Monday, October 19, 2009

CCNA2 Chapter 7-9 note

Chapter 7 RIP2
Some of these enhanced features include:
Next-hop addresses included in the routing updates
Use of multicast addresses in sending updates
Authentication option available

R2(config-router)#redistribute static
we want the RIP process on R2 to redistribute our static route ( by importing the route into RIP and then sending it to R1 and R3 using the RIP process.

R2(config)#ip route Null0

The address space represented by the static summary route does not actually exist. In order to simulate this static route, we use a null interface as the exit interface.

RIPv1 cannot support discontiguous networks, VLSM, or Classless Inter-Domain Routing (CIDR) supernets.
The autosumarization sometime will be the big problem for the routing table

RIPv1 either summarizes the subnets to the classful boundary or uses the subnet mask of the outgoing interface to determine which subnets to advertise.

all subnets must use the same subnet mask when a classful routing protocol is implemented in the network. If the subnets mask not the same, the route wont send or updates to other router.

R1(config)#router rip
R1(config-router)#version 2
R1(config-router)#version 1

By default, RIPv2 automatically summarizes networks at major network boundaries, just like RIPv1.

R1(config-router)#no auto-summary

This command important! Automatic summarization must be disabled to support discontiguous networks. After this, the route will send updates with individual subnet mask.

debug ip rip

A supernet is a block of contiguous classful networks that is addressed as a single network.

Supernets have masks that are smaller than the classful mask (/16 here, instead of the classful /24).

What is the default behaviour of RIP if no version type specified?
-send version 1 updates only, receive version 1 and updates

tip: remember if RIP1, and it using VLSM, the 1st thing is to "version 2"

The maximum network parameter permitted by default of RIPv2 is 15

A discontiguous network will have two or more subnetworks of a classful network connected together by different classful networks. It occurs when a classful major network address , such as, is seperated by one or more other major networks, like


Chapter 8 Routing Table

Level 1 route is a route with a subnet mask equal to or less than the classful mask of the network address.

A level 1 route can function as a:
Default route - A default route is a static route with the address
Supernet route - A supernet route is a network address with a mask less than the classful mask.
Network route - A network route is a route that has a subnet mask equal to that of the classful mask.

An ultimate route is a route that includes:
either a next-hop IP address (another path)
and/or an exit interface

A level 1 parent route is a network route that does not contain a next-hop IP address or exit interface for any network.

A parent route is actually a heading that indicates the presence of level 2 routes, also known as child routes.

A level 2 route is a route that is a subnet of a classful network address.

level 1 parent route exists only when there is at least one level 2 child route.

Regardless of the addressing scheme used by the network (classless or classful), the routing table will use a classful scheme.

Step in the route lookup process:
After child route search, it will be :

Classful routing behavior: If classful routing behavior is in effect, terminate the lookup process and drop the packet.

ultimate--> parent-->child--> drop

Classless routing behavior: If classless routing behavior is in effect, continue searching level 1 supernet routes in the routing table for a match, including the default route, if there is one.

ultimate--> parent-->child-->level 1 supernet-->default route--> drop

Remember that the route lookup process will need to do a recursive lookup on any route that references only a next-hop IP address and not an exit interface.

Using VSLM does not change the lookup process.

Routing behaviors
no ip classless
ip classless
ip classless

this 2 commands determine the address lookup behavior of the routing process

In IOS versions 11.3 and later, the command ip classless is the default, implementing a classless route lookup process.

A common error is to assume that a default route will always be used if the router does not have a better route. But for Classful routing behaviors, R2's default route is not examined nor used, although it is a match. This is often a very surprising result when a network administrator does not understand the difference between classful and classless routing behavior.


Chapter 9 EIGRP
The main purpose in Cisco's development of EIGRP was to create a classless version of IGRP.

EIGRP has a default administrative distance of 90 for internal routes and 170 for routes imported from an external source, such as default routes.

These features include:
Reliable Transport Protocol (RTP)
Bounded Updates
Diffusing Update Algorithm (DUAL)
Establishing Adjacencies
Neighbor and Topology Tables

RTP and the tracking of neighbor adjacencies set the stage for the EIGRP workhorse, the Diffusing Update Algorithm (DUAL).

As the computational engine that drives EIGRP, DUAL resides at the center of the routing protocol, guaranteeing loop-free paths and backup paths throughout the routing domain.

Instead of hop count, both IGRP and EIGRP use metrics composed of bandwidth, delay, reliability, and load. By default, both routing protocols use only bandwidth and delay.

Loop-free means that the neighbor does not have a route to the destination network that passes through this router.

EIGRP does not use holddown timers. Instead, loop-free paths are achieved through a system of route calculations (diffusing computations) that are performed in a coordinated fashion among the routers.

EIGRP packet header opcode:
Update (1)
Query (3)
Reply (4)
Hello (5)

In the IP packet header, the protocol field is set to 88 to indicate EIGRP, and the destination address is set to the multicast If the EIGRP packet is encapsulated in an Ethernet frame, the destination MAC address is also a multicast address: 01-00-5E-00-00-0A.

In TLV field, By default, only bandwidth and delay are weighted. Both are equally weighted, therefore, the K1 field for bandwidth and the K3 field for delay are both set to 1. The other K values are set to zero.

The Hold Time is the amount of time the EIGRP neighbor receiving this message should wait before considering the advertising router to be down

If the hold time expires, EIGRP will declare the route as down and DUAL will search for a new path by sending out queries.

The IP External message is used when external routes are imported into the EIGRP routing process.

The Destination field stores the address of the destination network. Although only 24 bits are shown in this figure, this field varies based on the value of the network portion of the 32-bit network address. For example, the network portion of is 10.1. Therefore, the Destination field stores the first 16 bits. Because the minimum length of this field is 24 bits, the remainder of the field is padded with zeros. If a network address is longer than 24 bits (, for example), then the Destination field is extended for another 32 bits (for a total of 56 bits) and the unused bits are padded with zeros.

Protocol dependent modules are responsible for the specific routing tasks for each Network layer protocol.

Reliable RTP requires an acknowledgement to be returned by the receiver to the sender. An unreliable RTP packet does not require an acknowledgement.

Hello packets
normal network - sent every 5 seconds.
nonbroadcast multiaccess network(NBMA) eg X25,Frame relay,ATM T1 - sent 60 second

hold time
normal - 15 second
NBMA - 180 seconds
An autonomous system (AS) is a collection of networks under the administrative control of a single entity that presents a common routing policy to the Internet. In the figure, companies A, B, C, and D are all under the administrative control of ISP1. ISP1 "presents a common routing policy" for all of these companies when advertising routes to ISP2.

The ISP is responsible for the routing of packets within its autonomous system and between other autonomous systems.

Although EIGRP refers to the parameter as an "autonomous-system" number, it actually functions as a process ID. This number is not associated with an autonomous system number discussed previously and can be assigned any 16-bit value.

Router1(config)#router eigrp 1
Router2(config)#router eigrp 1
Router3(config)#router eigrp 1

In order to establish neighbor adjacencies, EIGRP requires all routers in the same routing domain to be configured with the same process ID.

The autonomous system parameter is a number chosen by the network administrator between 1 and 65535

To configure EIGRP to advertise specific subnets only, use the wildcard-mask option with the network command:

Router(config-router)#network network-address [wildcard-mask]

show ip eigrp neighbors

By default, EIGRP automatically summarizes routes at the major network boundary. We can disable the automatic summarization with the "no auto-summary" command, just as we did in RIPv2.

Note: EIGRP automatically includes a null0 summary route as a child route whenever both of following conditions exist:
There is at least one subnet that was learned via EIGRP.
Automatic summarization is enabled.

By default, K1 and K3 are set to 1, and K2, K4, and K5 are set to 0.

Router(config-router)#metric weights tos k1 k2 k3 k4 k5

tos is 0 for eigrp

default mteric = k1*bandwidth + k3*delay

Use the interface command bandwidth to modify the bandwidth metric:

Router(config-if)#bandwidth kilobits

That bandwidth is used for the (10,000,000/bandwidth) * 256 portion of the formula. Next, determine the delay value for each outgoing interface on the way to the destination. Sum the delay values and divide by 10 (sum of delay/10) and then multiply by 256 (* 256). Add the bandwidth and sum of delay values to obtain the EIGRP metric.

EIGRP uses the slowest bandwidth in its metric calculation
EIGRP uses the cumulative sum of delay metrics of all of the outgoing interfaces.

DUAL determines the best loop-free path and loop-free backup paths.

The feasibility condition (FC) is met when a neighbor's reported distance (RD) to a network is less than the local router's feasible distance to the same destination network. (refer to the screenshot)

R2#show ip eigrp topology
more specific
R2#show ip eigrp topology

A feasible successor (FS) is a neighbor who has a loop-free backup path to the same network as the successor by satisfying the feasibility condition.

The show ip eigrp topology all-links command shows all possible paths to a network including successors, feasible successors, and even those routes that are not feasible successors

This finite state machine contains all of the logic used to calculate and compare routes in an EIGRP network.

#debug eigrp fsm

When the successor is no longer available and there is no feasible successor, DUAL will put the route into active state. . DUAL will send EIGRP queries asking other routers for a path to this network.

The Null0 summary:
Regardless of whether classful or classless routing behavior is being used, the null0 summary will be used and therefore denying the use of any supernet or default route.

example :Even if a default route was configured, R1 would still discard the packet because it matches the Null0 summary route to

To establish EIGRP manual summarization on all interfaces that send EIGRP packets, use the following interface command:

Router(config-if)#ip summary-address eigrp  as-number network-address subnet-mask

Because R3 has two EIGRP neighbors, the EIGRP manual summarization in configured on both Serial 0/0/0 and Serial 0/0/1.

EIGRP requires the use of the redistribute static command to include this static default route with its EIGRP routing updates.

Note: There is another method to propagate a default route in EIGRP, using the ip default-network comman

Router(config-if)#ip bandwidth-percent eigrp as-number percent
used to configure the percentage of bandwidth that may be used by EIGRP on an interface.

In our example, if bandwidtth is 64kbps,we are limiting EIGRP to no more than 50 percent of the link's bandwidth. Therefore, EIGRP will never use more the 32kbps of the link's bandwidth for EIGRP packet traffic.

Router(config-if)#ip hello-interval eigrp as-number seconds

If you change the hello interval, make sure that you also change the hold time to a value equal to or greater than the hello interval

Router(config-if)#ip hold-time eigrp as-number seconds

What is the purpose of EIGRP neighbor and topology table ?
the neighbour and topology tables are used by DUAL to building table

topology table : tables that contains successor and feasible successor

routing table: contain succssors only

1 comment:

  1. extra note (for EIGRP)

    EIGRP metric mean the COST of the route. And we will take the metric with low value since it mean the cost of the route is lower

    wildcard mask to specify the route

    if the route in routing table, the route is sucessor

    feasible successor
    -loop free backup route

    can R2 FD vs R1 FD(RD)-->

    if r2 FD > RD then
    FC = fulfilled --> R1 can be R2 FS

    #sh ip eigrp topology
    can see what is sucessor and fesible sucessor

    #show ip eigrp toplogy all-links

    5 packet type:

    hello (unreliable)
    achknowledge (unreliable)