Routing and Routing Protocols

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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

• 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

• Path Determination
• Router uses the routing table to determine the
  best path.
• 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
ip route 0.0.0.0 0.0.0.0 192.168.1.1
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

• 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

• Maintenance of a routing table
• 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
• Uses hop count as its metric
• Each router the packet goes through is 1 hop

Configuration Example
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

• Link-state advertisements (LSAs)
• A topological database
• The SPF algorithm, and the resulting SPF tree
• 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?

1. Also known as Bellman-Ford algorithms
2. Flood routing information to all routers
3. Requests routing information from directly
   connected neighbors
4. Complete view of the internetwork topology
5. Decisions based upon information provided by
   neighbors  
6. Use fewer system resources
7. When a network link changes state LSA are flooded
   through network
8. Less errors, but they use more system resources
9. Calculate the shortest path to all known sites on
   the network
10. Small update packets contain only changes
11. Slower convergence
12. OSPF and IS-IS
13. Do not scale well to larger systems.
14. Because they converge more quickly less prone to
    routing loops
15. Event-triggered updates, so convergence is fast
16. Based on finding the number of hops and direction
    to a link
17. Passes copies of complete routing table on a
    periodic basis
18. Each router simply inform its neighbors of its
    routing table
19. RIP and IGRP

DV
LS
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DV
DV
LS
LS
LS
LS
DV
LS
DV
LS
LS
DV
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LS
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Routing Protocols
Protocol Features
Distance vector, hop count metric, maximum 15 hops, broadcasts updates every 30 secs.
Cisco proprietary distance vector, bandwidth / load / delay / reliability composite metric, broadcast updates every 90 secs.
Cisco proprietary, enhanced distance vector (hybrid), load balancing, uses DUAL to calculate shortest path. Routing updates are triggered by topology changes.
Link-state, open standard, Uses SPF algorithm. Routing updates are sent as topology changes occur.
Distance vector exterior routing protocol, used between ISPs, used to route traffic between ASs.
RIP
IGRP
EIGRP
OSPF
BGP
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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.
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EGP and IGP

• _______________________ are designed for use in
  networks whose parts are under the control of a
  single organization.
• Exterior routing protocol is designed for use
  between different networks that are under the
  control of ___________________.
• ______ are typically used between ISPs or between
  a company and an ISP.
• EGPs require the following
• A list of _________________ with which to
  exchange updates.
• A list of _________ to advertise as directly
  reachable.
• The ____________________ number of the local
  router.
• An exterior routing protocol must isolate ____.

Interior Gateway Protocols
different organizations
EGP
neighbor routers
networks
autonomous system
ASs
EGP