Introducing OSPF Flashcards
- Discovering remote networks
- Maintaining up-to-date routing information
- Choosing the best path to destination networks
- Finding a new best path if the current path is no longer available
A dynamic routing protocol has these purposes
Otherwise known as a routing domain, is a collection of routers under a common administration, such as an internal company network or an Internet service provider (ISP) network.
autonomous system (AS)
An IGP routing protocol is used to exchange routing information within an AS. EIGRP, Intermediate System-to-Intermediate System (IS-IS), OSPF, and the legacy routing protocol, Routing Information Protocol (RIP) are examples of IGPs for IP version 4 (IPv4).
IGP
An Exterior Gateway Protocol (EGP) routing protocol is used to route between autonomous systems. Border Gateway Protocol (BGP) is the EGP used today for IPv4.
EGP Exterior Gateway Protocol
The distance vector routing approach determines the direction (vector) and distance (such as router hops) to any link in the internetwork. Some distance vector protocols periodically send complete routing tables to all connected neighbors.
Distance vector
The link-state approach, which uses the shortest path first (SPF) algorithm, creates an abstract of the exact topology of the entire internetwork, or at least of the partition in which the router is situated. A link-state routing protocol is like having a complete map of the network topology.
Link state
RIP version 2 (RIPv2), EIGRP, OSPF, IS-IS and BGP are classless routing protocols and can be considered second-generation protocols because they are designed to address the limitations of classful routing protocols. A classless routing protocol is a protocol that advertises subnet mask information in the routing updates for the networks advertised to neighbors.
Classless routing protocol
Classful routing protocols such as RIP version 1 (RIPv1) and Interior Gateway Routing Protocol (IGRP) are legacy protocols and not used today. They do not advertise the subnet mask information within the routing updates.
Classful routing protocol
Connected interface (and static routes via interface)
0
Static route (via next hop address)
1
External Border Gateway Protocol (EBGP)
20
EIGRP
90
OSPF
110
IS-IS
115
RIP
120
External EIGRP
170
Internal Border Gateway Protocol (IBGP)
200
Unreachable
255 (will not be used to pass traffic)
The router ID is a 32-bit number that uniquely identifies the router; it must be unique on each router in the network. The router ID is, by default, the highest IPv4 address on a loopback interface, if there is one configured. If a loopback interface with an IPv4 address is not configured, the router ID is the highest IPv4 address on any active interface. You can also manually configure the router ID by using the router-id command.
Router ID
The hello interval specifies the frequency in seconds at which a router sends hello packets to its OSPF neighbors. The default hello interval on broadcast and point-to-point links is 10 seconds. The dead interval is the time in seconds that a router waits to hear from a neighbor before declaring the neighboring router out of service. By default, the dead interval is four times the hello interval.
Hello and dead intervals
The Neighbors field lists the adjacent routers form which the router has received a hello packet. Bidirectional communication occurs when the router recognizes itself in the Neighbors field of the hello packet from the neighbor.
Neighbors
To communicate, two routers must share a common segment and their interfaces must belong to the same OSPF area on this segment. The neighbors must also be on the same subnet (with the same subnet mask). These routers in the same area will all have the same LSDB information for that area.
Area ID
The router priority is an 8-bit number. OSPF uses the priority to select a designated router (DR) and backup designated router (BDR). In certain types of networks, OSPF elects DRs and BDRs. The DR acts as a central exchange point to reduce traffic between routers.
Router priority
These addresses are the IPv4 addresses of the DR and BDR for the specific network, if they are known.
DR and BDR IPv4 addresses
If router authentication is enabled, two routers must exchange the same authentication data. Authentication is not required, but if it is enabled, all peer routers must have the same key configured.
Authentication data
A stub area is a special area. Designating a stub area is a technique that reduces routing updates by replacing them with a default route. Two routers have to agree on the stub area flag in the hello packets to become neighbors.
Stub area flag
The hello packet discovers and maintains neighbors.
Type 1 / Hello
The DBD packets describe the summary of the LSDB and contain the LSA headers that help routers build the LSDB.
Type 2 / DBD
After DBD packets are exchanged, each router checks the LSA headers against its own database. If it does not have current information for any LSA, it generates an LSR packet and sends it to its neighbor to request updated LSAs.
Type 3 / LSR
The LSU packets contain the requested LSAs that should be updated. This packet is often used in flooding.
Type 4 / LSU
LSAck packets help to ensure a reliable transmission of OSPF packets. Each DBD, LSR and LSU is explicitly acknowledged.
Type 5 / LSAck
