OSPF Lab Concepts Flashcards
6.12 - What happens when you create a VL on R6 with connections in Area 1 amd Area 2 to R1 which has connections to Area 1 and Area 0?
R6 becomes an ABR since the OSPF_VL interface created on R6 is an Area 0 interface.
6.12 - What happens when you change the RID on R1 if its associated with a VL config on R6?
It breaks the VL and the VL interface goes down and R6 looses OSPF adjacency over the VL. The only way to fix it is to reconfigure the RID on R6 to bring it back up.
6.12 - What are two ways that the RID on R1 could change (affecting the VL to R6)?
- by creating a loopback with a higher IP
2. by changing the router-id under OSPF process.
6.13 - What does the Transit Capability allow an ABR to do (an ABR has an interface in Area 0)?
When enabled (it is by default) it allows the ABR to find and use a shorter path through a non-backbone.
6.13 - If there is no VL between R6,R1, and with the Transit Capability enabled, will R1 ignore the LSA 3 from R4 that advertises the shorter path through Area 1 to R5?
Yes, it will ignore LSA 3s from R4 in Area 1, and vice versa because they are both ABRs (split-horizon loop prevention). This causes traffic to take a higher cost route to R5 through R3.
6.13 - Why does creating a VL between R6,R1 in Area 1 allow R6 to use the shorter path to R5 that is advertised by R4 in Area 1?
The VL gives R6 an interface in Area 0, so it becomes and ABR. It has interfaces in Areas 1 and 2 as well as transit capability enabled. Since it’s an ABR and has transit capability enabled it can consider shorter paths through non-backbone areas.
6.14 - Why doesn’t traffic from Area 2 to Area 51 flow across the P2P between R3 and R2 when the cost of that link is configured for 1?
Because the link is in Area 5, and by default all Inter-Area routing in OSPF has to traverse Area 0. LSA 5 says “Adv Router is not-reachable”
6.14 - Why does creating an area 5 VL across the P2P serial between R3 and R2 force traffic to flow from Area 2 to Area 51 across the P2P?
the VL will cause R3 to advertise a metric 1 Area 0 route across the P2P to R2 for Area 51 routes. R2 and R3 serial P2P are now essentially in Area 0.
6.15 - What does the command “ip ospf demand-circuit” do when configured on an interface?
Prevents “paranoid flooding” from occurring out that interface because the DoNotAge LSA is allowed (it ignores the MaxAge timer for that interface).
6.15 - When demand circuit is configured on a serial interface, what would you expect to see in the output of a “show ip ospf neighbor”?
The neighbor will be listed with a “-“ null value for the Dead Timer value for the adjacent neighbor on that interface.
6.16 - How would you turn on the flood-reduction feature to stop “paranoid updates”?
By configuring “ip ospf flood-reduction” under all interfaces for a particular area. All devices in that area must be configured that way. It prevents the MaxAge timer from expiring for devices in that area. All devices must agree on OSPF timer settings.
6.16 - When “ip ospf flood-reduction” is configured, what would you expect to see in the output of “show ip ospf database”?
The Age column will have (DNA) listed for LSAs in that area.
6.17 - How would you configure clear text authentication on all interfaces on a device in Area 2 using the password CLEARKEY with a single command?
Under router ospf 1:
area 2 authentication
Under all interfaces in Area 2:
ip ospf authentication-key CLEARKEY
6.17 - How would you configure clear text authentication on all interfaces on a device in Area 2 using the password CLEARKEY without modifying the router ospf 1 process?
Under all interfaces in Area 2:
area 2 authentication
ip ospf authentication-key CLEARKEY
6.18 - How would you configure OSPF MD5 Authentication on all interfaces in Area 0 using password MD5KEY, with a single command?
Under router ospf 1:
area 0 authentication message-digest
Under all interfaces in Area 0:
ip ospf message-digest-key 1 md5 MD5KEY
