Volume 1 - Chapter 21: Understanding OSPF Concepts Flashcards
What are the 3 ways that routers can learn/add routes to their routing table?
Static routes, connected routes, dynamic routes
Describe the difference between a routing protocol and a routed/routable protocol?
A routing protocol is an algorithm/program used for devices to learn and exchange routes.
A routed/routable protocol refers to a protocol that defines packets that can be forwarded (routed) - E.I IPv4 and IPv6
When a router learns of 2 routes to the same destination, the route that is chosen is the route with the lowest ___.
Metric
Describe an Interior Gateway Protocol (IGP).
A routing protocol designed for use within a single autonomous system (AS).
Describe an Exterior Gateway Protocol (EGP).
A routing protocol designed for use between different autonomous systems (AS).
What routing protocol is an example of an EGP?
BGP (Border Gateway Protocol) is the only currently used exterior gateway protocol.
A network under the administrative control of a single organization is referred to as ___.
An Autonomous System (AS).
What organization is responsible for assigning ASNs? (Autonomous System Numbers)
The Internet Assigned Numbers Authority (IANA).
What are the 3 branches of routing protocol algorithms that exist for IGP routing protocols?
Distance Vector
Advanced Distance Vector
Link-State
A distance vector protocol is sometimes also referred to as ___?
Bellman-Ford algorithm
An advanced distance vector protocol is sometimes also referred to as ___?
Balanced Hybrid algorithm
What are the main problems caused by using distance vector IGPs?
Slow convergence and the potential for creating routing loops.
The following are examples of?
- Routing Information Protocol (RIP)
- IGRP (Interior Gateway Routing Protocol)
Distance Vector protocols.
The following are examples of?
- OSPF (Open Shortest Path First)
- IS-IS (Intermediate System to Intermediate System)
Link-State protocols.
What is the main downside of using link-state protocols over distance vector?
Additional CPU and memory resources being required.
EIGRP (Enhanced Interior Gateway Routing Protocol) is an example of?
An Advanced Distance Vector IGP/
What metric is used by RIPv2?
Hop count:
The number of routers (hops) between a router and the destination subnet/
What metric is used by OSPF?
Cost:
The sum of all interface cost settings for all links in a route.
What metric is used by EIGRP?
Bandwidth and delay calculation:
Calculated based on the slowest link and cumulative delay with each interface in the route.
Which dynamic routing protocols are Cisco proprietary?
IGRP and EIGRP.
Which older protocols do not support the use of VLSM and route summarization?
RIPv1 and IGRP.
What IGPs use broadcast messages to send and receive routing updates?
RIPv1 and IGRP.
What IGPs use multicast messages to send and receive routing updates?
RIPv2, EIGRP, and OSPF.
OSPF exchanges data about the network in data structures called ___.
Link-State Advertisements (LSAs).
The collection of LSA objects is referred to as?
The Link-State Database (LSDB).
Routes do not reflood LSAs until what occurs?
Information changes or a network change occurs.
Or when the LSA aging timer expires.
What is the default time for the LSA aging timer?
30 minutes.
What algorithm do all link-state protocols use to process information in the LSDB?
The Dijkstra Shortest Path First (SPF) algorithm.
Describe the requirements for 2 routers to attempt to become OSPF neighbors.
They must both use OSPF and connect to the same data link.
Describe the purpose of the OSPF Hello message.
Used for routers to exchange information about themselves to see if they are compatible to become neighbors.
What is the information within the OSPF Hello message used to identify the router?
The Router ID (RID).
What is the size and format of an OSPF RID?
32 bits, in dotted decimal (IP) format.
A RID can be manually configured, but if it isn’t, the RID that is chosen by default is ___?
An IP address of one of the router’s interfaces.
What IP protocol type is used in OSPF hello messages?
IP Protocol Type 89.
What multicast address are OSPF hello messages sent to?
224.0.0.5
When routers R1 & R2 first connect, and R2 receives a hello from R1, what state is R2 in?
The interim INIT state
When R2 replies to R1’s hello and R1 receives it, what state does R1 move to?
The 2-way state
When R1 replies back to R2 that it has received the hello, what state does R1 move to?
The 2-way state
Describe what has happened between 2 routers that have reached the 2-way state.
Each router has received the RID of their potential neighbor
The routers have performed the necessary settings checks and are ready to become neighbors
The routers are ready to exchange their LSDBs
What state do the routes move to when they have fully exchanged their LSDBs?
The full state.
What specific LSAs do reach router request from each other?
Only the LSAs that they do not already have
What type of message does a router send when it wants additional LSA from neighbor?
A Link-State Request (LSR).
Routers R1 and R2 have reached a 2-way state and are ready to exchange LSDBs. R1 begins by sending a Database Description (DD) to R2. What state is R1 in?
ExStart.
Routers R1 and R2 have reached a 2-way state and are ready to exchange LSDBs. R2 has received a database description from R1 and sends back its own Database Description (DD). What state is R2 in?
Exchange.
R2 is currently in the Exchange state and sends its Database Description which is received by R1. What state does R1 reach when it has received R2’s Database Description?
Exchange
Describe the what is indicated by a router reaching the Exchange state.
The router has received the Database Description (DD) from its neighboring router.
Routers R1 and R2 have both sent and received each others Database Descriptions (DD) and begin exchanging LSUs. What states are both routers in?
Loading.
What state are routers R1 and R2 in after they have completing sharing their LSUs?
Full.
What is the default Dead Interval?
4 times the hello interval.
Multiple routers connected to the same VLAN will elect a ___ and ___.
Designated Router (DR) and Backup Designated Router (BDR).
Describe the database exchange flow process when multiple OSPF routers are connected on the same VLAN.
The database exchanges do not happen every router. Each router will forward their updates to the DR which then distributes them to the other routers in the topology.
Describe the difference of usage between the following multicast addresses:
224.0.0.5 & 224.0.0.6
224.0.0.5 is used to relay information to all SPF-enabled routers, whereas the 224.0.0.6 address is only used to relay information to the DRs and BDRs.
Routers that are neither a DR nor a BDR are referred to as?
DROthers
What type of OSPF router can reach a full state with every other router in the VLAN?
DRs and BDRs will form a full adjacency with every other router in the VLAN.
What type of OSPF router will only ever reach the 2-way state with routers of the same type?
DROthers
DROthers do not form full adjacencies with each other, they only form full adjacencies with the DR and BDR
Why do DROthers never reach a full state between each other?
They do not each LSDBs
Describe the difference between a neighbor and adjacent/fully adjacent neighbor?
Neighbors will reach a 2-way state because they do not exchange databases.
Fully adjacent neighbors have exchanged LSDBs.
When using a single-area OSPF design, where do all routers reside?
In area 0.
Describe the purpose of area 0 in a multi-are OSPF design.
Area 0 is the backbone of the topology and is used for connecting different areas together.
Describe the function of an ABR.
An Area Border Router (ABR) connects to both the backbone, and another OSPF area.
In muti-area OSPF networks, routers that connect to area 0 (including ABRs) are referred to as?
Backbone routers.
In muti-area OSPF networks, routers that do not connect to the backbone are referred to as?
Internal Routers.
What is the purpose of splitting the OSPF network into multiple areas?
Routers in different areas do not share LSDBs, which improves efficiency.
Describe the difference between an intra-area versus an interarea route.
Intra-area routes are for routes in the same area. Interarea routes are for routes between areas
When using a multi-area design in OSPF, how do routes learn about routes in other areas?
Routers will receive summary LSAs that contain an overview of the routers in other areas. These require less processing by SPF.
Routers to processing the smaller per-area LSDB saves ___ ___,
CPU Cycles.
The smaller per-area LSDB requires ___ ___.
Less Memory.
Describe the difference between the router LSA and the network LSA.
Router LSAs are used for each router in the area. They point to each router specifically.
Network LSAs are created and sent by the DR to inform other routers of the networks connected to the DR.
One ____ LSA is sent by the ABR for each subnet ID that exists in a different area
Summary.
A Type 1 LSA is also referred to as a?
Router LSA.
A Type 2 LSA is also referred to as a?
Network LSA.
A Type 3 LSA is also referred to as a?
Summary LSA.
