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Routing and Routing Protocols
CCNA 2 v3 – Module 6
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Introducing Routing
In order to forward packets correctly, routers must learn the
direction to remote networks.
Two types of routing:
1.
2.
Dynamic routing - information is learned from other routers,
and routing protocols adjust routes automatically.
Static routing - network administrator configures information
about remote networks manually. They are used to reduce
overhead and for security.
Because of the extra administrative requirements, static routing
does not have the scalability of dynamic routing.
In most networks static routes are often used in conjunction with a
dynamic routing protocol.
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Routing and Routed Protocols
Routing Protocols allow the routers to communicate with
other routers to update and maintain tables.
Examples:
RIP, IGRP, EIGRP and OSPF
Routed Protocols provide enough information in their network
layer address to allow packets to be forwarded from one
host to another host based on the addressing scheme.
Examples:
IP, IPX, AppleTalk
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Routing Protocols – Path Determination
Routing consists of two basic mechanisms:
1. Path Determination
Router uses the routing table to determine the best path.
2. Switching (forwarding)
Accept a packet on one interface and forward it to a second interface
Routing protocols create and maintain routing tables:
Simplified routing table.
The Default router entry
sends packets for any
other destinations out S1.
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Static Routes
Static routes between networks are manually configured by an administrator.
Static routes are added with the following command:
Router(config)# ip route 192.168.2.0 255.255.255.0 E0
Network Address
Subnet Mask
Gateway
This command sets a default route on a router:
Router(config)# ip route 0.0.0.0 0.0.0.0 192.168.1.1
Static routes to next hop addresses have administrative distance of 1.
Static routes out interfaces have an administrative distance of 0.
You can specify a non-default administrative distance for a static route:
Router(config)# ip route 0.0.0.0 0.0.0.0 192.168.1.1 130
Do the 2 e-Labs of 6.1.2
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Configuring Static Routes
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Step 1 Determine all desired prefixes, masks, and
addresses. The address can be either a local interface or a
next hop address that leads to the desired destination.
Step 2 Enter global configuration mode.
Step 3 Type the ip route command with a prefix and mask
followed by the corresponding address from Step 1. The
administrative distance is optional.
Step 4 Repeat Step 3 for all the destination networks that
were defined in Step 1.
Step 5 Exit global configuration mode.
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Configuring Static Routes: An Example
Do 6.1.3 e-Lab
Do 6.1.5 e-Lab
or 172.16.2.1
or 172.16.4.2
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Configuring Default Route Forwarding

Default routes are used to route packets with destinations
that do not match any of the other routes in the routing table.
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Dynamic Routing – Distance Vector and Link-State
The success of dynamic routing depends on two basic router
functions:
1.
Maintenance of a routing table
2.
Timely distribution of knowledge, in the form of routing
updates, to other routers.
Dynamic routing relies on the routing protocol.
Routing Protocols can be Distant Vector or Link-State.
Hybrid protocols (like EIGRP) contain some elements of both.
Different routing protocols use different metrics to determine
the best route to a network.
Administrative Distances are used to rate the trustworthiness
of the various routing protocols.
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Distance Vector Routing Protocols
The distance-vector routing algorithm passes complete routing
tables to neighbor routers.
The neighbor routers combine the received routing table with their
own routing tables.
RIP is a distance vector routing protocol:
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Uses hop count as its metric
Each router the packet goes through is 1 hop
A
Configuration Example:
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1
3
B
2
Router(config)# router rip
Router(config-router)# network 172.16.0.0
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Link State Routing Protocols
Link-state routing algorithms (Shortest Path First algorithms),
maintain a complex database of topology information.
Link-state routing uses:
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Link-state advertisements (LSAs)
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A topological database
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The SPF algorithm, and the resulting SPF tree
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A routing table of paths and ports to each network
Link-state routing requires more memory and processing power
than distance vector, and bandwidth requirements are often
higher as well.
OSPF is the most commonly used Link-State Protocol.
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Distance Vector or Link State?
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Also known as Bellman-Ford algorithms
Flood routing information to all routers
Requests routing information from directly connected neighbors
Complete view of the internetwork topology
Decisions based upon information provided by neighbors
Use fewer system resources
When a network link changes state LSA are flooded through network
Less errors, but they use more system resources
Calculate the shortest path to all known sites on the network
Small update packets contain only changes
Slower convergence
OSPF and IS-IS
Do not scale well to larger systems.
Because they converge more quickly less prone to routing loops
Event-triggered updates, so convergence is fast
Based on finding the number of hops and direction to a link
Passes copies of complete routing table on a periodic basis
Each router simply inform its neighbors of its routing table
RIP and IGRP
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more reliable, easier to debug, and less bandwidth-intensive
Routing Protocols
Protocol
RIP
Features
Distance vector, hop count metric, maximum 15 hops,
broadcasts updates every 30 secs.
IGRP Cisco proprietary distance vector, bandwidth / load / delay /
reliability composite metric, broadcast updates every 90 secs.
Cisco proprietary, enhanced distance vector (hybrid), load
EIGRP balancing, uses DUAL to calculate shortest path.
Routing updates are triggered by topology changes.
OSPF Link-state, open standard, Uses SPF algorithm. Routing
updates are sent as topology changes occur.
BGP
Distance vector exterior routing protocol, used between
ISPs, used to route traffic between ASs.
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Autonomous Systems
AS is a collection of networks under a common administration
and sharing a common routing strategy.
ARIN, ISP, or an administrator assigns the 16 bit AS number.
IGRP, EIGRP and BGP require assignment of a unique AS
number.
ASs divide the global internetwork into smaller, more
manageable networks.
Each AS has its own set
of rules and policies.
The AS number uniquely
distinguish it from other
ASs around the world.
AS 10
AS 20
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EGP and IGP
1. Interior
_______________________
Gateway Protocols are designed for use in networks
whose parts are under the control of a single organization.
2. Exterior routing protocol is designed for use between different
organizations
networks that are under the control of different
___________________.
EGP are typically used between ISPs or between a
3. ______
company and an ISP.
4. EGPs require the following:
neighbor routers with which to exchange updates.
 A list of _________________
networks to advertise as directly reachable.
 A list of _________
autonomous system number of the local router.
 The ____________________
5. An exterior routing protocol must isolate ____.
ASs
IGP
EGP
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IGP
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