CCNA 2 Chapter 8

If the router ID is identical on 2 neighboring routers, what error message does the router display?

%OSPF-4-DUP_RTRID1: Detected router with duplicate router ID.

What 4 steps does an OSPF attempt to do when initially connected to a network?

* Create adjacencies with neighbors * Exchange routing information * Calculate the best routes * Reach convergence

While attempting to reach convergence, what 7 states does OSPF go thru?

* down state * Init state * two-way state * exStart state * exchange state * loading state * full state

What command prevents the transmission of routing messages out a specific interface, but still allows that network to be advertised to other routers?

* use default passive-interfaces to make all interfaces passive

What are the 7 features of OSPF?

1. AD = 110 2. Classless - supports VLSM & CIDR 3. Efficient - routing changes trigger routing updates 4. Fast Convergence 5. Scalable 6. Secure - supports MD5 7. Dijkstra Algorithm chooses best path

What 3 databases does OSPF create and keep in RAM?

1. Adjacency - neighbor table 2. Link-state (LSDB) - topology table 3. Forwarding - routing table

To reach a state of convergence, what 4 steps does OSPF complete?

1. Establish Neighbor Adjacencies - hello packets are sent out all OSPF-enabled interfaces 2. Exchange Link-State Advertisements (LSAs) - routers flood LSAs to adjacent routers which contain the state, cost, and sequence # of each directly connected link 3. Build the Topology Table - based on the LSAs a router receives - eventually holds all info about networks topology 4. Execute the SPF Algorithm - which then creates the SPF tree

How does the router determine its router ID?

1. configure router-id command 2. router chooses highest IPv4 address of any configured lookback interfaces 3. if no loopback interfaces are configured, router chooses the highest active IPv4 address of any of its physical interfaces - least recommended

Why is a DR and BDR election necessary?

1. creating adjacencies with every router on the same network leads to excessive #'s of LSAs exchanged between routers 2. preventing excessive flooding of LSAs

List 3 things the OSPF Hello Packet is used for?

1. discover OSPF neighbors & establish adjacencies 2. advertise parameters on which 2 routers must agree to become neighbors 3. elect designated router (DR) & backup designated router (BDR) on multiaccess networks like Ethernet & Frame Relay

A Multi-area OSPF is used in larger networks to dived a large AS (autonomous system) into smaller areas. What are 3 characteristics of a multi-area OSPF?

1. it lightens the load on routers 2. all areas must connect to the backbone (0) area 3. interconnecting routers are called Area Border Routers (ABRs).

How does Dijkstra's algorithm work?

1. places each router at root of SPF tree and calculates the shortest path to each node 2. SPF tree calculates the best routes 3. OSPF places best routes in forwarding database to make the routing table

What are 4 advantages of Multi-area OSPFs?

1. smaller routing tables because network addresses can be summarized between areas 2. reduced link-state update overhead - uses less processing and memory 3. reduced frequency of SPF calculation - LSA flooding stops at the area boundary 4. link failure affects only the local area

What 4 conditions prevent 2 routers from forming an OSPF adjacency?

1. subnets don't match - different network 2. OSPF Hello or Dead timers don't match 3. OSPF Network Types don't match 4. Missing or incorrect OSPF network command

Sending unneeded messages out all OSPF-enabled ports to non-OSPF routers in a LAN affects the network negatively in what 3 ways?

1. wastes bandwidth 2. wastes resources - all devices on the LAN have to process the message and then discard it 3. increases security risks - advertising updates on a broadcast network subjects messages to packet sniffing software

Commands needed to assign a router ID:

10 = process ID

The OSPF hello timer has been set to 15 seconds on a router in a point-to-point network. By default, what is the dead interval on this router?

60 seconds - By default, the dead interval is calculated as 4 times the hello interval.

What does the OSPF Adjacency database contain?

A Neighbor table that lists all neighbor routers - can be viewed using the show ip ospf neighbor command. The neighbor table is unique for each router.

What is an OSPF area?

A group of routers that share the same link-state info in their LSDBs.

At which OSPF state are neighbor routers converged and able to exchange routing updates?

Full

When OSPF is enabled on an interface, the router must look for OSPF neighbors on the link by forwarding what kind of packet containing its router ID (IP address) out all OSPF-enabled interfaces.

Hello packet

Which 9 abbreviations should you know for OSPF routers?

LSA - link-state advertisements SPF - shortest path first DBD - database description LSR - link state request LSU - link state update LSAck - link state acknowledgment LSDB - link state database DR - designated router BDR - backup designated router

Do point-to-point links require DR or BDR?

NO

If the router chooses the highest IPv4 address for the router ID, does the interface need to be OSPF-enabled?

NO

Can a neighbor adjacency be formed over a passive interface?

NO - because link-state packets can't be send or achnowledged

What happens if the Dead Interval expires before the routers receive a Hello packet?

OSPF removes that neighbor from its LSDB and the router floods the LSDB with info about the down neighbor out of all OSPF-enabled interfaces

What is displayed in the neighbor adjacency table?

Priority- used for DR & BDR elections - means no DR or BDR is required dead time = amount of time remaining for a router to wait to receive an OSPF Hello packet before declaring neighbor down

What is OSPFv3?

Same as OSPFv2, but used for IPv6. If you use the address families feature, OSPFv3 works for both IPv6 & IPv4.

Because OSPF is classless, a wildcard mask is required. How do you calculate a wildcard mask?

Subtract the subnet mask from 255.255.255.255

How often are OSPF Hello packets transmitted?

every 10 seconds on multiaccess & point-to-point networks every 30 seconds on nonbroadcast multiaccess (NBMA) networks such as Frame Relay

After the topology databases are synchronized when are LSUs sent to neighbors?

every 30 minutes, or when a change is perceived

Cisco uses a default of 4X the Hello interval, so how often do Cisco routers transmit Hello packets?

every 40 seconds on multiaccess & point-to-point networks every 120 seconds on nonbroadcast multiaccess (NBMA) networks

What does an LSU contain?

link state advertisements (LSAs)

What kind of IPv6 addresses are automatically created when an IPv6 global unicast address is assigned to the interface?

link-local * by default, Cisco routers create the link-local address using FE80::/10 prefix and the EUI-64 process

In the ExStart state, what kind of relationship is created between each router and its adjacent DR and BDR?

the DR becomes the master, and the BDR the slave to exchange DBD packets

What is the Dijkstra SPF algorithm based on?

the cumulative cost from one router to reach a destination

In a nutshell, how does EUI-64 work?

uses the 48-bit Ethernet MAC address, inserts FFFE in the middle, and flips the seventh bit.

When does an OSPF router transition to the two-way state?

when it receives a Hello packet containing its own Router ID in the list of neighbors

What is a Single-area OSPF?

Used in a small network - all routers are within area 0 aka the backbone area.

What does the OSPF Forwarding Database contain?

A routing table lists the routes generated when an algorithm is run. Each router's table is unique & contains info on how and where to send packets. Can be viewed using the show ip route command.

What does the OSPF LSDB contain?

A topology table lists the network topology, info about all other routers on the network. All routers within an area have identical LSDB. Can be viewed using the show ip ospf database command.

OSPF uses cost as a metric. A higher bandwidth indicates a lower cost. What is the formula to calculate OSPF cost?

Cost = 100,000,000 bps / interface bandwidth in bps

If 2 OSPF routers are connected over a multi-access Ethernet network what must be elected?

DR - router with the highest router ID BDR - router with 2nd highest ID

Can a dual-stacked network have both OSPFv2 & OSPFv3 enabled?

YES

After all LSRs have been satisfied for a router, what state are its adjacent routers considered to be in?

a full state - synchronized

The router ID looks like an IP address, but is not included in the routing table, why is that?

because it is not a routable address

What 4 part command clears the OSPF process if a Router ID needs to be reset?

clear ip ospf process

List a few show OSPF commands:

show ip ospf interface brief show ip ospf

1. configure router-id command 2. router chooses highest IPv4 address of any configured lookback interfaces 3. if no loopback interfaces are configured, router chooses the highest active IPv4 address of any of its physical interfaces - least recommended

show ip protocols