Cisco^® CCNA-ICND2^™ Exam Notes

A. Routing Protocols

1. Given below are some important features of classful and classless routing protocols:

Classful routing protocols: RIP v1, IGRP are examples of classful routing protocols. It is important to know that classful routing protocols do not exchange subnet information during routing information exchanges. The summarization is always done automatically at major network boundaries. Classless routing protocols: RIP v2, EIGRP, OSPF, BGP v4, and IS-IS are examples of classless routing protocols. In classless routing protocols, subnet information is exchanged during routing updates. This results in more efficient utilization of IP addresses. The summarization in classless networks is manually controlled.

i. RIP

1. RIP takes only hop count into account when computing routing entries. Shortest hop count is the winner. Therefore, if RIP is used as routing protocol, a packet will travel from A to B using the 56KBPS link. When both EIGRP and OSPF are configured, EIGRP route takes precedence over OSPF because EIGRP has an administrative distance of 90, whereas OSPF has an administrative distance of 110. Therefore, the route discovered by EIGRP is entered into the routing table. OSPF determines the route by taking only bandwidth into account.

ii. EIGRP

1. EIGRP (as well as IGRP) uses Bandwidth and Delay as default criteria to determine the best path. The description of the terms is given below:

1. Bandwidth: This is the smallest bandwidth between the source and destination.
2. Delay: This is the cumulative interface delay along the path.
3. Reliability: This is the worst case reliability between source and destination based on keep alives.
4. Loading: This is the worst case load on a link between source and destination based on bps.
5. MTU: Smallest MTU in path.

2. EIGRP uses multicasts to send queries to neighbor routers.

3. EIGRP uses auto summarization of routes at major network boundaries.

4. The following are main features of route summarization in EIGRP:

1. By default, EIGRP summarizes routes at the major network boundaries (classful boundaries). 
2. To enable summarization at any level other than major network boundary, you need to disable auto summarization using the command: No auto-summary 
3. The following command enables summarization at an arbitrary network boundary: Ip summary-address <as-number> <address-mask>
4. Note that you need to specify the IP address and routing mask of the summary route. No need to specify the metrics.
5. Some of the important terms used in Enhanced IGRP are: 
Successor: A route (or routes) selected as the primary route(s) used to transport packets to reach destination. Note that successor entries are kept in the routing table of the router. 
Feasible successor: A route (or routes) selected as backup route(s) used to transport packets to reach destination. Note that feasible successor entries are kept in the topology table of a router. There can be up to 6 (six) feasible successors for IOS version 11.0 or later. The default is 4 feasible successors. 
DUAL (Diffusing Update Algorithm): Enhanced IGRP uses DUAL algorithm to calculate the best route to a destination.
6. For IGRP routing, you need to provide the AS (Autonomous System) number in the command. Routers need AS number to exchange routing information. Routers belonging to same AS exchange routing information.
7. By giving the command "show ip route igrp", we can see the routes found by igrp. A route discovered by igrp is denoted by letter "I" before start of the entry.
8. EIGRP:
· EIGRP update packet is sent every 90 seconds by default. This is 30 Sec for RIP.
· IGRP, and OSPF use Autonomous System (AS) numbers. An IGRP AS is a complex network that has diverse bandwidth and delay characteristics.
9. RIP (and IGRP) always summarizes routing information by major network numbers. This is called classful routing.
· RIP and IGRP are examples of routing protocols that use distance vector. In RIP, the maximum hop count allowed is 15 hops. A hop count of 16 is considered as unreachable. An RIP router determines the path to the destination based on the on the amount of hops it takes to reach the destination. If it had two different ways to reach the destination, it will simply send the packet via the shortest path (minimum hop count), regardless of the connection speed. This is commonly known as pinhole congestion.
· RIP sends its complete routing table out to all active interfaces at regular intervals (every 30 seconds by default) and when the network topology changes. RIP routers maintain only the best route (the route with the lowest metric value) to a destination. After updating its routing table, the router immediately begins transmitting routing updates to inform neighbors of the change. These updates are sent independently of the regularly scheduled updates that RIP routers send. In RIP, update packets are sent to the immediate neighbors.
10. The following are main features of route summarization in EIGRP:
· By default, EIGRP summarizes routes at the major network boundaries (classful boundaries). 
· To enable summarization at any level other than major network boundary, you need to disable auto summarization using the command:
“No auto-summary”
· The following command enables summarization at an arbitrary network boundary:
“Ip summary-address <as-number> <address-mask>”
· Note that you need to specify the IP address and routing mask of the summary route. No need to specify the metrics.
11. The default administrative distances for various protocols are as below:

iii. OSPF

1. OSPF is a link state technology that uses Dijkstra algorithm to compute routing information. It has the following advantages over Distance Vector protocols such as RIP: 

1. Faster convergence: OSPF network converges faster because routing changes are flooded immediately and computer in parallel. 
2. Support for VLSM: OSPF supports VLSM. However, please note that RIP version2 also supports VLSM. 
3. Network Reachability: RIP networks are limited to 15 hops. Therefore, networks with more than 15 hops can not be reached by RIP by normal means. On the other hand, OSPF has practically no reachability limitation. 
4. Metric: RIP uses only hop count for making routing decisions. This may lead to severe problems in some cases, for example, that a route is nearer but is very slow compared to another route with plenty of bandwidth available. OSPF uses "cost" metric to choose best path. Cisco uses "bandwidth" as metric to choose best route. 
5. Efficiency: RIP uses routing updates every 30 seconds. OSPF multicasts link-state updates and sends the updates only when there is a change in the network.

2. OSPF determines the router ID using the following criteria: 

1. Use the address configured by the ospf router-id command 
2. Use the highest numbered IP address of a loopback interface 
3. Use the highest IP address of any physical interface 
4. If no interface exists, set the router-ID to 0.0.0.0 

3. If no OSPF router ID is explicitly configured, OSPF computes the router-ID based on the items 2, 3, and 4 and restarts OSPF (if the process is enabled and router-ID has changed).

4. OSPF keeps up to six equal-cost route entries in the routing table for load balancing. Further, OSPF uses Dijkstra algorithm to calculate lowest cost route. The algorithm adds up the total costs between the local router and each destination network. The lowest cost route is always preferred when there are multiple paths to a given destination.

5. OSPF process identifier is locally significant. Two neighboring router interfaces can have same or different process ids. It is required to identify a unique instance of OSPF database.

6. The command 'show ip ospf interfaces' shows the summary information for OSPF interfaces. The information provided include autonomous system number, router id, designated router id, backup designated router id, hello time, and adjacent neighbors.

7. The command "show ip ospf database" displays the contents of the topological database maintained by the router. This command also displays router id and the ospf process id.

8. The command that is used for configuring OSPF in NBMA mode is: “ip ospf network non-broadcast”. However, note that NBMA mode is used by default.

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