OSPF Flashcards
What layer 4 protocol does OSPF use?
IP protocol 89
What address does OSPF send to?
224.0.0.5
On a segment with a DR, routers send multicast updates to 224.0.0.6.
What unit belongs to an area?
A link (interface)
What is area zero?
Area zero is the backbone. Updates to other areas must flow through area zero to be transmitted.
What is a type 1 LSA?
A type 1 router LSA is sent by all routers to describe their links. It is flooded to all routers in an area.
What happens if a router receives a new LSA?
It must rebuild its database and re-run the SPF algorithm.
What is an ABR?
Area border routers (ABR) span multiple areas, one of which must be the backbone area zero.
What is the LSDB?
The link state database (LSDB) is the name of the OSPF topology database.
How are the LSDBs of routers in the same area related?
They must all match within an area.
How do routers learn of routes in other areas?
The ABR floods a type 3 summary LSA into the area.
How is the OSPF process id significant?
It is only locally significant and does not have to match between neighbors.
What happens if a route matches multiple network statements with different areas?
The most specific network statement will win.
What command will show the areas that interfaces belong to?
sh ip ospf interfaces
How is the router-id determined?
The highest up/up loopback interface IP address is used, or the highest up/up physical address if no loopback is available.
What will make a router-id change go into effect?
If the router has no neighbors, it becomes immediately active.
Otherwise, the ospf process or the router must be reloaded.
What does “2WAY” neighborship indicate?
The routers are adjacent but have not exchanged topology information. This will occur on networks with a DR.
What side effect does adding an OSPF command to an interface have?
It will create the router configuration for that OSPF process if none exists.
What five things must match for a full adjacency to form?
Hello and dead interval Area ID Subnet mask and MTU of the shared link Stub area flag Authentication
On a multiaccess link, what additional information do Hellos include?
The hello packet includes the router-ids of all neighboring routers and the IP addresses of the DR and BDR.
Give the command to show the OSPF timers on an interface.
sh ip ospf interface s0/0
Give the commands to set the hello and dead intervals on an interface.
interface Serial0/0
ip ospf hello-interval #
ip ospf dead-interval #
How can sub-second hello timers be configured?
interface Serial0/0
ip ospf dead-interval minimal hello-multiplier $x
Where $x is the number of Hellos per second.
What is a better option for sub-second convergence than sub-second hello timers?
Bidirectional forwarding detection (BFD)
Can two routers with the same router-id become neighbors?
No, and they will not accept each others’ LSAs.
What behavior will result from a mismatch in MTU sizes?
The routers will get as far as EXSTART and then go down before they exchange LSAs. A log message will state the neighborship went from “EXSTART to DOWN: too many retransmissions.”
Where is the MTU for a link advertised?
In the database description packets sent after the Hellos during adjacency formation.
Give the OSPF states completed to form full adjacency.
Down Attempt (static neighbor only) Init 2-way Exstart Exchange Loading Full
Do I need 2 entertain every leaving faculty?
List the OSPF authentication types.
0 - no authentication
1 - plain text
2 - MD5
What error message is displayed for mismatched authentication?
No error is displayed, only debug messages.
Which OSPF packets include authentication information?
The authentication information is in the header of every packet.
What determines the OSPF network type?
layer 2 encapsulation
What is the default network type for Ethernet?
broadcast
What is the default network type for a physical interface configured with frame-relay encapsulation?
non-broadcast (multiaccess)
What network types are available for frame relay sub-interfaces?
“point-to-point” or “multipoint” (point-to-multipoint NBMA)
What is the metric cost for a T1 link?
64
What is the metric cost for an ethernet link?
10
What is the metric cost for a fast ethernet link?
1
How is metric cost calculated?
Reference-bandwidth divided by interface bandwidth
10^8 (100,000,000) / bandwidth in bits
Name the five OSPF packet types.
- Hello
- Database description
- Link state request
- Link state update
- Link state acknowledgement
What is contained in a hello packet?
router-id, router priority, area, neighbors, DR and BDR, hello and dead intervals, authentication, and stub area info
How is the DR determined?
By the highest router priority, then highest router-id
What is the default router priority?
1
Give the command to set the router priority.
interface Ethernet0/0
ip ospf priority #
Give the command to view all type 2 LSAs.
sh ip ospf database network
What is the first LSA sequence number?
80 million (80,000,000) in hex
Give the command to limit the number of LSAs.
router ospf 1
max-lsa #
What happens if the maximum number of LSAs is reached?
The router will send a notification message when the maximum is reached, then a warning when it’s exceeded. After one minute of exceeded LSAs, it will tear down all adjacencies and go silent on all OSPF interfaces for that process for the duration of the ignore-time configured.