OSPF Flashcards
What are the 7 states OSPF neighbors go through?
- Down
- Attempt / Init
- 2-way
- EXSTART
- Exchange
- Loading
- Full
In OSPF, what are the 5 types of LSAs?
- Type 1 - Router LSA
- Type 2 - Network LSA
- Type 3 - Summary LSA
- Type 4 - Summary ASBR LSA
- Type 5 - External LSA
Which 5 items in the OSPF Hello protocol must match in order for an adjacency to form?
- Hello and Dead Timers
- Area ID
- OSPF Area Type
- Subnet
- Authentication (if used)
What are the 4 Router Roles?
- Backbone Routers
- ASBR - Autonomous System Boundary Router
- ABR - Area Border Router (also considered a Backbone Router)
- Internal Router
What are the 3 Packet Types that are used during Database Synchronization?
- DD - Database Description packets
- LSR - Link-state Request packets
- LSU - Link-state Update packets
What is a Type 1 LSA?
- Name: Router LSA
- Flooded to: Other routers in SAME AREA
- Contains: Status of all the router’s Links in that area
What is a Type 2 LSA?
- Name: Network LSA
- Flooded by: DRs
- Flooded to: SAME AREA
- Contains: the state of other routers on the SAME NETWORK
What is a Type 3 LSA?
- Name: Summary LSA
- Flooded by: ABRs
- Flooded to: an area that is EXTERNAL to the one where they were generated.
- Contains: Networks and Inter-area Routes, IP address and RID of the sending ABR
What is a Type 4 LSA?
- Name: Summary ASBR LSA
- Flooded by: ABR
- Flooded to: All areas EXTERNAL to where the LSA was generated.
- Contains: Info letting all other OSPF Areas know how to get to the ASBR
What is a Type 5 LSA?
- Name: External LSA
- Flooded by: ASBR
- Flooded to: everywhere
- Contains: routes that are EXTERNAL to the OSPF Autonomous System
What are 3 features of an OSPF Hello packet itself?
- follows IP packet header with protocol 89
- sent via multicast 224.0.0.5 (all OSPF routers)
- OSPF routers listen for them to find out about new neighbors
What 3 conditions have been met when a pair of neighbors reach 2-Way State?
- They’ve exchanged Hellos
- The parameters listed in the Hellos match.
- They appear in each others neighbor table.
What is an LSA?
- An LSA is a data structure, NOT a packet
- Link State Advertisement
- LSAs are sent to other routers in Link State Update packets (LSUs)
What is an Inter-area route?
A route to a subnet that is NOT in the local routers AREA.
What is an Intra-area route?
A route to a subnet that IS in the local routers AREA.
What is happening in the Down State?
- Router initializing OSPF and chooses RID
- Runs OSPF on interfaces configured for OSPF
- Collects info to put in a Hello packet
What is happening in the Attempt / Init state?
- router has sent out Hello’s and is waiting for a response
- if a neighbor goes down the router will try to re-establish neighborship by sending another Hello. This is Attempt.
What is happening in the 2-Way State?
- Both routers have exchanged HELLOs and matched config parameters
- Both have exchanged Neighbor Tables and see each other
- They’ve added each other to their Neighbor Tables
What is happening in the EXStart State?
- Master / Slave relationship created. Highest RID is Master. Master has to initiate exchange process.
- Agree on initial sequence numbers
What is happening in EXCHANGE State?
- Routers send each other a list of LSAs in DD packets
- The router checks the list of LSAs looking for LSAs that it doesn’t have.
- LSRs are exchanged asking for networks they don’t already have
What is happening during the Loading State?
- Actual routing information is exchanged.
- LSAs are exchanged in LSU packets
What happens in the Full State?
- Both routers are done exchanging LSAs
- LSDBs on both routers are identical
How is a Default Route configured in OSPF?
- The route has to be in the routing table.
- Configuration:
- router ospf 10
- default-information originate
- router ospf 10
How can OSPF advertise a Default Route regardless of whether the route is in the routing table?
- router ospf 10
- default-information originate always
What is the OSPF formula for calculate an interface cost?
100 MB / bandwidth
So, a serial interface running at 1MB would have an interface cost of 100. (100/1=100)
