Cisco Certified Network Associate (CCNA™)
E. Routing Protocols
1. 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.
Maximum hop count supported by RIP is 15.
2. Routed and Routing Protocols: Routing protocols job is to maintain routing tables and route packets appropriately. Examples of routing protocols are RIP, IGRP, EIGRP, OSPF. Routers can support multiple independent routing protocols and can update and maintain routing tables for each protocol independently.
Routed protocols are used to transport user traffic from source node to destination node. Examples of routed protocols are IP, IPX, and AppleTalk.
3. There are broadly three types of routing protocols:
1. Distance Vector (Number of hops) - Distance vector routing determines the direction (vector) and distance to any link in the internetwork. Typically, the smaller the metric, the better the path. EX: Examples of distance vector protocols are RIP and IGRP. Distance vector routing is useful for smaller networks. The limitation is that any route which is greater than 15 hops is considered unreachable. Distance vector protocols listen to second hand information to learn routing tables whereas, Link state protocols build routing tables from first hand information. Routers with distance vector protocols send its entire routing table to each of its adjacent neighbors.
2. Link State Routing: Link State algorithms are also known as Shortest Path First (SPF) algorithms. SPF generates the exact topology of the entire network for route computation, by listening to the first hand information. Link State protocols take bandwidth into account using a cost metric. Link State protocols only send updates when a change occurs, which makes them more efficient for larger networks. Bandwidth and delay are the most widely used metrics when using Link-State protocols. EX: OSPF and NLSP.
Benefits of Link State protocols:
Allows for a larger scalable network
Reduces convergence time
The metric limit for link-state protocols is 65,533
3. Balanced Hybrid - Balanced Hybrid combines some aspects of Link State and Distance Vector routing protocols. Balanced Hybrid uses distance vectors with more accurate metrics to determine the best paths to destination networks. Ex: EIGRP
4. Distance vector protocol depends only on Hop count to determine the nearest next hop for forwarding a packet. One obvious disadvantage is that, if you have a destination connected through two hops via T1 lines, and if the same destination is also connected through a single hop through a 64KBPS line, RIP assumes that the link through 64KBPS is the best path!
5. RIP (and IGRP) always summarizes routing information by major network numbers. This is called classful routing.
6. RIP, RIP2, and IGRP use distance vector algorithms.
RIP2 transmits the subnet mask with each route. This feature allows VLSM (Variable Length Subnet Masks) by passing the mask along with each route so that the subnet is exactly defined.
7. IP RIP based networks send the complete routing table during update. The default update interval is 30 seconds. IGRP update packet is sent every 90 seconds by default.
8. Default administrative distances some important routing protocols are as below:
Directly connect Interface
An administrative distance of 0 represents highest trustworthiness of the route.
An administrative distance of 255 represents the lowest trustworthiness of the route.
9. The port numbers used by different programs are as below:
FTP : Port #21
Telnet : Port #23
SMTP : Port #25
SNMP : Port #161
It is important to know that FTP, Telnet, SMTP uses TCP; whereas TFTP, SNMP use UDP.
10. Address Resolution Protocol (ARP) is used to resolve or map a known IP address to a MAC sub layer address to allow communication on a multi-access medium such as Ethernet. Reverse ARP (RARP) is used to obtain an IP address using an RARP broadcast. RARP can be used to boot diskless workstations over a network.
1. 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.
2. Routing metrics used by IGRP:Bandwidth, MTU, Reliability, Delay, and Load.
Bandwidth: This is represents the maximum throughput of a link.
MTU (Maximum Transmission Unit): This is the maximum message length that is acceptable to all links on the path. The larger MTU means faster transmission of packets.
Reliability: This is a measurement of reliability of a network link. It is assigned by the administrator or can be calculated by using protocol statistics.
Delay: This is affected by the band width and queuing delay.
Load: Load is based among many things, CPU usage, packets processed per sec.
3. 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. OSPF, and IGRP use AS numbers.
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:
Faster convergence: OSPF network converges faster because routing changes are flooded immediately and computer in parallel.
Support for VLSM: OSPF supports VLSM. However, please note that RIP version2 also supports VLSM.
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.
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.
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. An OSPF area is a collection of networks and routers that have the same area identification.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.
3. 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.
4. OSPF determines the router ID using the following criteria:
Use the address configured by the ospf router-id command
Use the highest numbered IP address of a loopback interface
Use the highest IP address of any physical interface
If no interface exists, set the router-ID to 0.0.0.0
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).
5. When two or more routers are contending to be a DR (designated Router) on a network segment, the router with the highest OSPF priority will become the DR for that segment. The same process is repeated for the BDR. In case of a tie, the router with the highest RID will win. The default for the interface OSPF priority is one. Remember that the DR and BDR concepts are per multiaccess segment. Setting the ospf priority on an interface is performed using the ip ospf priority <value> interface command.A priority value of zero indicates an interface which is not to be elected as DR or BDR. The state of the interface with priority zero will be DROTHER.
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