NCC QUIZ 4 Dynamic Routing

Scalability:

Defines a routing protocols ability to support larger networks and growth.

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.

Equal and Unequal Cost Load Balancing

- EIGRP supports equal cost load balancing and unequal cost load balancing, which allows administrators to better distribute traffic flow in their networks.

The router eigrp Command:

The autonomous-system argument can be assigned to any 16-bit value between the number 1 and 65,535. All routers within the EIGRP routing domain must use the same autonomous system number. • The router eigrp autonomous-system command does not start the EIGRP process itself, rather, this command only provides access to configure the EIGRP settings. To completely remove the EIGRP routing process from a device, use the no router eigrp autonomous-system global configuration mode command, which stops the EIGRP process and removes all existing EIGRP router configurations.

EIGRP Diffusing Update Algorithm

- As the computational engine that drives EIGRP, (DUAL) resides at the center of the routing protocol. DUAL guarantees loop-free and backup paths throughout the routing domain. Using DUAL, EIGRP stores all available backup routes for destinations so that it can quickly adapt to alternate routes when necessary.

Partial and Bounded Updates

- EIGRP uses the terms partial and bounded when referring to its updates. Unlike RIP, EIGRP does not send periodic updates and route entries do not age out. The term partial means that the update only includes information about the route changes, such as a new link or a link becoming unavailable. The term bounded refers to the propagation of partial updates that are sent only to those routers that the changes affect. This minimizes the bandwidth that is required to send EIGRP updates.

Interior or Exterior Routing:

-An Autonomous System (AS) represents a collection of routers under a common administrator, otherwise known as a routing domain. -Interior Gateway Protocol (IGP): An IGP exchanges routes between routers in a single autonomous system. Common IGPs include OSPF and EIGRP. Although less popular, RIP and IS-IS are also considered IGPs. Also, be aware that BGP is used as an EGP; however, you can use interior BGP (iBGP) within an AS. IGPs have the following characteristics: ▪ Support small, medium-sized, and large organizations, but scalability is limited. ▪ Fast convergence, and basic functionality is not complex to configure. ▪ Used for routing within an AS. It is also referred to as intra-AS routing. Companies, organizations, and even service providers use an IGP on their internal networks. IGPs include RIP, EIGRP, OSPF, and IS-IS. -Exterior Gateway Protocol (EGP): Today, the only EGP in use is BGP. However, from a historical perspective, be aware that there was once another EGP, which was actually named Exterior Gateway Protocol (EGP). An EGP is used to exchange routes between different autonomous systems, also known as inter-AS routing.

Establishing Neighbor Adjacencies

-Establishes relationships with directly connected routers that are also enabled for EIGRP. They are used to track the status of these neighbors.

Routing Information Protocol (RIP):

A distance-vector routing protocol that uses a metric of hop count. The maximum number of hops between two routers in an this protocol's network is 15. Therefore, a hop count of 16 is considered to be infinite. It is an IGP. Three primary versions exist. v1 periodically broadcasts its entire IP routing table, and it supports only fixed-length subnet masks. v2 supports variable-length subnet masks, and it uses multicasts (to a multicast address of 224.0.0.9) to advertise its IP routing table, as opposed to broadcasts. RIP next generation (RIPng) supports the routing of IPv6 networks, while RIPv1 and RIPv2 support the routing of IPv4 networks. RIP uses hop count, the number of routers, as the metric. o If a device has two paths to the destination network, the path with fewer hops will be chosen as the path to forward traffic. o If a network is 16 or more hops away, the router considers it unreachable.

Routing Protocol Characteristics:

All routing protocols can be compared based on their characteristics. Knowing the characteristics of a specific routing protocol will help determine the best routing protocol use designing network. All routing protocols share the following characteristics: Speed of convergence: Deines how quickly routers in the network share routing information and reach a consistent state of knowledge. Scalability: Defines a routing protocols ability to support larger networks and growth. Class: The ability of a routing protocol to support VLSM. Resource Useage: Includes requirements such as RAM, CPU, and link bandwidth utilization. Implementation and Maintenance: Level of knowledge required for a network administrator to implement and maintain the network based on the routing protocol deployed Routing protocols can be classified into different groups based on their purpose, operation, and behavior. Purpose: Interior Gateway Protocol or Exterior Gateway Protocol. Operation: Distance Vector, Link-state, or Path Vector. Behavior: Classful or Classless.

Use the show ip eigrp topology command to view

the topology table. The topology table lists all successors and FSs that DUAL has calculated to destination networks. Only the successor is installed into the IP routing table.

The Delay(DLY) Metric:

Is the measure of the time it takes for a packet to traverse a route. It is a static value based on the type of link to which the interface is connected and is expressed in microseconds. Delay is not measured dynamically. In other words, the router does not actually track how long packets take to reach the destination. The delay value, much like the bandwidth value, is a default value that can be changed by the network administrator. When used to determine the EIGRP metric, delay is the cumulative (sum) of all interface delays along the path (measured in tens of microseconds). The table in the figure shows the default delay values for various interfaces. Notice that the default value is 20,000 microseconds for serial interfaces and 10 microseconds for GigabitEthernet interfaces. • Use the show interfaces command to verify the delay value on an interface

Composite Metric

By default, EIGRP uses the following values in this Metric to calculate the preferred path to a network: • Bandwidth - The slowest bandwidth among all of the outgoing interfaces, along the path from source to destination. • Delay - The cumulative (sum) of all interface delay along the path (in tens of microseconds). The following values can be used, but are not recommended, because they typically result in frequent recalculation of the topology table: • Reliability - Represents the worst reliability between the source and destination, which is based on keepalives. • Load - Represents the worst load on a link between the source and destination, which is computed based on the packet rate and the configured bandwidth of the interface. Although the MTU is included in the routing table updates, it is not a routing metric used by EIGRP. The figure below shows the composite metric formula used by EIGRP. The formula consists of values K1 to K5, known as EIGRP metric weights. K1 and K3 represent bandwidth and delay, respectively. K2 represents load, and K4 and K5 represent reliability. By default, K1 and K3 are set to 1, and K2, K4, and K5 are set to 0. The result is that only the bandwidth and delay values are used in the computation of the default composite metric. The metric calculation method (k values) and the EIGRP autonomous system number must match between EIGRP neighbors. If they do not match, the routers do not form an adjacency. The show ip protocols command is used to verify the k values.

Router(config-router)# no auto-summary

By default, RIPv2 automatically summarizes networks at major network boundaries, summarizing routes to the classful network address. When route summarization is disabled, the software sends subnet routing information across classful network boundaries.

Classful Routing vs Classless Routing

Classful Routing: The biggest distinction between classful and classless routing protocols is that classful routing protocols do not send subnet mask information in their routing updates. Classless routing protocols include subnet mask information in the routing updates. The two original IPv4 routing protocols developed were RIPv1 and IGRP. They were created when network addresses were allocated based on classes (i.e., class A, B, or C). At that time, a routing protocol did not need to include the subnet mask in the routing update, because the network mask could be determined based on the first octet of the network address. The biggest distinction between classful and classless routing protocols is that classful routing protocols do not send subnet mask information in their routing updates. Classless routing protocols include subnet mask information in the routing updates. The two original IPv4 routing protocols developed were RIPv1 and IGRP. They were created when network addresses were allocated based on classes (i.e., class A, B, or C). At that time, a routing protocol did not need to include the subnet mask in the routing update, because the network mask could be determined based on the first octet of the network address. The fact that RIPv1 and IGRP do not include subnet mask information in their updates means that they cannot provide variable-length subnet masks (VLSMs) and Classless Inter-Domain Routing (CIDR). Classful routing protocols also create problems in discontiguous networks. A discontiguous network is when subnets from the same classful major network address are separated by a different classful network address. When there are two entries with identical metrics in the routing table, the router shares the load of the traffic equally among the two links. This is known as load balancing. Classless Routing Protocols: Modern networks no longer use classful IP addressing, in which the subnet mask cannot be determined by the value of the first octet. Classless routing protocols support VLSM and CIDR. This is referred to as the child route. Parent routes never include an exit interface or next-hop IP address.

EIGRP Metrics

Composite Metric The Bandwidth Metric: The Delay Metric:

EIGRP uses the DUAL convergence algorithm

Convergence is critical to a network to avoid routing loops. Routing loops, even temporary ones, can be detrimental to network performance. Distance vector routing protocols, such as RIP, prevent routing loops with hold-down timers and split horizon. Although EIGRP uses both of these techniques, it uses them somewhat differently; the primary way that EIGRP prevents routing loops is with the DUAL algorithm. The DUAL algorithm is used to obtain loop-freedom at every instance throughout a route computation. This allows all routers involved in a topology change to synchronize at the same time. Routers that are not affected by the topology changes are not involved in the re-computation. This method provides EIGRP with faster convergence times than other distance vector routing protocols. The decision process for all route computations is done by the DUAL Finite State Machine (FSM). An FSM is a workflow model, similar to a flow chart, which is composed of the following: • A finite number of stages (states) • Transitions between those stages • Operations The DUAL FSM tracks all routes and uses EIGRP metrics to select efficient, loop-free paths, and to identify the routes with the least-cost path to be inserted into the routing table. Re-computation of the DUAL algorithm can be processor-intensive. EIGRP avoids re-computation whenever possible by maintaining a list of backup routes that DUAL has already determined to be loop-free. If the primary route in the routing table fails, the best backup route is immediately added to the routing table. A successor is a neighboring router that is used for packet forwarding and is the least-cost route to the destination network. The IP address of a successor is shown in a routing table entry right after the word "via".

These backup paths are known as Feasible Successors (FS)

DUAL can converge quickly after a change in the topology because it can use backup paths to other networks without re-computing DUAL is a neighbor that has a loop-free backup path to the same network as the successor, and it satisfies the Feasibility Condition (FC)

Speed of convergence:

Defines how quickly routers in the network share routing information and reach a consistent state of knowledge.

Routing Protocol Categories:

Distance-Vector Protocols: The distance vector routing approach determines the distance (metric) and vector (direction) to any destination network. Unlike link-state protocols, which build an entire map of the topology, the only information that a router knows about a remote network is the metric (distance) to reach this network and which path or interface to use to get there (vector). A distance-vector routing protocol sends a full copy of its routing table to its directly attached neighbors. This is a periodic advertisement, meaning that even if there have been no topological changes, a distance-vector routing protocol will, at regular intervals, re-advertise its full routing table to its neighbors. Obviously, this periodic advertisement of redundant information is inefficient. Ideally, you want a full exchange of route information to occur only once and subsequent updates to be triggered by topological changes. Link-State Protocols: Rather than having neighboring routers exchange their full routing tables with one another, a link-state routing protocol allows routers to build a topological map of a network. Then, similar to a global positioning system (GPS) in a car, a router can execute an algorithm to calculate an optimal path (or paths) to a destination network. Routers send link-state advertisements (LSA) to advertise the networks they know how to reach. Routers then use those LSAs to construct the topological map of a network, referred to as a link-state database (LSDB). The algorithm run against this topological map is Dijkstra's Shortest Path First algorithm. Unlike distance-vector routing protocols, link-state routing protocols exchange full routing information only when two routers initially form their adjacency. Then, routing updates are sent in response to changes in the network, as opposed to being sent periodically. Also, link-state routing protocols benefit from shorter convergence times, as compared to distance-vector routing protocols (although convergence times are comparable to EIGRP). Path-Vector Protocols: A path-vector routing protocol includes information about the exact path packets take to reach a specific destination network. This path information typically consists of a series of autonomous systems through which packets travel to reach their destination. Border Gateway Protocol (BGP) is the only path-vector protocol you are likely to encounter in a modern network. BGP's path selection is not solely based on AS hops, however. BGP has a variety of other parameters that it can consider. Interestingly, none of those parameters are based on link speed. Also, although BGP is incredibly scalable, it does not quickly converge in the event of a topological change. The current version of BGP is BGP version 4 (BGP-4). However, an enhancement to BGP-4, called Multiprotocol BGP (MP-BGP), supports the routing of multiple routed protocols, such as IPv4 and IPv6.

Initial Route Discovery: Exchanging routing Updates

EIGRP updates contain networks that are reachable from the router sending the update. As EIGRP updates are exchanged between neighbors, the receiving router adds these entries to its EIGRP topology table. Each EIGRP router maintains a topology table for each routed protocol configured, such as IPv4 and IPv6. The topology table includes route entries for every destination that the router learns from its directly connected EIGRP neighbors. The figure below shows the continuation of the initial route discovery process. It now shows the update of the topology table. When a router receives an EIGRP routing update, it adds the routing information to its EIGRP topology table and replies with an EIGRP acknowledgment. 1. R1 receives the EIGRP update from neighbor R2 that includes information about the routes that the neighbor is advertising, including the metric to each destination. R1 adds all update entries to its topology table. The topology table includes all destinations advertised by neighboring (adjacent) routers and the cost (metric) to reach each network. 2. EIGRP update packets use reliable delivery; therefore, R1 replies with an EIGRP acknowledgment packet informing R2 that it has received the update. 3. R1 sends an EIGRP update to R2 advertising the routes that it is aware of, except those learned from R2 (split horizon). 4. R2 receives the EIGRP update from neighbor R1 and adds this information to its own topology table. 5. R2 responds to R1's EIGRP update packet with an EIGRP acknowledgment.

Administrative Distance (AD)

Is the trustworthiness of the source of the routing information. ▪ is the first criterion that a router uses to determine which routing protocol to use if two protocols provide route information to the same destination. Lower AD is more trusted. AD Can be changed manually to shape traffic, this should only be done with knowledge of topology.

Implementation and Maintenance:

Level of knowledge required for a network administrator to implement and maintain the network based on the routing protocol deployed

EIGRP 5 Packet Types:

Hello packets - Used for neighbor discovery and to maintain neighbor adjacencies. An EIGRP router assumes that as long as it receives Hello packets from a neighbor, the neighbor and its routes remain viable. EIGRP uses a Hold timer to determine the maximum time the router should wait to receive the next Hello before declaring that neighbor as unreachable. By default, the hold time is three times the Hello interval. If the hold time expires, EIGRP declares the route as down and DUAL searches for a new path by sending out queries. o Sent with unreliable delivery o Multicast (on most network types) Update packets - Propagates routing information to EIGRP neighbors. Update packets are sent only when necessary. EIGRP updates contain only the routing information needed and are sent only to those routers that require it. Unlike the older distance vector routing protocol RIP, EIGRP does not send periodic updates and route entries do not age out. Instead, EIGRP sends incremental updates only when the state of a destination changes. This may include when a new network becomes available, an existing network becomes unavailable, or a change occurs in the routing metric for an existing network. EIGRP uses the terms partial update and bounded update when referring to its updates. A partial update means that the update only includes information about route changes. A bounded update refers to the sending of partial updates only to the routers that are affected by the changes. Bounded updates help EIGRP minimize the bandwidth that is required to send EIGRP updates. o Sent with reliable delivery o Unicast or multicast Acknowledgment packets - Used to acknowledge the receipt of an EIGRP message that was sent using reliable delivery. An EIGRP acknowledgment is an EIGRP Hello packet without any data. RTP uses reliable delivery for Update, Query, and Reply packets. Unreliable delivery makes sense; otherwise, there would be an endless loop of acknowledgments. o Sent with unreliable delivery o Unicast Query packets - Used to query routes from neighbors. o Sent with reliable delivery o Unicast or multicast Reply packets - Sent in response to an EIGRP query. o Sent with reliable delivery o Unicast

Resource Usage

Includes requirements such as RAM, CPU, and link bandwidth utilization.

The Bandwidth Metric:

Is a static value used by some routing protocols, such as EIGRP and OSPF, to calculate their routing metric. The bandwidth is displayed in kilobits per second (kb/s). Always verify bandwidth with the show interfaces command. The default value of the bandwidth may or may not reflect the actual physical bandwidth of the interface. If actual bandwidth of the link differs from the default bandwidth value, the bandwidth value should be modified. Configuring the Bandwidth Parameter Because both EIGRP and OSPF use bandwidth in default metric calculations, a correct value for bandwidth is very important to the accuracy of routing information. Use the following interface configuration mode command to modify the bandwidth metric: Router(config-if)# bandwidth kilobits-bandwidth-value Use the no bandwidth command to restore the default value. Verifying the Bandwidth Parameter Use the show interfaces command to verify the new bandwidth parameters. Modifying the bandwidth value does not change the actual bandwidth of the link. The bandwidth command only modifies the bandwidth metric used by routing protocols, such as EIGRP and OSPF.

Feasible Distance (FD)

Is the lowest calculated metric to reach the destination network. Is the metric listed in the routing table entry as the second number inside the brackets. As with other routing protocols, this is also known as the metric for the route.

Routing Protocol Metrics

Metric: - measurable value that is assigned by the routing protocol to different routes based on the usefulness of that route. When a routing protocol learns of more than one route to the same destination, it must select the best path. Routing metrics allow the protocol to evaluate and differentiate between the available paths. Different routing protocols use different metrics. The metric used by one protocol is not comparable to the metric used by another. Two different routing protocols may choose different paths to a destination in the same scenario. RIP - Hop count OSPF - Cost EIGRP - Composite metric (bandwidth, delay, load, reliability, MTU) Administrative Distance (AD): - Is the trustworthiness of the source of the routing information. ▪ Admin distance is the first criterion that a router uses to determine which routing protocol to use if two protocols provide route information to the same destination. Lower AD is more trusted. AD Can be changed manually to shape traffic, this should only be done with knowledge of topology.

Classless Routing Protocols

Modern networks no longer use classful IP addressing, in which the subnet mask cannot be determined by the value of the first octet. Classless routing protocols support VLSM and CIDR. This is referred to as the child route. Parent routes never include an exit interface or next-hop IP

EIGRP Timers

One of the slower components of the EIGRP convergence process relates to the timers that EIGRP neighbors use to recognize that a neighborship has failed. If the interface over which the neighbor is reachable fails, and Cisco IOS changes the interface state to anything other than "up/up," a router immediately knows that the neighborship should fail. However, in some cases, an interface state might stay "up/up" during times when the link is not usable. In such cases, EIGRP convergence relies on the Hold Timer to expire, which by default, on LANs, means a 15-second wait. (The default EIGRP Hold time on interfaces/subinterfaces with a bandwidth of T1 or lower, with an encapsulation type of Frame Relay, is 180 seconds.) The basic operation of these two timers is relatively simple. EIGRP uses the Hello messages in part as a confirmation that the link between the neighbors still works. If a router does not receive a Hello from a neighbor for one entire Hold time, that router considers the neighbor to be unavailable. For example, with a default LAN setting of Hello = 5 and Hold = 15, the local router sends Hellos every 5 seconds. The neighbor resets its downward-counting Hold Timer to 15 upon receiving a Hello from that neighbor. Under normal operation on a LAN, with defaults, the Hold Timer for a neighbor would vary from 15, down to 10, and then be reset to 15. However, if the Hellos were no longer received for 15 seconds, the neighborship would fail, driving convergence.

Link-State Protocols:

Rather than having neighboring routers exchange their full routing tables with one another, this routing protocol allows routers to build a topological map of a network. Then, similar to a global positioning system (GPS) in a car, a router can execute an algorithm to calculate an optimal path (or paths) to a destination network. Routers send link-state advertisements (LSA) to advertise the networks they know how to reach. Routers then use those LSAs to construct the topological map of a network, referred to as a link-state database (LSDB). The algorithm run against this topological map is Dijkstra's Shortest Path First algorithm. Unlike distance-vector routing protocols, link-state routing protocols exchange full routing information only when two routers initially form their adjacency. Then, routing updates are sent in response to changes in the network, as opposed to being sent periodically. Also, link-state routing protocols benefit from shorter convergence times, as compared to distance-vector routing protocols (although convergence times are comparable to EIGRP).

Class:

The ability of a routing protocol to support VLSM.

DUAL Finite State Machine

The centerpiece of EIGRP is DUAL and its EIGRP route-calculation engine. The actual name of this technology is DUAL(FSM). This FSM contains all of the logic used to calculate and compare routes in an EIGRP network. The figure shows a simplified version of the DUAL FSM. Occasionally, the path to the successor fails and there are no FSs. In this instance, DUAL does not have a guaranteed loop-free backup path to the network, so the path is not in the topology table as an FS. If there are no FSs in the topology table, DUAL puts the network into the active state. DUAL actively queries its neighbors for a new successor.

Initial Route Discovery: EIGRP Neighbor Adjacency

The goal of any dynamic routing protocol is to learn about remote networks from other routers and to reach convergence in the routing domain. Before any EIGRP update packets can be exchanged between routers, EIGRP must first discover its neighbors. EIGRP neighbors are other routers running EIGRP on directly connected networks. EIGRP uses Hello packets to establish and maintain neighbor adjacencies. For two EIGRP routers to become neighbors, several parameters between the two routers must match. For example, two EIGRP routers must use the same EIGRP metric parameters and both must be configured using the same autonomous system number. Each EIGRP router maintains a neighbor table, which contains a list of routers on shared links that have an EIGRP adjacency with this router. The neighbor table is used to track the status of these EIGRP neighbors. The figure below shows two EIGRP routers exchanging initial EIGRP Hello packets. When an EIGRP enabled router receives a Hello packet on an interface, it adds that router to its neighbor table. 1. A new router (R1) comes up on the link and sends an EIGRP Hello packet through all of its EIGRP-configured interfaces. 2. Router R2 receives the Hello packet on an EIGRP-enabled interface. R2 replies with an EIGRP update packet that contains all the routes it has in its routing table, except those learned through that interface (split horizon). However, the neighbor adjacency is not established until R2 also sends an EIGRP Hello packet to R1. 3. After both routers have exchanged Hellos, the neighbor adjacency is established. R1 and R2 update their EIGRP neighbor tables adding the adjacent router as a neighbor.

All links can be displayed using the show ip eigrp topology all-links command

This command displays links whether they satisfy the FC or not.

Metric

measurable value that is assigned by the routing protocol to different routes based on the usefulness of that route. When a routing protocol learns of more than one route to the same destination, it must select the best path. Routing metrics allow the protocol to evaluate and differentiate between the available paths. Different routing protocols use different metrics. The metric used by one protocol is not comparable to the metric used by another. Two different routing protocols may choose different paths to a destination in the same scenario. RIP - Hop count OSPF - Cost EIGRP - Composite metric (bandwidth, delay, load, reliability, MTU)

Distance-Vector Protocols:

This routing approach determines the distance (metric) and vector (direction) to any destination network. Unlike link-state protocols, which build an entire map of the topology, the only information that a router knows about a remote network is the metric (distance) to reach this network and which path or interface to use to get there (vector). This routing protocol sends a full copy of its routing table to its directly attached neighbors. This is a periodic advertisement, meaning that even if there have been no topological changes, a distance-vector routing protocol will, at regular intervals, re-advertise its full routing table to its neighbors. Obviously, this periodic advertisement of redundant information is inefficient. Ideally, you want a full exchange of route information to occur only once and subsequent updates to be triggered by topological changes.

Path-Vector Protocols:

This routing protocol includes information about the exact path packets take to reach a specific destination network. This path information typically consists of a series of autonomous systems through which packets travel to reach their destination. Border Gateway Protocol (BGP) is the only path-vector protocol you are likely to encounter in a modern network. BGP's path selection is not solely based on AS hops, however. BGP has a variety of other parameters that it can consider. Interestingly, none of those parameters are based on link speed. Also, although BGP is incredibly scalable, it does not quickly converge in the event of a topological change. The current version of BGP is BGP version 4 (BGP-4). However, an enhancement to BGP-4, called Multiprotocol BGP (MP-BGP), supports the routing of multiple routed protocols, such as IPv4 and IPv6.

Router(config-router)# version {1 | 2}

Using the Version 1 or Version 2 command controls default behavior of RIP on a global basis. You can override that bevahior by configuring a specific interface to behave differently Router(config-if)# ip rip send version 1 Router(config-if)# ip rip send version 2 Router(config-if)# ip rip send version 1 2 Same concept applies to receiving (Replace send with receive) RIP is also capable of load balancing traffic over equal-cost paths. o The default is four equal-cost paths. o If the maximum number of paths is set to one, load balancing is disabled.

Interior Gateway Protocol (IGP):

exchanges routes between routers in a single autonomous system. Include OSPF and EIGRP. Although less popular, RIP and IS-IS are also considered. Also, be aware that BGP is used as an EGP; however, you can use interior BGP (iBGP) within an AS. They have the following characteristics: ▪ Support small, medium-sized, and large organizations, but scalability is limited. ▪ Fast convergence, and basic functionality is not complex to configure. ▪ Used for routing within an AS. It is also referred to as intra-AS routing. Companies, organizations, and even service providers use an IGP on their internal networks. IGPs include RIP, EIGRP, OSPF, and IS-IS

A successor

is a neighboring router that is used for packet forwarding and is the least-cost route to the destination network. The IP address of a successor is shown in a routing table entry right after the word "via".

Reported Distance (RD)

is the metric that a router reports to a neighbor about its own cost to that network. If the reported distance is less, it represents a loop-free path. The reported distance is simply an EIGRP neighbor's feasible distance to the same destination network.

Reliable Transport Protocol (RTP)

is unique to EIGRP and provides delivery of EIGRP packets to neighbors. This protocol and the tracking of neighbor adjacencies set the stage for DUAL. o Although "reliable" is part of its name, it includes both reliable delivery and unreliable delivery of EIGRP packets, similar to TCP and UDP, respectively. Reliable requires an acknowledgment to be returned by the receiver to the sender. An unreliable packet does not require an acknowledgment. For example, an EIGRP update packet is sent reliably over and requires an acknowledgment. An EIGRP Hello packet is also sent over, but unreliably. This means that EIGRP Hello packets do not require an acknowledgment.

ip summary-address rip ip-address network-mask interface command

is used to summarize an address or subnet under a specific interface. Router(config-if)# ip summary-address rip 10.11.0.0 255.255.0.0

The show ip protocols command

is useful to identify the parameters and other information about the current state of any active IPv4 routing protocol processes configured on the router. Information includes: 1. Routing protocol in use 2. Router ID 3. Administrative Distance 4. State of auto summarization 5. Routing information Sources 6. Networks being routed

The show ip eirgp topology displays

the topology table. The topology table contains all routes to reachable networks. Routes marked with a "P" are passive, meaning they are not being recalculated, and are useable routes. Routes maked with an "A" are active, meaning they actively being recalculated, and not available for use. 1. "Passive" routes are located in the IP routing table • The topology table lists all successors and FSs that DUAL has calculated to destination networks. Only the successor is installed into the IP routing table.

Passive Interface

o As soon as a new interface is enabled within the EIGRP network, EIGRP attempts to form a neighbor adjacency with any neighboring routers to send and receive EIGRP updates. o At times it may be necessary, or advantageous, to include a directly connected network in the EIGRP routing update, but not allow any neighbor adjacencies off of that interface to form. The passive-interface command can be used to prevent the neighbor adjacencies. There are two primary reasons for enabling the passive-interface command: • To suppress unnecessary update traffic, such as when an interface is a LAN interface, with no other routers connected. (Stub network) • To increase security controls, such as preventing unknown rogue routing devices from receiving EIGRP updates o The passive-interface router configuration mode command disables the transmission and receipt of EIGRP Hello packets on these interfaces. • Router(config)#router eigrp as-number • Router(config-router)#passive-interface interface-type interface-number • To configure all interfaces as passive, use the passive-interface default command. To disable an interface as passive, use the no passive-interface interface-type interface-number command

The show ip eigrp neighbors command displays information about all current EIGRP neighbors.

o Neighbors address o Interface o Hold timer o Up time

The network Command and Wildcard Mask:

o The network command has the same function as in all IGP routing protocols. The network command in EIGRP: Enables any interface on this router that matches the network address in the network router configuration mode command to send and receive EIGRP updates. The network of the interfaces is included in EIGRP routing updates. By default, when using the network command and an IPv4 network address, such as 172.16.0.0, all interfaces on the router that belong to that classful network address are enabled for EIGRP. However, there may be times when the network administrator does not want to include all interfaces within a network when enabling EIGRP. 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] • Ex - Router(config-router)#network 192.168.1.0 0.0.0.255 A wildcard mask is similar to the inverse of a subnet mask. In a subnet mask, binary 1s are significant while binary 0s are not. In a wildcard mask, binary 0s are significant, while binary 1s are not. For example, the inverse of subnet mask 255.255.255.252 is 0.0.0.3. Calculating a wildcard mask may seem daunting at first but it's actually pretty easy to do. To calculate the inverse of the subnet mask, subtract the subnet mask from 255.255.255.255 as follows: 255.255.255.255 - 255.255.255.252 --------------- 0. 0. 0. 3 Wildcard mask • The latest Cisco IOS allows you to enter a subnet mask or a wildcard mask when entering EIGRP network statements

EIGRP Router ID

o is used to uniquely identify each router in the EIGRP routing domain. o is used in both EIGRP and OSPF routing protocols. However, the role of the router ID is more significant in OSPF. In EIGRP IPv4 implementations, the use of the router ID is not that apparent. EIGRP for IPv4 uses the 32-bit router ID to identify the originating router for redistribution of external routes. o If the network administrator does not explicitly configure a router ID using the eigrp router-id command, EIGRP generates its own router ID using either a loopback or physical IPv4 address. A loopback address is a virtual interface and is automatically in the up state when configured. The interface does not need to be enabled for EIGRP, meaning that it does not need to be included in one of the EIGRP network commands. However, the interface must be in the up/up state. o The eigrp router-id ipv4-address router configuration command is the preferred method used to configure the EIGRP router ID.

Configuring EIGRP •Autonomous System Numbers:

o used for EIGRP configuration is only significant to the EIGRP routing domain. It functions as a process ID to help routers keep track of multiple running instances of EIGRP. This is required because it is possible to have more than one instance of EIGRP running on a network. Each instance of EIGRP can be configured to support and exchange routing updates for different networks.

An Autonomous System (AS)

represents a collection of routers under a common administrator, otherwise known as a routing domain.

DUAL's method of guaranteeing

that a neighbor has a loop-free path is that the neighbor's metric must satisfy the FC. By ensuring that the RD of the neighbor is less than its own FD, the router can assume that this neighboring router is not part of its own advertised route; thus, always avoiding the potential for a loop. EIGRP is a distance vector routing protocol, without the ability to see a complete, loop-free topological map of the network.

Initial Route Discovery: Updating the Routing Table

the final steps of the initial route discovery process. 1. After receiving the EIGRP update packets from R2, using the information in the topology table, R1 updates its IP routing table with the best path to each destination, including the metric and the next-hop router. 2. Similar to R1, R2 updates its IP routing table with the best path routes to each network. At this point, EIGRP on both routers is considered to be in the converged state. R2 has lost connectivity to the LAN and it sends out queries to all EIGRP neighbors searching for any possible routes to the LAN. Because queries use reliable delivery, the receiving router must return an EIGRP acknowledgment. The acknowledgment informs the sender of the query that it has received the query message. To keep this example simple, acknowledgments were omitted in the graphic. All neighbors must send a reply, regardless of whether or not they have a route to the downed network. Because replies also use reliable delivery, routers, such as R2, must send an acknowledgment. It may not be obvious why R2 would send out a query for a network it knows is down. Actually, only R2's interface that is attached to the network is down. Another router could be attached to the same LAN and have an alternate path to this same network. Therefore, R2 queries for such a router before completely removing the network from its topology table.

Classful Routing

these do not send subnet mask information in their routing updates. these routing protocols include subnet mask information in the routing updates. The two original IPv4 routing protocols developed were RIPv1 and IGRP. They were created when network addresses were allocated based on classes (i.e., class A, B, or C). At that time, a routing protocol did not need to include the subnet mask in the routing update, because the network mask could be determined based on the first octet of the network address. The biggest distinction between classful and classless routing protocols is that classful routing protocols do not send subnet mask information in their routing updates. Classless routing protocols include subnet mask information in the routing updates. The two original IPv4 routing protocols developed were RIPv1 and IGRP. They were created when network addresses were allocated based on classes (i.e., class A, B, or C). At that time, a routing protocol did not need to include the subnet mask in the routing update, because the network mask could be determined based on the first octet of the network address. The fact that RIPv1 and IGRP do not include subnet mask information in their updates means that they cannot provide variable-length subnet masks (VLSMs) and Classless Inter-Domain Routing (CIDR). Classful routing protocols also create problems in discontiguous networks. A discontiguous network is when subnets from the same classful major network address are separated by a different classful network address.

Enhanced Interior Gateway Routing Protocol

was a Cisco-proprietary protocol until early 2013, has been popular in Cisco-only networks; however, other vendors can now implement it on their routers. • is classified as an advanced distance-vector routing protocol, because it improves on the fundamental characteristics of a distance-vector routing protocol and Combines the advantages of link-state and distance vector routing protocols • does not periodically send out its entire IP routing table to its neighbors. Instead it uses triggered updates, and it converges quickly. • can support multiple routed protocols (for example, IPv4 and IPv6) by using protocol-dependent modules (PDMs). PDMs are responsible for the specific routing tasks for each network layer protocol, including: Maintaining the neighbor and topology tables of EIGRP routers that belong to that protocol suite Building and translating protocol-specific packets for DUAL Interfacing DUAL to the protocol-specific routing table Computing the metric and passing this information to DUAL Implementing filtering and access lists Performing redistribution functions to and from other routing protocols Redistributing routes that are learned by other routing protocols • When a router discovers a new neighbor, it records the neighbor's address and interface as an entry in the neighbor table. One neighbor table exists for each protocol-dependent module, such as IPv4. EIGRP also maintains a topology table. The topology table contains all destinations that are advertised by neighboring routers. There is also a separate topology table for each PDM. • By default, EIGRP uses bandwidth and delay in its metric calculation; however, other parameters can be considered. These optional parameters include reliability, load, and maximum transmission unit (MTU) size.

RIP implements Split Horizon

which prevents routing information from being sent out the same interface from which it was received. Split horizon with route poisoning is a similar technique but sends the update with a metric of 16, which is considered unreachable by RIP.

Features of EIGRP

• Diffusing Update Algorithm - As the computational engine that drives EIGRP, the Diffusing Update Algorithm (DUAL) resides at the center of the routing protocol. DUAL guarantees loop-free and backup paths throughout the routing domain. Using DUAL, EIGRP stores all available backup routes for destinations so that it can quickly adapt to alternate routes when necessary. • Establishing Neighbor Adjacencies - EIGRP establishes relationships with directly connected routers that are also enabled for EIGRP. Neighbor adjacencies are used to track the status of these neighbors. • Reliable Transport Protocol - The Reliable Transport Protocol (RTP) is unique to EIGRP and provides delivery of EIGRP packets to neighbors. RTP and the tracking of neighbor adjacencies set the stage for DUAL. o Although "reliable" is part of its name, RTP includes both reliable delivery and unreliable delivery of EIGRP packets, similar to TCP and UDP, respectively. Reliable RTP requires an acknowledgment to be returned by the receiver to the sender. An unreliable RTP packet does not require an acknowledgment. For example, an EIGRP update packet is sent reliably over RTP and requires an acknowledgment. An EIGRP Hello packet is also sent over RTP, but unreliably. This means that EIGRP Hello packets do not require an acknowledgment. • Partial and Bounded Updates - EIGRP uses the terms partial and bounded when referring to its updates. Unlike RIP, EIGRP does not send periodic updates and route entries do not age out. The term partial means that the update only includes information about the route changes, such as a new link or a link becoming unavailable. The term bounded refers to the propagation of partial updates that are sent only to those routers that the changes affect. This minimizes the bandwidth that is required to send EIGRP updates. • Equal and Unequal Cost Load Balancing - EIGRP supports equal cost load balancing and unequal cost load balancing, which allows administrators to better distribute traffic flow in their networks.