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Chapter 5 IP RoutingRouting Protocol
vs.
Routed Protocol
2
Topics• Review
– Internetworking
– Path determination
– Router
– IP Address
• Routed and Routing Protocols– Network protocols
• Routed
• Routing
– Interior Protocols vs. Exterior Protocols
• Routing Protocol Activity
Review
Router and IP Address
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Internetworking
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Path determination
• Path determination is the process that the router uses to choose the next hop in the path for the packet to travel to its destination based on the link bandwidth, hop, delay ...
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Router• A router is a type of internetworking device
that passes data packets between networks, based on Layer 3 addresses.
• A router has the ability to make intelligent decisions regarding the best path for delivery of data on the network.
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IP addresses• IP addresses are implemented in software, and
refer to the network on which a device is located.
• IP addressing scheme, according to their geographical location, department, or floor within a building.
• Because they are implemented in software, IP addresses are fairly easy to change.
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Router and Bridge
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Router connections• Routers connect two or more networks, each of
which must have a unique network number in order for routing to be successful.
• The unique network number is incorporated into the IP address that is assigned to each device attached to that network.
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Router Interface
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Router function
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Router function (cont.1)
Strips off the data link header, carried by the frame.
(The data link header contains the MAC addresses of the source and destination.)
Strips off the data link header, carried by the frame.
(The data link header contains the MAC addresses of the source and destination.)
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Router function (cont.2)
Examines the network layer address to determine the destination network.
Examines the network layer address to determine the destination network.
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Router function (cont.3)
Consults its routing tables to determine which of its interfaces it will use to send the data, in order for it to reach its destination network.
Consults its routing tables to determine which of its interfaces it will use to send the data, in order for it to reach its destination network.
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Router function (cont.4)
Send the data out interface B1, the router would encapsulate the data in the appropriate data link frame.
Send the data out interface B1, the router would encapsulate the data in the appropriate data link frame.
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Router Interface example
• Interface is a router’s attachment to a network, it may also be referred to as a port. In IP routing.
• Each interface must have a separate, unique network address.
ROUTED AND ROUTING
PROTOCOLS
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Network protocols
• In order to allow two host communicate together through internetwork, they need a same network protocol.
• Protocols are like languages. • IP is a network layer protocol.
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Network protocol operation
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Routed protocol• Protocols that provide
support for the network layer are called routed or routable protocols.
• IP is a network layer protocol, and because of that, it can be routed over an internetwork.
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Protocol addressing variations
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Three important routed protocols• TCP/IP: 04 bytes
– Class A: 1 byte network + 3 bytes host– Class B: 2 bytes network + 2 bytes host– Class C: 3 bytes network + 1 byte host
• IPX/SPX: 10 bytes– 4 bytes network + 6 bytes host
• AppleTalk: 03 bytes– 2 bytes network + 1 byte host
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Non-routable protocol• Non-routable protocols are
protocols that do not support Layer 3.
• The most common of these non-routable protocols is NetBEUI.
• NetBEUI is a small, fast, and efficient protocol that is limited to running on one segment.
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Addressing of a routable protocol
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Routing table
131.108.1.0 E0
131.108.2.0 E1
131.108.3.0 E2
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Multi-protocol routing
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Classification #1: Static and Dynamic• Static routes:
– The network administrator manually enter the routing information in the router.
• Dynamic routes:– Routers can learn the information from each other
on the fly.– Using routing protocol to update routing
information.– RIP, IGRP, EIGRP, OSPF …
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Static routes
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Dynamic routes
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Static vs. dynamic routes• Static routes:
– For hiding parts of an internetwork.
– To test a particular link in a network.
– For maintaining routing tables whenever there is only one path to a destination network.
• Dynamic routes:– Maintenance of routing table.
– Timely distribution of information in the form of routing updates.
– Relies on routing protocol to share knowledge.
– Routers can adjust to changing network conditions.
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Routing protocol• Routing protocols determine
the paths that routed protocols follow to their destinations.
• Routing protocols enable routers that are connected to create a map, internally, of other routers in the network or on the Internet.
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Routed vs. Routing protocol
Routing protocols
Routing protocols
determine how ro
uted
determine how ro
uted
protocols are routed
protocols are routed
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Classification #2: IGP and EGP • Dynamic routes.• Interior Gateway Protocols (RIP, IGRP, EIGRP, OSPF):
– Be used within an autonomous system, a network of routers under one administration, like a corporate network, a school district's network, or a government agency's network.
• Exterior Gateway Protocols (EGP, BGP):– Be used to route packets between autonomous systems.
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IGPIGP
IGPIGP
IGP vs. EGP
EGPEGP
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Classification #3: DVP and LSP• Distance-Vector Protocols (RIP, IGRP):
– View network topology from neighbor’s perspective.– Add distance vectors from router to router.– Frequent, periodic updates.– Pass copy of routing tables to neighbor routers.
• Link State Protocols (OSPF):– Gets common view of entire network topology.– Calculates the shortest path to other routers.– Event-triggered updates.– Passes link state routing updates to other routers.
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Distance vector routing
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Link state routing
Part II
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© 2004 Cisco Systems, Inc. All rights reserved. ICND v2.2—3-39
Distance Vector Routing
Distance Vector Routing Protocols
• Dynamic routing protocols help the network administrator overcome the time-consuming and exacting process of configuring and maintaining static routes.
• Examples of Distance Vector routing protocols: Routing Information Protocol (RIP)
–RFC 1058.–Hop count is used as the metric for path selection. –If the hop count for a network is greater than 15, RIP cannot supply a route to that network.–Routing updates are broadcast or multicast every 30 seconds, by default.
Interior Gateway Routing Protocol (IGRP) –proprietary protocol developed by Cisco. –Bandwidth, delay, load and reliability are used to create a composite metric. –Routing updates are broadcast every 90 seconds, by default. –IGRP is the predecessor of EIGRP and is now obsolete.
Enhanced Interior Gateway Routing Protocol (EIGRP) –Cisco proprietary distance vector routing protocol. –It can perform unequal cost load balancing. –It uses Diffusing Update Algorithm (DUAL) to calculate the shortest path. –There are no periodic updates as with RIP and IGRP. Routing updates are sent only when there is a change in the topology.
Distance Vector Routing Protocols
• The Meaning of Distance Vector:
–A router using distance vector routing protocols knows 2 things:
Distance to final destination
The distance or how far it is to the destination network
Vector, or direction, traffic should be directed
The direction or interface in which packets should be forwarded
For example, in the figure, R1 knows that the distance to reach network 172.16.3.0/24 is 1 hop and that the direction is out the interface S0/0/0 toward R2.
Distance Vector Routing Protocols• Characteristics of Distance Vector routing protocols:
Periodic updates• Periodic Updates sent at regular intervals (30 seconds for RIP).
Even if the topology has not changed in several days, Neighbors
The router is only aware of the network addresses of its own interfaces and the remote network addresses it can reach through its neighbors.
It has no broader knowledge of the network topology Broadcast updates
Broadcast Updates are sent to 255.255.255.255. Some distance vector routing protocols use multicast addresses
instead of broadcast addresses. Entire routing table is included with routing update
Entire Routing Table Updates are sent, with some exceptions to be discussed later, periodically to all neighbors.
Neighbors receiving these updates must process the entire update to find pertinent information and discard the rest.
Some distance vector routing protocols like EIGRP do not send periodic routing table updates.
Distance Vector Routing Protocols Routing Protocol Algorithm:
–The algorithm is used to calculate the best paths and then send that information to the neighbors.
–Different routing protocols use different algorithms to install routes in the routing table, send updates to neighbors, and make path determination decisions.
Distance Vector Routing ProtocolsRouting Protocol Characteristics
–Criteria used to compare routing protocols includes Time to convergence
Time to convergence defines how quickly the routers in the network topology share routing information and reach a state of consistent knowledge. The faster the convergence, the more preferable the protocol.
Scalability Scalability defines how large a network can become based on the routing protocol that is deployed. The larger the network is, the more scalable the routing protocol needs to be.
Resource usage Resource usage includes the requirements of a routing protocol such as memory space, CPU utilization, and link bandwidth utilization. Higher resource requirements necessitate more powerful hardware to support the routing protocol operation
Classless (Use of VLSM) or ClassfulClassless routing protocols include the subnet mask in the updates. This feature supports the use of Variable Length Subnet Masking (VLSM) and better route summarization.
Implementation & maintenanceImplementation and maintenance describes the level of knowledge that is required for a network administrator to implement and maintain the network based on the routing protocol deployed.
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• Routers pass periodic copies of their routing table to neighboring routers and accumulate distance vectors.
Distance Vector Routing Protocols
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• Routers discover the best path to destinations from each neighbor.
Sources of Information and Discovering Routes
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Selecting the Best Route with Metrics
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• Updates proceed step by step from router to router.
Maintaining Routing Information
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• Each node maintains the distance from itself to each possible destination network.
Inconsistent Routing Entries
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• Slow convergence produces inconsistent routing.
Inconsistent Routing Entries (Cont.)
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• Router C concludes that the best path to network 10.4.0.0 is through router B.
Inconsistent Routing Entries (Cont.)
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• Router A updates its table to reflect the new but erroneous hop count.
Inconsistent Routing Entries (Cont.)
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• The hop count for network 10.4.0.0 counts to infinity.
Count to Infinity
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• Packets for network 10.4.0.0 bounce (loop) between routers B and C.
Routing Loops
Routing Loops
• Routing loops can eliminate
–Defining a maximum metric to prevent count to infinity
–Holddown timers
–Split horizon
–Route poisoning or poison reverse
–Triggered updates
• Note: The IP protocol has its own mechanism to prevent the possibility of a packet traversing the network endlessly. IP has a Time-to-Live (TTL) field and its value is decremented by 1 at each router.
–If the TTL is zero, the router drops the packet.
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• A limit is set on the number of hops to prevent infinite loops.
Defining a Maximum
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• The router sends updates when a change in its routing table occurs.
Triggered Updates
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• Routers advertise the distance of routes that have gone down to infinity.
Route Poisoning
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• It is never useful to send information about a route back in the direction from which the original information came.
Split Horizon
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• Poison reverse overrides split horizon.
Poison Reverse
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• The router keeps an entry for the “possibly down state” in the network, allowing time for other routers to recompute for this topology change.
Holddown Timers
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© 2004 Cisco Systems, Inc. All rights reserved. ICND v2.2—3-62
Link-State Routing
Link-State Routing• Link state routing protocols
– -Also known as shortest path first algorithms
-These protocols built around Dijkstra’s SPF
OSPF will be discussed in Chapter 11, and IS-IS will be discussed in CCNP.
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• After initial flood of LSAs, link-state routers pass small event-triggered link-state updates to all other routers.
Link-State Routing Protocols
Link-State Routing
• Dikjstra’s algorithm also known as the shortest path first (SPF) algorithm
–This algorithm accumulates costs along each path, from source to destination.
Link-State Routing: Step 1 – Learn about directly connected Networks
• Link
This is an interface on a router
• Link state
This is the information about the state of the
links
Link-State Routing: step 2 - Sending Hello Packets to Neighbors
• Link state routing protocols use a hello protocolPurpose of a hello protocol:
-To discover neighbors (that use the same link state routing protocol) on its link
• Connected interfaces that are using the same link state routing protocols will exchange hello packets.
• Once routers learn it has neighbors they form an adjacency
– 2 adjacent neighbors will exchange hello packets
– These packets will serve as a keep alive function
Link-State Routing: step 2 - Sending Hello Packets to Neighbors
• Contents of LSP:– State of each directly connected link– Includes information about neighbors such as neighbor ID, link type, & bandwidth.
• A simplified version of the LSPs from R1 is:
–1. R1; Ethernet network 10.1.0.0/16; Cost 2–2. R1 -> R2; Serial point-to-point network; 10.2.0.0/16; Cost 20–3. R1 -> R3; Serial point-to-point network; 10.3.0.0/16; Cost 5–4. R1 -> R4; Serial point-to-point network; 10.4.0.0/16; Cost 20
Link-State Routing: step 3 - Building the Link State Packet (LSP)
• Once LSP are created they are forwarded out to neighbors.
–Each router floods its link-state information to all other link-state routers in the routing area.
–Whenever a router receives an LSP from a neighboring router, it immediately sends that LSP out all other interfaces except the interface that received the LSP.
–This process creates a flooding effect of LSPs from all routers throughout the routing area.
Link-State Routing: step 4 - Flooding LSPs to Neighbors
• LSPs are sent out under the following conditions– Initial router start up or routing process– When there is a change in topology
• including a link going down or coming up, or a neighbor adjacency being established or broken
Link-State Routing: step 4 - Flooding LSPs to Neighbors
• Routers use a database to construct a topology map of the network
–After each router has propagated its own LSPs using the link-state flooding process, each router will then have an LSP from every link-state router in the routing area.
–These LSPs are stored in the link-state database.
–Each router in the routing area can now use the SPF algorithm to construct the SPF trees that you saw earlier.
Link-State Routing: step 5 - Constructing a link state data base
Link-State Routing: step 5 - Constructing a link state data base
router R1 has learned the link-state information for each router in its routing area.
With a complete link-state database, R1 can now use the database and the shortest path first (SPF) algorithm to calculate the preferred path or shortest path to each network.
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Drawbacks to Link-State Routing Protocols
• Initial discovery may cause flooding.
• Link-state routing is memory- and processor-intensive.
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How Routing Information Is Maintained
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Link-State Routing Protocol Algorithms
Link-State Routing ProtocolsAdvantages of a Link-State Routing Protocol
Routingprotocol
BuildsTopological
map
Router can independently determine the shortest path
to every network.
ConvergenceEvent driven
routing updates
Useof
LSP
Distance vector
No No Slow Generally No No
Link State Yes Yes Fast Generally Yes Yes
Link-State Routing Protocols• There are several advantages of link-state routing protocols compared to distance vector routing
protocols.
• Builds a Topological Map• Link-state routing protocols create a topological map, or SPF tree of the network topology.
• Using the SPF tree, each router can independently determine the shortest path to every network.
• Distance vector routing protocols do not have a topological map of the network.
• Routers implementing a distance vector routing protocol only have a list of networks, which includes the cost (distance) and next-hop routers (direction) to those networks.
• Fast Convergence• When receiving a Link-state Packet (LSP), link-state routing protocols immediately flood the LSP out all
interfaces except for the interface from which the LSP was received.
• A router using a distance vector routing protocol needs to process each routing update and update its routing table before flooding them out other interfaces, even with triggered updates.
• Event-driven Updates• After the initial flooding of LSPs, link-state routing protocols only send out an LSP when there is a change in the
topology. The LSP contains only the information regarding the affected link.
• Unlike some distance vector routing protocols, link-state routing protocols do not send periodic updates.
• Hierarchical Design• Link-state routing protocols such as OSPF and IS-IS use the concept of areas. Multiple areas create a hierarchical
design to networks, allowing for better route aggregation (summarization) and the isolation of routing issues within an area.
Link-State Routing Protocols
Requirements for using a link state routing protocol• Memory requirements
– Typically link state routing protocols use more memory • Processing Requirements
– More CPU processing is required of link state routing protocols• Bandwidth Requirements
– Initial startup of link state routing protocols can consume lots of bandwidth– This should only occur during initial startup of routers, but can also be an issue on unstable networks.
Link-State Routing Protocols• Modern link-state routing protocols are designed to
minimize the effects on memory, CPU, and bandwidth. • The use and configuration of multiple areas can reduce
the size of the link-state databases. Multiple areas can also limit the amount of link-state information flooding in a routing domain and send LSPs only to those routers that need them.
• For example, when there is a change in the topology, only those routers in the affected area receive the LSP and run the SPF algorithm.
• This can help isolate an unstable link to a specific area in the routing domain.
• In the figure, If a network in Area 51 goes down, the LSP with the information about this downed link is only flooded to other routers in that area.
• Routers in other areas will learn that this route is down, but this will be done with a type of link-state packet that does not cause them to rerun their SPF algorithm. Note: Multiple areas with
OSPF and IS-IS are discussed in CCNP
Link-State Routing Protocols
• 2 link state routing protocols used for routing IP -Open Shortest Path First (OSPF)-Intermediate System-Intermediate System (IS-IS)
Routing ProtocolRouting Protocol
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RIP • Most popular.
• Interior Gateway Protocol.
• Distance Vector Protocol.
• Only metric is number of hops.
• Maximum number of hops is 15.
• Updates every 30 seconds.
• Doesn’t always select fastest path.
• Generates lots of network traffic.
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IGRP and EIGRP • Cisco proprietary.• Interior Gateway Protocol.• Distance Vector Protocol.• Metric is compose of bandwidth, load, delay
and reliability.• Maximum number of hops is 255.• Updates every 90 seconds.• EIGRP is an advanced version of IGRP, that is
hybrid routing protocol.
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OSPF • Open Shortest Path First.
• Interior Gateway Protocol.
• Link State Protocol.
• Metric is compose of cost, speed, traffic, reliability, and security.
• Event-triggered updates.
End Chapter V