs. Bridging cont.Routing vs. Bridging cont.Routing vs. Bridging cont.

• Routing is involved with the first three layers of the OIS model (Physical, Data link, network).

• Routers are aware and make use of IP addresses, are are able to communicate between various network and protocol types.

• Using a routing table, routers are able to keep track of complex network topologies, and thus

efficiently choose paths through the network.

• Routing is involved with the first three layers of the OIS model (Physical, Data link, network).

• Routers are aware and make use of IP addresses, are are able to communicate between various network and protocol types.

• Using a routing table, routers are able to keep track of complex network topologies, and thus

efficiently choose paths through the network.



So which one should I use?So which one should I use?So which one should I use?So which one should I use?

• Bridges are best suited for small networks of a single type and, when necessary, communication between small networks of the same type.

• Routers, though more difficult to configure, are usually the most optimal solution as they deal in layers 1-3 and, due to their routing tables, are able to better choose routes through a network, or networks, which are not necessarily of the same

type.

• Bridges are best suited for small networks of a single type and, when necessary, communication between small networks of the same type.

• Routers, though more difficult to configure, are usually the most optimal solution as they deal in layers 1-3 and, due to their routing tables, are able to better choose routes through a network, or networks, which are not necessarily of the same

type.



How does a router know where to How does a router know where to send a packet?send a packet?How does a router know where to How does a router know where to send a packet?send a packet?

•Routing Protocols

There are two types of Routing Protocols:

- Distance Vector

- Link State

•Routing Protocols

There are two types of Routing Protocols:

- Distance Vector

- Link State



Distance-Vector RoutingDistance-Vector RoutingDistance-Vector RoutingDistance-Vector Routing• Distance vector protocols count the number of devices data

must flow through to reach a destination. Each device is referred to as a 'hop', so the total number of hops between source and destination is the 'hop count'. Routes with lower hop counts are preferred

• Distance Vector algorithms require very little overhead in terms of processor power and memory, compared to Link State. Distance vector algorithms calculate only the hop count to destinations and choose best routes according to the hop count metric.

(http://www.inetdaemon.com/tutorials/internet/routing/dv_vs_ls.html)

• Distance vector protocols count the number of devices data must flow through to reach a destination. Each device is referred to as a 'hop', so the total number of hops between source and destination is the 'hop count'. Routes with lower hop counts are preferred

• Distance Vector algorithms require very little overhead in terms of processor power and memory, compared to Link State. Distance vector algorithms calculate only the hop count to destinations and choose best routes according to the hop count metric.




Example of a Distant-Vector Routing Protocol:

RIP – Routing Information Protocol

Example of a Distant-Vector Routing Protocol:

RIP – Routing Information Protocol



Where is RIP used?

RIP is used on smaller networks, with a maximum path of 15 hops. (Also known as a TTL or Time To Live of 15)

– Generally, a metric (cost) of 1 is applied to each interface on a router running RIP

Why is RIP used?

– To recognize changes in network topology automatically and advertise these changes to other routers in the network, with a quick and simple configuration.

Where is RIP used?

RIP is used on smaller networks, with a maximum path of 15 hops. (Also known as a TTL or Time To Live of 15)

– Generally, a metric (cost) of 1 is applied to each interface on a router running RIP

Why is RIP used?

– To recognize changes in network topology automatically and advertise these changes to other routers in the network, with a quick and simple configuration.



• How it works:

– Each RIP-enabled router keeps it’s own store of the routing information received through normal updates.

– Each RIP-enabled router advertises all of it’s learned and configured addresses on each interface at every normal update interval.(A configurable interval, but the default is 30 seconds)

– When a new route is received from a neighboring router, a triggered update is sent on the router’s other interfaces indicating this new route and a newly calculated metric.

– Eventually each router on the network learns the existence of every other router.

• How it works:

– Each RIP-enabled router keeps it’s own store of the routing information received through normal updates.

– Each RIP-enabled router advertises all of it’s learned and configured addresses on each interface at every normal update interval.(A configurable interval, but the default is 30 seconds)

– When a new route is received from a neighboring router, a triggered update is sent on the router’s other interfaces indicating this new route and a newly calculated metric.

– Eventually each router on the network learns the existence of every other router.



Calculating MetricCalculating MetricCalculating MetricCalculating Metric• How it works

– Each RIP-enabled router keeps it’s own store of the routing information received through normal updates, including where it learned the route from and the cost (metric) of that route

– Each time a RIP router receives a new route it transmits that information on its’ other interfaces.

– Eventually each RIP-enabled router will have a routing table which lists the cost from it self to destinations in the network.

• How it works– Each RIP-enabled router keeps

it’s own store of the routing information received through normal updates, including where it learned the route from and the cost (metric) of that route

– Each time a RIP router receives a new route it transmits that information on its’ other interfaces.

– Eventually each RIP-enabled router will have a routing table which lists the cost from it self to destinations in the network.

• G# are hosts

• R# are RIP enabled routers

• N# are individual networks

• Each outgoing interface has a metric of 1

• G# are hosts




R2 R3

R1N2

N3

N1 N4

R4

G3

G4

G1

G2



Disadvantages of RIPDisadvantages of RIPDisadvantages of RIPDisadvantages of RIP

• Not suited for large networks – due to the 15 hop max path

• Slow convergence time – It takes a while for problems to be detected and propagated throughout the network

• Counting to infinity – 16 is infinity in RIP. If a path becomes unavailable it won’t be removed from a router’s routing table until its cost has reached 16, this requires a lot of traffic and a lot of time. There are two common solutions to this : Split Horizon and Split Horizon with Poison Reverse.

• Not suited for large networks – due to the 15 hop max path

• Slow convergence time – It takes a while for problems to be detected and propagated throughout the network

• Counting to infinity – 16 is infinity in RIP. If a path becomes unavailable it won’t be removed from a router’s routing table until its cost has reached 16, this requires a lot of traffic and a lot of time. There are two common solutions to this : Split Horizon and Split Horizon with Poison Reverse.



Split HorizonSplit HorizonSplit HorizonSplit Horizon• Split Horizon prevents routing loops

from occurring in the network

• R4 joins the network and tells router R1 of its connection to N4 with a metric of 1 (the outgoing interface of R4 to R3)

• R1 notifies R2 of the newly learned route with a newly calculated metric of the original 1, plus the outgoing interface’s metric, which is also 1 = 2

• R1 does not send a message to R4 advertising this route, as R1 learned the route from R4

• Split Horizon prevents routing loops from occurring in the network



• R1 does not send a message to R4 advertising this route, as R1 learned the route from R4

• G# are hosts




• G# are hosts




R2 R3

R1N2

N3

N1 N4

R4

G3

G4

G1

G2



Split Horizon with Poisoned ReverseSplit Horizon with Poisoned ReverseSplit Horizon with Poisoned ReverseSplit Horizon with Poisoned Reverse• Split Horizon with Poisoned Reverse

further prevents routing loops from occurring in the network



• R1 also sends a triggered response to R4 that its connection to N3 is unreachable (metric 16)

• Split Horizon with Poisoned Reverse further prevents routing loops from occurring in the network



• R1 also sends a triggered response to R4 that its connection to N3 is unreachable (metric 16)

• G# are hosts




• G# are hosts




R2 R3

R1N2

N3

N1 N4

R4

G3

G4

G1

G2

© UNIVERSITY of NEW HAMPSHIRE INTEROPERABILITY LABORATORY

IntermissionIntermission

Insert cheesy movie theater music…



Link State RoutingLink State RoutingLink State RoutingLink State Routing• Link State protocols track the status and connection type (and

therefore speed) of each link, and produce a calculated metric based on these and other factors, including some set by the network administrator. Distance Vector protocols differ from link state protocols because they see all hops as equal and only the number of hops matters. Link State protocols will take a path which has more hops, but that uses a faster medium over a path using a slower medium with fewer hops.

• Because of their awareness of media types and other factors, link state protocols require more processing power (more circuit logic in the case of ASICs) and memory. Distance vector algorithms being simpler require simpler hardware.


• Link State protocols track the status and connection type (and therefore speed) of each link, and produce a calculated metric based on these and other factors, including some set by the network administrator. Distance Vector protocols differ from link state protocols because they see all hops as equal and only the number of hops matters. Link State protocols will take a path which has more hops, but that uses a faster medium over a path using a slower medium with fewer hops.

• Because of their awareness of media types and other factors, link state protocols require more processing power (more circuit logic in the case of ASICs) and memory. Distance vector algorithms being simpler require simpler hardware.




Example of a Link State Routing Protocol

OSPF – Open Shortest Path First

Example of a Link State Routing Protocol

OSPF – Open Shortest Path First



OSPF TerminologyOSPF TerminologyOSPF TerminologyOSPF Terminology

• Hello Packets: a periodic message broadcasted by a router which allows others routers to know that it is running OSPF

• Hello Interval: The time between Hello packets. By default, this is set to 10 seconds.

• Router Dead Interval: If a router does not send out a hello packet within this interval, other routers on the same network will assume that the router is no longer on that network. By default, this is set to 40 seconds.

• Router ID: A number in the form of an IP address that routers use to identify each other.

• Flooding: When a router broadcasts an LSA from another network

• AS: Autonomous System – a group of routers running the same interior gateway protocol and are controlled by an administrator.

• ASBR: Autonomous System Border Router – when a router is located between an OSPF autonomous system and a non-OSPF network.

• Hello Packets: a periodic message broadcasted by a router which allows others routers to know that it is running OSPF

• Hello Interval: The time between Hello packets. By default, this is set to 10 seconds.

• Router Dead Interval: If a router does not send out a hello packet within this interval, other routers on the same network will assume that the router is no longer on that network. By default, this is set to 40 seconds.

• Router ID: A number in the form of an IP address that routers use to identify each other.

• Flooding: When a router broadcasts an LSA from another network

• AS: Autonomous System – a group of routers running the same interior gateway protocol and are controlled by an administrator.

• ASBR: Autonomous System Border Router – when a router is located between an OSPF autonomous system and a non-OSPF network.



•What is OSPF used for?

– It advertises routing information between routers within a single AS. The OSPF protocol is based on link-state or SPF (shortest path first) technology.

– In a link-state routing protocol, each router maintains a database describing the AS’s topology. This database is referred to as the link-state database. Each participating router has an identical database.

•What is OSPF used for?

– It advertises routing information between routers within a single AS. The OSPF protocol is based on link-state or SPF (shortest path first) technology.

– In a link-state routing protocol, each router maintains a database describing the AS’s topology. This database is referred to as the link-state database. Each participating router has an identical database.

AS 1

Router A Router B Router C

AS 2

Router D Router E



What is OSPF used for Cont.

– All routers run the same algorithm, in parallel. Each router constructs a tree of shortest paths with itself as root. This shortest-path tree gives the route to each destination in the AS.

– When there are multiple routes to a destination with the same preference, traffic is equally distributed between them. One measure of preference is cost. The cost of a route is described by a single dimensionless metric.


– All routers run the same algorithm, in parallel. Each router constructs a tree of shortest paths with itself as root. This shortest-path tree gives the route to each destination in the AS.

– When there are multiple routes to a destination with the same preference, traffic is equally distributed between them. One measure of preference is cost. The cost of a route is described by a single dimensionless metric.1 1

2 2

1

1

2

2

Router A

network 1

network 4

Router C

network 3

Router DPC 2

PC 1 PC 3

PC 4

network 2

Router B




– OSPF allows sets of networks to be grouped together. The group of networks is called an area. The topology of the area is hidden from the rest of the AS. This reduces traffic that is transmitted within an AS.


– OSPF allows sets of networks to be grouped together. The group of networks is called an area. The topology of the area is hidden from the rest of the AS. This reduces traffic that is transmitted within an AS.

AS 1

Router A Router B

network 0

network 1

Router C

network 2

Router D

network 3

Router C

Area 1

Area 0

Area 2



LSA - Link State AdvertisementLSA - Link State AdvertisementLSA - Link State AdvertisementLSA - Link State Advertisement

• What is an LSA? An advertisement sent to describe the local state of a router or

network. For a router, this includes the state of the router’s

interfaces and adjacencies. • There are five common types of LSA’s:

1 Router2 network3 summary-network4 summary-ASBR5 AS-external

• The other types of LSA’s are: Group-membership, NSSA and external-attributes

• What is an LSA? An advertisement sent to describe the local state of a router or

network. For a router, this includes the state of the router’s

interfaces and adjacencies. • There are five common types of LSA’s:

1 Router2 network3 summary-network4 summary-ASBR5 AS-external

• The other types of LSA’s are: Group-membership, NSSA and external-attributes



The five common LSA’sThe five common LSA’sThe five common LSA’sThe five common LSA’s• Router LSA: Advertises a router’s active interfaces and neighbors • Network LSA: Advertises a network segment and the routers currently attached to

the network

• Summary-Network LSA: Advertises a route to a network

• Summary-ASBR LSA: Advertises a route to an ASBR

• AS-external LSA: Describes links that are learned outside of the AS

• Router LSA: Advertises a router’s active interfaces and neighbors • Network LSA: Advertises a network segment and the routers currently attached to

the network

• Summary-Network LSA: Advertises a route to a network

• Summary-ASBR LSA: Advertises a route to an ASBR

• AS-external LSA: Describes links that are learned outside of the AS



Hello PacketsHello PacketsHello PacketsHello Packets

What are Hello packets used for?

Hello packets allow routers to form adjacencies (become neighbors).

A Hello packet advertises a router’s router ID, hello interval, router dead interval, neighbor(s) (routers on the same network as itself), subnet mask, area ID, and the DR and BDR on the subnet.

A router will only recognize and make adjacencies with routers that have the same hello interval, router dead interval, subnet mask and area ID. If any of the conditions are different, the routers will ignore each other.

What are Hello packets used for?

Hello packets allow routers to form adjacencies (become neighbors).

A Hello packet advertises a router’s router ID, hello interval, router dead interval, neighbor(s) (routers on the same network as itself), subnet mask, area ID, and the DR and BDR on the subnet.

A router will only recognize and make adjacencies with routers that have the same hello interval, router dead interval, subnet mask and area ID. If any of the conditions are different, the routers will ignore each other.



DR – Designated RouterDR – Designated RouterDR – Designated RouterDR – Designated Router

What is the purpose of a DR?

The DR has two main functions: 1) The DR originates a network-LSA on behalf of the

network. This LSA lists the set of routers (including the DR itself) currently attached to the network.

2) The DR becomes adjacent to all other routers on the network. Since the link state databases are synchronized across adjacencies, the DR plays a central part in the synchronization process.

What is the purpose of a DR?

The DR has two main functions: 1) The DR originates a network-LSA on behalf of the

network. This LSA lists the set of routers (including the DR itself) currently attached to the network.

2) The DR becomes adjacent to all other routers on the network. Since the link state databases are synchronized across adjacencies, the DR plays a central part in the synchronization process.



BDR – Backup Designated RouterBDR – Backup Designated RouterBDR – Backup Designated RouterBDR – Backup Designated Router

• What is the purpose of the BDR? In order to make the transition to a new DR smoother,

there is a BDR for each network. The BDR is also adjacent to all routers on the network, and becomes DR when the previous DR fails. If there were no BDR, when a new DR became necessary, new adjacencies would have to be formed between the new DR and all other routers attached to the network. During this time, the network would not be available for transit data traffic. The BDR makes the need to form these adjacencies unnecessary, since they already exist. This means the period of disruption in transit traffic lasts only as long as it takes to flood the new LSAs (which announce the new DR).

• What is the purpose of the BDR? In order to make the transition to a new DR smoother,

there is a BDR for each network. The BDR is also adjacent to all routers on the network, and becomes DR when the previous DR fails. If there were no BDR, when a new DR became necessary, new adjacencies would have to be formed between the new DR and all other routers attached to the network. During this time, the network would not be available for transit data traffic. The BDR makes the need to form these adjacencies unnecessary, since they already exist. This means the period of disruption in transit traffic lasts only as long as it takes to flood the new LSAs (which announce the new DR).



How is a Routing Table formed?How is a Routing Table formed?How is a Routing Table formed?How is a Routing Table formed?

•A routing table is formed by advertisements that a router receives from its neighbors. Consider the diagram show below:

•A routing table is formed by advertisements that a router receives from its neighbors. Consider the diagram show below:

Router C Router D

network 3

network 4 network 2

network 1

Router A Router B1 1

3 3

4

4

2

2

PC 1

PC 2

PC 3

PC 4



Since Router A and Router B are both connected on network 0, they are able to learn each other’s routes in their routing table. They send advertisements that tell their neighbors what is in their routing table.

Why would Router A choose the path to PC 4 with a gateway of Router B???

Since Router A and Router B are both connected on network 0, they are able to learn each other’s routes in their routing table. They send advertisements that tell their neighbors what is in their routing table.

Why would Router A choose the path to PC 4 with a gateway of Router B???



Sample Routing TableSample Routing TableSample Routing TableSample Routing Table

• ERX-00-0b-e0#show ip route

• Protocol/Route type codes:

• I1- ISIS level 1, I2- ISIS level2,

• I- route type intra, IA- route type inter, E- route type external,

• i- metric type internal, e- metric type external,

• O- OSPF, E1- external type 1, E2- external type2,

• N1- NSSA external type1, N2- NSSA external type2

• L- MPLS label, V- VRF

• Prefix/Length Type Next Hop Dst/Met Intf

• ------------------ ------- --------------- ---------- ------------------------

• 10.0.0.0/8 Static 10.10.10.2 1/0 FastEthernet5/0


• 10.10.10.0/24 Connect 10.10.10.13 0/0 FastEthernet5/0




• ERX-00-0b-e0#show ip route

• Protocol/Route type codes:

• I1- ISIS level 1, I2- ISIS level2,

• I- route type intra, IA- route type inter, E- route type external,

• i- metric type internal, e- metric type external,

• O- OSPF, E1- external type 1, E2- external type2,

• N1- NSSA external type1, N2- NSSA external type2

• L- MPLS label, V- VRF

• Prefix/Length Type Next Hop Dst/Met Intf

• ------------------ ------- --------------- ---------- ------------------------









In SummaryIn SummaryIn SummaryIn Summary

• Routing protocols are used to determine the best path for traffic through a network.

• There are two types of routing protocols: Distance-Vector and Link-State

• RIP is a small, easily configurable Distance-Vector protocol, best suited for small networks.

• OSPF is a example of a Link-State protocol. It keeps a database of the topology of the entire network (which requires a large amount of data storage space), allowing for well informed calculations of the shortest path, and making it well suited for large networks which require quick convergence times.

• Routing protocols are used to determine the best path for traffic through a network.

• There are two types of routing protocols: Distance-Vector and Link-State

• RIP is a small, easily configurable Distance-Vector protocol, best suited for small networks.

• OSPF is a example of a Link-State protocol. It keeps a database of the topology of the entire network (which requires a large amount of data storage space), allowing for well informed calculations of the shortest path, and making it well suited for large networks which require quick convergence times.



ReferencesReferencesReferencesReferences

• http://www.webopedia.com

• http://www.inetdaemon.com/tutorials/internet/routing/dv_vs_ls.html

• OSPF_eric.ppt, Eric Barrett, UNH-IOL Routing Consortium

• Routing Information Protocol Version 2.ppt, Joe Scholefield, UNH-IOL Routing

Consortium

• Routing1999.ppt, Kimo Johnso, UNH-IOL Routing Consortium

• RFC 2328 (OSPFv2)

• RFC 2453 (RIPv2)

• IPtraining1.ppt and IPtraining2.ppt, Ben Schultz, UNH-IOL Routing

Consortium

© university of new hampshire interoperability laboratory ipv4 routing summer training 2004 kari...

Documents