OSPF Message Encapsulation
Data link frame header
  Contains – Source MAC address and Destination   MAC address

Hello Protocol
OSPF Hello Packet
Purpose of Hello Packet
  Discover OSPF neighbors & establish adjacencies
  Advertise guidelines on which routers must agree to become neighbors
  Used by multi-access networks to elect a designated router and a backup designated router

Hello Packets continued
Contents of a Hello Packet
 router ID of transmitting router
OSPF Hello Intervals
Usually multicast (224.0.0.5)
Sent every 30 seconds for NBMA segments
OSPF Dead Intervals
This is the time that must transpire
  before the neighbor is considered
  down
Default time is 4 times
  the hello interval

Hello protocol packets contain information that is used in electing
-Designated Router (DR)
 DR is responsible for updating all other OSPF routers
-Backup Designated Router (BDR)
 This router takes over DR’s responsibilities if DR fails

OSPF Link-state Updates
Purpose of a Link State Update (LSU)
Used to deliver link state advertisements
Purpose of a Link State Advertisement (LSA)
Contains information about neighbors & path costs
OSPF Algorithm
OSPF routers build & maintain link-state database containing LSA received from other routers
Information found in database is utilized upon execution of  Dijkstra SPF  algorithm
SPF algorithm used to create SPF tree
SPF tree used to populate routing table

Administrative Distance
Default Administrative Distance for OSPF is 110

OSPF Authentication
Purpose is to encrypt & authenticate routing information
This is an interface specific configuration
Routers will only accept routing information from other routers that have been configured with the same password or authentication information

Lab Topology
Topology used for this chapter
Discontiguous IP addressing scheme
Since OSPF is a classless routing protocol the subnet mask is configured in

The router ospf command
To enable OSPF on a router use the following command 
R1(config)#router ospf process-id
Process id
 A locally significant number between 1 and 65535
 -this means it does not have to match other OSPF    routers

OSPF network command
-Requires entering: network address
        wildcard mask – the inverse of the subnet     mask
                   area-id – area-id refers to the OSPF area.      OSPF area  is a group of routers that     share link state information
-Example:  Router(config-router)#network network-address     wildcard-ask area area-id 

Router ID
This is an IP address used to identify a router
3 criteria for deriving the router ID
Use IP address configured with OSPF router-id command
-Takes precedence over loopback and physical interface addresses
If router-id command not used then router chooses highest IP address of any loopback interfaces
If no loopback interfaces are configured then the highest IP address on any active interface is used

OSPF Router ID
Commands used to verify current router ID
Show ip protocols
Show ip ospf
Show ip ospf interface

OSPF Router ID
Router ID & Loopback addresses
-Highest loopback address will be used as router ID if router-id command isn’t used
-Advantage of using loopback address
 the loopback interface cannot fail  OSPF stability
The OSPF router-id command
Introduced in IOS 12.0
Command syntax
Router(config)#router ospfprocess-id
Router(config-router)#router-idip-address
Modifying the Router ID
Use the command  Router#clear ip ospf process

Verifying OSPF
Use the show ip ospf command to verify & trouble shoot OSPF networks
Command will display the following:
 Neighbor adjacency
    -No adjacency indicated by -
Neighboring router’s Router ID is not displayed
A state of full is not displayed
    -Consequence of no adjacency-
No link state information exchanged
Inaccurate SPF trees & routing tables
OSPF uses cost as the metric for determining the best route
-The best route will have the lowest cost
-Cost is based on bandwidth of an interface
Cost is calculated using the formula
  108 / bandwidth
-Reference bandwidth
defaults to 100Mbps
can be modified using
auto-cost reference-bandwidth command

Examining the routing table
Use the show ip route command to display the routing table
-An “O’ at the beginning of a route indicates that the router source is OSPF
-Note OSPF does not automatically summarize at major network boundaries

OSPF uses cost as the metric for determining the best route
-The best route will have the lowest cost
-Cost is based on bandwidth of an interface
Cost is calculated using the formula
  108 / bandwidth
-Reference bandwidth
defaults to 100Mbps
can be modified using
auto-cost reference-bandwidth command
COST of an OSPF route
Is the accumulated value from one router to the next

Usually the actual speed of a link is different than the default bandwidth
This makes it imperative that the bandwidth value reflects link’s actual speed
Reason: so routing table has best path information
The show interface command will display interface’s bandwidth
-Most serial link default to 1.544Mbps
Modifying the Cost of a link
Both sides of a serial link should be configured with the same bandwidth
Commands used to modify bandwidth value
Bandwidth command
Example: Router(config-if)#bandwidthbandwidth-kbps
ip ospf cost command – allows you to directly specify interface cost
-Example:R1(config)#interface serial 0/0/0
           R1(config-if)#ip ospf cost 1562
      

Modifying the Cost of the link
Difference between bandwidth command & the ip ospf cost command
Ip ospf cost command
Sets cost to a specific value
Bandwidth command
Link cost is calculated

Challenges in Multiaccess Networks
OSPF defines five network types:
Point-to-point
Broadcast Multiaccess
Nonbroadcast Multiaccess (NBMA)
Point-to-multipoint
Virtual links
2 challenges presented by multiaccess networks
Multiple adjacencies
Extensive LSA flooding
Extensive flooding of LSAs
For every LSA sent out there must be an acknowledgement of receipt sent back to transmitting router.
 consequence:  lots of bandwidth consumed and chaotic traffic

Solution to LSA flooding issue is the use of
Designated router (DR)
Backup designated router (BDR)
DR & BDR selection
Routers are elected to send & receive LSA
Sending & Receiving LSA
DRothers send LSAs via multicast 224.0.0.6 to DR & BDR
DR forward LSA via multicast address 224.0.0.5 to all other routers

DR/BDR Election Process
DR/BDR elections DO NOT occur in point to point networks
DR/BDR elections will take place on multiaccess networks as shown below
Criteria for getting elected DR/BDR
DR: Router with the highest OSPF interface priority.
2. BDR: Router with the second highest OSPF  interface priority.
3. If OSPF interface priorities are equal, the  
   highest router ID is used to break

Timing of DR/BDR Election
Occurs as soon as 1st router has its interface enabled on multiaccess network
When a DR is elected it remains as the DR until one of the following occurs
-The DR fails.
-The OSPF process on the DR fails.
-The multiaccess interface on the DR fails.

Manipulating the election process
-If you want to influence the election of DR & BDR then do one of the following
Boot up the DR first, followed by the BDR, and then boot all other routers,
 OR
Shut down the interface on all routers, followed by a no shutdown on the DR, then the BDR, and then all other routers.
OSPF Interface Priority
Manipulating the DR/BDR election process continued
Use the ip ospf priority interface command.
Example:Router(config-if)#ip ospf priority {0 – 255}
Priority number range 0 to 255
0 means the router cannot become the DR or BDR
1 is the default priority value
Redistributing an OSPF Default Route
Topology includes a link to ISP
Router connected to ISP
Called an autonomous system border router
Used to propagate a default route
Example of static default route
 R1(config)#ip route 0.0.0.0 0.0.0.0 loopback 1
Requires the use of the default-information originate command
Example of default-information originate command
 R1(config-router)#default-information originate
Fine-Tuning OSPF
Since link speeds are getting faster it may be necessary to change reference bandwidth values
Do this using the auto-cost reference-bandwidth command
Example:
 R1(config-router)#auto-cost reference-bandwidth 10000
Fine-Tuning OSPF
Modifying OSPF timers
Reason to modify timers
Faster detection of network failures
Manually modifying Hello & Dead intervals
Router(config-if)#ip ospf hello-interval  seconds
Router(config-if)#ip ospf dead-interval seconds
Point to be made
Hello & Dead intervals must be the same between neighbors

RFC 2328 describes OSPF link state concepts and operations
OSPF Characteristics
A commonly deployed link state routing protocol
Employs DRs & BDRs on multi-access networks
DRs & BDRs are elected
DR & BDRs are used to transmit and receive LSAs
Uses 5 packet types:
            1: HELLO
2: DATABASE DESCRIPTION
3: LINK STATE  REQUEST
4: LINK STATE  UPDATE                  
5: LINK STATE ACKNOWLEDGEMENT

Dikjstra’s algorithm also known as the shortest path first (SPF) algorithm
The shortest path to a destination is not necessarily the path with the least number of hops

Link-State Routing Process
How routers using Link State Routing Protocols reach convergence
-Each routers learns about its own directly connected networks
-Link state routers exchange hello packet to “meet” other directly
connected link state routers.
-Each router builds its own Link State Packet (LSP) which includes information about neighbors such as neighbor ID, link type, & bandwidth.
-After the LSP is created the router floods it to all neighbors who then store the information and then forward it until all routers have the same information.
-Once all the routers have received all the LSPs, the routers then construct a topological map of the network which is used to determine the best routes to a destination
Directly Connected Networks
Link
  This is an interface on a  router
Link state
  This is the information  about the state of the  links

Sending Hello Packets to Neighbors
Link state routing protocols use a hello protocol
  Purpose of a hello protocol:
   -To discover neighbors (that use the same   link state routing protocol) on its link

Connected interfaces that are using the same link state routing protocols will exchange hello packets.
Once routers learn it has neighbors they form an adjacency
  -2 adjacent neighbors will  exchange hello packets
  -These packets will serve as a  keep alive function
Building the Link State Packet
Each router builds its own Link State Packet (LSP)
 Contents of LSP:
  -State of each directly  connected link
  -Includes information  about neighbors such as  neighbor ID, link type, &  bandwidth.

Flooding LSPs to Neighbors
Once LSP are created they are forwarded out to neighbors.
-After receiving the LSP the neighbor continues to forward it throughout routing area.

LSPs are sent out under the following conditions
  -Initial router start up or routing process
  -When there is a change in topology
Constructing a link state data base
Routers use a database to construct a topology map of the network
Building a portion of the SPF tree
 Process begins by examining R2’s LSP information
  -R1 ignores 1st LSP
   Reason:  R1 already knows it’s connected to R2
  
Building a portion of the SPF tree
  -R1 uses 2nd LSP
   Reason:  R1 can create a link from R2 to R5.    This information is added to R1’s SPF tree

Building a portion of the SPF tree
  -R1 uses 3rd LSP
Reason: R1 learns that R2 is connected to    10.5.0.0/16.
This link is added to R1’s SPF tree.
Determining the shortest path
The shortest path to a destination determined by adding the costs & finding the lowest cost

Once the SPF algorithm has determined the shortest path routes, these routes are placed in the routing table.

Requirements for using a link state routing protocol
Memory requirements
  Typically link state routing protocols use more  memory
Processing Requirements
  More CPU processing is required of link state  routing protocols
Bandwidth Requirements
  Initial startup of link state routing protocols can  consume lots of bandwidth

2 link state routing protocols used for routing IP
  -Open Shortest Path First (OSPF)
  -Intermediate System-Intermediate System (IS-IS)

TLV: IP internal contains
Metric field
Subnet mask field
Destination field
TLV: IP external contains
Fields used when external
  routes are imported into
  EIGRP routing process

Protocol Dependent Modules (PDM)
EIGRP uses PDM to route several different protocols i.e. IP, IPX & AppleTalk
PDMs are responsible for the specific routing task for each network layer protocol

Reliable Transport Protocol (RTP)
Purpose of RTP
Used by EIGRP to transmit and receive EIGRP packets
Characteristics of RTP
Involves both reliable & unreliable delivery of EIGRP packet
Reliable delivery requires acknowledgment from destination
Unreliable delivery does not require an acknowledgement from destination
Packets can be sent
Unicast
Multicast
Using address 224.0.0.10

Hello packets
Used to discover & form adjacencies with neighbors
Update packets
Used to propagate routing information
Acknowledgement packets
Used to acknowledge receipt of update, query & reply packets

Query & Reply packets
Used by DUAL for searching for networks
Query packets
-Can use
Unicast
Multicast
Reply packet
-Use only
unicast

Purpose of Hello Protocol
To discover & establish adjacencies with neighbor routers
Characteristics of hello protocol
Time interval for sending hello packet
Most networks it is every 5 seconds
Multipoint non broadcast multi-access networks
Unicast every 60 seconds

Holdtime
This is the maximum time router should wait before declaring a neighbor down
Default holdtime
3 times hello interval
EIGRP Bounded Updates
EIGRP only sends update when there is a change in route status
Partial update
A partial update includes only the route information that has changed – the whole routing table is NOT sent
Bounded update
When a route changes, only those devices that are impacted will be notified of the change
EIGRP’s use of partial bounded updates minimizes use of bandwidth

Diffusing Update Algorithm (DUAL)
Purpose
EIGRP’s primary method for preventing routing loops
Advantage of using DUAL
Provides for fast convergence time by keeping a list of loop-free backup routes

Administrative Distance (AD)
Defined as the trustworthiness of the source route
EIGRP default administrative distances
Summary routes  = 5
Internal routes     = 90
Imported routes   = 170
EIGRP can
Encrypt routing information
Authenticate routing information

Topology used is the same as previous chapters with the addition of an ISP router

EIGRP will automatically summarize routes at classful boundaries

Autonomous System (AS) & Process IDs
This is a collection of networks under the control of a single authority (reference RFC 1930)
AS Numbers are assigned by IANA
Entities needing AS numbers
ISP
Internet Backbone prodiers
Institutions connecting to other institutions using AS numbers
EIGRP autonomous system number actually functions as a process ID
Process ID represents an instance of the routing protocol running on a router
Example
Router(config)#router         
                eigrp autonomous-system

The router eigrp command
The global command that enables eigrp is
router eigrp autonomous-system
-All routers in the EIGRP routing domain must use the same process ID number (autonomous-system number)

The Network Command
Functions of the network command
Enables interfaces to transmit & receive EIGRP updates
Includes network or subnet in EIGRP updates
Example
Router(config-router)#network network-address
The network Command with a Wildcard Mask
-This option is used when you want to configure EIGRP to advertise specific subnets
-Example
    Router(config-router)#network network-address [wildcard-mask]
Verifying EIGRP
EIGRP routers must establish adjacencies with their neighbors before any updates can be sent or received
Command used to view neighbor table and verify that EIGRP has established adjacencies with neighbors is
  show ip eigrp neighbors

The show ip protocols command is also used to verify that EIGRP is enabled

Examining the Routing Table
The show ip route command is also used to verify EIGRP
EIGRP routes are denoted in a routing table by the letter “D”
By default , EIGRP automatically summarizes routes at major network boundary
Introducing the Null0 Summary Route
Null0 is not a physical interface
In the routing table summary routes are sourced from Null0
Reason:  routes are used for advertisement purposes
EIGRP will automatically include a null0 summary route as child route when 2 conditions are met
At least one subnet is learned via EIGRP
Automatic summarization is enabled

R3’s routing table shows that the 172.16.0.0/16 network is automatically summarized by R1 & R3
EIGRP Composite Metric & the K Values
EIGRP uses the following values in its composite metric
-Bandwidth, delay, reliability, and load
The composite metric used by EIGRP
 formula used  has values K1 K5
K1 & K3           = 1
all other K values = 0
 
Use the sh ip protocols command to verify the K values

EIGRP Metrics
Use the show interfaces command to view metrics
EIGRP Metrics
Bandwidth – EIGRP uses a static bandwidth to calculate metric
Most serial interfaces use a default bandwidth value of 1.544Mbos (T1)

EIGRP Metrics
Delay is the defined as the measure of time it takes for a packet to traverse a route
-it is a static value based on link type to which interface is connected

Reliability (not a default EIGRP metric)
-A measure of the likelihood that a link will fail
-Measure dynamically & expressed as a fraction of 255
 the higher the fraction the better the reliability
Load (not a default EIGRP metric)
 A number that reflects how much traffic is using a link
 Number is determined dynamically and is expressed as a fraction of 255
The lower the fraction the less the load on the link

Using the Bandwidth Command
Modifying the interface bandwidth
-Use the bandwidth command
-Example
 Router(config-if)#bandwidth kilobits
Verifying bandwidth
Use the show interface command
Note – bandwidth command
  does not change the
  link’s physical
  bandwidth
The EIGRP metric can be determined by examining the
  bandwidth delay

EIGRP uses the lowest bandwidth (BW)in its metric calculation
       Calculated BW = reference BW / lowest BW(kbps)
Delay – EIGRP uses the cumulative sum of all outgoing interfaces
Calculated Delay = the sum of outgoing interface delays
EIGRP Metric = calculated BW + calculated delay

Disabling Automatic Summarization
The auto-summary command permits EIGRP to automatically summarize at major network boundaries
The no auto-summary command is used to disable automatic summarization
This causes all EIGRP neighbors to send updates that will not be automatically summarized
this will cause changes to appear in both
-routing tables
-topology tables
Manual Summarization
Manual summarization can include supernets
Reason: EIGRP is a classless routing protocol & include subnet mask in update
Command used to configure manual summarization
Router(config-if)#ip summary-address eigrp  as-number   network-address subnet-mask
EIGRP Default Routes
“quad zero” static default route
-Can be used with any currently supported routing protocol
-Is usually configured on a router that is connected a network outside the EIGRP domain
EIGRP & the “Quad zero” static default route
Requires the use of the redistribute static command to disseminate default route in EIGRP updates
Fine-Tuning EIGRP
EIGRP bandwidth utilization
-By default, EIGRP uses only up to 50% of interface bandwidth for EIGRP information
-The command to change the percentage of bandwidth used by EIGRP is
  Router(config-if)#ip bandwidth-percent eigrp as-   number percent

Configuring Hello Intervals and Hold Times
-Hello intervals and hold times are configurable on a per-interface basis
-The command to configure hello interval is
  Router(config-if)#ip hello-interval eigrp as-number seconds
Changing the hello interval also requires changing the hold time to a value greater than or equal to the hello interval
-The command to configure hold time value is
 Router(config-if)#ip hold-time eigrp as-number seconds

§Level 1 Routes

-Ultimate Route

§Includes either:

  -A next-hop address

       OR

  -An exit interface

Parent and Child Routes
 -A parent route is a level 1 route
 -A parent route does not contain any next-hop IP address or exit interface information

Automatic creation of parent routes
-Occurs any time a subnet is added to the routing table
Child routes
  -Child routes are level  2 routes
  -Child routes are a  subnet of a classful  network address

Level 2 child routes contain route source & the network address of the route
Level 2 child routes are also considered ultimate routes
  Reason:  they contain the next hop address  &/or exit interface

Both child routes have the same subnet mask
 -This means the parent route maintains the /24 mask

In classless networks, child routes do not have to share the same subnet mask

The Route Lookup Process
Examine level 1 routes
 -If best match a level 1 ultimate route and is not a parent route this route is used to forward packet
Router examines level 2 (child) routes
 -If there is a match with level 2 child route then
  that subnet is used to forward packet
 -If no match then
 determine routing behavior type
Router determines classful or classless routing behavior
 -If classful then
  packet is dropped
 -If classless then router searches level one supernet and default routes
 -If there exists a level 1 supernet or default route match then Packet is forwarded.  If not packet is dropped

Longest Match: Level 1 Network Routes
Best match is also known as the longest match
The best match is the one that has the most number of left most bits matching between the destination IP address and the route in the routing table.

Level 1 Parent & Level 2 Child Routes
Before level 2 child routes are examined
  -There must be a match between classful level one  parent route and destination IP address.

 After the match with parent route has been made Level 2 child routes will be examined for a match
   -Route lookup process searches for child     routes with a match with destination IP

How a router finds a match with one of the level 2 child routes
  -First router examines parent routes for a match
  -If a match exists then:
Child routes are examined
Child route chosen is the one with the     longest match

Classful & classless routing protocols
   Influence how routing table is populated
Classful & classless routing behaviors
  Determines how routing table is searched after it is  filled

Classful Routing Behavior:  no ip classless
What happens if there is not a match with any level 2 child routes of the parent?
 -Router must determine if the routing behavior is classless or classful
 -If router is utilizing  classful routing behavior then
  -Lookup process is   terminated and   packet is dropped

Classful Routing Behavior – Search Process
An example of when classful routing behavior is in effect and why the router drops the Packet
-The destination’s subnet mask is a /24 and none of the child routes left most bits match the first 24 bits.  This means packet is dropped

Classful Routing Behavior – Search Process
The reason why the router will not search beyond the child routes
Originally networks were all classful
This meant an organization could subnet a major network address and “enlighten” all the  organization’s routers about the subnetting
Therefore, if the subnet was not in the routing table, the subnet did not exist and packet was dropped

ip Classless
Beginning with IOS 11.3, ip classless was configured by default
Classless routing behavior works for   
  -Discontiguous networks
         And
  -CIDR supernets

Classless Routing Behavior: ip classless
Route lookup process when ip classless is in use
  -If classless routing behavior in effect then
Search level 1 routes
Supernet routes Checked first
  -If a match exists then forward packet
Default routes Checked second
  If there is no match or no default      route then the
Packet is dropped

Classless Routing Behavior – Search Process
Router begins search process by finding a match between destination IP and parent route
  After finding the above mentioned match, then  there is a search of the child route

Classless Routing Behavior – Search Process
If no match is found in child routes of previous slide then
  Router continues to search the routing table for a  match that may have fewer bits in the match

Classful vs. Classless Routing Behavior
  -It is recommended to use classless routing  behavior
Reason: so supernet and default routes can             be used whenever needed

Similarities between RIPv1 & RIPv2
  -Use of timers to prevent routing loops
  -Use of split horizon or split horizon with poison  reverse
  -Use of triggered updates
  -Maximum hop count of 15
Scenario:
3 router set up
Topology is discontiguous
There exists a static summary route
Static route information can be injected into routing table updates using redistribution.
Routers 1 & 3 contain VLSM networks
Ripv1 Limitations
Scenario Continued
VLSM
 -Recall this is sub netting the subnet
Private IP addresses are on LAN links
Public IP addresses are used on WAN links
Loopback interfaces
 -These are virtual interfaces that can be pinged and added to routing table
Null Interfaces
This is a virtual interface that does not need to be  created or configured
   -Traffic sent to a null interface is discarded
   -Null interfaces do not send or receive traffic
Static routes and null interfaces
null interfaces will serve as the exit interface for static route
  -Example of configuring a static supernet route with a   null interface
  -R2(config)#ip route 192.168.0.0 255.255.0.0 Null0

Route redistribution
  -Redistribution command is way to disseminate a  static route from one router to another via a routing  protocol
  -Example
   R2(config-router)#redistribute static

Verifying and Testing Connectivity
Use the following commands:
 show ip interfaces brief
 ping
 traceroute

RIPv1 – a classful routing protocol
  -Subnet mask are not sent in updates
  -Summarizes networks at major network boundaries
  -if network is discontiguous and RIPv1 configured  convergence                                                                       will not be reached

Examining the routing tables
  -To examine the contents of  routing updates use the  
  debug ip rip command      
  -If RIPv1 is     configured then
  Subnet masks will not be   included with the     network address

RIPv1 does not support VLSM
  Reason: RIPv1 does not  send subnet mask   in routing updates
RIPv1 does summarize routes to the Classful boundary
       Or uses the Subnet mask  of the outgoing interface  to determine which  subnets to advertise

No CIDR Support
In the diagram R2 will not include the static route in its update
 Reason: Classful routing protocols do not support CIDR routes that are summarized with a smaller mask than the classful subnet mask
-RIPv2 Message format is similar to RIPv1 but has 2  extensions
1st extension is the subnet mask field
2nd extension is the addition of next hop    address
Enabling and Verifying RIPv2
Configuring RIP on a Cisco router
  By default it is running RIPv1

Configuring RIPv2 on a Cisco router
 -Requires using the version 2 command
 -RIPv2 ignores RIPv1 updates
To verify RIPv2 is configured use the
 show ip protocols       command

Auto-Summary & RIPv2
RIPv2 will automatically summarize routes at major network boundaries and can also summarize routes with a subnet mask that is smaller than the classful subnet mask

Disabling Auto-Summary in RIPv2
To disable automatic summarization issue the no auto-summary command
Verifying RIPv2 Updates
When using RIPv2 with automatic summarization turned off
  Each subnet and mask has its own specific entry, along  with the exit interface and next-hop address to reach that  subnet.
To verify information being sent by RIPv2 use the
  debug ip rip command
RIPv2 and VLSM
Networks using a VLSM IP addressing scheme
  Use classless  routing protocols (i.e.  RIPv2) to  disseminate  network addresses  and their subnet  masks

CIDR uses Supernetting
  Supernetting is a bunch of contiguous classful   networks that

To verify that supernets are being sent and received use the following commands
  -Show ip route
  -Debug ip rip
Basic Troubleshooting steps
  -Check the status of all links
  -Check cabling
  -Check IP address & subnet mask configuration 
  -Remove any unneeded configuration commands
Commands used to verify proper operation of RIPv2
Show ip interfaces brief
Show ip protocols
Debug ip rip
Show ip route
Common RIPv2 Issues
When trouble shooting RIPv2 examine the following issues:
Version
   Check to make sure you are using version 2
Network statements
   Network statements may be incorrectly typed    or missing
Automatic summarization
   If summarized routes are not needed then disable   automatic summarization
Reasons why it’s good to authenticate routing information
  -Prevent the possibility of accepting invalid routing  updates
  -Contents of routing updates are encrypted
Types of routing protocols that can use authentication
   -RIPv2
   -EIGRP
   -OSPF
   -IS-IS
   -BGP

Classful IP addressing
As of January 2007, there are over 433 million hosts on internet
Initiatives to conserve IPv4 address space include:
  -VLSM & CIDR notation (1993, RFC 1519)
  -Network Address Translation (1994, RFC 1631)
  -Private Addressing (1996, RFC 1918)

Class A address begin with a 0 bit
   Range of class A addresses = 0.0.0.0 to 127.255.255.255
  Class B address begin with a 1 bit and a 0 bit
   Range of class B addresses = 128.0.0.0 to 191.255.255.255
  Class C addresses begin with two 1 bits & a 0 bit
   Range of class C addresses = 192.0.0.0 to 223.255.255.255.

An IP address has 2 parts:
   -The network portion
    Found on the left side of an IP address
   -The host portion
    Found on the right side of an IP address

Classful Routing Updates
  -Recall that classful routing protocols (i.e. RIPv1)  do not send subnet masks in their routing updates   The reason is that the Subnet mask is   directly related to the network address

Advantage of CIDR :
    -More efficient use of IPv4 address                       space
             -Route summarization
Requires subnet mask to be included in routing update     because address class is meaningless
   Recall purpose of a subnet mask:
    -To determine the network and host portion    of an IP address

Classless IP Addressing
CIDR & Route Summarization
  -Variable Length Subnet Masking (VLSM)
  -Allows a subnet to be further sub-netted    according to individual needs
  -Prefix Aggregation a.k.a. Route Summarization
  -CIDR allows for routes to be summarized as a  single route

Classless Routing Protocol
Characteristics of classless routing protocols:
   -Routing updates include the subnet mask
   -Supports VLSM
   Supports Route Summarization

Classful routing
 -only allows for one  subnet mask for all  networks
VLSM & classless routing
  -This is the process  of subnetting a subnet
  -More than one  subnet mask can be  used
  -More efficient use of IP  addresses as compared  to classful IP  addressing

VLSM – the process of sub-netting a subnet to fit your needs
 -Example:
 Subnet 10.1.0.0/16, 8    more bits are borrowed again, to create 256 subnets with a /24 mask.
 -Mask allows for 254 host addresses per subnet
 -Subnets range from:  10.1.0.0 / 24 to 10.1.255.0 / 24

Route summarization done by CIDR
  -Routes are summarized with masks that are less  than that of the default classful mask
  -Example:
   172.16.0.0 / 13 is the summarized     route for the 172.16.0.0 / 16 to     172.23.0.0 / 16 classful networks

Steps to calculate a route summary
  -List networks in binary  format
  -Count number of left  most matching bits to  determine summary  route’s mask
  -Copy the matching  bits and add zero bits  to determine the  summarized  network address

   -A classful, Distance Vector (DV) routing protocol

  -Metric =  hop count

  -Routes with a hop count > 15 are unreachable

  -Updates are broadcast every 30 seconds

RIP Message Format
RIP header – divided into 3 fields
  -Command field
  -Version field
  -Must be zero
Route Entry – composed of 3 fields
-Address family identifier
-IP address
-Metric

RIP uses 2 message types:
Request message
   -This is sent out on startup by each RIP    enabled interface
   -Requests all RIP enabled neighbors to send   routing table
Response message
   -Message sent to requesting router     containing routing table

IP addresses initially divided into classes
  -Class A
  -Class B
  -Class C
RIP is a classful routing protocol
  -Does not send subnet  masks in routing updates

RIP’s default administrative distance is 120

A typical topology suitable for use by RIPv1 includes:
-Three router set up
-No PCs attached to LANs
    -Use of 5 different IP           subnets

Specifying Networks
Use the network command to:
  -Enable RIP on all   interfaces that    belong to this    network
  -Advertise this    network in RIP    updates     sent to other    routers     every 30 seconds

To verify and troubleshoot routing
    -Use the following    
    commands:
  -show ip route
  -show ip protocols 
  -debug ip rip

show ip protocols command
  Displays  routing  protocol  configured  on  router

Debug ip rip command
  -Used to display RIP routing updates as they are  happening

Passive interface command
  -Used to prevent a router from sending updates through  an interface

To remove the RIP routing  process use the following  command
   No router rip
  To check the configuration  use the following command
   Show run

RIP automatically summarizes classful networks
Boundary routers summarize RIP subnets from one major network to another.

2 rules govern RIPv1 updates:
  -If a routing update and the interface it’s  received on belong to the same  network then
   The subnet mask of the    interface is applied to the    network in the routing update
  -If a routing update and the interface it’s  received on belong to a different  network then
   The classful subnet mask of the   network is applied to  the    network in the routing update

Advantages of automatic summarization:
  -The size of  routing updates is  reduced
  -Single routes are  used to represent  multiple routes  which results in  faster lookup in the  routing table.

Disadvantage of Automatic Summarization:
  -Does not support discontiguous networks
  

Propagating the Default Route in RIPv1
Default-information originate command
-This command is used to specify that the router is to originate default information, by propagating the static default route in RIP update.

The Meaning of Distance Vector:
A router using distance vector routing protocols knows 2 things:
Distance to final destination
Vector, or direction, traffic should be directed

Characteristics of Distance Vector routing protocols:
 Periodic updates
 Neighbors    
 Broadcast updates
 Entire routing table is included with routing update

Routing Protocol Characteristics
Criteria used to compare routing protocols includes
-Time to convergence
-Scalability
-Resource usage
-Implementation & maintenance

Initial Exchange of Routing Information
If a routing protocol is configured then
   -Routers will exchange routing information
Routing updates received from other routers
  -Router checks update for new information
If there is new information:
          -Metric is updated
          -New information is
   stored in routing table

Router convergence is reached when
   -All routing tables in the network contain the same   network information
Routers continue to exchange routing information
  -If no new information is found then Convergence is  reached

§Convergence must be reached before a network is considered completely operable §Speed of achieving convergence consists of 2 interdependent categories

  -Speed of broadcasting routing information

  -Speed of calculating routes 

Convergence must be reached before a network is considered completely operable
Speed of achieving convergence consists of 2 interdependent categories
  -Speed of broadcasting routing information
  -Speed of calculating routes

RIP uses 4 timers
-Update timer
-Invalid timer
-Holddown timer
-Flush timer

EIGRP routing updates are
  -Partial updates
  -Triggered by topology changes
  -Bounded
  -Non periodic

Triggered Updates
Conditions in which triggered updates are sent
  -Interface changes state
  -Route becomes unreachable
  -Route is placed in routing table

Random Jitter
Synchronized updates
A condition where multiple routers on multi access LAN segments transmit routing updates at the same time.
Problems with synchronized updates
-Bandwidth consumption
-Packet collisions
Solution to problems with
  synchronized updates
  - Used of random variable
  called RIP_JITTER

Routing loops are
  A condition in  which a packet is  continuously  transmitted  within  a series of routers  without ever  reaching its  destination

Routing loops may be caused by:
  -Incorrectly configured static routes
  -Incorrectly configured route redistribution
  -Slow convergence
  -Incorrectly configured discard routes
Routing loops can create the following issues
  -Excess use of bandwidth
  -CPU resources may be strained
  -Network convergence is degraded
  -Routing updates may be lost or not processed in a timely  manner

Preventing loops with holddown timers
-Holddown timers allow a router to not accept any changes to a route for a specified period of time.
-Point of using holddown timers
Allows routing updates to propagate through network with the most current information.

The Split Horizon Rule is used to prevent routing loops
Split Horizon rule:
  A router should not advertise a network through the  interface from which the update came.

IP & TTL
Purpose of the TTL field
   The TTL field is found in an IP header and is   used to prevent packets from endlessly    traveling on a network
How the TTL field works
  -TTL field contains a numeric value
   The numeric value is decreased by one by   every router on the route to the destination.
    If numeric value reaches 0 then Packet    is discarded.

Factors used to determine whether to use RIP or EIGRP include
  -Network size
  -Compatibility between models of routers
  -Administrative knowledge

Features of RIP:
   -Supports split horizon & split horizon with   poison reverse
   -Capable of load balancing
   -Easy to configure
   -Works in a multi vendor router environment

The purpose of a dynamic routing protocol is to:
-Discover remote networks
-Maintaining up-to-date routing information

Advantages of static routing
-It can backup multiple interfaces/networks on a router
-Easy to configure
-No extra resources are needed
-More secure
Disadvantages of static routing
-Network changes require manual reconfiguration
-Does not scale well in large topologies

Dynamic routing protocols are grouped according to characteristics.  Examples include:
-RIP
-IGRP
-EIGRP
-OSPF
-IS-IS
-BGP
-Choosing the best path to destination networks
-Ability to find a new best path if the current path is no longer available

Interior Gateway Routing Protocols (IGP)
-Used for routing inside an autonomous system & used to route within the individual networks themselves.
-Examples: RIP, EIGRP, OSPF
Exterior Routing Protocols (EGP)
-Used for routing between autonomous systems
-Example: BGPv4

Distance vector
 routes are advertised as vectors
 of distance & direction.
 incomplete view of network
 topology.
Generally, periodic
   updates.
Link state
 complete view of network
 topology is created.
 updates are not
    periodic.

Classful routing protocols
Do NOT  send subnet mask in routing updates
Classless routing protocols
Do send subnet mask in
routing updates.

Convergence is defined as when all routers’ routing tables are at a state of consistency

Metric
A value used by a routing protocol to determine which routes are better than others.

Metrics used in IP routing protocols
-Bandwidth
-Cost
-Delay
-Hop count
-Load
-Reliability

Metric used for each routing protocol
-RIP – hop count
-IGRP & EIGRP – Bandwidth (used by default), Delay (used by default), Load, Reliability
-IS-IS & OSPF – Cost, Bandwidth (Cisco’s implementation)

Load balancing
This is the ability of a router to distribute packets among multiple same cost paths

Purpose of a metric
It’s a calculated value used to determine the best path to a destination
Purpose of Administrative Distance
It’s a numeric value that specifies the preference of a particular route

Directly connected routes
Have a default AD of 0
Static Routes
Administrative distance of a static route has a default value of 1

Best Path Selections

Forwarding packets to destination

§Connections of a Router for WAN

-A router has a DB-60 port that can support 5 different cabling standards

§Connections of a Router for Ethernet

-2 types of connectors can be used: Straight through and Cross-over

Straight through used to connect:

-Switch-to-Router, Switch-to-PC, Router-to-Server, Hub-to-PC, Hub-to-Server

Cross-over used to connect:

-Switch-to-Switch, PC-to-PC, Switch-to-Hub, Hub-to-Hub, Router-to-Router

Show IP router command – used to view routing table

Show Interfaces command – used to show status of an           interface

Show IP Interface brief command – used to show a portion of            the interface information

Show running-config command – used to show configuration                   file in RAM

By default all serial and Ethernet interfaces are down

To enable an interface use the No Shutdown command

Show interfaces for fastEthernet 0/0 – command used to show   status of fast Ethernet   port

Show ip interface brief

Show running-config

-Enter interface configuration mode

-Enter in the ip address and subnet mask

-Enter in the no shutdown command

§Example:

-R1(config)#interface serial 0/0

-R1(config-if)#ip address 172.16.2.1 255.255.255.0

-R1(config-if)#no shutdown

A WAN Physical Layer connection has sides:

Data Circuit-terminating Equipment (DCE) – This is the service provider.  CSU/DSU is a DCE device.  §Data Terminal Equipment (DTE) – Typically the router is the DTE device. 

A WAN Physical Layer connection has sides:
Data Circuit-terminating Equipment (DCE) – This is the service provider.  CSU/DSU is a DCE device. 
Data Terminal Equipment (DTE) – Typically the router is the DTE device.

Purpose of the debug ip routing command
Allows you to view changes that the router performs when adding or removing routes
Example:
-R2#debug ip routing
-IP routing debugging is on

To configure an Ethernet interface:
-R2(config)#interface fastethernet 0/0
-R2(config-if)#ip address 172.16.1.1 255.255.255.0
-R2(config-if)#no shutdown

Show cdp neighbors command
Displays the following information:
Neighbor device ID
Local interface
Holdtime value, in seconds
Neighbor device capability code
Neighbor hardware platform
Neighbor remote port ID

Configuring a default static route
Similar to configuring a static route.  Except that destination IP address and subnet mask are all zeros
Example:
-Router(config)#ip route 0.0.0.0 0.0.0.0 [exit-interface | ip-address ]

Tools that can be used to isolate routing problems include:
-Ping– tests end to end connectivity
-Traceroute– used to discover all of the hops (routers) along the path between 2 points
-Show IP route– used to display routing table & ascertain forwarding process
-Show ip interface brief- used to show status of router interfaces
-Show cdp neighbors detail– used to gather configuration information about directly connected neighbors