What is the Router LSA?
LSA Type 1 that each router creates to represent itself for each area it connects to. The LSDB for an area has one Type 1 LSA per router per area that lists the RID and all interface IP addresses on that router that are in that area. Represents stub networks too.
What is the Network LSA?
LSA Type 2 that is one per transit network. It is created by the DR on the subnet and represents the subnet and router interfaces connected to the subnet.
What is the Network Summary LSA?
LSA Type 3 created by ABRs to represent subnets listed in one area’s type 1 and type 2 LSAs when being advertised into another area. Defines the links in the origin area, and cost, but no topology data.
What is the ASBR Summary LSA?
LSA Type 4 that advertises a host route used to reach an ASBR, like a type 3 LSA.
What is the NSSA External LSA?
LSA Type 7 that is created by ASBRs inside an NSSA area, instead of a type 5 LSA.
What is listed in Type 1 LSAs?
- Router’s interface subnet number/mask and interface OSPF cost (in which no DR has been elected).
- IP address of the DR and a notation that the link attaches to a transit network (for each interface which a DR has been elected).
- Lists the neighbor’s RID, for each interface with no DR, but for which a neighbor is reachable.
What are the two main reasons OSPF uses a DR in a particular subnet?
- To create and flood a Type 2 network LSA for that subnet.
- To aid in the detailed process of database exchange over that subnet.
What rules do routers follow when no DR exists at the time?
- Choose the router with the highest priority.
- If tied on priority, choose the router with the highest RID.
- Choose a BDR, based on next-best priority, or if a tie, next-best (highest) RID.
How is a Type 2 LSA created?
DR in a subnet creates the Type 2 LSA for that subnet. Then identifies LSA by assigning an LSID for the DR’s interface IP address for that subnet. The type 2 LSA also lists the DR’s RID as the router advertising the LSA.
What is the concept of LSA Type 3?
ABRs don’t forward Type 1 and Type 2 LSAs from one area into another area, and vice versa. ABRs generate a Type 3 LSA for each subnet in one area, and advertises each Type 3 LSA into the other areas. Type 3 LSA appears to be another subnet connected to the ABR that created and advertised the Type 3 LSA.
What does an ABR do with a Type 3 LSA?
The ABR creates and floods each Type 3 LSA into the next area. The ABR assigns an LSID of the subnet number being advertised. It also adds its own RID to the LSA so that routers know which ABR advertised the route. It also includes the subnet mask. The correlation between the advertising router’s RID and the LSID (subnet number) allows the OSPF processes to create the part of the topology.
How do you limit the number of LSAs?
Trick question. You can’t. You can set a maximum number of LSAs learned from other routers with the max-lsa number OSPF subcommand.
When router hits the maximum LSA in max-lsa number it will issue log messages for a time period, repeatedly. In the end, the router will close all neighborships, discard its LSDB, and then starts adding neighbors again.
What is the summary of internal LSA types?
show ip ospf database router – Type 1 Router LSA representing a router. LSID is equal to the RID of the router. Each router creates its own.
show ip ospf database network – Type 2 Network LSA representing a subnet in which a DR exists. LSID is equal to the DR’s IP address in the subnet. Created by the DR in that subnet.
show ip ospf database summary – Type 3 Summary LSA that represents a subnet in another area. LSID is equal to the subnet number and is created by an ABR.
What is the Hello OSPF message type?
Used to discover neighbors, supply info used to confirm two routers should be allowed to become neighbors, to bring a neighbor relationship to a 2-way state, and to monitor a neighbor’s responsiveness in case it fails.
What is a Database Description (DD or DBD) OSPF message type?
Used to exchange brief versions of each LSA, typically on initial topology exchange, so that a router knows a list of that neighbor’s known LSAs.
What is a Link-State Request (LSR) OSPF message type?
A packet that lists the LSIDs of LSAs the sender of the LSR would like the receiver of the LSR to supply during database exchange.
What is a Link-State Update (LSU) OSPF message type?
A packet that contains fully detailed LSAs, typically sent in response to an LSR message.
What is a Link-State Acknowledgement (LSAck) OSPF message type?
Sent to confirm receipt of an LSU message.
What are the OSPF Neighbor States?
Down – no Hellos have been received from neighbor for more than the dead interval.
Attempt – Used when the neighbor is defined with the neighbor command, after sending a Hello, but before receiving a Hello from that neighbor.
Init – A Hello has been received from the neighbor, but it did not have the local router’s RID in it or lists parameters that do not pass the neighbor verification checks. This is a permanent state when Hello parameters do not match.
2Way – A Hello has been received from the neighbor, it has the router’s RID in it, and all neighbor verification checks passed.
ExStart – Currently negotiating the DD sequence numbers and master/slave logic used for DD packets.
Exchange – Finished negotiating the DD process particulars, and currently exchanging DD packets.
Loading – All DD packets are exchanged, and the routers are currently sending LSR, LSU, and LSAck packets to exchange full LSAs.
Full – Neighbors are fully adjacent, meaning they believe that their LSDBs for that area are identical. Routing table (re)calculations can begin.
When do routers proceed from Init to 2Way?
When a router receives a Hello that lists its own RID as having been seen by the other router.
What is the general process when no DR exists?
- Discover the LSAs known to the neighbor but unknown to me.
- Discover the LSAs known by both routers, but the neighbor’s LSA is more up to date.
- Ask the neighbor for a copy of all the LSAs identified in the first two steps.
How do routers learn the list of LSAs known by the neighbor?
- Multicast database description packets to 126.96.36.199.
- When sending the first DD message, transition to the ExStart state until one router, the one with the higher RID, becomes master in a master/slave relationship.
- After electing a master, transition the neighbor to the Exchange state.
- Continue multicasting DD messages to each other until both routers have the same shared view of the LSIDs known collectively by both routers, in that area.
What are the mechanics of exchanging LSAs?
- Transition neighbor state to Loading.
- For any missing LSAs, send an LSU, listing the LSID of the requested LSA.
- Respond to any LSR messages with an LSU, listing one or more LSAs in each message.
- Acknowledge receipt by sending an LSAck message or by sending the same LSA that was received back to the other router in an LSU message.
- When all LSAs have been sent, received, and acknowledged, transition the neighborship to the FULL state.
What is the role of the DR?
- Receive messages on 188.8.131.52 All DR routers multicast address.
- Perform database exchange with the same messages but to 184.108.40.206 all SPF routers multicast address.
- Neighbors that do not reach FULL neighbor state remain in 2Way state as Drothers.
Why does OSPF flood the LSDB?
To prevent looping of LSAs as a side-effect of the database exchange process. By requesting only unknown LSAs or new versions of the old LSAs, routers prevent the LSAs advertisements from looping.
How often does OSPF reflood each LSA?
30 minutes based on each LSA’s age variable.
How does a router pick the best route?
- Analyzes the LSDB to find all possible routes to reach the subnet.
- For each possible route, add the OSPF interface cost for all outgoing interfaces in that route.
- Pick the route with the lowest total cost.
What is the process to calculate the cost of Intra-Area routes?
- Finds all subnets inside the area, based on the stub interfaces listed in the Type 1 LSAs and based on any Type 2 network LSAs.
- Runs SPF to find all possible paths through the areas topology, from itself to each subnet.
- Calculates the OSPF interface costs for all outgoing interfaces in each route, picking the lowest total cost route for each subnet as the best route.
What is the process to calculate the cost of an interarea route?
- Calculate the intra-area cost from that router to the ABR listed in the type 3 LSA.
- Add the cost value listed in the Type 3 LSA. (represents the cost from the ABR to the destination subnet).
What are the special rules concerning intra-area and interarea routes on ABRs?
- When choosing the best route, an intra-area route is always better than a competing interarea route, regardless of metric.
- If an ABR learns a Type 3 LSA inside a non backbone area, the ABR ignores that LSA when calculating its own routes.
How do you configure cost directly?
Use the ip ospf cost value interface subcommand.