Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Personal Cisco > OSPF > Flashcards

OSPF Flashcards

1
Q

OSPF Traits

A

1) Open Shortest Path First
2) link-state protocol
3) v2 only supports IPv4; v3 supports both IPv4 and IPv6
4) Dijkstra algorithm
5) triggered incremental updates
6) Classless
7) Protocol 89

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

OSPF Autonomous System (AS)

A
  • a collection of networks under a common administration that shares a common routing strategy.
  • sometimes called a domain
  • can be divided into several areas
  • if either the redistribute or default-information originate commands are used, that makes it an ASBR
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

OSPF Area

A
  • group of contiguous networks and attached hosts
  • routers in the same area will have the same link-state info
  • identified using a 32-bit area ID, expressed either as a dotted decimal or a decimal
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q
A
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

router ospf <process-id></process-id>

A
  • enters router config mode for OSPF
  • process-id is only locally significant
  • best practice is to use process-id 1, and count up from there if you need more
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

OSPF router-id

A
  • 32-bit number written like an IP address
  • best practice is to hard-code the loopback as the router-id
  • must be unique within an AS
  • selected once you enter the router ospf command, so best to configure the loopback before running that command
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

OSPF Router-ID Selection

A

1) Manually configured using the router-id a.b.c.d command
2) Highest IP of an active loopback interface
3) Highest IP of an active physical interface

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

OSPF Neighbor Adjacency Requirements

A

1) Hello/Dead intervals
2) Area ID
3) Authentication
4) Stub Area Flag
5) *MTU; adjacency will form, but DBD won’t negotiate

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

OSPF Multicast Address

A

224.0.0.5 (AllSPFRouters)
224.0.0.6 (AllDRRouters)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

OSPF Hello Packet Contents

A

1) Router-id
2) Hello/dead intervals
3) Neighbors
4) Area ID
5) Router priority
6) DR & BDR IP addresses
7) Authentication data
8) Stub Area Flag

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

OSPF Hello & Dead Intervals

A

1) Hello intervals specifies frequency to send hello packets; default is 10s
2) Dead intervals is how long to wait for hello before declaring the neighbor dead; default is 4x hello timer, or 40s

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

OSPF Neighbors

A

1) lists the adjacent routers with established bidi communication
2) Considered bidi when the router sees itself in the neighbor field of the neighbor’s hello packet

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

OSPF Router Priority

A
  • 8-bit number that indicates the priority to select the DR and BDR
  • a 0 value means the router doesn’t participate in the election
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

OSPF States

A

1) Down - no info received on segment
2) Init - interface detected a hello from a neighbor, but bidi not established yet
3) 2-way - bidi to neighbor established; DR/BDR election after this stage, if needed
4) Exstart - routers establish initial sequence number. One router with the higher router ID becomes primary, the other becomes secondary. The primary polls the secondary for info and only the primary can increment the sequence. Neighbors with mismatched MTU get stuck here.
5) Exchange - routers describe their entire LSDB by sending DBDs. Packets may be flooded to other interfaces on the router
6) Loading - finalizing info exchange. Incomplete or outdated info will be put on request list. Updates sent without ACKs get put into a retrasmit list.
7) Full - adjacency is complete and adjacent routers will ahve similar LSDBs

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

OSPF Packet Types

A

1) Hello - discovers and maintains neighbors; contains list of known neighbors
2) DBD - contains the LSA headers to help routers build the LSDB; includes all known router IDs and their last seq numbers
3) LSR - used to request more current info if DBD shows newer info; contains type of LSU needed, and router ID that has the needed LSU
4) LSU - contains a list of LSAs that should be updated; often used in flooding; multiple LSA entries can fit in one LSU
5) LSAck - ensures reliable transmission; packet is empty

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

OSPF Exchange Protocol

A

1) RoutersA & B exchange DBDs (or Type2 OSPF) packets.
2) RouterA acknowledges receipt of DBD with LSAck
3) If received DBD has more up-to-date link-state info, RouterA sends a LSR to RouterB to request the updated link-state entry
4) RouterB responds with updated entry in LSU packet. RouterA adds the entry to its LSDB
5) RouterA sends an LSAck after getting the LSU

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

OSPF DBD

A
  • Database Descriptor
  • includes info about the LSA entry header in the LSDB
  • each LSA entry header includes info about link-state type, addr of advertising router, cost of the link, and sequence number, which determines the newness of the info
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

OSPF Packet Fields

A

1) Version Number - v2 for IPv4, v3 for IPv4 & IPv6
2) Type - differentiates the five OSPF packet types
3) Packet Length - length of packet in bytes
4) Router ID - defines which router is the src of the packet
5) Area ID - defines which area the packet originated
6) Checksum - packet header error detection
7) Authentication Type - either none, clear text, or MD5
8) Authentication
9) Data - each packet type has different data

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

OSPF LSA Types

A

1) Router
2) Network
3) Summary
4) ASBR Summary
5) Autonomous System
6) Specialized LSA used in multicast OSPF
7) Used in NSSA for external routes
8 & 9) OSPFv3 Link-local addresses and intra-area prefixes
10 & 11) Generic/Opaque LSAs for future use

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

OSPF Router LSA

A
  • Type1
  • describe the state of the router links to the area
  • only flooded within area
  • link-state ID is the originating router ID
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

OSPF Network LSA

A
  • Type2
  • describes the set of routers that are attached to a particular multiaccess network
  • flooded in the area that contains the network
  • link-state ID is the DR IP
21
Q

OSPF Summary LSA

A
  • Type3
  • an ABR takes info learned in one area, and summarizes it for another area
  • not on by default
  • link-state ID is the destination network
22
Q

OSPF ASBR Summary LSA

A
  • Type4
  • informs the rest of the OSPF domain how to get to the ASBR
  • link-state ID includes the router ID of the described ASBR
23
Q

OSPF Autonomous System LSA

A
  • Type5
  • AS external link advertisements; describes routes to destinations that are external to the AS
  • generated by ASBRs
  • flooded everywhere except to special areas
  • link-state ID is the external network number
24
Q

Multiarea OSPF Advantages

A

1) make routing tables smaller with route summarization
2) divides routers into separate areas to limit propagation and processing of LSAs

25
Q

OSPF Area Structure

A

1) Area0 is the backbone
2) Non-Backbone areas should all be attached to the backbone
3) Inter-Area traffic must traverse the backbone

26
Q

OSPF Area Best Practices

A

1) minimize the number of routers in one area
2) areas with unstable links should be smaller
3) one router should not be in more than 3 areas

27
Q

OSPF Network Types

A

1) Point-to-point
2) Broadcast
3) Nonbroadcast
4) Point-to-multipoint
5) Point-to-multipoint nonbroadcast
6) Loopback
7) Can be changed with the ip ospf network <network_type> command</network_type>

28
Q

OSPF Network Type - Point-to-Point

A

1) No DR/BDR
2) 10s Hello
3) Uses multicast to dynamically discover neighbors
4) No more than 2 routers allowed in subnet
5) default type for serial links and Frame Relay subinterfaces

29
Q

OSPF Network Type - Broadcast

A

1) Uses DR/BDR
2) 10s Hello
3) Uses multicast to dynamically discover neighbors
4) More than 2 routers allowed in subnet
5) default type for Ethernet

30
Q

OSPF Network Type - Nonbroadcast

A

1) Uses DR/BDR
2) 30s Hello
3) Must statically configure neighbors
4) More than 2 routers allowed in subnet
5) default type for all physical interfaces, and multipoint subinterfaces using Frame Relay encapsulation

31
Q

OSPF Network Type - Point-to-Multipoint

A

1) No DR/BDR
2) 30s Hello
3) Uses multicast to dynamically discover neighbors
4) More than 2 routers allowed in subnet
5) Treated as a logical collection of point-to-point links

32
Q

OSPF Network Type - Point-to-Multipoint Nonbroadcast

A

1) No DR/BDR
2) 30s Hello
3) Must statically configure neighbors
4) More than 2 routers allowed in subnet
5) Cisco extension for cases where multicast and broadcast aren’t supported
6) Unicast used for communication

33
Q

OSPF Network Type - Loopback

A

1) No DR/BDR
2) No Hello
3) No Dynamic Neighbor Discovery
4) More than 2 Routers not allowed in the subnet
5) Default type for loopback interfaces

34
Q

OSPF DR/BDR

A
  • DR & BDR need full mesh adjacency to all other routers
  • BDR does nothing while DR is active
  • when DR fails, BDR is promoted and a new BDR election occurs
35
Q

OSPF DR/BDR Election Priority

A
  • router with highest priority becomes DR, and 2nd highest becomes BDR
  • if priorities are equal, it uses router ID instead
  • on Broadcast networks, routers use 224.0.0.6 to talk with the DR, and DR uses 224.0.0.5 to talk to all non-DR routers
  • on NBMA networks, DR and adjacent routers use unicast
36
Q

debug ip ospf hello

A
  • used to see a mismatch in hello timers
37
Q

OSPF Timers

A
  • when changing only the OSPF hello timer, the dead interval is automatically changed to 4x the hello unless otherwise specified
38
Q

OSPF Cost Formula

A

cost = reference_bandwidth (bps) divided by interface_bandwidth (bps)

default reference bw = 10^8 (100 Mbps)

39
Q

OSPF Route Summarization

A

1) ABRs use the area range command
2) ASBRs use the summary-address command
3) ABRs summarize type3 LSAs
4) ASBRs summarize type5 LSAs

40
Q

OSPF Route Filtering Tools

A

1) most filtering methods do not remove networks from the LSDB
2) the routes are removed from the routing table, which prevents the local router from using them to forward traffic
3) Distribute Lists - filter routes that the router’s SPF process is attempting to add to its routing table without affecting the LSDB; can be applied to Type3 and Type5 LSAs
4) Prefix Lists
5) Route Maps
6) ABR Type3 LSA filtering using the filter-list command; prevents an ABR from creating certain Type3 summary LSAs
7) area range not-advertise command: prevents an ABR from creating specific Type3 summary LSAs
8) summary-address not-advertise command: prevents an ASBR from creating specific Type5 External LSAs

41
Q

Distribute List

A
  • allows you to apply an ACL to routing updates
  • can be applied to teh transmitted, received, or redistributed routing updates
  • oubound applies only to redistributed routes and only on an ASBR
  • inbound does not filter inbound LSAs; it instead filters the routes that SPF chooses to add to its own local routing table
42
Q

Prefix Lists

A

1) uses ACL or prefix list to filter routing updates
2) ACLs

43
Q

Route Maps

A

1) Route fitlering during redistribution - distribute lists work too, but route-maps can manipulate routing metrics with set commands
2) Policy-based routing
3) BGP - control routes and path attributes
4) multiple match criteria in the same line are processed with a logical or

44
Q

Route Map Configuration

A

1) Define the route-map name
2) Define the match
3) Define the set
4) Attach the route-map

45
Q

router ospf commands

A

1) router ospf <process-id> - for IPv4
2) ipv6 router ospf <process-id> for ipv6 only; peering over link-local addresses
3) router ospfv3 <process-id> - for both IPv4 and IPv6; transported over IPv6 (link-local addrs) so no peering with traditional OSPFv2 routers</process-id></process-id></process-id>

46
Q

OSPF Renamed LSAs

A

1) Interarea prefix LSAs for ABRs (Type3)
2) Interarea router LSAs for ASBRs (Type4)

47
Q

Interarea Prefix LSAs for ABRs (Type3)

A

1) these advertise internal networks to routers in other areas (interarea routes)
2) only ABRs generate summary LSAs
3) addresses for these LSAs are expressed as prefix/prefix-length instead of an address and mask
4) the default route is expressed as a prefix with length 0

48
Q

Interarea Router LSAs for ASBRs (Type4)

A

1) advertise the location of an ASBR
2) the ABR originates a separate interarea Router LSA for each ASBR it advertises

49
Q

OSPF New LSAs

A

1) Link LSAs (Type8)
2) Intra-area prefix LSAs (Type9)

50
Q

Link LSAs (Type8)

A

1) have local-link flooding scope and are never flooded beyond the link with which they are associated
2) Link LSAs provide the link-local address of the router to all other routers that are attached to the link
3) Inform other routers that are attached to the link a list of IPv6 prefixes to associate with the link
4) Allow the router to assert a collection of option bits to associate with the network LSA that will be originated for the link

Personal Cisco (25 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions