CCNA

  The following table lists the differences between the three most popular interior routing protocols:   Feature RIP EIGRP OSPF Type distance vector hybrid distance vector Metric hop count bandwidth and delay cost Speed of convergence slow fast fast Routing classful (RIPv1), classless(RIPv2) classless classless Updates periodical broadcast (RIPv1), multicast(RIPv2) multicast multicast Manual summarization no (RIPv1), yes (RIPv2) yes yes Supported on non-Cisco routers yes no yes Configuration complexity easy medium hard

  The following table lists the differences between OSPF and EIGRP:

  Here is a list of the most important OSPF features: link state routing protocol classless routing protocol supports VLSM (Variable Length Subnet Mask) converges fast uses the concept of areas uses multicast address for routing updates (224.0.0.5 and 224.0.0.6) sends partial routing updates supports only equal cost load balancing manual summarization can be done only on ABRs and ASBRs uses interface cost as a metric uses SPF algorithm to calculate the best paths open standard each router inside an area has the same topology table supports clear-text and MD5 authentication

  Route summarization helps reduce OSPF traffic and route computation. OSPF, unlike EIGRP, doesn’t support automatic summarization. Also, unlike EIGRP, where you can summarize routes on every router in an EIGRP network, OSFP can summarize routes only on ABRs and ASBRs. The following command is used for OSPF summarization: (config-router) area AREA_ID range IP_ADDRESS MASK To better understand OSPF summarization, consider the following example network: All three routers are running OSPF and exchanging routers. Before OSPF summarization is configured, the router R1 inside the backbone area has two entries for the networks 11.0.0.0/24 and 11.0.1.0/24 in its routing table. We could summarize these two subnets on R2, so that R1 receive only one routing update for both subnets. To do that, the following command can be used on R2: Now, R1 has only one entry in its routing table for R3’s directly connected subnets: NOTE Be careful with summarization. In this case, router R1 thinks that R2 ...

  OSPF can be configured to authenticate every OSPF message. This is usually done to prevent a rogue router from injecting false routing information and therefore causing a Denial-of-Service attack. Two types of authentication can be used: 1.     clear text authentication  – clear text passwords are used 2.     MD5 authentication  – MD5 authentication is used. This type of authentication is more secure because the password doesn’t go in clear-text over the network. NOTE With OSPF authentication turned on, routers must pass the authentication process before becoming OSPF neighbors.   To configure clear text authentication, the following steps are required: configure the OSPF password on the interface by using the  ip ospf authentication-key PASSWORD  interface command configure the interface to use OSPF clear-text authentication by using the  ip ospf authentication  interface command   In the following e...

  Based on the network type, OSPF router can elect one router to be a  Designated Ruter (DR)   and one router to be a   Backup Designated Router (BDR) . For example, on multiaccess broadcast networks (such as LANs) routers defaults to elect a DR and BDR. DR and BDR serve as the central point for exchanging OSPF routing information. Each non-DR or non-BDR router will exchange routing information only with the DR and BDR, instead of exchanging updates with every router on the network segment. DR will then distribute topology information to every other router inside the same area, which greatly reduces OSPF traffic. To send routing information to a DR or BDR the multicast address of 224.0.0.6 is used. DR sends routing updates to the multicast address of 224.0.0.5. If DR fails, BDR takes over its role of redistributing routing information. Every router on a network segment will establish a full neighbor relationship with the DR and BDR. Non-DR and non-BDR routers will es...

  Configuring OSPF 1 OSPF basic configuration is very simple. Just like with other routing protocols covered so far (RIP, EIGRP) first you need to enable OSPF on a router. This is done by using the  router   ospf PROCESS-ID  global configuration command. Next, you need to define on which interfaces OSPF will run and what networks will be advertised. This is done by using the  network IP_ADDRESS WILDCARD_MASK AREA_ID  command from the ospf configuration mode. NOTE The OSPF process number doesn’t have to be the same on all routers in order to establish a neighbor relationship, but the Area ID has to be the same on all neighboring routers in order for routers to become neighbors.   Let’s get started with some basic OSPF configuration. We will use the following network topology: First, we need to enable OSPF on both routers. Then we need to define what network will be advertised into OSPF. This can be done by using the following sequence of commands on bot...

  OSPF (Open Shortest Path First)   is a link state routing protocol. Because it is an open standard, it is implemented by a variety of network vendors. OSPF will run on most routers that doesn’t necessarily have to be Cisco routers (unlike EIGRP which can be run only on Cisco routers). Here are the most important features of OSPF: a classless routing protocol supports VLSM, CIDR, manual route summarization, equal cost load balancing incremental updates are supported uses only one parameter as the metric – the interface cost. the administrative distance of OSPF routes is, by default, 110. uses multicast addresses 224.0.0.5 and 224.0.0.6 for routing updates.   Routers running OSPF have to establish neighbor relationships before exchanging routes. Because OSPF is a link state routing protocol, neighbors don’t exchange routing tables. Instead, they exchange information about network topology. Each OSFP router then runs SFP algorithm to calculate the best routes and adds...

  NOTE This topic is not included in the latest version of the CCNA exam (200-301). If you are studying for the exam feel free to skip this article.   Here is a list of the most important EIGRP features: advanced distance vector routing protocol classless routing protocol supports VLSM (Variable Length Subnet Mask) converges fast supports multiple Network layer protocols (IPv4, IPv6, IPX, AppleTalk…) uses multicast address of 224.0.0.10 for routing updates sends partial routing updates supports equal and unequal-cost load balancing supports manual summarization on any router within a network by default, uses bandwidth and delay to calculate its metric Cisco proprietary supports MD5 authentication

  NOTE This topic is not included in the latest version of the CCNA exam (200-301). If you are studying for the exam feel free to skip this article.   Diffusing Update Algorithm (DUAL)  is an algorithm used by EIGRP to select and maintain the best route to each remote network. DUAL is also used for the following purposes: discover a backup route if there is one available. support for variable length subnet masks (VLSMs). perform dynamic route recoveries. query neighbors for unknown alternate routes. send out queries for alternate routes.   EIGRP stores all routes advertised by all EIGRP neighbors. The metric of these routes is used by DUAL to select the efficient and loop free paths. DUAL selects routes that will be inserted into the routing table. If a route fails, and there is no feasible successor, DUAL chooses a replacement route, which usually takes a couple of seconds. The following requirements must be met in order for DUAL to work properly: EIGRP neighbors mu...

  NOTE This topic is not included in the latest version of the CCNA exam (200-301). If you are studying for the exam feel free to skip this article.   EIGRP doesn’t send messages with UDP or TCP; instead, a Cisco’s protocol called  Reliable Transport Protocol (RTP)  is used for communication between EIGRP-speaking routers. As the name implies, reliability is a key feature of this protocol, and it is designed to enable quick delivery of updates and tracking of data reception. Five different packets types are used by EIGRP: Update  – contains route information. When routing updates are sent in response to the metric or topology changes, reliable multicasts are used. In the event that only one router needs an update, for example when a new neighbor is discovered, unicasts are used. Query  – a request for specific routes that always uses the reliable multicast method. Routers send queries when they realize they’ve lost the path to a particular network and are l...

  NOTE This topic is not included in the latest version of the CCNA exam (200-301). If you are studying for the exam feel free to skip this article.   EIGRP authentication EIGRP authentication is used to prevent an attacker from forming the EIGRP neighbor relationship with your router and advertising incorrect routing information. By using the same  preshared key (PSK)  on all routers you can force EIGRP to authenticate each EIGRP message. That way you can ensure that your router accepts routing updates only from the trusted sources. To authenticate every message, the MD5 (Message Digest 5) algorithm is used. Three steps are required to configure EIGRP authentication: 1. creating a keychain 2. specifying a key string for a key 3. configuring EIGRP to use authentication   EIGRP uses the concept of key chains. Each key chain can have many keys, just like in real life. You can specify a different lifetime interval of each key. That way the second key in a key chain...

  NOTE This topic is not included in the latest version of the CCNA exam (200-301). If you are studying for the exam feel free to skip this article.   Route summarization is a method of representing multiple networks with a single summary address. It is often use in large networks with many subnets because it reduces the number of routes that a router must maintain and minimizes the traffic used for routing updates. Two methods for summarizing routes exist: automatic summarization and manual summarization. EIGRP automatic summarization By default, EIGRP has the auto summary feature enabled. Because of this, routes are summarized to classful address at network boundaries in the routing updates. To better understand the concept of auto-summarization, consider the following example. Router R1 and R2 are running EIGRP. Router R1 has the locally connected subnet 10.0.1.0/24 that is advertised to the router R2. Because of the auto summary feature, the router R1 summarizes the networ...

  NOTE This topic is not included in the latest version of the CCNA exam (200-301). If you are studying for the exam feel free to skip this article.   The  network  command in EIGRP uses a classful network as the parameter by default, which means that all interfaces inside the classful network will participate in the EIGRP process. We can enable EIGRP only for specific networks using  wildcard masks . The syntax of the command is: (router-eigrp)#network IP_ADDRESS WILDCARD_MASK We will use the following example network: The router is directly connected to three subnets. Let’s say that we want to advertise only the 10.0.0.0/24 subnet in EIGRP. We can use the wildcard mask of 0.0.0.255 to do this: R1(config-router)#network 10.0.0.0 0.0.0.255 Using the  show ip protocols  command we can verify that only the subnet 10.0.0.0/24 is included in EIGRP: R1#show ip protocols Routing Protocol is "eigrp 1 " Outgoing update filter list for all interfaces is not s...

  NOTE This topic is not included in the latest version of the CCNA exam (200-301). If you are studying for the exam feel free to skip this article.   Configuring EIGRP 1 EIGRP configuration closely resembles RIP configuration. Only two steps are required: enabling EIGRP by using the  router eigrp ASN_NUMBER  command telling EIGRP which networks to advertise by using one or more  network  statements The first command,  router eigrp ASN_NUMBER , enables EIGRP on a router.  ASN_NUMBER  represents an autonomous system number and has to be the same on all routers running EIGRP, otherwise routers won’t become neighbors. The second command,  network SUBNET , enables EIGRP on selected interfaces and specifies which networks will be advertised. By default, the  network  command takes a classful network number as the parameter. To illustrate a configuration of EIGRP, we will use the following network: The network depicted above consists...