Describe Cisco Dynamic IPv4 Routing with OSPF Flashcards
1
Q
In OSPF, what is an RID and what is it used for?
A
- Stands for Router ID
- Uniquely identifies a router on the network
- Routes and their costs associated with a particular RID
- Duplicate RIDs can cause connectivity issues
2
Q
How is a default route configured?
A
- Statically: Manually configured by an admin
- DHCP: If a router interface IP is dynamically configured, the default route may change and can be configured through DHCP
- Redistribution: Redistributed via a dynamic routing protocol such as OSPF
3
Q
What is the importance of the OSPF process ID?
A
- A process ID is needed to enable OSPF on a router
- Only locally significant on the router and does not need to match the ID configured on other routers
- It is best practice to run one process ID per router at a time but there may be cases where multiple process IDs can be used.
4
Q
Describe the OSPF adjacency process and its purpose
A
- Forms adjacencies between OSPF routers which allows them to trust and exchange routing information between each other
- INIT: routers are powered on and OSPF is enabled. Routers begin sending out OSPF Hellos.
- 2-way: Routers receive hellos from the others and include the other RIDs in their messages. Routers become “aware” of the others
- ExStart/Exchange: routers negotiate communication details. Which router will start sharing DBDs (DataBase Descripters). Router with the higher RID or Priority starts. These are not full LSAs but contain information about remote networks. DBDs are sent
- Loading: Based on exchanged DBDs routers request LSAs for networks they don’t know (LS Requests) and those LSAs are communicated (LS Updates)
- Full Adjacency: LSDBs match and routers freely communicate and trust LSAs from the others
5
Q
What command displays the reference bandwidth used in OSPF link cost calculations?
A
show ip ospf
6
Q
Describe a LSA Type 2?
A
- Only generated by the DR in a broadcast or non-broadcast multi-access (NBMA) network
- Contains information about all routers adjacent to the DR as part of the segment
- Only advertised in the local area
7
Q
What is the purpose of default route distribution?
A
- Allows a default route to be configured on one router and distributed to others in the same area
- Eliminates the need to manually configure a default route on all routers
8
Q
What is an area in OSPF, where are they defined and what is the advantage of using areas?
A
- Areas are used to demarcate where certain LSAs are propagated.
- Reduces overhead by limiting the size of the LSAs for a given area.
- Areas are defined at the interface, not at the router. The router interface and the network it connects to are part of a particular area. ABRs, for example, will have multiple interfaces in multiple areas.
9
Q
What are the two primary types of dynamic routing protocols and what distinguishes them from the other?
A
- Distance Vector: vector, indicating the direction in which to send traffic addressed for a certain network. Distance in this case refers to hop count.
- Link State: routers running the same link state routing protocol update each other with information regarding links to directly connected networks, using Link State Advertisements (LSAs). Information like link status (up or down) and cost. This information is used by all routers in the same group to maintain link state databases (LSDBs) that are used to make forwarding decisions. Cost used to determine shortest path which is preferred.
- Distance Vector is ok and can update on dynamic route changes but is not as sophisticated as link state. Forwarding decisions are based purely on hop-count and do not consider congestion.
10
Q
What is a static route? Why is it important to understand and know how to configure static routes? What challenges exist with static routing?
A
- A static route is a route manually configured by a network admin.
- It is important to understand how static routes work because, to an extent, that is how routing works in general. There are also cases where configuring a static route is necessary or preferable over a dynamic route, such as setting up default routes in key places.
- Static routes are difficult to implement across complex networks.
- Requires manual changes when network environment changes which can be difficult to plan and execute.
11
Q
By default, what is the OSPF link cost calculation for an ethernet interface with a bandwidth of 10Mbps?
A
- The default calculation would yield a link cost of 10
12
Q
On a Cisco router, what command is used to enable OSPF?
A
R1 (config)# router ospf 1
- 1 represents the OSPF process ID. This can be any number and is only locally significant
13
Q
How does a dynamic routing protocol make routing decisions when there are multiple paths to the same network?
A
- Ideally there should be multiple paths for redundancy
- Routing protocols use different metrics to determine path cost, typically preferring lower cost paths.
- Different protocols use different metrics
14
Q
What is a default route?
A
- Route of last resort
- If the router receives a packet destined for a network not in its routing table, the packet is sent to the default route if configured
15
Q
What commands verify that OSPF is running on your router?
A
- show ip ospf
- show ip protocols
16
Q
In OSPF, how is an RID determined?
A
- Can be assigned automatically or manually by an admin with the router preferring the manually configured RID first
- Configured manually with the “router-id A.B.C.D” command in OSPF config mode
- If no RID is manually configured, one is automatically configured using the highest IP assigned to the loopback interfaces
- If there are no loopback interfaces, the RID is configured with the highest IP of an active interface.
17
Q
What message types are used by OSPF routers?
A
- Unicast: one router to one router
- Multicast to 224.0.0.5: General OSPF communication
- Multicast to 224.0.0.6: Used by DR/BDR
18
Q
What is an ASBR?
A
- Autonomous System Boundary Router (ASBR)
- A router used between networks running different routing protocols including static, EIGRP, RIP and BGP
19
Q
What commands can be used to verify OSPF Adjacencies?
A
- show ip ospf interface brief
- show ip ospf neighbor
20
Q
What is a dynamic routing protocol?
A
- A protocol that communicates routing information between routers running the same protocol.
21
Q
What command is used to distribute a default route to other OSPF routers?
A
- default-information originate
- issued from OSPF router configuration mode
22
Q
What configuration advertises a connected network in OSPF?
A
- Once OSPF is enabled (router ospf 1) use the “network A.A.A.A B.B.B.B area X” command from OSPF router configuration mode
- A.A.A.A represents the network address you want to advertise
- B.B.B.B represents the wildcard mask you want to use. This identifies the portion of the network address that you want advertised.
- A.A.A.A B.B.B.B can be as specific or as general as you like. Any interfaces that fall within the range of IPs set by your configuration will be advertised by OSPF. This can simplify configuration but be careful you don’t advertise something you didn’t intend to.
- area X represents the area you want the advertised networks to be a part of
23
Q
What command changes the reference bandwidth used in OSPF link cost calculations? Where must this configuration be made?
A
- From the OSPF router configuration mode, issue the command “auto-cost reference-bandwidth [bandwidth]”
- Must be issued on every router to ensure matching cost calculations
24
Q
What command verifies that the networks you want advertised are in fact being advertised?
A
- show ip ospf interface
- show ip ospf interface brief
25
What are the key components necessary for successfully negotiating an adjacency between two OSPF routers? What happens if any of these components are off?
- Subnet: For the link between the routers, be it multi-access or point-to-point, they must agree on the network address and subnet mask
- Area: For that same link, both routers must have that network configured with the same area number
- Hello interval: If both routers are not using the same timer for OSPF hello messages the adjacency will fail
- Authentication: If used, both routers must use the same OSPF authentication credentials
- If any of these components are not configured identically on both routers the adjacency will fail and routing information will not be exchanged
26
How are DRs and BDRs determined?
If all routers connected to a segment are brought up simultaneously
- The router with an interface connected to the segment configured with the highest priority determines the DR. Default priority is 1 for every router.
- Second highest priority becomes the BDR.
- All other routers become DR Other
- If priorities are all the same, the highest RID becomes DR while second highest becomes BDR. All others become DR Others
27
What command can be used to change the OSPF Hello interval?
- ip ospf hello-interval X
| - issued from global config
28
In OSPF, what is an IR?
- Known as an Internal Router (IR)
| - Limited to a single area and only receives LSAs for that area
29
What metric does OSPF use to determine the shortest path to a destination network? How is this metric calculated?
- OSPF uses a metric called cost to determine the shortest path. When sending out an LSA, OSPF routers include their own cost to a destination network which is added to another routers cost for reaching that network.
- Cost is related to the egress interface bandwidth, that is the bandwidth of the interface the router would use to forward traffic to the destination network.
- Cost is calculated by dividing a benchmark bandwidth by the bandwidth of the egress interface.
- By default the benchmark bandwidth is 100 Mbps, however this can be changed to match more modern LAN speeds.
30
What are the three OSPF network types?
- Broadcast: multi-access network in which broadcasts are allowed. Requires DR/BDR
- Non-broadcast: multi-access network that does not have broadcasts, such as with ATM and Frame-Relay. Requires DR/BDR
- Point-to-Point: Direct connection between one router interface to another, such as with a serial link. No DR/BDR established.
31
Describe an LSA Type 1?
- Router LSA generated by every OSPF router.
- Advertises directly connected networks, interface, interface cost and RID for the respective router connected to those networks.
- Only advertised in the local area
32
In OSPF, what is an ABR?
- Known as an Area Border Router (ABR)
- Router that has interfaces in more than one area and thus maintains LSDBs for those areas.
- Packets from one area destined for another will go through an ABR
33
What are DRs and BDRs? What is their purpose?
- Stands for Designated Router (DR) and Backup Designated Router (BDR)
- Created when more than one OSPF router is on the same network segment / broadcast domain
- Full Adjacencies are formed only between DR/BDR and other routers on the segment. Non DR/BDR routers do not form full adjacencies with each other
- Reduces overhead for maintaining full adjacency between every router
34
When is an OSPF adjacency formed between two routers?
- When OSPF is enabled on each routers' interfaces connected to a network segment shared by those interfaces
35
What happens when two routers form a new adjacency? What is required for two routers to be fully adjacent?
- Each router sends the other summary LSAs, briefly describing known networks in their own LSDBs
- If one router sees that other has routing information it doesn't know about, it will request the full LSAs from the other router.
- Routers are fully adjacent when all LSA information is exchanged and LSDBs match
36
True or False: Routers belong to areas
False: Router interfaces and the connected network belong to an area. Although a router can have all interfaces in a single area, it can also have multiple interfaces in multiple areas. A router with the latter configuration is known as an Area Border Router (ABR)
37
What command, issued on a router, shows the contents of the Link State Database?
show ip ospf database
38
What command, issued on a router, allows you to see OSPF enabled interfaces, advertised networks and the DR state for those interfaces?
show ip ospf interface
| show ip ospf interface brief
39
What command, issued on a router, shows the OSPF adjacency state with a neighbor as well as that neighbor's DR state?
show ip ospf neighbor
40
What command, issued on a router, shows you only the Type 2 LSAs in the LSDB?
show ip ospf database network
41
What is an ABR?
- Stands for Area Border Router
| - A router with interfaces in two or more areas
42
Describe an LSA Type 3
- Summary LSA
- Generated by ABRs
- Advertises networks from one area into another area (works both ways)
- Summary LSAs, not the full LSA as would be flooded in the orginating area
- Single LSA Type 3 generated for every network in the other area (100 networks = 100 LSA Type 3)
43
How can you determine whether a router is an ABR?
- show ip ospf interface brief
If the router has interfaces in multiple areas, it is an ABR
- show ip ospf
Shows router's ABR designation
44
How can you determine whether a route was learned from an LSA Type 3?
- show ip route
Any route learned from a Summary LSA will be identified with "O IA" in the routing table. IA standing for inter-area
- show ip ospf database summary
45
What is redistribution in OSPF?
- When a router advertises routes from non-OSPF sources (or from another OSPF process)
- Can come from other routing protocols such as BGP, EIGRP, RIP or from statically configured routes
46
What happens when a router redistributes routes into OSPF?
- It becomes a Autonomous System Boundary Router (ASBR)
- Floods all regular areas with LSA Type 5, advertising the external routes and designating itself as the gateway to those routes
47
What command redistributes static routes configured on a router into OSPF?
conf t
router ospf 1
redistribute static subnets
*subnets option ensures routes are advertised using classless boundaries in the case that subnetting is being used
48
What is an ASBR?
- Autonomous Systems Boundary Router (ASBR)
| - Advertises in OSPF, routes learned statically or from other routing protocols
49
Describe an LSA Type 5
- External LSA
- Generated by an ASBR
- Describes routes learned from non-OSPF sources such as static routes or those learned from RIP
- Advertised to every area, identifying the ASBR as the source for those routes
50
Describe an LSA Type 4
- Summary ASBR LSA
- Generated by ABRs and advertised into areas not shared with originating ASBR
- Routers in other areas will receive type 5 LSAs describing external routes and the ASBR that sourced them but will not know how to reach the ASBR. ABR generates type 4 LSAs, designating itself as the destination to reach the ASBR
51
Why must all areas connect to backbone area 0?
- OSPF uses distance vector for routing between areas and link state for routing within an area.
- Distance vector protocols are prone to the "counting to infinity" problem, where if a network goes does the router connected to that network might mistake upstream routers as having a legitimate routes to the down network - even though they're using the connected router to connect to the down network.
52
What command can be used to observe and verify the OSPF adjacency process?
debug ip ospf adjacency
53
What two ways can be used to advertise a network in OSPF? What commands are used for each method?
- From router OSPF configuration mode
conf t
router ospf 1
network A.B.C.D W.W.W.W area X
*advertises all networks that fall within the boundaries of the network address and wildcard mask
- From the configuration mode of the interface you want to advertise. OSPF must already be enable and process ID used must match
conf t
interface gi0/0
ip ospf 1 area X
54
What happens when you make an area stubby?
- Reduces LSDB size within the stubby area by eliminating Type 4 (ASBR summary) and Type 5 (External) LSAs
- ABR becomes the default gateway for any routes going through the ASBR
55
How to configure a stubby area
conf t
router ospf 1
area X stub
- must be configured on every router connected to the area, otherwise adjacencies won't form.
56
What happens when you create a totally stubby area?
- Similar to a stubby area, all Type 4 (ASBR summary) and Type 5 (external) LSAs are eliminated
- Most Type 3 (summary) LSAs are also eliminated
- ABR becomes default gateway for all non-intra-area routes
- The only Type 3 LSA is a default route pointing to the ABR
57
How to configure a totally stubby area
conf t
router ospf 1
area X stub no-summary
- Same as a regular stub, must be configured on all routers connected to the area
58
How is priority for DR/BDR election configured?
- On the interface connected to the broadcast/NBMA network segment
conf t
interface gi0/0
ip ospf priority X
59
What happens when you configure an interface with OSPF priority of 0?
The router will never become a DR/BDR for that segment
60
What is the purpose of administrative distance?
- A router might learn multiple routes to the same network through different routing protocols
- Administrative distance determines which routes from which sources are prioritized and added to the routing table
- The lower the administrative distance, the higher priority so the lowest administrative distance route gets added to the routing table
61
How can administrative distance for OSPF be modified? What are the configuration commands?
Two ways
- Change administrative distance for all OSPF learned routes
conf t
router ospf 1
distance <1-255>
- Change administrative distance for routes learned from a specific RID
conf t
router ospf 1
distance <1-255> A.B.C.D M.M.M.M
* ACLs can be used to limit this adjustment to specific routes
62
How can OSPF interface cost be changed? What are the configuration commands?
Two ways
- Change the reference bandwidth used to calculate interface costs
conf t
router ospf 1
auto-cost reference bandwidth X
- Hard set an interface cost for a specific interface
conf t
interface gi0/0
ip ospf cost X
63
On what types of networks do OSPF routers do DR/BDR elections?
- Broadcast networks. Ethernet networks such as those connected to a switch
- Non-Broadcast Multiple Access networks such as Frame Relay networks
64
On what types of networks do OSPF router NOT do DR/BDR elections?
- Point-to-Point networks such as serial connections
- Point-to-Multipoint networks
* In a P2MP network, full adjacencies are formed between every router
65
How does a router determine the network type and whether to attempt the DR/BDR election process?
- Encapsulation at the interface indicates the network type and whether DR/BDR needs to be elected
- Ethernet encapsulation indicates a broadcast network so a DR and BDR will be elected
- Serial connections used PPP or HDLC encapsulation, indicating a Point-to-Point connection where no DR/BDR will be elected.
66
How can you manually change the OSPF network type? What commands are used?
- Changed at the interface connected to the network for which you want to change the network type
conf t
interface gi0/0
ip ospf network
67
What is the purpose of OSPF authentication?
Prevents the malicious injection of incorrect routing information
68
What are the levels of OSPF authentication?
- 0: No authentication
- 1: credentials stored and transmitted in plain-text
- 2: credentials stored and transmitted with an MD5 hash
69
How is OSPF authentication configured?
conf t
router ospf 1
area X authentication
* enables OSPF authentication for all interfaces in the designated area. Keys must be configured on the interface
* message-digest option enables level 2 MD5 encrypted authentication.
conf t
interface gi0/0
ip ospf authentication (if not already enabled from router configuration mode)
ip ospf message-digest-key md5
70
What is the purpose of a default route?
- The last resort when a destination network is not found in the routing table of a router
71
What commands configure a default route?
- From global configuration mode
conf t
ip route 0.0.0.0 0.0.0.0
72
True or False: in OSPF it is necessary to configure a default route on every router?
False: A default route can be configured on the router connected to an external network and propagated from that router using OSPF.
73
What commands redistribute a default route configured on an OSPF router? What happens when these configurations are made?
conf t
router ospf 1
default-info orginate
- redistributes the default route to all routers apart of the autonomous system
- directs all traffic with an unknown destination to the router that originated the default route
- always option directs any default traffic to the originating router, even if a default route has not yet been configured
- default metric and metric type can be configured
74
Where is route summarization performed? What is the purpose of summarization and what challenges are there when summarizing routes?
- Summarization can be performed on ABRs or ASBRs
- Summarization reduces the size and number of summary LSAs (or external LSAs) advertised into other areas
- For example, if an area has multiple networks using the 10.0.X.X format, these may be summarized by a single LSA advertising 10.0.0.0/16 (or a different mask depending on how you want to summarize)
- Challenging in that it requires an adequate understanding of subnetting to create a summary address that incorporates all networks efficiently or doesn't cover too much
75
What commands are used to create a route summarization in OSPF?
conf t
router ospf 1
area X range A.B.C.D M.M.M.M
*use the actual mask for the range you're trying to cover; not a wildcard mask
76
What is route filtering?
- prevents a route learned in one area from being advertised in another, or in the case of ASBR, external routes
77
What methods are there for filtering routes?
- Simple method. Effective but can only filter routes for areas directly connected to an ABR
- Prefix-List/Filter-List
- Local route filtering
78
What is the simplest way of filtering routes?
- At the ABR. Can only be used for routes learned from areas connected to the ABR
conf t
router ospf 1
area X range A.B.C.D M.M.M.M not-advertise
79
How are prefix-lists used to filter routes?
- Referenced by filter-lists in OSPF router configuration mode
- similar to the syntax of an ACL, whether a route is permitted or denied is configured at the prefix-list
- implicit "deny all" at the end of every prefix-list. Important to be aware of this to not filter too much
- uses slash notation
80
How is a prefix-list configured for OSPF route filtering?
conf t
ip prefix-list deny A.B.C.D/nn
ip prefix-list permit 0.0.0.0/0 le 32
* first line denies routes that fall within the range of the defined network. This can be repeated for multiple network ranges
* last line allows all other routes you don't want to be filtered, if not filtered by previous lines.
* if you want multiple routes to be referenced by the same prefix list, use the same name
81
How are filter-lists used to filter routes?
- Reference prefix-lists which define the network ranges for routes you want to filter
- Tells OSPF to follow guidelines for denying or permitting routes that fall within the ranges configured in the prefix-list
- Tells OSPF to filter routes being advertised OUT of an area or IN to an area
82
How is a filter-list configured?
conf t
router ospf 1
area X filter-list prefix
83
How does local route filtering work?
- Only keeps route out of particular router's routing table.
| - Router still has LSAs in LSDB but kept out of routing table
