Download Routing Protocol vs. Routed Protocol

Chapter 5 IP Routing
Routing Protocol
vs.
Routed Protocol
Topics
• Review
–
–
–
–
Internetworking
Path determination
Router
IP Address
• Routed and Routing Protocols
– Network protocols
• Routed
• Routing
– Interior Protocols vs. Exterior Protocols
• Routing Protocol Activity
2
Review
Router and IP Address
Internetworking
4
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 ...
5
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.
6
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
8
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
10
Router function
11
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.)
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Router function (cont.2)
Examines the network layer
address to determine the
destination network.
13
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.
14
Router function (cont.4)
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
16
network address.
ROUTED AND ROUTING
PROTOCOLS
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.
18
Network protocol operation
19
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.
20
Protocol addressing variations
21
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
24
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
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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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IGP vs. EGP
IGP
EGP
34
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
36
Link state routing
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Part II
Distance Vector Routing
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© 2004 Cisco Systems, Inc. All rights reserved.
ICND v2.2—3-39
•
•
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 Protocols
Routing 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 Classful
Classless 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 & maintenance
Implementation 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.
Distance Vector Routing Protocols
• Routers pass periodic copies of their routing table to neighboring
routers and accumulate distance vectors.
45
Sources of Information and
Discovering Routes
• Routers discover the best path to destinations from each neighbor.
46
Selecting the
Best Route with Metrics
47
Maintaining Routing Information
• Updates proceed step by step from router to router.
48
Inconsistent Routing Entries
• Each node maintains the distance from itself to each possible
destination network.
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Inconsistent Routing Entries
(Cont.)
• Slow convergence produces inconsistent routing.
50
Inconsistent Routing Entries
(Cont.)
• Router C concludes that the best path to network 10.4.0.0 is
through router B.
51
Inconsistent Routing Entries
(Cont.)
• Router A updates its table to reflect the new but erroneous
hop count.
52
Count to Infinity
• The hop count for network 10.4.0.0 counts to infinity.
53
Routing Loops
• Packets for network 10.4.0.0 bounce (loop) between
routers B and C.
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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.
Defining a Maximum
• A limit is set on the number of hops to prevent infinite loops.
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Triggered Updates
• The router sends updates when a change in its routing
table occurs.
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Route Poisoning
• Routers advertise the distance of routes that have gone down to
infinity.
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Split Horizon
• It is never useful to send information about a route back in the
direction from which the original information came.
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Poison Reverse
• Poison reverse overrides split horizon.
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Holddown Timers
• The router keeps an entry for the “possibly down state” in the
network, allowing time for other routers to recompute for this
topology change.
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Link-State Routing
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© 2004 Cisco Systems, Inc. All rights reserved.
ICND v2.2—3-62
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.
Link-State Routing Protocols
• After initial flood of LSAs, link-state routers pass small event-triggered link64
state updates to all other routers.
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 protocol
Purpose of a hello protocol:
-To discover neighbors (that use the same
link state routing protocol) on its link
Link-State Routing:
step 2 - Sending Hello Packets to Neighbors
• 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 3 - Building the Link State Packet (LSP)
• 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 4 - Flooding LSPs to Neighbors
• 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 5 - Constructing a link state data base
• 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 linkstate 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
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.
Drawbacks to Link-State Routing Protocols
• Initial discovery may cause flooding.
• Link-state routing is memory- and processorintensive.
74
How Routing Information Is
Maintained
75
Link-State Routing Protocol
Algorithms
76
Link-State Routing Protocols
Advantages of a Link-State Routing Protocol
Routing
protocol
Builds
Topological
map
Router can
independently
determine the
shortest path
to every
network.
Convergence
Event driven
routing
updates
Use
of
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
•
•
•
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
•
•
•
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.
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 Protocol
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.
83
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.
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End Chapter V