CCNA

Distance Vector
Link State
Hybrid
Distance Vector vs. Link State
Route table
Topology
Incremental Update
Periodic Update
Routing by rumor
A
B
C
D
X
E
Distance Vector vs. Link State
Distance Vector
Updates frequently
Each router is "aware" only of its immediate neighbors
Slow convergence
Prone to routing loops
Easy to configure
Link State
Updates are event triggered
Each router is "aware" of all other routers in the "area"
Fast convergence
Less subject to routing loops
More difficult to configure
Comparison Continued
Distance Vector
Fewer router resources required
Updates require more bandwidth
Does not "understand" the topology of the network
Link State
More router resource intensive
Updates require less bandwidth
Has detailed knowledge of distant networks and routers
Link State Routing
Link State
Example
OSPF
IS-IS

OSPF is used for corporate networks
IS-IS is used for ISP’s
Open Shortest Path First (OSPF)
OSPF is an open standards routing protocol
This works by using the Dijkstra algorithm
OSPF provides the following features:
Minimizes routing update traffic
Allows scalability (e.g. RIP is limited to 15 hops)
Has unlimited hop count
Supports VLSM/CIDR
Allows multi-vendor deployment (open standard)
Link State
There are two types of Packets
Hello
LSA’s

OSPF Hello
When router A starts it send Hello packet – uses 224.0.0.5
Hello packets are received by all neighbors
B will write A’s name in its neighbor table
C also process the same way


A
B
C
"Hello" Packets
Small frequently issued packets
Discover neighbours and negotiate "adjacencies"
Verify continued availability of adjacent neighbours
Hello packets and Link State Advertisements (LSAs) build and maintain the topological database
Hello packets are addressed to 224.0.0.5.
Link State Advertisement
(LSA)
An OSPF data packet containing link state and routing information that is shared among OSPF routers

LSAs are shared only with routers with whom it has formed adjacencies

LSA packets are used to update and maintain the topology database.
Link State
There are three type of tables
Neighbor
Topology
Routing

Tables
Neighbor
Contain information about the neighbors
Neighbor is a router which shares a link on same network
Another relationship is adjacency
Not necessarily all neighbors
LSA updates are only when adjacency is established

Tables
Topology
Contain information about all network and path to reach any network
All LSA’s are entered in to topology table
When topology changes LSA’s are generated and send new LSA’s
On topology table an algorithm is run to create a shortest path, this algorithm is known as SPF or dijkstra algorithm

Tables
Routing Table
Also knows as forwarding database
Generated when an algorithm is run on the topology database
Routing table for each router is unique
OSPF Terms
Link
Router ID
Neighbours
Adjacency
OSPF Area
Backbone area
Internal routers
Area Border Router (ABR)
Autonomous System Boundary Router (ASBR)
Link
A network or router interface assigned to a given network
Link (interface) will have "state" information associated with it
Status (up or down)
IP Address
Network type (e.g. Fast Ethernet)
Bandwidth
Addresses of other routers attached to this interface
OSPF Term: Link
A link is a network or router interface assigned to any given network
This link, or interface, will have state information associated with it (up or down) as well as one or more IP addresses
OSPF Term: Link State
Status of a link between two routers
Information is shared between directly connected routers.
This information propagates throughout the network unchanged and is also used to create a shortest path first (SPF) tree.
Router ID
The Router ID (RID) is an IP address used to identify the router

Cisco chooses the Router ID by using the highest IP address of all configured loopback interfaces

If no loopback interfaces are configured with addresses, OSPF will choose the highest IP address of all active physical interfaces.

You can manually assign the router ID.

The RID interface MUST always be up, therefore loopbacks are preferred
Neighbours
Neighbours are two or more routers that have an interface on a common network
E.g. two routers connected on a serial link
E.g. several routers connected on a common Ethernet or Frame relay network
Communication takes place between / among neighbours
neighbours form "adjacencies"
Adjacency
A relationship between two routers that permits the direct exchange of route updates
Not all neighbours will form adjacencies
This is done for reasons of efficiency – more later
OSPF Design
Each router connects to the backbone called area 0, or the backbone area.

Routers that connect other areas to the backbone within an AS are called Area Border Routers (ABRs). One interface must be in area 0.

OSPF runs inside an autonomous system, but can also connect multiple autonomous systems together. The router that connects these ASes together is called an Autonomous System Boundary Router (ASBR).
OSPF Areas
An OSPF area is a grouping of contiguous networks and routers
Share a common area ID
A router can be a member of more than one area (area border router)
All routers in the same area have the same topology database
When multiple areas exist, there must always be an area 0 (the backbone) to which other areas connect
Why areas?
Decreases routing overhead
Compare to multiple smaller broadcast domains instead of one large one

Speeds convergence

Confines network instability (e.g. route "flapping") to single area of the network

Adds considerably to the complexity of setting up OSPF
CCNA certification deals only with single-area OSPF
Area Terminology
LSA’s in Area
LSAs communicate with adjacent routers in the same OSPF area

Subsequently, a change in a link state is "flooded" to all area routers via LSAs

In larger networks, multiple areas may be created
LSAs are sent only to adjacent routers in the same area
"Area border routers" connect areas, passing summarized route information between
Path Calculation
Changes to the topological database of a router trigger a recalculation to re-establish the best route(s) to known networks
Uses the SPF (shortest path first) algorithm developed by a computer scientist named Dijkstra
This is done by each individual router using its detailed "knowledge" of the whole network
Leads to rapid and accurate convergence
Based on detailed knowledge of every link in the area and the OSPF "cost" of each
builds an OSPF tree with itself at the route
Terminology: Cost
Various criteria can be selected by the administrator to determine the metric
Usually,
OSPF cost=108/bandwidth
Do not forget to
configure the
bandwidth` command on serial links to ensure correct
default OSPF cost
Pros and Cons
Note that OSPF is a more sophisticated routing protocol
Converges rapidly and accurately
Can use a metric calculation that effectively selects the "best" route(s) primarily based on bandwidth, although an OSPF cost can be administratively assigned
Use of OSPF requires
More powerful routing hardware
More detailed knowledge by the administrator, especially when large multi-area networks are used
Types of Neighbors
OSPF can be defined for three type of neighbors
Broadcast Multi Access (BMA) ex- Ethernet
Point to Point
Non-Broadcast Multi Access (NBMA)


OSPF Network Types
Adjacencies
Point to Point all routers form adjacencies
BMA & NBMA one router is elected as DR
DR establish adjacency with every neighbor router
LSA updates are exchanged only to DR
DR is the router which has highest priority
All CISCO routers has priority 1
If priority is same then router id is seen
The RID is highest IP address of all interfaces
Point-to-Point Links
Usually a serial interface running either PPP
or HDLC
No DR or BDR election required
OSPF autodetects this interface type
OSPF packets are sent using multicast 224.0.0.5
All routers form adjacencies
Multi-access Broadcast Network
Generally LAN technologies like Ethernet and Token Ring
DR and BDR selection required
All neighbor routers form full adjacencies with the DR and
BDR only
Packets to the DR use 224.0.0.6
Packets from DR to all other routers use 224.0.0.5
Electing the DR and BDR
Hello packets are exchanged via IP multicast.
The router with the highest priority is
selected as the DR.
If Priority is same then Router ID is seen
Use the OSPF router ID as the tie breaker.
Terminology: DRs and BDRs
The designated router (DR) is responsible for generating LSAs on behalf of all routers connected to the same segment
DR Responsibility
When a router sees a new or changed link-state, it sends an LSA to its DR using a particular multicast address

The DR then forwards the LSA to all the other routers with whom it is adjacent
Minimizes the number of formal adjacencies that must be formed and therefore the amount of LSU (link state update) packet traffic in a multi-router network
OSPF Summary
AD -100
Hop count is unlimited
Metric = Cost – 108/BW
Classless, VLSM
Load balance up to SIX routers
Require more processing power
Basic OSPF Configuration
Router(config)# router ospf 1
The number 1 in this example is a process-id # that begins an OSPF process in the router
More than one process can be launched in a router, but this is rarely necessary
Usually the same process-id is used throughout the entire network, but this is not required
The process-id # can actually be any value from 1 to "very large integer“
The process-id # cannot be ZERO
This is NOT the same as the AS# used in IGRP and EIGRP
Configuring OSPF Areas
After identifying the OSPF process, you need to identify the interfaces that you want to activate OSPF communications
Lab_A#config t
Lab_A(config)#router ospf 1
Lab_A(config-router)#network 10.0.0.0 0.255.255.255
area ?
<0-4294967295> OSPF area ID as a decimal value
A.B.C.D OSPF area ID in IP address format
Lab_A(config-router)#network 10.0.0.0 0.255.255.255
area 0
Every OSPF network must have an area 0 (the backbone area) to which other areas connect
So in a multiple area network, there must be an area 0
The wildcard mask represents the set of hosts supported by the network and is really just the inverse of the subnet mask.
OSPF Configuration
OSPF Process ID number is irrelevant. It can be the same on every router on the network
The arguments of the network command are the network number (10.0.0.0) and the wildcard mask (0.255.255.255)
Wildcards - A 0 octet in the wildcard mask indicates that the corresponding octet in the network must match exactly
A 255 indicates that you don’t care what the corresponding octet is in the network number
A network and wildcard mask combination of 1.1.1.1 0.0.0.0 would match 1.1.1.1 only, and nothing else.
The network and wildcard mask combination of 1.1.0.0 0.0.255.255 would match anything in the range 1.1.0.0–1.1.255.255
OSPF Configuration -1
10.0.0.1
20.0.0.1
20.0.0.2
30.0.0.1
30.0.0.2
40.0.0.1
10.0.0.2
40.0.0.2
OSPF Configuration -1
R1#config t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#router ospf 1
R1(config-router)#network 10.0.0.0 0.255.255.255 area 0
R1(config-router)#network 20.0.0.0 0.255.255.255 area 0
R1(config-router)#^Z
OSPF Configuration -2
200.0.0.16/28
200.0.0.8/30
200.0.0.12/30
200.0.0.32/27
OSPF Configuration -2
200.0.0.17
200.0.0.9
200.0.0.10
200.0.0.13
200.0.0.14
200.0.0.33
200.0.0.18
200.0.0.34
255.255.255.240
255.255.255.252
255.255.255.252
255.255.255.224
OSPF Configuration -2
200.0.0.17
200.0.0.9
200.0.0.10
200.0.0.13
200.0.0.14
200.0.0.33
200.0.0.18
200.0.0.34
255.255.255.240
255.255.255.252
255.255.255.252
255.255.255.224
R1#config t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#router ospf 1
R1(config-router)#network 200.0.0.16 0.0.0.15 area 0
R1(config-router)#network 200.0.0. 8 0.0.0.3 area 0
R1(config-router)#^Z
R3#config t
Enter configuration commands, one per line. End with CNTL/Z.
R3(config)#router ospf 1
R3(config-router)#network 200.0.0. 32 0.0.0.31 area 0
R3(config-router)#network 200.0.0. 12 0.0.0.3 area 0
R3(config-router)#^Z
OSPF and Loopback Interfaces
Configuring loopback interfaces when using the OSPF routing protocol is important
Cisco suggests using them whenever you configure OSPF on a router
Loopback interfaces are logical interfaces, which are virtual, software-only interfaces; they are not real router interfaces
Using loopback interfaces with your OSPF configuration ensures that an interface is always active for OSPF processes.
The highest IP address on a router will become that router’s RID
The RID is used to advertise the routes as well as elect the DR and BDR.
If you configure serial interface of your router with highest IP Address this Address becomes RID of t is the RID of the router because e router
If this interface goes down, then a re-election must occur
It can have an big impact when the above link is flapping
Configuring Loopback Interfaces
R1#config t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#int loopback 0
R1(config-if)#ip address 172.16.10.1 255.255.255.255
R1(config-if)#no shut
R1(config-if)#^Z
R1#
show ip protocols
Router#
Verifies the configured IP routing protocol processes, parameters and statistics
Verifying OSPF Operation
show ip route ospf
Router#
Displays all OSPF routes learned by the router
show ip ospf interface
Router#
Displays the OSPF router ID, area ID and adjacency information
show ip ospf
Router#
Displays the OSPF router ID, timers, and statistics

Verifying OSPF Operation (Cont.)
show ip ospf neighbor [detail]
Router#
Displays information about the OSPF neighbors, including Designated Router (DR) and Backup Designated Router (BDR) information on broadcast networks
The show ip route ospf Command
RouterA# show ip route ospf

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile,
B - BGP, D - EIGRP, EX - EIGRP external, O - OSPF,
IA - OSPF inter area, E1 - OSPF external type 1,
E2 - OSPF external type 2, E - EGP, i - IS-IS, L1 - IS-IS
level-1, L2 - IS-IS level-2, * - candidate default

Gateway of last resort is not set
10.0.0.0 255.255.255.0 is subnetted, 2 subnets
O 10.2.1.0 [110/10] via 10.64.0.2, 00:00:50, Ethernet0
The show ip ospf interface Command
RouterA# show ip ospf interface e0

Ethernet0 is up, line protocol is up
Internet Address 10.64.0.1/24, Area 0
Process ID 1, Router ID 10.64.0.1, Network Type BROADCAST, Cost: 10
Transmit Delay is 1 sec, State DROTHER, Priority 1
Designated Router (ID) 10.64.0.2, Interface address 10.64.0.2
Backup Designated router (ID) 10.64.0.1, Interface address 10.64.0.1
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:04
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 10.64.0.2 (Designated Router)
Suppress hello for 0 neighbor(s)
The show ip ospf neighbor Command
RouterB# show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface
10.64.1.1 1 FULL/BDR 00:00:31 10.64.1.1 Ethernet0
10.2.1.1 1 FULL/- 00:00:38 10.2.1.1 Serial0
show ip ospf neighbor detail
show ip ospf database
Setting Priority for DR Election
ip ospf priority number
This interface configuration command assigns the OSPF priority to an interface.
Different interfaces on a router may be assigned different values.
The default priority is 1. The range is from 0 to 255.
0 means the router is a DROTHER; it can’t be the DR or BDR.
Router(config-if)#
EIGRP
IGRP
DV
Easy to configure
Neighbor
Advanced Metric
Periodic
Broadcast
OSPF
LS
Incremental Updates
Multicast
Open Standard
EIGRP
Hybrid
DUAL
Topology Database
Rapid Convergence
Reliable
Overview
Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco-proprietary routing protocol based on Interior Gateway Routing Protocol (IGRP).

Released in 1994, Unlike IGRP, which is a classful routing protocol, EIGRP supports CIDR and VLSM.

it is probably one of the two most popular routing protocols in use today.

Compared to IGRP, EIGRP boasts faster convergence times, improved scalability, and superior handling of routing loops.

EIGRP is often described as a hybrid routing protocol, offering the best of distance vector and link-state algorithms.
Comparing EIGRP with IGRP
IGRP and EIGRP are compatible with each other.
EIGRP offers multiprotocol support, but IGRP does not.
Communication via Reliable Transport Protocol (RTP)
Best path selection via Diffusing Update Algorithm (DUAL)
Improved convergence time
Reduced network overhead

Introducing EIGRP
EIGRP supports:
Rapid convergence
Reduced bandwidth usage
Multiple network-layer protocols
EIGRP Tables
EIGRP maintains 3 tables

Neighbor table
Topology table
Routing table
Neighbor Discovery
There are three conditions that must be met for neighborship establishment
Hello or ACK received
AS numbers match
Identical metrics (K values)

Hello
? AS
? K
K1 – BW
K2- Delay
K3-Load
K3-Reliability
K5-MTU
The metrics used by EIGRP in making routing decisions are (lower the metric the better):
bandwidth
delay
load
Reliability
MTU

By default, EIGRP uses only:
Bandwidth
Delay

Analogies:
Think of bandwidth as the width of the pipe
and
delay as the length of the pipe.

Bandwidth is the carrying capacity
Delay is the end-to-end travel time.
Metric Calculation
Neighbor Table
The neighbor table is the most important table in EIGRP

Stores address and interface of neighbor

Topology Table
Give me information about all routes
Network
Topology Table
The topology table is made up of all the EIGRP routing tables in the autonomous system.

DUAL takes the information and calculates the lowest cost routes to each destination.

By tracking this information, EIGRP routers can identify and switch to alternate routes quickly.

The information that the router learns from the DUAL is used to determine the successor route, which is the term used to identify the primary or best route.

Every EIGRP router maintains a topology table. All learned routes to a destination are maintained in the topology table.
Routing Tables
A successor is a route selected as the primary route to use to reach a destination.
DUAL calculates Successor (Primary Route) and places it in the routing table (and topology table)
Can have up to 4 successors of equal or unequal value
DUAL calculates Feasible Successor (Backup Route) and places it in the Topology Table.
Promoted to successor if the route goes down if it has a lower cost than current successor
If no FS in Table - Send query
Multiple feasible successors for a destination can be retained in the topology table although it is not mandatory
EIGRP Concepts & Terminology
EIGRP routers that belong to different autonomous systems (ASes) don’t automatically share routing information

The only time EIGRP advertises its entire routing table is when it discovers a new neighbor and forms an adjacency with it through the exchange of Hello packets

When this happens, both neighbors advertise their entire routing tables to one another

After each has learned its neighbor’s routes, only changes to the routing table are propagated
172.16.100.0
1.544Mbps
56Kbps
1.544Mbps
Dist to 172.16.100.0 =100
Dist to 172.16.100.0 =100
Dist to 172.16.100.0 =350
10Mbps
10Mbps – 100
1,544Mbps – 250
56Kbps -1000
Chennai receives an update from Mumbai with a cost of 100, which is Mumbai`s cost to reach 172.16.100.0, This cost is referred to as the reported distance (RD)
Bangalore will report its cost to reach 172.16.100.0. Bangalore`s RD is 350
Chennai will compute its cost to reach 172.16.100.0 via Mumbai and Bangalore and compare the metrics for the two paths
Chennai`s cost via Mumbai is 1100. Chennai`s cost via Bangalore is 600. The lowest cost to reach a destination is referred to as the feasible distance (FD) for that destination
Chennai`s FD to 172.16.100.0 is 600. The next-hop router in the lowest-cost path to the destination is referred to as the successor.
A feasible successor is a path whose reported distance is less than the feasible distance, and it is considered a backup route.
EIGRP Terms
Feasible distance (FD) - This is the lowest calculated metric to reach destination. This is the route that you will find in the routing table, because it is considered the best path

Reported distance (RD) - The distance reported by an adjacent neighbor to a specific destination.

Interface information - The interface through which the destination can be reached.

Route status - The status of a route. Routes are identified as being either passive, which means that the route is stable and ready for use, or active, which means that the route is in the process of being recomputed by DUAL
Successor – Current Route
A successor is a route selected as the primary route to use to reach a destination.
Successors are the entries kept in the routing table.

Feasible Successor - A backup route
A feasible successor is a backup route.
These routes are selected at the same time the successors are identified, but they are kept in the topology table.
Multiple feasible successors for a destination can be retained in the topology table.
EIGRP Terminology and Operations
Reliable Transport Protocol (RTP)
Used by EIGRP for its routing updates in place of TCP
EIGRP can call on RTP to provide reliable or unreliable service
EIGRP uses reliable service for route updates
Unreliable for Hellos

Reliable Transport Protocol (RTP) is a transport layer protocol that guarantees ordered delivery of EIGRP packets to all neighbors.
On an IP network, hosts use TCP to sequence packets and ensure their timely delivery. RIP uses UDP
However, EIGRP is protocol-independent and does not rely on TCP/IP to exchange routing information the way that RIP, IGRP, and OSPF do.
EIGRP uses RTP as its own proprietary transport layer protocol to guarantee delivery of routing information.
With RTP, EIGRP can multicast and unicast to different peers simultaneously.
Diffusing Update Algorithm (DUAL)
All route computations in EIGRP are handled by DUAL
One of DUAL`s tasks is maintaining a table of loop-free paths to every destination.
This table is referred to as the topology table
DUAL saves all paths in the topology table
The least-cost path(s) is copied from the topology table to the routing table
In the event of a failure, the topology table allows for very quick convergence if another loop-free path is available
If a loop-free path is not found in the topology table, a route recomputation must occur
DUAL queries its neighbors, who, in turn, may query their neighbors, and so on...
Hence the name "Diffusing" Update Algorithm
VLSM Support
EIGRP supports the use of Variable- Length Subnet Masks

Can use 30-bit subnet masks for point-to-point networks

Because the subnet mask is propagated with every route update, EIGRP also supports the use of discontiguous subnets

Discontiguous network is the one that has two or more subnetworks of a classful network connected together by different classful networks
Discontiguous Network
EIGRP & IGRP Metric Calculation
Configuring EIGRP
Router(config-router)#network network-number
Selects participating attached networks
Router(config)#router eigrp autonomous-system
Defines EIGRP as the IP routing protocol
EIGRP Configuration Example
EIGRP Configuration
200.0.0.17
200.0.0.9
200.0.0.10
200.0.0.13
200.0.0.14
200.0.0.33
200.0.0.18
200.0.0.34
255.255.255.240
255.255.255.252
255.255.255.252
255.255.255.224
R1#config t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#router eigrp 10
R1(config-router)#network 200.0.0.16
R1(config-router)#network 200.0.0. 8
R1(config-router)#^Z
R3#config t
Enter configuration commands, one per line. End with CNTL/Z.
R3(config)#router eigrp 10
R3(config-router)#network 200.0.0. 32
R3(config-router)#network 200.0.0. 12
R3(config-router)#^Z
Verifying the EIGRP Configuration
To verify the EIGRP configuration a number of show and debug commands are available.

These commands are shown on the next few slides.
show ip eigrp topology
show ip eigrp topology
[active | pending | successors]
show ip eigrp topology
all-links
show ip eigrp traffic
Administrative Distances
TELNET
Getting information about remote device
Can connect to remote device and configure a device
Password must be set
R1(config)# line vty 0 4
Password cisco
login
© 2002, Cisco Systems, Inc. All rights reserved.
86
Discovering Neighbors on the Network
Cisco Discovery Protocol
CDP is a proprietary utility that gives you a summary of directly connected switches, routers, and other Cisco devices.
CDP discovers neighboring devices regardless of which protocol suite they are running.
Runs on the Data link layer
Physical media must support the Subnetwork Access Protocol (SNAP) encapsulation.
Only give directly connected device
By default enabled, you can enable or disable
Discovering Neighbors with CDP
CDP runs on routers with Cisco IOS®
software Release 10.3 or later and on Cisco switches.
Show CDP ?
Summary information
includes:
Device ID
Local Interface
Port ID
Capabilities list
Platform
CDP
CDP timer is how often CDP packets are transmitted to all active interfaces.

Router(config)#cdp timer 90

CDP holdtime is the amount of time that the device will hold packets received from neighbor devices.
Router(config)#cdp holdtime 240
Using CDP
Using the show cdp
neighbors Command
The show cdp neighbor command (sh cdp nei for short) delivers information about directly connected devices.

CDP
show cdp neighbor detail

This command can be run on both routers and switches, and it displays detailed information about each device connected to the device
Using the show cdp entry Command
The show cdp entry * command displays the same information as the show cdp
neighbor details command.

Additional CDP Commands
The show cdp traffic command displays information about interface traffic, including the number of CDP packets sent and received and the errors with CDP.
CDP Commands
To disable the CDP on particular interface use the "no cdp enable" command



To disable CDP on the entire router use the "no cdp run" in global configuration mode.
Summary
Cisco Discovery Protocol is an information-gathering tool used by network administrators to get information about directly connected devices.

CDP exchanges hardware and software device information with its directly connected CDP neighbors.

You can enable or disable CDP on a router as a whole or on a port-by-port basis.

The show cdp neighbors command displays information about a router’s CDP neighbors.

The show cdp entry, show cdp traffic, and show cdp interface commands display detailed CDP information on a Cisco device.
Manage IP traffic as network access grows
Filter packets as they pass through the router
Why Use Access Lists?
What are ACLs?
ACLs are lists of conditions that are applied to traffic traveling across a router`s interface. 

These lists tell the router what types of packets to accept or deny.

Acceptance and denial can be based on specified conditions.

ACLs can be configured at the router to control access to a network or subnet.

Some ACL decision points are source and destination addresses, protocols, and upper-layer port numbers.

Reasons to Create ACLs
The following are some of the primary reasons to create ACLs:

Limit network traffic and increase network performance.
Provide traffic flow control.
Provide a basic level of security for network access.
Decide which types of traffic are forwarded or blocked at the router interfaces
For example: Permit e-mail traffic to be routed, but block all telnet traffic.
If ACLs are not configured on the router, all packets passing through the router will be allowed onto all parts of the network.
ACL’s
Different access list for Telnet
When configuring ISDN you need to use access list
Implicit deny at bottom
All restricted statements should be on first
There are two types
Standard
Extended

Network
N1
N2
N3
N4
N5
N6
192.168.12.0
A
B
C
192.168.34.0
192.168.56.0
192.168.12.2
192.168.12.3
IP Packet
SRC IP Address
DEST IP Address
Protocol type
SRC Port
DEST Port
The first 2 bytes in the TCP/UDP header are the source port number
The next 2 bytes in the TCP/UDP header are the Destination port number

Standard
Checks source address
Permits or denies entire protocol suite
Extended
Checks source and destination address
Generally permits or denies specific protocols
Types of Access Lists
How to Identify Access Lists
Standard IP lists (1-99) test conditions of all IP packets from
source addresses.
Extended IP lists (100-199) test conditions of source and destination addresses, specific TCP/IP protocols, and destination ports.
Standard IP lists (1300-1999) (expanded range).
Extended IP lists (2000-2699) (expanded range).
Standard ACLs
The full syntax of the standard ACL command is:

Router(config)#access-list access-list-number {deny | permit} source [source-wildcard ]

The no form of this command is used to remove a standard ACL. This is the syntax:
Router(config)#no access-list access-list-number
Config# Access-list 1 deny 192.168.1.0 0.0.0.255
Config# access-list 1 permit any
Wildcard Mask
Access-list 99 permit 192.168.1.1 wildcard mask
All 32 bits of an IP Address can be filtered
Wildcard inverse mask
0=must match
1= ignore
The ANY and HOST keyword
Access-list 1 permit 200.0.0.9 0.0.0.0
Or
permit host 200.0.0.9
Access-list 1 permit 0.0.0.0 255.255.255.255
Or
permit any
Testing Packets with
Standard Access Lists
Outbound ACL Operation
If no access list statement matches, then discard the packet.
Reading an ACL
First Hit or Best Fit?
Access-list 99 deny host 192.168.1.1 0.0.0.0
access-list 99 permit any 255.255.255.255

Access-list 99 permit 192.168.1.0 0.0.0.255
Access-list 99 deny host 192.168.1.1
access-list 99 permit any

Access-list 99 deny host 192.168.1.1

Implicit deny at the end of every ACL


Creating ACLs
ACLs are created in the global configuration mode. There are many different types of ACLs including standard, extended, IPX, AppleTalk, and others. When configuring ACLs on a router, each ACL must be uniquely identified by assigning a number to it. This number identifies the type of access list created and must fall within the specific range of numbers that is valid for that type of list.
Since IP is by far the most popular routed protocol, addition ACL numbers have been added to newer router IOSs.
Standard IP: 1300-1999
Extended IP: 2000-2699
The ip access-group command
Exercise – Standard Access List
Account should be denied access to Sales

To steps to configure
Create a standard Access list
Apply ACL to proper interface inbound or outbound
S0
S0
E0
E0
192.168.0.18
255.255.255.248
S0
S1
192.168.0.17
255.255.255.248
192.168.0.5
255.255.255.252
192.168.0.6
255.255.255.252
192.168.0.9
255.255.255.252
192.168.0.10
255.255.255.252
192.168.0.33
255.255.255.240
192.168.0.34
255.255.255.240
Exercise – Standard Access List
S0
S0
E0
E0
192.168.0.18
255.255.255.248
S0
S1
192.168.0.17
255.255.255.248
192.168.0.5
255.255.255.252
192.168.0.6
255.255.255.252
192.168.0.9
255.255.255.252
192.168.0.10
255.255.255.252
192.168.0.33
255.255.255.240
192.168.0.34
255.255.255.240
Config# Access-list 1 deny 192.168.0.18 0.0.0.7
Config# access-list 1 permit any
Config#int e 0
Config-if# ip access-group 1 out
Extended ACLs
Extended ACLs are used more often than standard ACLs because they provide a greater range of control.

Extended ACLs check the source and destination packet addresses as well as being able to check for protocols and port numbers.

At the end of the extended ACL statement, additional precision is gained from a field that specifies the optional Transmission Control Protocol (TCP) or User Datagram Protocol (UDP) port number.

Logical operations may be specified such as, equal (eq), not equal (neq), greater than (gt), and less than (lt), that the extended ACL will perform on specific protocols.

Extended ACLs use an access-list-number in the range 100 to 199 (also from 2000 to 2699 in recent IOS).
Configuration
Access-list acl# {permit/Deny}
Protocol
Src IP src WCM
Dst IP dst WCM
Opetrator port
Protocol
OSPF
EIGRP
ICMP
TCP
UDP
RP
If you need to Block a routing protocol
IP
Operator
eq
gt
lt
neq

Testing Packets with
Extended Access Lists
Extended ACL Syntax
Extended ACL LAB
Account should be denied Sales Web site
Config# Access-list 100 deny tcp 200.0.0.10 0.0.0.7 200.0.0.18 0.0.0.15 eq www
Config# access-list 100 permit IP any any
Config#int Fastethernet 0/0
Config-if# ip access-group 100 IN
Extended ACL LAB -2
S0
S0
E0
E0
192.168.0.34 should be denied FTP of 192.168.0.18

On Router R1
Config# Access-list 100 deny tcp 192.168.0.34 0.0.0.0 192.168.0.18 0.0.0.0 eq 21
Config# access-list 100 permit IP any any

Config#int s0
Config-if# ip access-group 100 IN
192.168.0.18 should be denied website of 192.168.0.34

On Router R3
Config# Access-list 100 deny tcp 192.168. 0.18 0.0.0.0 192.168.0.34 0.0.0.0 eq 80
Config# access-list 100 permit IP any any

Config#int s0
Config-if# ip access-group 100 IN
S1
S0
192.168.0.18
255.255.255.248
192.168.0.17
255.255.255.248
192.168.0.5
255.255.255.252
192.168.0.6
255.255.255.252
192.168.0.9
255.255.255.252
192.168.0.10
255.255.255.252
192.168.0.33
255.255.255.240
192.168.0.34
255.255.255.240
Deny FTP
access-list 101 deny tcp any any eq 21

access-list 101 permit ip any any

or

access-list 101 deny tcp any any eq ftp

access-list 101 permit ip any any
Rules
For extended access list apply near to the source
For standard access list apply near to the destination
Named ACLs
IP named ACLs were introduced in Cisco IOS Software Release 11.2, allowing standard and extended ACLs to be given names instead of numbers.

The characteristics of named accesslist:
Identify an ACL using an alphanumeric name.
You can delete individual statements in a named access list
Named access lists must be specified as standard or extended
You can use the ip access-list command to create named access lists.

Named ACLs are not compatible with Cisco IOS releases prior to Release 11.2.

The same name may not be used for multiple ACLs.

Named ACL’s
Numbered Access list did not give you any hint, What is filtered

Named ACL’s are both basic and advanced filtering tool

Name cannot start with a number or !

Cannot have space in the name

Should not have ? Character anywhere in the name

Name is case sensitive
Named ACL Example
R1(config)#ip access-list standard blocksales
R1(config-std-nacl)#deny 172.16.40.0 0.0.0.255
R1(config-std-nacl)#permit any
R1(config-std-nacl)#exit
R1(config)#^Z
R1#

#Int e 0
#Ip access-group blocksales out


Verify Access List
Basic Rules for ACLs
Standard IP access lists should be applied closest to the destination.
Extended IP access lists should be applied closest to the source.
Use the inbound or outbound interface reference as if looking at the port from inside the router.
Statements are processed sequentially from the top of list to the bottom until a match is found, if no match is found then the packet is denied.
There is an implicit deny at the end of all access lists. This will not appear in the configuration listing.
Access list entries should filter in the order from specific to general. Specific hosts should be denied first, and groups or general filters should come last.
Never work with an access list that is actively applied.
New lines are always added to the end of the access list.
A no access-list x command will remove the whole list. It is not possible to selectively add and remove lines with numbered ACLs.
Outbound filters do not affect traffic originating from the local router.