Dynamic Routing Protocols part2

CH2 Outline Dynamic Routing Protocols Distance Vector Dynamic Routing Link-State Dynamic Routing RIP OSPF

CH2 p3 Outline Link State Routing Protocols Link-State Routing Process Advantages and disadvantages of link state routing protocols OSPF Routing Protocol Components of OSPF OSPF Terminologies OSPF Operation OSPF Operational State Dijkstra’s Algorithm

Types of Routing Protocols 7.1.4.1 4 Types of Routing Protocols

Link-State Routing Protocols In contrast to distance vector routing protocol operation, a router configured with a link-state routing protocol can create a complete view or topology of the network by gathering information from all of the other routers. A link-state router uses the link-state information to create a topology map and to select the best path to all destination networks in the topology A link-state routing protocol is like having a complete map of the network topology. The sign posts along the way from source to destination are not necessary, because all link-state routers are using an identical map of the network. A link-state router uses the link-state information to create a topology map and to select the best path to all destination networks in the topology.

Link-State Routing Process 7.4.2.1 6 Link-State Routing Process

7.4.2.2 7 Link and Link-State The first step in the link-state routing process is that each router learns about its own links, its own directly connected networks. 

7.4.2.3 8 Say Hello The second step in the link-state routing process is that each router is responsible for meeting its neighbors on directly connected networks.

7.4.2.4 9 Link State Updates The third step in the link-state routing process is that each router builds a link-state packet (LSP) containing the state of each directly connected link.

Flooding the LSP and Building the Link-State Database 7.4.2.5 10 Flooding the LSP and Building the Link-State Database The fourth step in the link-state routing process is that each router floods the LSP to all neighbors, who then store all LSPs received in a database.

Computing the Best Path 7.4.2.6 11 Computing the Best Path The final step in the link-state routing process is that each router uses the database to construct a complete map of the topology and computes the best path to each destination network.

Adding Routes to the Routing Table 7.4.2.7 12 Adding Routes to the Routing Table The best paths are inserted into the routing table

Why Use Link-State Protocols? 7.4.3.1 13 Why Use Link-State Protocols? Disadvantages compared to distance vector routing protocols: Memory Requirements  Processing Requirements  Bandwidth Requirements

Protocols that Use Link-State 7.4.3.3 14 Protocols that Use Link-State Only two link-state routing protocols: Open Shortest Path First (OSPF) most popular two current versions OSPFv2 - OSPF for IPv4 networks OSPFv3 - OSPF for IPv6 networks IS-IS was designed by ISO popular in provider networks

OSPF Operational State OSPF Routing Protocol Components of OSPF OSPF Terminologies OSPF Operation OSPF Operational State Dijkstra’s Algorithm

OSPF OSPF is link-state routing protocol OSPF is an IGP routing protocol. It is a Link State routing Protocol based on SPF technology. OSPF has fast convergence OSPF supports VLSM and CIDR Cisco’s OSPF metric is based on bandwidth OSPF only sends out changes when they occur. periodic updates (link-state refresh) every 30 minutes. OSPF also uses the concept of areas to implement hierarchical routing OSPF is link-state routing protocol RIP is distance-vector routing protocol, susceptible to routing loops, split-horizon, and other issues. OSPF has fast convergence OSPF supports VLSM and CIDR RIPv1 does not Cisco’s OSPF metric is based on bandwidth RIP is based on hop count OSPF only sends out changes when they occur. RIP sends entire routing table every 30 seconds. periodic updates (link-state refresh) every 30 minutes. OSPF also uses the concept of areas to implement hierarchical routing

Link State Routing In link state routing, each router shares its knowledge about its neighborhood with every router in the area. The three features: Sharing knowledge about the neighborhood. Sharing with every other router. Sharing when there is a change.

OSPF Operational State OSPF Routing Protocol Components of OSPF OSPF Terminologies OSPF Operation OSPF Operational State Dijkstra’s Algorithm

From CH2 p1 What are the components of OSPF?

8.1.1.3 20 Components of OSPF

Components of OSPF 21 8.1.1.3 OSPF Routers Exchange Packets These packets are used to discover neighboring routers and also to exchange routing information to maintain accurate information about the network.

7.4.1.2 22 Components of OSPF OSPF Routers run Dijkstra’s Algorithm to compute the best path to each destination network.

OSPF Operational State OSPF Routing Protocol Components of OSPF OSPF Terminologies OSPF Operation OSPF Operational State Dijkstra’s Algorithm Link, Link State and LSDB. Area. OSPF Route Types OSPF Routers Classifications. OSPF Packets

Link and Link State Link: Interface on a router Link state: Description of an interface and of its relationship to its neighboring routers, including: IP address/mask of the interface, The type of network it is connected to The routers connected to that network The metric (cost) of that link The collection of all the link-states would form a link-state database (LSDB).

Networks Supported by OSPF OSPF supports the following types of physical networks

Area OSPF allows the grouping of routers into a set, called an area. An area is a collection of networks, hosts, and routers all contained within an AS. An AS can be divided into many different areas. All networks inside area must be connected.

Area Routers inside an area flood the area with routing information. This technique minimizes the routing traffic required for the protocol.

Area The topology of an area is hidden from the rest of the AS Inside an area, each router has an identical LSDB. Each area has its own copy of the topological database. At the border of an area, special routers called area border routers summarize the information about the area and send it to other areas.

Area Among the areas inside an AS is a special area called the backbone. All the areas inside an AS must be connected to the backbone. The routers inside the backbone are called the backbone routers. Note that a backbone router can also be an area border router. In other words, the backbone serves as a primary area and the other areas as secondary areas.

Area Each area has an area identification. The area identification of the backbone is zero.

Area With multiarea, routing within the AS takes place on two levels, depending on whether the route to the destination lies entirely within an area (intra-area routing) or in another area (inter-area routing). When a packet must be routed between two areas, the backbone is used. The packet is first routed to the Area Border Router. The packet is then routed through the backbone to another area border router acting for the destination area. The packet is finally routed through the destination area to the specific destination

OSPF Route Types

OSPF Routers Classifications OSPF routers can be classified into four overlapping types: Internal routers, Area Border routers, Backbone routers, and Autonomous system boundary routers Area 0 Area 2 Area 3 IR ABR/BR To another AS ASBR

OSPF Routers Classifications

Types of OSPF Packets OSPF routers exchange packets. These packets are used to discover neighboring routers and also to exchange routing information to maintain accurate information about the network.

OSPF Operational State OSPF Routing Protocol Components of OSPF OSPF Terminologies OSPF Operation OSPF Operational State Dijkstra’s Algorithm

OSPF Operation To maintain routing information, OSPF routers complete the following generic link-state routing process to reach a state of convergence Exchanging Hello packets Exchanging LSAs Creating SPF Tree Updating routing table 1 2 3 and 4 5

1. Establish Neighbor Adjacencies OSPF-enabled routers must recognize each other on the network before they can share information. 38 1. Establish Neighbor Adjacencies An OSPF-enabled router sends Hello packets out all OSPF-enabled interfaces to determine if neighbors are present on those links. If a neighbor is present, the OSPF-enabled router attempts to establish a neighbor adjacency with that neighbor.

Establish Neighbor Adjacencies OSPF creates adjacencies between neighboring routers. The reason for forming adjacencies is to exchange topological information. Not every router needs to become adjacent to every other router. Adjacencies are established and maintained with hello packets. These packets are sent periodically.

2- Exchanging Link State Advertisements 8.1.1.4 After adjacencies are established, routers then exchange link-state advertisements (LSAs). 40 2- Exchanging Link State Advertisements LSAs contain the state and cost of each directly connected link. Routers flood their LSAs to adjacent neighbors. Adjacent neighbors receiving the LSA immediately flood the LSA to other directly connected neighbors, until all routers in the area have all LSAs.

3. Build the Topology Table After LSAs are received, OSPF-enabled routers build the topology table (LSDB) based on the received LSAs. This database eventually holds all the information about the topology of the network.

4. Execute the SPF Algorithm Routers then execute the SPF algorithm that creates the SPF tree.

5- Updating routing table From the SPF tree, the best paths are inserted into the routing table. Routing decisions are made based on the entries in the routing table.

OSPF Operational State Route calculation and Dijkstra’s Algorithm OSPF Routing Protocol Components of OSPF OSPF Terminologies OSPF Operation OSPF Operational State Route calculation and Dijkstra’s Algorithm

OSPF Operational States 8.1.3.1 45 OSPF Operational States When an OSPF router is initially connected to a network, it attempts to: Create adjacencies with neighbors Exchange routing information Calculate the best routes Reach convergence OSPF router progresses through several states while attempting to reach convergence.

Establish Neighbor Adjacencies OSPF-enabled routers must form adjacencies with their neighbor before they can share information with that neighbor. When OSPF is enabled on an interface, the router must determine if there is another OSPF neighbor on the link. To accomplish this, the router forwards a Hello packet that contains its router ID out all OSPF-enabled interfaces to determine whether neighbors are present on those links. If a neighbor is present, the OSPF-enabled router attempts to establish a neighbor adjacency with that neighbor. The OSPF router ID is used by the OSPF process to uniquely identify each router in the OSPF area. A router ID is an IP address assigned to identify a specific router among OSPF peers.

Establishing Neighbor Adjacencies An OSPF adjacency is established in several steps and OSPF router goes through the following states:

Down State This is the first OSPF neighbor adjacency state. It means that no information (Hellos) has been received, but Hello packets can still be sent to the neighbor in this state.

Down to Init State In the first step, routers that intend to establish an OSPF neighbor adjacency exchange a Hello packets. When OSPF is enabled, the enabled Gigabit Ethernet 0/0 interface transitions from the Down state to the Init state. Refer to R1 in Figure 1. When OSPF is enabled, the enabled Gigabit Ethernet 0/0 interface transitions from the Down state to the Init state. R1 starts sending Hello packets out all OSPF-enabled interfaces to discover OSPF neighbors to develop adjacencies with. * A Cisco router includes the Router IDs of all neighbors in the init (or higher) state in its Hello packets.

Init State When a router receives a Hello packet with a router ID that is not within its neighbor list, the receiving router attempts to establish an adjacency with the initiating router. 1- adds the R1 router ID to its neighbor list In Figure 2, R2 receives the Hello packet from R1 and adds the R1 router ID to its neighbor list. R2 then sends a Hello packet to R1. The packet contains the R2 Router ID and the R1 Router ID in its list of neighbors on the same interface. 2- sends a Hello packet to R1

A router transit to Init State when It is in Down state and it starts sending Hello packet It receives a Hello packet with a router ID that is not within its neighbor list

Init State Init state specifies that the router has received a Hello packet from its neighbor, but the receiving router's ID was not included in the hello packet. When a router receives a Hello from the neighbor but has not yet seen its own router ID in the neighbor Hello packet, it will transit to the Init state. In this state, the router will record all neighbor router IDs and start including them in Hellos sent to the neighbors.

2-Way State When the router sees its own router ID in the Hello packet received from the neighbor, it will transit to the 2-Way state. This means that bidirectional communication with the neighbor has been established. In Figure 3, R1 receives the Hello and adds the R2 Router ID in its list of OSPF neighbors. It also notices its own Router ID in the Hello packet’s list of neighbors. When a router receives a Hello packet with its Router ID listed in the list of neighbors, the router transitions from the Init state to the Two-Way state This state designates that bi-directional communication has been established between two routers. Bi-directional means that each router has seen the other's hello packet. This state is attained when the router receiving the hello packet sees its own Router ID within the received hello packet's neighbor field. 1- adds the R2 Router ID in its list of OSPF neighbors. 2- its own Router ID in the Hello packet

When a router receives a Hello packet with its Router ID listed in the list of neighbors, it will transit from the Init state to the Two-Way state its Router ID not listed in the list of neighbors, it will transit to the Init state *The transtion to 2-Way state happens if the router is in the Init state

2-Way State The action performed in Two-Way state depends on the type of inter-connection between the adjacent routers: If the link is a point-to-point link, then they immediately transition from the Two-Way state to the database synchronization phase. If the routers are interconnected over a multiaccess network, then a designated router(DR) and a backup designated router (BDR) must be elected. broadcast media and non-broadcast multiaccess networks

Why a DR and a BDR Multiaccess networks can create two challenges : Creation of multiple adjacencies Extensive flooding of LSAs

Why a DR and a BDR The solution to managing the number of adjacencies and the flooding of LSAs on a multiaccess network is the DR. On multiaccess networks, OSPF elects a DR to be the collection and distribution point for LSAs sent and received. A BDR is also elected in case the DR fails. All other routers become DROTHERs. A DROTHER is a router that is neither the DR nor the BDR. On broadcast links, OSPF neighbors first determine the designated router (DR) and backup designated router (BDR) roles, which optimize the exchange of information in broadcast segments.

DR and BDR The DR and BDR act as a central point of contact for link-state information exchange on a multiaccess network. Each router must establish a full adjacency with the DR and the BDR only. Each router, rather than exchanging LSA with every other router on the segment, sends the LSA to the DR and BDR only.

DR and BDR DR router performs the following tasks: Network Links Advertisement The DR originates the network LSA for the network. Managing LSDB synchronization: The DR and BDR ensure that the other routers on the network have the same link-state information about the common segment.

DR and BDR When the DR is operating, the BDR does not perform any DR functions. Instead, the BDR receives all the information, but the DR performs the LSA forwarding and LSDB synchronization tasks. The BDR performs the DR tasks only if the DR fails. When the DR fails, the BDR automatically becomes the new DR, and a new BDR election occurs.

Synchronizing OSPF Databases After the Two-Way state, routers transition to database synchronization states.

Synchronizing OSPF Databases While the Hello packet was used to establish neighbor adjacencies, the other four types of OSPF packets are used during the process of exchanging and synchronizing LSDBs.

ExStart state In the ExStart state, a master and slave relationship is created between each router and its adjacent DR and BDR. The router with the higher router ID acts as the master for the Exchange state.

Exchange state In the Exchange state, the master and slave routers exchange one or more DBD packets. DBD packets is an abbreviated list of the sending router’s LSDB and is used by receiving routers to check against the local LSDB. The LSDB must be identical on all OSPF routers within an area to construct an accurate SPF tree.

Loading State When a router receives a DBD packet, it compares the information received with the information it has in its own LSDB. If the DBD packet has a more current LSA or has an LSA that is not in its LSDB, the router transitions to the Loading state.

the router adds an entry to its Link State Request list

Loading State In this state, the actual exchange of link state information occurs. Based on the information provided by the DBDs, routers send link-state request (LSR) packets. The neighbor then provides the requested link-state information in link-state update (LSU) packets. During the adjacency, if a router receives an outdated or missing LSA, it requests that LSA by sending a LSR packet. All link-state update packets are acknowledged.

Full State After all LSRs have been satisfied for a given router, the adjacent routers are considered synchronized (have identical LSDBs ) and in a full state.

Establishing Bidirectional Communication 172.16.5.1/24 Port2 B A Port1 172.16.5.2/24 Down state hello I am router id 172.16.5.1, and I see no one To 224.0.0.5 Initial State Router B neighbor List 172.16.5.1/24,in Port2 Unicast to A hello I am router id 172.16.5.2, and I see 172.16.5.1 Router A neighbor List 172.16.5.2/24,in Port1 Two-way State

Discovering the Network Routes Port2 172.16.5.1/24 B A Port1 172.16.5.2/24 Exstart state DBD I will start exchange because I have router id 172.16.5.1 DBD No, I’ll start exchange because I have a higher RID exchange State DBD Here is a summary of my LSDB DBD Here is a summary of my LSDB

Adding the Link-State Entries Port2 172.16.5.1/24 B A Port1 172.16.5.2/24 LSAck LSAck Thanks for the information! Loading state LSR I need complete entry for network 172.16.6.0/24 LSU Here is the entry for network 172.16.6.0/24 LSAck Thanks for the information! Full State

OSPF Operational State Route calculation and Dijkstra’s Algorithm OSPF Routing Protocol Components of OSPF OSPF Terminologies OSPF Operation OSPF Operational State Route calculation and Dijkstra’s Algorithm

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