33 Routing Basics for cisco networking and

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© CSS Corp | Confidential | www.csscorp.com
Routing Basics

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• Routing
• Routing Protocol
• Classful Subnetting
• Classful and Classless Updates
• Dynamic Routing protocol
• Distance Vector Protocols
• Link State Routing Protocols
Learning Objective

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Routing
• To route, a router needs to know:
• Destination addresses
• Sources it can learn from
• Possible routes
• Best route
• Maintain and verify routing information

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Routing
Routers must learn destinations that are not directly connected

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Static Routes
Configure unidirectional static routes to and from a
stub network to allow communications to occur

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Static Route Example

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Default Route

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Routing Protocol
• Routing protocols are used
between routers to
determine paths and
maintain routing tables.
• Once the path is
determined a router can
route a routed protocol

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Autonomous Systems:
Interior or Exterior Routing Protocols
• An autonomous system is a collection of networks
under a common administrative domain
• IGPs operate within an autonomous system
• EGPs connect different autonomous systems
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Classes of Routing Protocols

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Distance Vector Routing Protocols
Pass periodic copies of routing table to neighbor routers
and accumulate distance vectors

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Distance Vector—Sources of Information
and Discovering Routes
Routers discover the best path to destinations from each neighbor

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Distance Vector—Selecting Best Route
with Metrics

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Maintaining Routing Information Problem
Each node maintains the distance from itself to each
possible destination network

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Slow convergence produces inconsistent routing

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Router C concludes that the best path to network
10.4.0.0 is through Router B

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Router A updates its table to reflect the new but
erroneous hop count

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Symptom: Counting to Infinity
• Packets for network 10.4.0.0 bounce between routers A, B, and C
• Hop count for network 10.4.0.0 counts to infinity

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Solution: Defining a Maximum
Define a limit on the number of hops to prevent infinite
loops

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Solution: Split Horizon
It is never useful to send information about a route back in
the direction from which the original packet came

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Solution: Route Poisoning
Routers set the distance of routes that have gone down to infinity

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Solution: Poison Reverse

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Solution: Hold-Down Timers
Router keeps an entry for the network possibly down
state, allowing time for other routers to recompute for this
topology change

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Solution: Triggered Updates
Router sends updates when a change in its routing table
occurs

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• Classful routing protocols do not include the subnet mask
with the route advertisement
• Within the same network, consistency of the subnet
masks is assumed
• Summary routes are exchanged between foreign networks
• Example of classful routing protocols:
 RIPv1 (RIP Version 1)
 IGRP
Classful Routing Review

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Classful Routes
• Sub network routes are shared by devices within the same
network
• Summary routes are exchanged between foreign networks
• Summary routes are automatically created at Class A, B, and C
network boundaries

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Classful Subnetting Requirements
• All router interfaces within the same network must have the same
subnet mask (Fixed-length subnet masking)
• This approach may not fully utilize available allocation of host
addresses
• All subnets of the same major network must be contiguous

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• Classless routing protocols include the subnet mask with the
route advertisement
• Classless routing protocols support Variable-length subnet
masking (VLSM)
• Summary routes can be manually controlled within the network
• Example of classless routing protocols:
 OSPF
 EIGRP
 RIPv2
 IS-IS
 BGP
Classless Routing Overview

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Classless Subnetting Requirements
• Router interfaces within the same network can have different subnet masks
• Variable-length subnet masking (VLSM) is supported
• This approach maximizes allocation of available host addresses

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Classful and Classless Updates

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• A procedure for passing reachability information about networks to
other routers.
• A procedure for receiving reachability information from other routers
• A procedure for determining optimal routes based on the
reachability information it has and for recording this information in a
route table
• A procedure for reacting to, compensating for, and advertising
topology changes in an internetwork
• Note: Most routing protocols fall into one of two classes: distance
vector or link state.
Dynamic Routing protocol

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• Distance vector algorithms are based on the work done of
R. E. Bellman, L. R. Ford, and D. R. Fulkerson and for this
reason occasionally are referred to as Bellman-Ford or
Ford-Fulkerson algorithms.
• The name distance vector is derived from the fact that
routes are advertised as vectors of (distance, direction)
• Router depends on its neighbors for information, which
the neighbors in turn may have learned from their
neighbors, and so on, distance vector routing is
sometimes facetiously referred to as "routing by rumor."
Distance Vector Routing Protocols

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Distance Vector Protocols
With Distance Vector routing protocols, routers broadcast
their routing tables to its adjacent neighbors at periodic
intervals.

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• Routing Information Protocol (RIP) for IP
• Xerox Networking System's XNS RIP
• Novell's IPX RIP
• Cisco's Internet Gateway Routing Protocol (IGRP)
• DEC's DNA Phase IV
• AppleTalk's Routing Table Maintenance Protocol (RTMP)
Distance vector routing protocols

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• Periodic Updates
• Neighbors
• Broadcast Updates
• Full Routing Table Updates
• Routing by Rumor
• Route Invalidation Timers
• Split Horizon
• Counting to Infinity
• Triggered Updates
• Holddown Timers
Distance vector Routing Protocols
Characteristics

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• Routers originates information about itself, its directly
connected links and the state of those links.
• This information is passed around from router to router, each
router making a copy of it, but never changing it.
• The ultimate objective is that every router has identical
information about the internetwork, and each router will
independently calculate its own best paths.
• Link state protocols, sometimes called shortest path first or
distributed database protocols, are built around a well-known
algorithm from graph theory, E. W. Dijkstra'a shortest path
algorithm.
Link-State Routing Protocols Overview

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• Open Shortest Path First (OSPF) for IP
• The ISO's Intermediate System to Intermediate
• System (IS-IS) for CLNS and IP
• DEC's DNA Phase V
• Novell's NetWare Link Services Protocol (NLSP)
Link State Routing Protocols

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Link-State Routing Protocols Overview

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• Link-state routers keep track of
 Their Neighbors
 All the routers in the network, or at least within the same area
 Best paths toward a destination
Basic Terms of Link-State Routing
Protocols

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• Neighbor table, formally known as an adjacency database
(list of neighbors we are aware of)
• Topology table, typically referred to as a link-state database
—LSDB (routers and links in the area/network)
• Routing table, commonly named a forwarding database (list
of best paths to destinations)
Basic Terms—Link-State Data
Structures

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• Routers find the best paths to destinations by applying the
Dijkstra Shortest Path First (SPF) algorithm to the link-state
database
 Every router in the area places itself into the root of the tree
that is built
 Best path is calculated with respect to the lowest
 Total cost of links to a specific destination
 Best routes are put into the forwarding database
Basic Terms—The Best Path Calculation

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Basic Terms—Link - State Environment

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• Link-state routing requires a hierarchical network structure
 Enforced by some LS protocols (for example, OSPF)
 Some LS protocols are more tolerant (IS-IS)
• Two level hierarchy—areas
 Backbone or level-2 area
 Non-backbone or level-1 area
Link-State Data Structure—Network
Hierarchy

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Link-State Data Structure—Network
Hierarchy Example
• Minimizes routing table entries
• Localizes impact of a topology change within an
area

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• The foundation for best-path calculation is the LS database
 LS database has to be identical on all the routers in
 the area—identical view
 Routers know everything about their respective area
 Routers know about the nearest exit point(s) to other
 areas or other routing domains
LS Data Structures Link-State Database

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• Routers discover neighbors by exchanging Hello packets
 Routers declare neighbors “UP” after checking some
parameters/options in the Hello packet
 Some routers become adjacent
 (tightly connected neighbors)—
 “good neighbors”
 Adjacent routers exchange topology information
LS Data Structures—Adjacency Database

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• Upon establishing neighbor relationship, routers exchange
their pictures of the network
 Full picture—link-state database (LSDB)—is built by
link-state packets (LS packets)
 LS packets report the states of links and routers
 LS packets are flooded reliably throughout the area/ network
 LS packets are sequenced, aged and periodically refreshed
LS Data Structures—Link-State Packets

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• Routers maintain the consistency of the link-state database
 Routers check their neighbor relationship by
sending and receiving periodic hello packets
 Routers report changes in the network (immediately/ depending
on timers)
 Receivers of a link-state packet normally flood it further
 Routers periodically resend their part of the “network map”
(even if no changes)
LS Data Structures—
Link-State Maintenance

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LS Data Structures—LS Advertisement
Operation

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• Benefits
 Fast convergence—changes reported immediately by
the source affected
 Robustness against routing loops:
— Routers know the topology
— LS packets are sequenced and acknowledged
 By careful (hierarchical) network design, resources
can be utilized optimally
Benefits of Link-State Routing

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• Routing protocols are used between routers to determine
paths and maintain routing tables.
• Once the path is determined a router can route a routed
protocol.
• An autonomous system is a collection of networks under a
common administrative domain
• Classless routing protocols support Variable-length subnet
masking (VLSM)
• A procedure for passing reachability information about
networks to other routers.
• Distance vector algorithms are occasionally are referred
to as Bellman-Ford or Ford-Fulkerson algorithms.
Conclusion

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© CSS Corp
The information contained herein is subject to change without
notice. All other trademarks mentioned herein are the property of
their respective owners.
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33 Routing Basics for cisco networking and

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