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CCNA – ROUTING By: Sameh El-Hakim Cyber Security Engineer

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CCNA – ROUTING By: Sameh El-Hakim

Cyber Security Engineer

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Routing is finding a path on which data can pass from the source to destination.

Routing protocols:Routing Information Protocol (RIP)

Open Shortest Path First (OSPF)

Interior Gateway Routing Protocol (IGRP)

Enhanced Interior Gateway Routing Protocol (EIGRP)

Border Gateway Protocol (BGP)

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ROUTING PROTOCOL vs ROUTED PROTOCOLA routing protocol such as RIP is used to route information packets over the Internet

A routed protocol such as IP (or IPX or AppleTalk) is the payload (contains data) that get routed from source to the destination.

Routing protocols types:Distance Vector Protocols: RIP

Link State Protocols: OSPF

Hybrid Protocols: IS-IS

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Routing Protocol Metric

RIPv2 Hop count

EIGRP Bandwidth, Delay, Load, Reliability, and MTU

OSPF Cost (Higher bandwidth indicates lower cost)

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Distance Vector routing protocols usually send their entire routing table to their nearest neighbors at regular intervals.

Link State routing protocols usually send only the routing changes to every other router within their area.

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CLASSFUL VS CLASSLESS ROUTING PROTOCOLS• Classful routing protocols DO NOT send the subnet mask along with their updates.

• Classless routing protocols DO send the subnet mask along with their updates.

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ROUTING INFORMATION PROTOCOL.RIP stands for Routing Information Protocol.

There are currently two versions of RIP protocol.RIPv1, and


Limitations of RIPv1: Hop Count Limit: Destination that is more than 15 hops away is considered unreachable

Classful Routing Only: RIP is a classful routing protocol. so it is not possible to subnet a network.

Metric limitation: The best route in RIP is determined by counting the number of hops required to reach the destination.

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RIPv2 a classless routing protocol.

Next Hop Specification: In RIPv2, each RIP entry includes a space where an explicit IP address can be entered as the next hop router for datagrams intended for the network in that entry.

Authentication: RIPv2 provides a basic authentication scheme.

Route Tag: Each RIPv2 entry includes a Route Tag field, where additional information about a route can be stored.

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The hop count of 16 still remains as unreachable, and the metric still remains hop count.

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* OSPF stands for Open Shortest Path First.

OSPF sends link-state advertisements (LSAs) to all other routers within the same area. Information on attached interfaces, metrics used, and other variables is included in OSPF LSAs.

OSPF routers use the SPF (Shortest Path First) algorithm to calculate the shortest path to each node. SPF algorithm is also known as Dijkstra algorithm.

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1. OSPF is an open standard, not related to any particular vendor.

2. OSPF is hierarchical routing protocol, using area 0 (Autonomous System) at the top of the hierarchy.

3. OSPF uses Link State Algorithm, and an OSPF network diameter can be much larger than that of RIP.

4. OSPF supports Variable Length Subnet Masks (VLSM), resulting in efficient use of networking resources.

5. OSPF uses multicasting within areas.

6. OSPF is classless.

7. Using areas, OSPF networks can be logically segmented to improve administration, and decrease the size of routing tables.

** Autonomous System: a backbone (Area 0) network that links all other smaller areas

within the hierarchy.

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1. OSPF is very processor intensive due to implementation of SPF algorithm. OSPF maintains multiple copies of routing information, increasing the amount of memory needed.

2. OSPF is a more complex protocol to implement compared to RIP.

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THE FOLLOWING ARE THE IMPORTANT COMPONENTS OF AN OSPF NETWORK1. Areas: An area consists of routers that have been administratively grouped

together. Usually, an area as a collection of contiguous IP subnetted networks. Routers that are totally within an area are called internal routers. All interfaces on internal routers are directly connected to networks within the area.

2. Area Border Routers: Routers that belong to more than one area are called area border routers (ABRs). ABRs maintain a separate topological database for each area to which they are connected.

3. AS Boundary Routers (ASBRs): Routers that exchange routing information with routers in other Autonomous Systems are called ASBRs. They advertise externally learned routes throughout the AS.

4. Within an area, all routers have identical topological databases.

5. Backbone Area: An OSPF backbone area consists of all routers in area 0, and all area border routers (ABRs). The backbone distributes routing information between different areas.

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Stub Areas: Stub areas are areas that do not propagate AS external advertisements. By not propagating AS external advertisements, the size of the topological databases is reduced on the internal routers of a stub area. This in turn reduces the processing power and the memory requirements of the internal routers.

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1. Type 1: Router link advertisements generated by each router for each area it belongs to. Type 1 LSAs are flooded to a single area only.

2. Type 2: Network link advertisements generated by designated routers (DRs) giving the set of routers attached to a particular network. Type 2 LSAs are flooded to the area that contains the network.

3. Type 3/4: These are summary link advertisements generated by ABRs describing inter-area routes. Type 3 describes routes to networks and is used for summarization. Type 4 describes routes to the ASBR.

4. Type 5: Generated by the ASBR and provides links external to the Autonomous System (AS). Type 5 LSAs are flooded to all areas except stub areas and totally stubby areas.

5. Type 6: Group membership link entry generated by multicast OSPF routers.

6. Type 7: NSSA external routes generated by ASBR. Only flooded to the NSSA. The ABR converts LSA type 7 into LSA type 5 before flooding them

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NeighborTwo routers that have an interface on a common network. Usually discovered by hello’s but can also be configured administratively

AdjacencyRelationship formed between selected neighbors in which routing information is exchanged. Not all neighbors are adjacentOnly Broadcast and Non-Broadcast network types have Designated and Backup Designated Routers!!!








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ROUTER ID (RID)Each router that is participating in OSPF needs to be uniquely identified. The method of identification that OSPF uses is Router IDs (RID).

32 bits that uniquely identifies an OSPF router

Highest IP address in router is RouterID

Overridden by Loopback interface if present

Even if Loopback address has lower value

Recommended to use loopback interface

Easier to manipulate this number

Always up

Interface loopback 0

Ip address

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•Routers elect a DR and BDR per network

•All routers set by default to priority 1 (0-255)

•Priority of zero (0) means router can not be elected as a DR

•Router with highest priority wins BDR (1 – 255), if no other router has a higher priority the BDR will then become the DR

•RouterID breaks tie, Router ID is either the Highest Loopback or Highest Configured IP address on any given active interface

•If DR fails, BDR promoted to DR and a new BDR is elected

•Existing DR will not be overthrown if “better” router is turned on after initial election

•DRs and BDRs listen to multicast traffic on both multicast address and is exclusively listed to by DRs

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ENHANCED INTERIOR GATEWAY ROUTING PROTOCOL (EIGRP)*(EIGRP) is a proprietary Cisco protocol that runs on Cisco routers.


Support for IP, IPX, and AppleTalk via protocol-dependent modules

Efficient neighbor discovery

Communication via Reliable Transport Protocol (RTP)

Best path selection via Diffusing update algorithm (DUAL)

• Enhanced IGRP (EIGRP) is a classless, EIGRP includes the subnet mask in its route updates.

* No updates. Route updates sent only when a change occurs – multicast on “Link State not distance vector”* Hello messages sent to neighbors every 5 seconds (60 seconds in most WANs)

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Neighbor Table—IP

Next Hop InterfaceRouter

Topology Table—IP

Destination 1 Successor

Destination 1 Feasible Successor

Routing Table—IP

Destination 1 Successor

** Note: A feasible successor is a backup route

and stored in the Topology table

** Successor route is used by EIGRP to forward traffic to a destination

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Redistribution is translating one type of routing protocol into another.

Router D

Router B

Router A

Router C


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* CCNA Routing and Switching Study Guide - Lammle, Todd

* http://searchnetworking.techtarget.com

* http://www.simulationexams.com/tutorials/ccna/routing-fundamentals.htm

* http://web.calstatela.edu/faculty/egean/cs447/lecture-notes-sybex2007/chapter7ccna.ppt

* Interview questions: http://computernetworkingnotes.com/basic-networking-interview/questions-and-answers.html

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