Chapter 7: EIGRP and OSPF

Chapter 7: EIGRP and OSPF
Sybex CCNA Chapter 7: EIGRP and OSPF

Chapter 7 Objectives Enhanced IGRP (EIGRP)
EIGRP tables Configuring EIGRP Verifying EIGRP Open Shortest Path First (OSPF) Configuring OSPF Verifying OSPF Configuring OSPF with wildcards

What is EIGRP? EIGRP is an advanced distance-vector routing protocol that relies on features commonly associated with link-state protocols. EIGRP uses Link State's partial updates and neighbor discovery. EIGRP's advanced features supports IP, IPX and AppleTalk. EIGRP uses RTP (Reliable Transport Protocol) to transport its routing updates

What Is EIGRP? Enhanced IGRP supports: Rapid convergence
IP Routing Protocols IP Routing Protocols AppleTalk Routing Protocol Enhanced IGRP AppleTalk Routing Protocol IPX Routing Protocols IPX Routing Protocols Enhanced IGRP supports: Rapid convergence Reduced bandwidth usage Multiple network-layer support EIGRP includes support for AppleTalk, IP, and Novell NetWare as well as IP and IP v.6. The AppleTalk implementation redistributes routes learned from the Routing Table Maintenance Protocol (RTMP). The IP implementation redistributes routes learned from OSPF, RIP, IS-IS, EGP and BGP. The Novell implementation redistributes routes learned from Novell RIP or Service Advertisement Protocol (SAP). Enhanced Interior Gateway Routing Protocol (EIGRP) is a proprietary Cisco protocol that runs on Cisco routers and internal route processors found in the Cisco Distribution and Core layer switches. In this section, you’ll see the many features of EIGRP and describe how it works, with particular focus on the unique way it discovers, selects, and advertises routes. There are a number of powerful features that make EIGRP a real stand out from IGRP and other protocols. The main ones are listed here: Support for IP, IPX, and AppleTalk via protocol-dependent modules Efficient neighbor discovery Communication via Reliable Transport Protocol (RTP) Best path selection via Diffusing update algorithm (DUAL)

What Is EIGRP? Enhanced IGRP supports:
IP Routing Protocols IP Routing Protocols AppleTalk Routing Protocol Enhanced IGRP AppleTalk Routing Protocol IPX Routing Protocols IPX Routing Protocols Enhanced IGRP supports: Uses Diffused Update Algorithm (DUAL) to select loop-free routes and enable fast convergence. DUAL enables EIGRP routers to determine whether a path advertised by a neighbor is looped or loop-free, and Allows a router running EIGRP to find alternate paths without waiting on updates from other routers. Up to 6 unequal paths to remote network, default = 4 Enhanced Interior Gateway Routing Protocol (EIGRP) is a proprietary Cisco protocol that runs on Cisco routers and internal route processors found in the Cisco Distribution and Core layer switches. In this section, you’ll see the many features of EIGRP and describe how it works, with particular focus on the unique way it discovers, selects, and advertises routes. There are a number of powerful features that make EIGRP a real stand out from IGRP and other protocols. The main ones are listed here: Support for IP, IPX, and AppleTalk via protocol-dependent modules Efficient neighbor discovery Communication via Reliable Transport Protocol (RTP) Best path selection via Diffusing update algorithm (DUAL)

Comparing EIGRP to IGRP
Both IGRP and EIGRP: Use “Autonomous Systems” (AS) to divide the internetwork [This is the number that you include when configuring the protocol; e.g.: “router (e)igrp 1” All routers in the same AS use at least one common protocol, share the same routing information and are “contiguous”. They also use the same metrics: bandwidth, delay, load and reliability; with MTU as a tiebreaker. Same load balancing properties Maximum hop count of 255 (100 default)

Comparing EIGRP to IGRP
But EIGRP: Includes the subnet mask information in its routing updates, which allows the use of VLSM. And helps EIGRP to differentiate between internal (within an AS) and external (between ASs) routes Also, it does not send “any periodic updates” (which IGRP sends every 90 seconds) EIGRP has improved convergence time – much faster than IGRP EIGRP also sharply reduces network overhead

EIGRP for IP Enhanced IGRP
No updates: Route updates sent only when a change occurs – multicast on “Hello” messages sent to neighbors every 5 seconds (60 seconds in most WANs) Enhanced IGRP EIGRP EIGRP doesn’t send link-state packets as OSPF does; instead, it sends traditional distance-vector updates containing information about networks plus the cost of reaching them from the perspective of the advertising router. And EIGRP has link-state characteristics as well—it synchronizes routing tables between neighbors at startup, and then sends specific updates only when topology changes occur. hello

EIGRP for IP EIGRP and PDMs (Protocol-dependent modules ):
Supports IP (and through IP (v.4 & 6), IPX, OSPF, IS-IS, RIP and RIP v.2, EGP (Exterior Gateway Protocol), and BGP (Border Gate Protocol), AppleTalk which gives you RTMP (Routing Table Maintenance Protocol), and Novell NetWare, supporting IPX, Novell RIP and SAP (Service Ad Protocol). EIGRP supports more protocols than any other routing protocol, (only IS-IS comes close), by using PDMs. Each PDM keeps it’s own set of routing tables. PDMs are responsible for network layer protocol-specific requirements. The IP-EIGRP module, for example, is responsible for sending and receiving EIGRP packets that are encapsulated in IP.

EIGRP for IP EIGRP (other features): Is “Classless”
Supports VLSM and CIDR. Supports “discontiguous” networks & “summarization” Uses RTP (Reliable Transport Protocol) (see ff) for communication – uses multicasts and unicasts for quick updates with receipts for tracking data. Uses the “DUAL” algorithm; unique and efficient.

EIGRP Terminology and Operation 1
EIGRP sends out five different types of packets— hello, update, query, reply, and acknowledge (ACK)— that establish the initial adjacency between neighbors and to keep the topology and routing tables current. When troubleshooting an EIGRP network, network administrators must understand what EIGRP packets are used for and how they are exchanged. For example, if routers running EIGRP do not form neighbor relationships, those routers cannot exchange EIGRP updates with each other and cannot connect to services across the internetwork.

The following terms are related to EIGRP: Neighbor table (contains neighbors) EIGRP routers use hello packets to discover neighbors. When a router discovers and forms an adjacency with a new neighbor, it includes the neighbor's address and the interface through which it can be reached in an entry in the neighbor table. This table is comparable to the neighborship (adjacency) database used by link-state routing protocols. It serves the same purpose—ensuring bidirectional communication between each of the directly connected neighbors. EIGRP keeps a neighbor table for each network protocol supported; in other words, the following tables could exist: an IP neighbor table, an IPX and an AppleTalk neighbor table.

Topology table (contains updates re: all routes) When the router dynamically discovers a new neighbor, it sends an update about the routes it knows to its new neighbor and receives the same back. These updates populate the topology table. The topology table contains all destinations advertised by neighboring routers; in other words, each router stores its neighbors' routing tables in its EIGRP topology table. If a neighbor is advertising a destination, it must be using that route to forward packets; this rule must be strictly followed by all distance vector protocols. An EIGRP router maintains a topology table for each network protocol configured (IP, IPX, and AppleTalk).

Advertised distance (AD) & feasible distance (FD) DUAL uses distance information, known as a metric or cost, to select efficient, loop-free paths. The lowest-cost route is calculated by adding the cost between the next-hop router and the destination—referred to as the advertised distance—to the cost between the local router and the next-hop router. The sum of these costs is referred to as the feasible distance. Successor Is a neighboring router that has a least-cost path to a destination (the lowest FD) that is guaranteed not to be part of a routing loop Successors are used for forwarding packets. Multiple successors can exist if they have the same FD.

Routing table (contains only the best routes) Holds the best routes to each destination and is used for forwarding packets. Successor routes are offered to the routing table. If a router learns more than one route to exactly the same destination from different routing sources, it uses the administrative distance to determine which route to keep in the routing table. By default, up to 4 routes to the same destination with the same metric can be added to the routing table (the table can hold up to 6 unequal cost paths). The router maintains one routing table for each network protocol configured.

Feasible successor (FS) Along with keeping least-cost paths, DUAL keeps backup paths to each destination. The next-hop router for a backup path is called the “feasible successor”. To qualify as a feasible successor, a next-hop router must have an AD less than the FD of the current successor route in other words, a feasible successor is a neighbor that is closer to the destination, but it is not the least-cost path and, thus, is not used to forward data. Feasible successors are selected at the same time as successors but are kept only in the topology table. The topology table can maintain multiple feasible successors for a destination.

If the route via the successor becomes invalid (because of a topology change for example) or if a neighbor changes the metric, DUAL checks for feasible successors to the destination. If a feasible successor is found, DUAL uses it, thereby avoiding recomputing the route. If no suitable feasible successor exists, a recomputation must occur to determine the new successor. Although recomputation is not processor-intensive, it does affect convergence time, so it is advantageous to avoid unnecessary recomputations.

IGRP and EIGRP Metric Calculation - 1
The composite metric is calculated with the following formula: By default, k1=k3=1 and k2=k4=k5=0. The default composite metric for EIGRP, adjusted for scaling factors, is as follows:

IGRP and EIGRP Metric Calculation - 2
BWmin is in kbps and the sum of delays are in 10s of microseconds. Example The bandwidth and delay for an Ethernet interface are 10 Mbps and 1ms, respectively. The calculated EIGRP BW metric is as follows: 256 × 107/BW = 256 × 107/10,000, = 256 x (10,000,000/10,000) = 256 × 1,000 =

EIGRP Neighbor Discovery -1
EIGRP routers actively establish relationships with their neighbors, similar to what Link State routers do. EIGRP routers establish “adjacencies” with neighbor routers by using small hello packets. The Hello protocol uses a multicast address of , and all routers periodically send hellos.

EIGRP Neighbor Discovery - 2
On hearing hellos, the router creates a table of its neighbors. The continued receipt of these packets maintains the neighbor table To become a neighbor, the following 3 conditions must be met: The router must hear a hello packet or an ACK from a neighbor. The AS number in the packet header must be the same as that of the receiving router. The neighbor’s metric settings must be the same. Note: Each Layer 3 protocol has its own neighbor table.

Neighbor Discovery - 3

EIGRP Timers EIGRP updates are set only when necessary and are sent only to neighboring routers. There is no periodic update timer. EIGRP use hello packets to learn of neighboring routes. The holdtime to maintain a neighbor adjacency is three times the hello time. For hello is not received with the holdtime, the neighbor is removed from the table.

Default Hello Intervals and Hold Time for EIGRP

Routing Concepts EIGRP relies on four fundamental concepts:
Topics now considered in more detail: EIGRP relies on four fundamental concepts: neighbor tables, topology tables, route states, and route tagging. Each of these is summarized in the slides that follow.

When a neighbor sends a hello packet, it advertises a hold time.
Routing Concepts: 1. Neighbor Tables One neighbor table exists for each protocol-dependent module. When a router discovers a new neighbor, it records the neighbor’s address and interface as an entry in the “neighbor table”. This is the amount of time that a router treats a neighbor as reachable and operational. When a neighbor sends a hello packet, it advertises a hold time. DUAL is informed of the topology change. If a hello packet is not received within the hold time, the hold time expires.

A neighbor-table entry includes info required by RTP.
Routing Concepts: 1. Neighbor Tables A neighbor-table entry includes info required by RTP. Sequence numbers are employed to match acknowledgments with data packets, and the last sequence number received from the neighbor is recorded so that out-of-order packets can be detected. A “transmission list” is used to queue packets for possible retransmission on a per-neighbor basis. “Round-trip timers” are kept in the neighbor-table entry to estimate an optimal retransmission interval.

What is in the Neighbor Table?
SRTT: “Smooth Round Trip timer”: Time for round trip to neighbor and back. RTO: “Retransmission Time Out”: Time EIGRP waits to send a packet from its retransmission queue to a neighbor. Q count - the number of EIGRP Packets that the software is waiting to send

Routing Concepts: 2. Topology Tables
The “topology table” contains all destinations advertised by neighboring routers. The protocol-dependent modules populate the table, and the table is acted on by the DUAL “finite-state” machine. Each entry in the topology table includes the destination address and a list of neighbors that have advertised the destination. For each neighbor, the entry records the advertised metric, which the neighbor stores in its routing table. An important rule that distance vector protocols must follow is that if the neighbor advertises this destination, it must use the route to forward packets.

Dual Terminology - 1 AD (Advertised distance) is the metric that is reported by the neighbor router(s). FD (Feasible Distance) – Feasible distance is the metric that is reported by neighbor router(s), plus the cost associated with the forwarding link from the local interface to the neighbor router(s). When multiple paths exist, the “local FD” is the lowest-cost metric to a remote network. Feasibility Condition – If the AD from a given neighbor is less than the locally calculated FD, that neighbor meets the criteria to become the feasible successor.

Dual Terminology - 2 Successor - A successor is a neighboring router that is currently being used for packet forwarding; it provides the least-cost route to the destination and is not part of a routing loop Feasible successor - A feasible successor is a backup route. Feasible successors provide the next lowest-cost path without introducing routing loops. Feasible successor routes can be used in case the existing route fails. Packets to the destination network are immediately forwarded to the feasible successor, which at that point is promoted to the status of successor

Successor routes Successor route is used by EIGRP to forward traffic to a destination A successor routes may be backed up by a feasible successor route Successor routes are stored in both the topology table and the routing table Topology Table—IP Destination Successor Destination Feasible Successor Successor route is used by EIGRP to forward traffic to a destination A successor routes may be backed up by a feasible successor route Successor routes are stored in both the topology table and the routing table Routing Table—IP Destination 1 Successor

EIGRP successors and feasible successors - 1

Dual Example – 1a

Dual Example – 1b In the previous slide, EIGRP's composite metric is replaced by a link cost to simplify calculations. RTA's topology table includes a list of all routes advertised by neighbors. For each network, RTA keeps the real (computed) cost of getting to that network and also keeps the advertised cost (reported distance) from its neighbor.

Dual Example – 1c RTY is the successor to network 24, by virtue of its lowest computed cost 31. This value is also the FD to Network 24. RTA follows a three-step process to select a feasible successor to become a successor for Network 24: Determine which neighbors have a reported distance (RD) (=AD) to Network 24 that is less than 31. RTX's RD is 30 < 31, meet FC and is a feasible successor. RTZ's RD is 220 > 31, not meet FC, and cannot be a FS.

Dual Example – 2a (a) Is the Destination Network

Dual Example – 2b In this example, (a) is the destination network,
From C’s point of view, if it goes to (a) via B, the FD is 3 and the AD is 1. Others entries are computed in the same manner. Note in the example that router D does not have a feasible successor identified. The FD for router D to router A is 2 and the AD via router C is 3. Because the AD is larger than the FD, no feasible successor is placed in the topology table.

Dual Example – 2c Router C has a feasible successor identified because the AD for the next hop router is less than the FD for the successor. How about router E?

EIGRP Convergence - 1 In the context of routing protocols, convergence refers to the speed and ability of a group of internetworking devices running a specific routing protocol to agree on the topology of an internetwork after a change in that topology. DUAL results in EIGRP's exceptionally fast convergence. Why? The FS provides the capability to make an immediate switchover to a backup route!

EIGRP Convergence - 2

EIGRP Neighbor Tables The most important table in EIGRP is the neighbor table and relationships tracked in the neighbor table are the basis for all the EIGRP routing update and convergence activity. The neighbor table contains information about adjacent neighboring EIGRP routers. A neighbor table is used to support reliable, sequenced delivery of packets. An EIGRP router can maintain neighbor tables, one for each PDM running (e.gmultiple ., IP, IPX, and AppleTalk) routed protocols.

EIGRP Packet Types - 1 Hello packets assist in the discovery of EIGRP neighbors. The packets are multicast to An acknowledgment packet acknowledges the reception of an update packet. An acknowledgment packet is a hello packet with no data. Acknowledgment packets are sent to the unicast address of the sender of the update packet.

EIGRP Packet Types - 2 Update packets contain the routing information of destinations. Update packets are unicast to newly discovered neighbors; otherwise, update packets are multicast to when a link metric changes. Update packets are acknowledged to ensure reliable transmission. Query packets are sent to ﬁnd feasible successors to a destination. Query packets are always multicast.

EIGRP Packet Types - 3 Reply packets are sent to respond to query packets. Reply packets provide a feasible successor to the sender of the query. Reply packets are unicast to the sender of the query packet.

Routing Concepts: 3. Route States
A topology-table entry for a destination can exist in one of two states: active or passive. A destination is in the passive state when the router is not performing a recomputation; it is in the active state when the router is. If feasible successors are always available, a destination never has to go into the active state, thereby avoiding a recomputation. A recomputation occurs when a destination has no feasible successors. The router initiates the recomputation by sending a query packet to each of its neighboring routers. After the router has received a reply from each neighboring router the router can select a successor.

Routing Concepts: 4. Route Tagging
EIGRP supports internal and external routes. Internal routes originate within an EIGRP AS. External routes are learned by another routing protocol or reside in the routing table as static routes. These routes are tagged individually with the identity of their origin. External routes are tagged with this information: Router ID of the router that redistributed the route AS number of the destination Configurable administrator tag ID of the external protocol Metric from the external protocol Bit flags for default routing

EIGRP Tables and Packets
The neighbor table and topology table are held in ram and are maintained through the use of hello and update packets. Enhanced IGRP EIGRP The neighbor table and topology table are held in ram and are maintained through the use of hello and update packets. hello To see all feasible successor routes known to a router, use the “show ip eigrp topology” command

Choosing Routes IP IP A B 19.2 AppleTalk T1 T1 AppleTalk IPX IPX T1 C D EIGRP uses a composite metric to pick the best path: bandwidth and delay of the line by default. EIGRP can load balance across six unequal cost paths to a remote network (4 by default) Like IGRP, EIGRP uses only bandwidth and delay of the line to determine the best path to a remote network by default. Cisco sometimes likes to call these path bandwidth value and cumulative line delay—go figure.

Configuring EIGRP for IP
AS=10 A C B Enable EIGRP Assign networks Router(config)#router eigrp 10 Router(config-router)#network Router(config-router)#network To start an EIGRP session on a router, use the router eigrp command followed by the autonomous system number of your network. You then enter the network numbers connected to the router using the network command followed by the network number. If you use the same AS number for EIGRP as IGRP, EIGRP will automatically redistribute IGRP into EIGRP

Redistribution Redistribution is translating one type of routing protocol into another. EIGRP IGRP Router B Router A Router D Redistribution is important, because if you want to use EIGRP and don’t have all Cisco router, you need to configure redistribution commands. If you are using IGRP and want to migrate to EIGRP (yes, you should do this), configure EIGRP with the same AS number and EIGRP automatically redistributed IGRP into EIGRP routes. These routes show up as external routes with an AS of 170. Router C IGRP and EIGRP translate automatically, as long as they are both using the same AS number. See another example - next slide:

Using EIGRP with IGRP

Route Path Assuming all default parameters, which route will RIP (v1 and v2) take, and which route(s) will IGRP and EIGRP take to get from Routers A to B? T1 T1 56K RIPv1 and RIPv2 use the same metric (hop count) and would find the 56K link the best path to the remote network. EIGRP and IGRP use the same metric as well (bandwidth and delay of the line) and would use the path through the LAN interfaces, not the serial T1’s. 10BaseT Router B Router A 100BaseT 100BaseT

Verifying EIGRP Operation
Router# show ip eigrp neighbors Displays the neighbors discovered by IP Enhanced IGRP Displays the IP Enhanced IGRP topology table Displays current Enhanced IGRP entries in the routing table Displays the parameters and current state of the active routing protocol process Displays the number of IP Enhanced IGRP packets sent and received Router# show ip eigrp topology Router# show ip route eigrp Router# show ip protocols Show ip route: Shows the entire routing table show ip route eigrp: Shows only EIGRP entries in the routing table show ip eigrp neighbors: Shows all EIGRP neighbors. show ip eigrp topology: Shows entries in the EIGRP topology table. Which EIGRP show command will provide you with the IP addresses of the devices with which the router has established an adjacency, as well as the transmit and queue counts for the adjacent routers? Which command will display all the EIGRP feasible successor routes known to a router? Router# show ip eigrp traffic

Show IP Route P1R1#sh ip route [output cut] Gateway of last resort is not set D /24 [90/2172] via ,00:04:36, Serial0/0 C /24 is directly connected, FastEthernet0/0 D /24 [90/2681] via ,00:04:36, Serial0/0 C /24 is directly connected, Serial0/0 D /24 [90/2707] via ,00:04:35, Serial0/0 P1R1# -D is for “DUAL” -[90/2172] is the administrative distance and cost of the route. The “cost” of the route is a composite metric comprised from the bandwidth and delay of the line The show ip route command, or the show ip route eigrp command, will show you the routing table the routes found by DUAL. -D is for “Dual” -[90/2172] is the administrative distance and cost of the route. The cost of the route is a composite metric comprised from the bandwidth and delay of the line

Some EIGRP Features Large Network support:
Support for multiple Autonomous Systems: This is one way to break up a large number of hosts. VLSM Support and Summarization: Support for “discontiguous networks”: This is a network in which 2 subnets of a classful network; say, and , which are both part of the “classful” network, are separated by a different classful network; say , or any subnet in that network. By default, EIGRP does not handle this configuration, (only OSPF can), but it can be configured to do so.

Some EIGRP Features Load Balancing:
EIGRP can handle equal or unequal load balancing By default, up to 4 links; up to 6 links can be configured with the “maximum paths” command.

EIGRP Configuration Initial Setup (pg 426, and Cisco command reference): Step Command Purpose 1 router eigrp autonomous-system Enable an EIGRP routing process in global config mode. network network-number Associate networks with an EIGRP routing process in router config mode. Create a Passive Interface This prohibits an interface from sending or Router(config-router)#passive-interface serial 0/1 receiving Hellos; so it will never form adjacencies. Redistribution and Set Metric values The following example takes redistributed Routing Information Protocol (RIP) metrics and translates them into EIGRP metrics with values as follows: bandwidth = 1000, delay = 100, reliability = 250, loading = 100, and MTU = 1500. router eigrp 1 network Command Syntax redistribute rip redistribute (IP) default-metric default-metric bandwidth delay reliability loading mtu

EIGRP Configuration (continued)
Load Balancing – This is automatic with EIGRP; the only time you need to configure it is when you want to vary the load over each of several links. In this case you would use the “traffice-share balanced” or the “variance” command: To control how traffic is distributed among routes when there are multiple routes for the same destination network that have different costs, use the traffic-share balanced command in router configuration mode. To disable this function, use the no form of the command. traffic-share balanced To control load balancing in an Enhanced Interior Gateway Routing Protocol (EIGRP) based internetwork, use the variance command in router configuration mode. To reset the variance to the default value, use the no form of this command. variance multiplier

Introducing OSPF (pg 444 ff)
Open Shortest Path First (OSPF) is an open standards routing protocol that’s been implemented by a wide variety of network vendors, including Cisco. If you have multiple routers, and not all of them are Cisco (what!) then you can’t use EIGRP now can you? So your remaining options are basically RIP, RIPv2 or OSPF. If it’s a large network, then really, your only options are OSPF, or something called route redistribution—a translation service between routing protocols. OSPF converges quickly, although perhaps not as quickly as EIGRP, and it supports multiple, equal-cost routes to the same destination. But unlike EIGRP, it only supports IP routing. Open standard Shortest path first (SPF) algorithm Link-state routing protocol (vs. distance vector) Can be used to route between AS’s

OSPF Hierarchical Routing
OSPF is supposed to be designed in a hierarchical fashion, which basically means that you can separate the larger internetwork into smaller Internetworks called areas. Consists of areas and autonomous systems Minimizes routing update traffic Supports VLSM Unlimited hop count

Link State Vs. Distance Vector
Provides common view of entire topology Calculates shortest path Utilizes event-triggered updates Can be used to route between AS’s Distance Vector: Exchanges routing tables with neighbors Utilizes frequent periodic updates This slides represents some important Link State characteristics, compared to distance vector.

Types of OSPF Routers Area 1 Backbone Area 0 Area 2
ABR and Backbone Router Backbone/ Internal Routers Internal Routers Internal Routers Notice how each router connects to the backbone—called area 0, or the backbone area. OSPF must have an area 0, and all routers should connect to this area if at all possible, but routers that connect other areas within an AS together are called Area Boundary Routers (ABRs). Still, at least one interface must be in area 0. OSPF runs inside an autonomous system, but can also connect multiple autonomous systems together. The router that connects these AS’s together is called an Autonomous System Boundary Router (ASBR). Area 0 is called the backbone area Hierarchical OSPF networks do not require multiple areas You must have an area 0 Multiple OSPF areas must connect to area 0 ASBR and Backbone Router ABR and Backbone Router External AS

Compare RIP to OSPF Feature RIP OSPF Algorithm Maximum Hops
Vector-distance Link-state Maximum Hops 15 hops. 16 hops is considered to be infinity, implying that the destination is unreachable Limited only by size of routing tables within routers Subsystem Segmentation Treats the autonomous system as a single subsystem Breaks the autonomous system into one or more areas with two levels of routing algorithms, intra-area, and inter-area. Metric Destination/hop Destination/cost/link identifier Integrity No authentication in RIP-1, Authentication has been added to RIP-2 Supports Authentication. Several authentication algorithms are available ranging from simple password operations to more complex cryptographic algorithms. Complexity Relatively Simple More Complex. Several more PDUs and exchanges are defined in the protocol. Routing tables are large and include not only destinations, but also a tree representation of local network. Acceptance Widely Available, BSD routed supports RIP Newer, published in RFCs Route Options Identifies a single route to a destination Supports multiple routes to a single destination. Facilitates load-balancing traffic distribution Types of Routes Host, network. RIP-2 adds the ability to transfer subnetwork route entries Host, network, and subnetwork routes

Configuring Single Area OSPF
Router(config)#router ospf <process-id> Defines OSPF as the IP routing protocol. Note: The process ID is locally significant and is needed to identify a unique instance of an OSPF database Router(config-router)#network address mask area <area-id> Configuring basic OSPF isn’t as simple as RIP, IGRP and EIGRP, and it can get can really complex once the many options that are allowed within OSPF are factored in. These two elements are the basic elements of OSPF configuration: -Enabling OSPF -Configuring OSPF areas The easiest, and also least scalable way to configure OSPF is to just use a single area. Doing this requires a minimum of two commands as shown in the next slide. The command you use to activate the OSPF routing process is: Lab_A(config)#router ospf ? < > A value in the range 1– identifies the OSPF Process ID. Process ID’s can be assigned any number from 0 to 65535 Area’s can be any number up to 2.4 billion Assigns networks to a specific OSPF area

OSPF Example R3 R2 R1 Area 0 hostname R3 router ospf 10 network area 0 network area 0 hostname R2 router ospf 20 network area 0 hostname R1 router ospf 30 network area 0 network area 0 There are various ways to configure OSPF. The configuration of R3 shows how the wildcard is used to place each interface individually into area 0 R2 show how two interface can be configured into area 0 with one wildcard network statement of R3 shows the wildcards of and It doesn’t matter how you configure the network statements, the results are the same. Remember, the process ID is irrelevant and can be the same on each router, or different on each router, as they are in this example.

Verifying the OSPF Configuration
Router#show ip protocols Verifies that OSPF is configured Router#show ip route Displays all the routes learned by the router Router#show ip ospf interface There are several ways to verify proper OSPF configuration and operation, and this slides shows some basic verification commands that you will use in the next hands-on labs. Displays area-ID and adjacency information Router#show ip ospf neighbor Displays OSPF-neighbor information on a per-interface basis

OSFP Neighbors OSPF uses hello packets to create adjacencies and maintain connectivity with neighbor routers OSPF uses the multicast address Hello? Neighbors Neighbors are two or more routers that have an interface on a common network, such as two routers connected on a point-to-point serial link. Adjacency An adjacency is a relationship between two OSPF routers that permits the direct exchange of route updates. OSPF is really picky about sharing routing information, unlike EIGRP that directly shares routes with all of its neighbors. Instead, OSPF directly shares routes only with neighbors that have also established adjacencies. Link State Advertisement A Link State Advertisement (LSA) is an OSPF data packet containing link-state and routing information that’s shared among OSPF routers. Hello packets provides dynamic neighbor discovery Hello Packets maintains neighbor relationships Hello packets and LSA’s from other routers help build & maintain the topological database

OSPF Terminology Neighbor: Adjacency Neighbors Adjacencies
Two routers that have an interface on a common network Usually discovered by hello’s but can also be configured administratively Adjacency Relationship formed between selected neighbors in which routing information is exchanged. Not all neighbors are adjacent Only Broadcast and Non-Broadcast network types have Designated and Backup Designated Routers!!! Neighbors Neighbor Two routers that have an interface on a common network Usually discovered by hello’s but can also be configured administratively Adjacency Relationship formed between selected neighbors in which routing information is exchanged. Not all neighbors are adjacent Only Broadcast and Non-Broadcast network types have Designated and Backup Designated Routers!!! ABR DR Adjacencies Non-DR Cost=6 BDR

OSPF Terminology – Table: part 1
Description Link state Information is shared between directly connected routers. This information propagates throughout the network unchanged and is also used to create a shortest path first (SPF) tree. Area A group of routers that share the same area ID. All OSPF routers require area assignments. Autonomous system (AS) A network under a common network administration. Cost The routing metric used by OSPF. Lower costs are always preferred. You can manually configure the cost with the ip ospf cost command. By default, the cost is calculated by using the formula cost = 108 / bandwidth. Router ID Each OSPF router requires a unique router ID, which is the highest IP address configured on a Cisco router or the highest numbered loopback address. You can manually assign the router ID. Adjacency When two OSPF routers have exchanged information between each other and have the same topology table. An adjacency can have the following different states or exchange states: 1. Init state —When Hello packets have been sent and are awaiting a reply to establish two-way communication. 2. Establish bi-directional (two-way) communication —Accomplished by the discovery of the Hello protocol routers and the election of a DR. 3. Exstart —Two neighbor routers form a master/slave relationship and agree upon a starting sequence to be incremented to ensure LSAs are acknowledged. 4. Exchange state —Database Description (DD) packets continue to flow as the slave router acknowledges the master's packets. OSPF is operational because the routers can send and receive LSAs between each other. DD packets contain information, such as the router ID, area ID, checksum, if authentication is used, link-state type, and the advertising router. LSA packets contain information, such as router ID also but in addition include MTU sizes, DD sequence numbering, and any options. 5. Loading state —Link-state requests are sent to neighbors asking for recent advertisements that have not yet been discovered. 6. Full state —Neighbor routers are fully adjacent because their link-state databases are fully synchronized. Routing tables begin to be populated.

OSPF Terminology – Table: part 2
Topology table Also called the link-state table. This table contains every link in the whole network. Designated router (DR) This router is responsible for ensuring adjacencies between all neighbors on a multiaccess network (such as Ethernet). This ensures all routers do not need to maintain full adjacencies with each other. The DR is selected based on the router priority. In a tie, the router with the highest router ID is selected. Backup DR A backup router designed to perform the same functions in case the DR fails. Link-state advertisement (LSA) A packet that contains all relevant information regarding a router's links and the state of those links. Priority Sets the router's priority so a DR or BDR can be correctly elected. Router links Describe the state and cost of the router's interfaces to the area. Router links use LSA type 1. Summary links Originated by area border routers (ABRs) and describe networks in the AS. Summary links use LSA types 3 and 4. Network links Originated by DRs. Network links use LSA type 2. External links Originated by autonomous system boundary routers (ASBRs) and describe external or default routes to the outside (that is, non- OSPF) devices for use with redistribution. External Links use the LSA type 5. Area border router (ABR) Router located on the border of one or more OSPF areas that connects those areas to the backbone network. Autonomous system boundary router (ASBR) ABR located between an OSPF autonomous system and a non-OSPF network.

Router ID (RID) Each router that is participating in OSPF needs to be uniquely identified. The method of identification that OSPF uses is Router IDs (RID). 32 bits that uniquely identifies an OSPF router Highest IP address in router is RouterID Overridden by Loopback interface if present Even if Loopback address has lower value Recommended to use loopback interface Easier to manipulate this number Always up Interface loopback 0 Ip address You can also Statically assign the Router ID in the OSPF router configuration mode: (config)# router ospf 1 (config-router)# router-id Do NOT use same loopback address on different routers Each router in OSPF needs to be uniquely identified to properly arrange them in the Neighbor tables.

Electing the DR and BDR OSPF sends Hellos which elect DRs and BDRs
Multicast Hellos are sent and compared Router with Highest Priority is Elected as DR Router with 2nd Highest Priority is Elected as BDR OSPF sends Hellos which elect DRs and BDRs Routers form adjacencies with DRs and BDRs in a multi-access environment The next slide covers loopback interfaces. The reason you would configure a loopback (a logical interface) is to assign it the highest priority interface on the router, thus ensuring that it will become the DR. This avoids the router selecting a physical interface as DR, which is sometimes undesirable because physical interfaces can go up and down and sometimes fail to provide a stable routing environment. The following outlines the process OSPF takes and rules that are followed when electing a Designated Router: Routers elect a DR and BDR per network All routers set by default to priority 1 (0-255) Priority of zero (0) means router can not be elected as a DR Router with highest priority wins BDR (1 – 255), if no other router has a higher priority the BDR will then become the DR RouterID breaks tie, Router ID is either the Highest Loopback or Highest Configured IP address on any given active interface If DR fails, BDR promoted to DR and a new BDR is elected Existing DR will not be overthrown if “better” router is turned on after initial election DRs and BDRs listen to multicast traffic on both multicast address and is exclusively listed to by DRs

Configuring Loopback Interfaces
Router ID (RID): Number by which the router is known to OSPF Default: The highest IP address on an active interface at the moment of OSPF process startup Can be overridden by a loopback interface: Highest IP address of any active loopback interface – also called a logical interface Configuring loopback interfaces when using the OSPF routing protocol is important and Cisco suggests using them whenever you configure OSPF on a router. Loopback interfaces are logical interfaces, which means they are not real router interfaces. They can be used for diagnostic purposes as well as OSPF configuration. The reason you want to configure a loopback interface on a router is because if you don’t, the highest IP address on a router will become that routers Router ID (RID). The RID is used to advertise the routes as well as elect the designated router (DR) and backup designated router (BDR).

What is the default OSPF interface priority?
Interface Priorities What is the default OSPF interface priority? Router# show ip ospf interface ethernet0/0 Ethernet0 is up, line protocol is up Internet Address /29, Area 4 Process ID 19, Router ID , Network Type BROADCAST, Cost: 10 Transmit Delay is 1 sec, State DR, Priority 1 Designated Router (ID) , Interface address No backup designated router on this network Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:06 Index 2/2, flood queue length 0 Next 0x0(0)/0x0(0) Last flood scan length is 0, maximum is 0 Last flood scan time is 0 msec, maximum is 0 msec Neighbor Count is 0, Adjacent neighbor count is 0 Suppress hello for 0 neighbor(s) Sometimes it is desirable for a router to be configured so that it is not eligible to become the DR or BDR. You can do this by setting the OSPF priority to zero with the ip ospf priority priority# interface subcommand. Router(config-if)# ip ospf priority {0 – 255} Change the priority of a router on an interface 0 means to not participate in election 1 is default, 255 is highest priority

Interface Priorities ip ospf priority ip ospf priority number-value
To set the router priority, which helps determine the designated router for this network, use the ip ospf priority command in interface configuration mode. To return to the default value, use the no form of this command. ip ospf priority number-value no ip ospf priority number-value Syntax Description number-value <A number value that specifies the priority of the router. The range is from 0 to 255>

Ensuring your DR First, what is the RID of each router? Which router is the default DR for the LAN? There are three options that will ensure that R2 will be the DR for the LAN segment /24: Configure the priority value of the Fa0/0 interface of the R2 router to a higher value than any other interface on the Ethernet network Configure a loopback interface on the R2 with an IP address higher than any IP address on the other routers Change the priority value of the Fa0/0 interface of R1 and R3 to zero Priority value overrides RID Loopback Interface IP override physical IP even the former is smaller What options can you configure that will ensure that R2 will be the DR of the LAN segment?

Configuring Wildcards
If you want to advertise a partial octet (subnet), you need to use wildcards. means all octets match exactly means that the first three match exactly, but the last octet can be any value After that, you must remember your block sizes…. This slides introduces the wildcards used in OSPF. These wildcards will also be used in access-list configurations. A 0 octet in the wildcard mask indicates that the corresponding octet in the network must match exactly. On the other hand, a 255 indicates that you don’t care what the corresponding octet is in the network number. A network and wildcard mask combination of would match only, and nothing else. This is really useful if you want to activate OSPF on a specific interface in a very clear and simple way. If you insist on matching a range of networks, the network and wildcard mask combination of would match anything in the range – Because of this, it’s simpler and safer to stick to using wildcard masks of and identify each OSPF interface individually.

Wildcard The wildcard address is always one less than the block size….
/30 = /28 = /27 = /26 = What the author means is that where, in the first line, you’ve borrowed 6 bits to get a /30 subnet mask, and this would give you 64 subnets with 4 hosts in each! The “4” is the “block size” that the author refers to. So, in the wildcard, the last number must be one less than 4, or “3”. Same thing in line 2: /28 means 4 bits borrowed; this gives you 16 subnets with 16 hosts in each. Block size is 16 and the wildcard is 16-1, or 15. This slides shows how to find a wildcard that can be used to configure a subnet in an octet.

Wildcard Configuration of the Lab_B Router
You need to understand wildcard configuration. Configure the Lab_B router using wildcards: Router ospf 1 Network area 0 Network area 0 Network area 0 NOTE: to remove a bad entry, use the following example: Router(config)#router ospf 1 Router(config-router)#no network area 0 Router(config-router)#network area 0 Lab_B E0: /24 S0: /30 S1: /30 Lab_C E0: /24 S1: /30 Lab_A E0: /24 S0: /30

Summary Go through all the written and review questions
Go over the answers with the class 83

Chapter 7: EIGRP and OSPF

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