OSPF Flashcards
How do OSPF-enabled routers establish neighbor adjacencies?
Via the exchange of Hello packets
What happens once OSPF-enabled routers have established adjacencies?
Routers exchange LSAs
What process occurs once LSAs have been received?
Routers build their topology table (LSDB)
Once the LSDB has been built, what process does the router perform?
Router runs SPF algorithm against LSDB to create SPF tree. The SPF tree contains the computed best paths to destination networks.
Once the SPF algorithm has been run, what happens next?
Computed routes are offered to the routing table
What is contained in the Datalink header of an OSPFv2 message?
Destination MAC (multicast 01-00-5e-00-00-05 or 06) Source MAC (sending interface)
What is contained in the IP packet header of an an OSPFv2 message?
Destination IP address (224.0.0.5 or .6)
Source IP address
Protocol field, with value 89 indicating OSPF message
What is contained within the OSPF packet header of an OSPFv2 message?
Type code indicating OSPF packet type
Router ID
Area ID
What is the purpose of a Type 1 OSPF packet?
Hello packet, used to establish and maintain adjacencies with other OSPF routers
What is the purpose of a Type 2 OSPF packet?
Database Description Packet (DBD, DD), which is an abbreviated list of the sending router’s LSDB.
What is the purpose of a Type 3 OSPF packet?
Link-state Request (LSR), used by routers that have received a Type 2 DBD (summary) to request additional information from their neighbor about specific LSDB entries)
What is the purpose of a Type 4 OSPF packet?
Link State Update, sent in response to an LSA, and to announce newly-learned information.
What is the purpose of a Type 5 OSPF packet?
Link State Acknowledgement (LSAck), sent in response to a received LSA to confirm receipt.
Which packets are involved in the DR/BDR election process?
Type 1 (Hello) packets
What fields are included in a Type 1 Hello Packet
Type, router ID, Area ID, network mask, hello/dead timer intervals, router priority, DR (if applicable), BDR (if applicable), list of neighbors
What determines whether two OSPF routers form an adjacency?
Hello/Dead interval timers must match, both devices must be on same subnet (subnet mask mis-match is a giveaway)
What is the default value of the Hello interval?
10 seconds on multiaccess and P2P networks, 30 seconds on NBMA networks (Frame Relay)
OSPFv2 Hello packets are sent to which address?
Multicast 224.0.0.5
What is the default value of the Dead Interval?
4x hello interval
List the OSPF operational states from Establishment through convergence
Down, Init, 2-way, ExStart, Exchange, Loading, Full
What OSPF operational states are expected as adjacencies are established?
Down, Init, 2-way
What OSPF operational states are expected routers exchange OSPF databases?
ExStart, Exchange, Loading, Full
Identify the OSPF State, and what will happen next
- No hello packets yet received
- Router sends hello packets
Down, transition to Init
Identify the OSPF State, and what will happen next
- Hello packets received from neighbors
- Sending router’s RID observed
Init, transition to 2-way
Identify the OSPF State, and what will happen next
- On Ethernet links, elect DR and BDR
2-way, transition to ExStart
Identify the OSPF State, and what will happen next
- Negotiate Master/Slave relationship
- Negotiate DBD sequence number
- Initiate DBD packet transmission
ExStart, transition to Exchange
Identify the OSPF State, and what will happen next
- Exchange of DBD packets
Exchange State. If additional router information is required, transition to Loading. Otherwise, transition to Full
Identify the OSPF State, and what will happen next
- LSRs and LSUs used to gain additional route information
- Routes are processed using SPF
Loading. Transition to Full
Identify the OSPF State, and what will happen next
- Routers have reached convergence
Full State (routers have reached convergence)
What determining factor will allow a router too transition from the Init State to the 2-way state?
Receiving a hello packet that includes its own router ID
Two routers on a common Ethernet network link exchange Hello packets and reach the 2-way state. What process must occur next?
DR/BDR election
What influences which devices will become DR/BDR?
Interface priority (higher is better, default 1), RouterID (higher is better)
What is the purpose of the DR?
Minimization of adjacencies and LSA flooding. The DR becomes the collection and dissemination point for LSAs. The BDR also collects LSAs in case in ends of becoming the DR.
What formula is used to calculate the potential number of adjacencies in a multiaccess broadcast network?
n (n-1) / 2
Two OSPF routers have reached the ExStart state. Which router will begin the process of exchanging DBD packets?
The one with the highest router ID
What is contained within DBD packets?
Summary information of the sending routers LSDB
Two routers in the Exchange state are exchanging DBDs. Router 1 notices that 1 entry has a more current Link-state entry. What happens next?
R1 transitions to Loading State, and sends an LSR to R2 asking for complete information regarding that entry. R2 responds with an LSU. R1 sends LSAck to R2 confirming receipt.
How often are LSUs sent to neighbors?
- When a topology change is received
2. Every 30 minutes
How does an OSPF router determine its RID?
- Explicit configuration
- Highest IP address of any configured loopbacks
- Highest IP address of any configured interfaces
What formula is used to calculate OSPF cost
Cost=Reference Bandwidth / Interface bandwidth (in bps)
What is the default reference value bandwith
100,000,000 bps (100Mb)
What is the default OSPF cost of a fast ethernet link?
1
What is the default OSPF cost of an Ethernet (10mb) link
10
What is the default OSPF cost of a 1.544Mbps serial link
64
An OSPF router with all interfaces in the same area
Internal router
All LSDBs on internal routers are…
identical
All routers in area 0 are considered…
Backbone routers
An OSPF router with interfaces attached to multiple areas
Area Border Routers
An OSPF router with at least one interface attached to an external Internetwork
Autonomous System Boundary Router
Another name for a Type 1 LSA
router link entry
Another name for a Type 2 LSA
network link entry
Another name for Type 3 and Type 4 LSAs
summary LSA
What is the purpose of a type 1 LSA?
Advertisement of directly-connected links
What is contained in a type 1 LSA
directly connected interfaces, link types, neighbors, link states
Where are Type 1 LSA flooded?
Only within their area of origin (ABRs advertise networks learned via Type 1 LSAs to other areas as Type 3 LSAs)
How are Type 1 LSA link IDs identified
by the router ID of the originating router
What is the purpose of a Type 2 LSA
To inform other routers about multiaccess networks in the same area
What is contained within a Type 2 LSA
Router ID and IP of the DR, and router IDs of all other routers on the multiaccess segment
Where are Type 2 LSA flooded?
Only within their area of origin
What is the link state ID of a Type 2 LSA?
Interface IP address of the DR that advertises it
What is the purpose of a Type 3 LSA?
Used by ABRs to advertise networks from other areas
What is the link-state ID of a Type 3 LSA?
The advertised network number and mask
A router receives a Type 3 LSA from an ABR. Will the SPF algorithm be required?
No. SPF calculations are only performed on advertisements within the same area
What is the purpose of a Type 4 LSA?
Identification and advertisement of ASBR routers
An ABR receives a Type 1 LSA with the “e bit” set. What happens next?
The ABR notices the “e bit”, which means the sending router has identified itself as an ASBR. The ABR must notify other areas that the ASBR exists. The ABR floods that advertisement to the backbone via a Type 4 LSA. The backbone propagates that Type 4 LSA to other areas, alerting them to existence of that ASBR.
What is the link-state ID of a Type 4 LSA?
The router ID of the ASBR
What is the purpose of a Type 5 LSA?
Describes routes to networks outside the OSPF routing domain
How are Type 5 LSAs propagated?
Type 5 LSAs are originated by the ASBR and flooded to the entire routing domain
A routing table entry lists a routing table code of “O” for a specific OSPF route. What LSA Type ultimately produced this entry in the routing table?
Type 1 or type 2
A routing table entry lists a routing table code of “O IA” for a specific OSPF route. What LSA Type ultimately produced this entry in the routing table?
Type 3
A routing table entry lists a routing table code of “O E1” or “O E2” for a specific OSPF route. What LSA Type ultimately produced this entry in the routing table?
Type 5
Each router uses the SPF algorithm to against the LSDB to build the OSPF tree. The SPF tree is used to determine the bas path(s) to add to the routing table. In what order are these best paths calculated?
- Intra-area routes
- Inter-area routes
- External routes
