ENSA Module 1: Single-Area OSPFv2 Concepts

Which commands shows the adjacency database? Which commands shows the link-state database? Which commands shows the forwarding database?

- "show ip ospf neighbor" - "show ip ospf database" - "show ip route"

Describe the OSPF Router ID.

- 32-bit number formatted like an IPv4 address and assigned to establish an adjacency with the initiating router - uniquely identifies each router in an OSPF area

Describe each of the 3 data structure tables that are in a router's RAM.

- Adjacency databases (makes the neighbor table) + list neighbor routers with which the router has established bidirectional communication - Link-state database (LSDB)(makes the topology table) + info about other network routers + same LSDB for all routers that are in the same area - Forwarding database (makes the routing table) + lists routes generated when an algorithm is run on the link-state database

Describe each of the database synchronization states.

1) Decide First Router +

Describe each of the 5 steps a router takes in link-state routing.

1) Establish Neighbor Adjacencies + Hello packets out all OSPF-enabled interfaces + router attempts to establish a neighbor adjacency with present neighbors 2) Exchange Link-State Advertisements (LSAs) + LSAs flooded directly-connected neighbors + LSA: state and cost of each directly connected neighbor 3) Build the Link State Database + OSPF-enabled routers use LSA received to build the topology table 4) Execute the SPF Algorithm + creates the SPF tree 5) Choose the Best Route, for inclusion in the routing table, unless there is lower-AD route (static route)

Describe each of the 11 OSPF LSAs. Which ones should you know at the CCNA level?

1) Router LSAs 2) Network LSAs 3 or 4) Summary LSAs 5) Autonomous System External LSAs 6) Acknowledges the other packet types 7) Defined for Not-So-Stubby Areas 8) External Attributes LSA for Border Gateway Protocol (BGP) 9) Intra-Area Prefix LSAs 10,11) Opaque - 1-5 you should know at the CCNA level

Describe each of the OSPF Operational States.

- Down State + Router sends Hello packets, but haven't received any yet + transitions to Init state - Init State + Hello packets are received from the neighbor + contains Router ID of the sending router + transitions to Two-Way state - Two-Way State + communication is bidirectional + DR and BDR elected, on multiaccess links + highest priority wins, default priority is 1, if equal priority highest Router ID (interface IP address) wins + transitions to ExStart state - ExStart State + elects which router is the first to send DBD packets, based on highest Router ID (interface IP address) + also decides the initial sequence number - Exchange State + routers exchange DBD packets + if additional router information is required, go to Loading State; otherwise, go to Full State - Loading State + LSRs and LSUs are used to gain additional route information + routes processed using the SPF algorithm transitions to Full state - Full State + link-state database of the router is fully synchronized

Describe the 3 primary benefits of the hierarchical-topology design used in multiarea OSPF.

- Smaller routing tables + network addresses are summarized between areas (must be enabled) - Reduced link-state update overhead + CPU processing and memory requirements - Reduced frequency of SPF calculations + because LSA flooding stops at the area boundary

What is the Dead Interval in an OSPF packet?

- Time that a router will wait to hear from another router before it declares that that router is dead and out of service. - is 4 X the Hello interval by default, thus commonly 40 seconds

In which IP packet field is the type of OSPF packet displayed? Describe each of the 5 types of OSPF packets.

- Type (ex. Type = 1) 1) Hello + to the reserved All OSPF Routers IPv4 multicast address 244.0.0.5 + contains router ID + sent out of all OSPF-enabled interfaces + discovers OSPF neighbors and builds adjacencies between them + advertises parameters on which two routers must agree to become neighbors + used, along with keep-alives, to keep track of the router + for multiaccess networks (not point-to-point, like serial), helps decide Designated Router (DR) and Backup Designated Router (BDR) 2) Database Description (DBD) + checks for database synchronization between routers + one router sends an abbreviated list of the Link-State Database, against which the receiving router checks its database to ensure that it is identical 3) Link-State Request (LSR) + requests specific link-state records from router to router about a specific entry in the DBD message 4) Link-State Update (LSU) + Sends specifically requested link-state records, to respond to a LSR + Can be: - OSPF created message regarding a new network or a network change - a response to a LSR that was received + can contain 11 types of LSA within 5) Link-State Acknowledgement (LSAck) + Acknowledges any of the other packet types, except Hello or Ack

Describe the router priority that is listed in a Hello packet.

- aids in the Designated Router (DR) and Backup Designated Router (BDR) election process - default is 1, but can be modified up to 255 - higher the better chance at becoming the designated router

Describe an OSPF area along with the 2 types of OSPF areas.

- group of routers that share the same link-state information in their LSDB - Single-Area OSPF + all routers in one area, normally called Area 0 - Multiarea OSPF + one large routing domain divided into smaller areas + supports hierarchical routing + allows CPU-intensive operations to be occur only within the specific area needing it + avoids having to rerun the SPF algorithm, update the SPF tree, and update the routing table for all routers every time the topology changes (rather it only updates the routing table with topology change updates) + all areas connect to backbone area (Area 0) + Area Border Routers (ABRs): routers that interconnect areas

How does a DR and BDR being used help in a converged network?

- having a DR and BDR elected inside of a multiaccess network means that a router not serving as a DR or BDR: + only keeps a full state with that DR and BDR instead of with every other router with which it is connected (even through switches) +only keeps a two-way state with other non-DR and non-BDR routers, not required to have the same topology table as each other routers - cuts down on messages - DR and BDR serving governs traffic flow within its area, and fewer messages must pass with the same information

Open Shortest Path First (OSPF)

- link-state routing protocol, as is the IS-IS protocol - either single-area or multiarea - designed to find the fastest available path for a packet from source to destination - offers faster convergence and scales to much larger network implementations

Which information is included in link-state information?

- network prefix - prefix length - cost

What are the List of Neighbor(s) in an OSPF packet?

- router IDs of all adjacent OSPF routers

How do routing protocol messages relate to data structures and routing algorithms? Describe routing protocol messages, data structures, and algorithms as they pertain to OSPF.

- routing protocol messages exchange route information to build data structures that are processed by routing algorithms - Routing protocol messages + used to discover neighboring routers and maintain accurate network information + 5 types of packets: + Hello packet + Database description packet + Link-state request packet + Link-state update packet + Link-state acknowledgement packet - Data Structures + 3 databases tables maintained in RAM that list neighboring routers IOT exchange routing information: + Adjacency database + Link-state database (LSDB) + Forwarding database - Algorithm + uses SPF (shortest-path first) algorithm, which places each router at the root of the tree and calculating the shortest path to each node + SPF tree gets used to calculate the best routes, to be put into the routing table

What is the hello interval in an OSPF Hello Packet?

- specifies the frequency in seconds that the router will send hello packets - default is 10 seconds, for multiaccess routers

Describe a link-state routing protocol.

- uses the concept of areas, dividing the routing domain into distinct areas that help control routing update traffic - links in OSPF + router interfaces + network segments connecting two routers + stub networks connected to a single router, such as an Ethernet LAN