What is the concept of OSPF Link State?
OSPF uses link state logic in three major steps:
- Neighbor discovery
- Topology database exchange
- Route computation
What are ABRs?
Boundary routers that advertise briefer information between areas, including info about subnets/masks, but info advertised into one area doesn’t include details about the topology of the other area.
What is a Link State Database?
The data structure held by an OSPF router for the purpose of storing topology data.
What is Shortest Path First (SPF)?
The name of the algorithm OSPF uses to analyze the LSDB. The analysis determines the best (lowest cost) route for each prefix/length.
What is Link State Update (LSU)?
The name of the OSPF packet that holds the detailed topology info, specifically LSAs.
What is a Link State Advertisement (LSA)?
The name of a class of OSPF data structures that hold topology info. LSAs are held in memory in the LSDB and communicated over the network in LSU messages.
What is an area?
A contiguous grouping of routers and router interfaces. Routers in an area strive to learn all topology info about the area, but they do not learn topology info about areas to which they do not connect.
What is an Area Border Router (ABR)?
A router that has interfaces connected to at least two different OSPF areas, including the backbone area. ABRs hold topology data for each area, and calculate routes for each area, and advertise about those routes between areas.
What is a Backbone router?
Any router that has at least one interface connected to the backbone area.
What is an Internal router?
A router that has interfaces connected to only one area, making the router completely internal to that one area.
What is a Designated Router (DR)?
On multiaccess data links like LANs, an OSPF router elected by the routers on that data link to perform special functions. These functions include the generation of LSAs representing the subnet, and playing a key role in the database exchange process.
What is a Backup Designated Router (BDR)?
A router on a multiaccess data link that monitors the DR and becomes prepared to take over for the DR, should the DR fail.
How is OSPF configured?
router ospf process-id
network net-id wildcard-mask area area-id
What are the rules for the OSPF configuration commands?
- Neighboring routers’ router ospf process-id commands do not have to be the same process-id.
- IOS only enables OSPF on interfaces matched by the OSPF network command. When enabled, the router does the following:
- Attempts to discover OSPF neighbors on that interface by sending multicast OSPF Hello messages.
- Includes the connected subnet in future topology database exchanges.
- To match an interface w/ the network command, IOS compares the net-id configured w/ each interface’s IP address, while using the configured wildcard-mask as an ACL wildcard mask.
- Regardless of the order in which the network commands are added to the config, IOS puts these commands into the config file w/ the most specific wildcard mask first. IOS lists the network commands in this sorted order in the configuration.
- The first network command that matches an interface, per the order shown in the output of the show running-config command, determines the OSPF area number associated with the interface.
How does OSPF determine the router ID?
- Use the router ID configured in the router-id x.x.x.x OSPF router subcommand.
- Use the highest IP address of any up/up loopback interface.
- Use the highest IP address of any up/up non-loopback interface.
What are the most commonly used OSPF show commands for verification?
show ip ospf interface brief – Lists the interfaces on which OSPF is enabled; omits passive interfaces.
show ip protocols – Lists the contents of the network configuration commands for each routing process, and a list of enabled but passive interfaces.
show ip ospf neighbors – Lists known neighbors, including neighbor state; doesn’t list neighbors for which some mismatched parameter is preventing a valid OSPF neighbor relationship.
show ip ospf database – Lists all LSAs for all connected areas.
show ip route – Lists the contents of the IP routing table, listing OSPF-learned routes with a code of O on the left side of the output.
What is the OSPF feature summary?
Transport – IP, protocol type 89.
Metric – Based on cumulative cost of all outgoing interfaces in a route.
Hello interval – Interval at which a router sends OSPF Hello messages on an interface.
Dead interval – Timer used to determine when a neighboring router has failed, based on a router not receiving any OSPF messages, including Hellos, in this timer period.
Update destination address – Normally sent to 188.8.131.52 (All SPF routers) and 184.108.40.206 (All DRs).
Full or partial updates – Full updates are used when new neighbors are discovered; otherwise, partial updates are used.
Authentication – Supports MD5 and clear-text authentication.
VLSM/classless – OSPF includes the mask w/ each route, also allowing it to support discontiguous networks and VLSM.
Route tags – Allows OSPF to tag routes as they are redistributed into OSPF.
Next-hop field – Supports the advertisement of routes w/ a different next-hop router than the advertising router.
Manual route summarization – Allows route summarization at ABR routers only.
What requirements must be met for OSPF to send multicast OSPF Hello messages?
- OSPF has been enabled on the interface, either through the network router subcommand or the ip ospf area interface subcommand.
- The interface has not been made passive by the passive-interface router subcommand.
What command impacts whether a router discovers potential neighbors using Hellos?
ip ospf process-id area area-id interface subcommand. This command enables OSPF directly on the interface and assigns the area number.
What settings must match for OSPF neighborship?
- Interfaces’ primary IP address must be in the same subnet.
- Must not be passive on the connected interface.
- Must be in the same area.
- Hello interval/timer and Dead timer must match.
- Router IDs must be unique.
- IP MTU must match.
- Must pass neighbor authentication.
How would you tune OSPF for faster convergence?
Set a lower Hello and Dead timer.
What does FULL refer to?
A fully adjacent neighbor, meaning the OSPF topology has been exchanged w/ that neighbor.
What does 2WAY refer to?
A normal, stable, working state for neighbors with which topology data was not exchanged directly.
What does OSPF authentication do?
Cause routers to authenticate every OSPF message.
What are the configuration steps to OSPF authentication?
- Authentication must be enabled, plus the authentication type must be selected, through one of two means:
- Enabling per interface using the ip ospf authentication [message-digest] interface subcommand
- Enabling on all interfaces in an area by change the area-wide authentication setting using the area area-no authentication [message-digest] subcommand under router ospf
- The authentication keys must be configured per interface.
What are the OSPF authentication types?
0 (none) – ip ospf authentication null
1 (clear text) – ip ospf authentication; ip ospf authentication-key key-value
2 (MD5) – ip ospf authentication message-digest; ip ospf message-digest-key key-number md5 key-value
How do you configure authentication for an area?
area num authentication (type 1)
area num authentication message-digest (type 2)
What command can be used to troubleshoot OSPF authentication?
debug ip ospf adj – explicitly states that a mismatch exists w/ the authentication key.
What are three important facts of OSPF network types?
- Whether the router can expect to discover neighbors using multicast Hello messages.
- Whether only two or more than two OSPF routers can exist in the subnet attached to the interface.
- Whether the router should attempt to elect an OSPF DR on that interface.