OSPF

Question 1

How do you configure a router so that OSPF show commands displays DNS name of routers instead of IP Addresses

Answer 1

1. Configure DNS entries for routers with the “ip host “ command
2. Use DNS name with OSPF with command “ip ospf name-lookup”

Question 2

How do you configure OSPF so that if a router doesnt receive an ACK for its LSA it waits 10 seconds before re-sending that LSA

Answer 2

This is configured under the interface with the command “ip ospf retransmit-interval”

Question 3

How do you configure OSPF so that for LSA1 and 2, only the part that has a topology change is recalculated instead of the whole area going through an SPT recalculation

Answer 3

Use Incremental SPF (iSPF) and configuration is:
router ospf 1
ispf

Question 4

How can you get around the rule in OSPF that requires two subnets to be in the same network?

Answer 4

Use IP unnumbered on the interface, OSPF makes an exception to the rule if interface is configured as unnumbered

Question 5

Name the different methods of filtering within OSPF for LSA1 and 2.

Answer 5

Distribute-list with route-map that calls prefix-list/ACL
- removes route from RIB but not from LSDB so if you want this done everywhere you have to apply to all relevant routers

Prefix suppression (works when going to another area or being translated)

• Prefix-suppression under interface individually
• prefix-suppression globally then disabled under interface you dont want surpressed

Distance to 255 to poison route:
access-list 
!
router ospf
  distance 255

Question 6

How do you configure OSPF so that a loopback interface with a /24 mask which is advertised in OSPF shows /24 instead of /32?

Answer 6

By default loopbacks are classified as network type loopback and assigned a /32. Configure the loopback as “ip ospf network point-to-point” and this will change network type and advertise its true subnet mask

Question 7

If there are multiple ABR’s doing LSA5/7 translation, which one is chosen? What can you do to influence which one the translator is?

Answer 7

The ABR with the highest RID is chosen. This can be changed with the command “area xx nssa type7 translate always”. This changes the translator but doesnt mean translator is in the datapath.

Question 8

When Type3/5/7 LSA’s are generated who is the next hop?

Answer 8

Type 3 - ABR which generates the LSA
Type 5 - Either ABR or ASBR that generated LSA5 depending if certain conditions are met
Type 7 - Router that generated the LSA7

Question 9

With LSA5 is the ABR or the ASBR (where redistribution occurs) chosen as the forwarding address and why?

Answer 9

ASBR is chosen if following conditions are met:

• OSPF is enabled on the ASBR’s next hop interface
• ASBR’s next hop interface is non-passive under OSPF
• ASBR’s next hop interface is not point-to-point
• ASBR’s next hop interface is not point-to-multipoint - - - ASBR’s next hop interface address falls under the network range specified in the router ospf command.
• forwarding address has to be a value so it wont work for redistributed connected or static which have no next hop value

If any of these arent satisfied then ABR advertises it self as the forwarding address (0.0.0.0)

Question 10

If you have just one router doing the Type7 LSA creation and this router has multiple interfaces in OSPF, then which interface see chosen as the forwarding address by other routers for the LSA7?

Answer 10

1. Highest Loopback
2. Loopback over normal interface
3. Highest IP normal interface if no loopback

Question 11

Name the different methods of filtering within OSPF for LSA3 and 4.

Answer 11

1. Filter-list w prefix-list/ACL which removes from LSDB. Configure on  ABR 
ip prefix-list test
!
router ospf
 area x filter-list prefix test in|out

2. area x range no-advertise
3. Final tricky one:
   When generating LSA3 the ABR walks the RIB not the LSDB, so if you have a prefix that you dont want an LSA3 generated for, invalidate it from the RIB by creating a route with a lower AD e.g. a static pointing to null0

Question 12

Name the different methods of filtering within OSPF for LSA5 and 7.

Answer 12

1. Filter-list
2. summary-address no-advertise
3. area x nssa no-redistribution

Question 13

How many ways can you use to filter out all LSA’s?

Answer 13

1. If you are sending unicast updates and neighbor command is used you can filter out LSA per neighbor
   “neighbor x.x.x.x database-filter all out”
2. Filter LSA’s going out of an interface
   int fx/x
   ip ospf database-filter all out