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Routing and Routing Protocols
PJC CCNA Semester 2 Ver. 3.0byWilliam Kelly
Routing Basics Routing is the process of forwarding packets
from a source to a destination based on the destination IP address of the packet
Routing occurs at layer 3 of the OSI model (network layer)
Routes can be entered by the administrator (static) or learned from other routers (dynamic)
The network layer provides a “best-effort” delivery system
The router uses a routing table to send packets from a source to a destination
How Routers Route Packets Consistent end-to-end Network
addressing is the key to routing Broadcasts are minimized and
bandwidth is conserved
The 2 Functions of a Router Best Path Determination – The router
determines the best path to a destination by examining the Network portion of an address and consulting it’s routing tables (a map of the network contained in each router)
Switching – A router accepts a packet on one interface and forwards it to another interface
Routed vs. Routing Protocols Routed protocols are network
protocols that provide enough information for a packet to be forwarded from one host to another
Routing protocols allow routers to communicate information about network paths and to maintain tables
Routed vs. Routing Protocols Routed protocols are network
protocols that provide enough information for a packet to be forwarded from one host to another
Routed protocols allow routers to communicate information about network paths and to maintain tables
Common Routed and Routing Protocols
Routed Protocols IP IPX Appletalk
Routing Protocols RIP IGRP OSPF EIGRP BGP
Static Routing Routing instructions manually entered
by an administrator Routes must be changed manually
when the network topology changes Static routing is labor intensive Static routes are used to enhance
security and to set up routes accessible by only one path
Network Layer Protocols
Application
Presentation
Session
Transport
Network
Data Link
Physical1
2
3
4
5
6
7
Network
Data Link
Physical1
2
3 Network
Data Link
Physical1
2
3
Application
Presentation
Session
Transport
Network
Data Link
Physical1
2
3
4
5
6
7
Host X Host Y
RouterA
RouterB
Configuring a Static RouteSyntax for entering a static route using the
interface:Router(config)# ip route 192.168.2.0 255.255.255.0 S1
Syntax for entering a static route using the next hop address:Router(config)# ip route 192.168.2.0 255.255.255.0 192.168.1.2
Syntax for entering a default route (sometimes called a quad route):Router(config)# ip route 0.0.0.0 0.0.0.0 S1
Syntax for entering a quad route with an administrative distance
Router(config)# ip route 0.0.0.0 0.0.0.0 192.168.1.2 4
Syntax for entering a static route using the interface:
Router(config)# ip route 192.168.2.0 255.255.255.0 192.168.1.2
Examining Static Route SyntaxSyntax for entering a static route using the interface:
Router(config)# ip route 192.168.2.0 255.255.255.0 S1
destination network address
next hop interface
next hop addressdestination network address
Common Administrative Distances
Application of Administrative distances in Static Routes
When using a next hop address the default administrative distance is 1
When using an outgoing interface the default administrative distance is 0
Dynamic Routing Dynamic routing allows routers to
automatically share routing (topology) information with other routers
Routing Protocols RIP – distance vector protocol, metric is hop
count, max hop count is 15, updates every 30 seconds
IGRP – distance vector protocol, metric is bandwidth, delay, load, and reliability, highly scalable for large networks, updates every 90 seconds
OSPF – link state protocol, uses triggered updates, VLSM compliant
EIGRP – Cisco proprietary hybrid protocol, VLSM compliant
BGP – exterior routing protocol that was designed to work between ISP’s
Commonly Used Metrics Bandwidth – data capacity of a link Delay – length of time for a packet to
travel through links Load – amount of network activity Reliability – error rate Hop Count – number of routers a
packet must traverse Ticks – each tick is 1/18 of a second Cost
Link State Basics Link State Protocols use the Link State
Algorithm (also called the Dijkstras algorithm)1. Link State Advertisements (LSA’s) are
transferred between routers to provide topology information
2. Each router maintains a topology database of LSA information
3. The Dijkstras algorithm computes the shortest paths to different networks
4. The routing table is then populated with the best paths
Link State Concerns Processor Overhead Memory requirements Bandwidth consumption – The initial
discovery process is bandwidth intensive; thereafter, minimal bandwidth is required
Hybrid Routing Protocols Similar to distance vector protocols in
that metrics are used but the metrics are more accurate.
Similar to Link State protocols in that triggered updates are used instead of period updates
Examples are IS-IS and EIGRP
Configuring a Router to Use Routing ProtocolsStarting routing
Router(config)# router protocol {options}ex: Router(config)# router rip
Entering Networks
Router(config)# network network-number ex: Router(config)# network 192.168.1.0
Note: A separate network line must be used to enter every network whose interface will participate in the sending and receiving of routing updates
Autonomous Systems Interior Gateway Protocols (IGP) were
designed to be used in a network designed to be administered by a single organization
Exterior Gateway Protocols (EGP) were designed to be used between networks that are controlled by two organizations. EGP’s such as BGP are used between ISP’s
Every autonomous system is assigned a 16 bit identifying number by the American Registry of Internet Numbers (ARIN)