Lecture 2 - Dijkstra, OSPF, Switches (Spanning Tree, VLAN) Flashcards
Which algorithm does OSPF use to compute the shortest route?
Dijstra’s Algorithm
3 Ways employed by OSPF to reduce signalling overheads.
- Areas
- HELLO/LSA messages are sent to a multicast address
- Flooding on all ports except for the incoming port
What are the disadvantages of flooding?
Increased bandwidth consumption, loops and congestion
What does BFD stand for?
Bidirectional Forwarding Detection
What problems does BFD overcome?
HELLO message sent out by ospf is too infrequenct to detect link failures. BFD addresses this problem by ensuring each network interface card carries out link failure detection.
Why is link route convergence important?
To ensure all routers have the same routing table or topological information as soon as possible
What does ECMP stand for?
Equal Cost Multipath Routing
What is the benefit of ECMP?
Load balancing.
Allow an OSPF router to forward packets onto multiple paths with equal cost to a destination
How does an ethernet switch learn the existance of a machine on a port?
Whenever the machine starts transmitting
Does each OSPF area run a separate copy of the Dijkstra algorithm?
Yes
In OSPF, how does having a designated + backup router help reduce signalling overheads?
Each non-DR or non-BDR router will exchange routing information only with the DR and BDR, instead of exchanging updates with every router on the network segment
What is a disadvantage of Flooding a network?
waste of bandwidth
creates congestion
possible looping of packets
How are hops measured?
number of intermediate devices (routers) through which data must pass between source and destination
Difference between Distance Vector and Link-state
Distance vector, each router knows little about network topology, router can’t see beyond neighbours. Link-state requires routers know about the paths reachable by all other routers in the network. Routing information is flooded within the routing domain.
What does OSPF stand for?
Open Shortest Path First
What is LSA?
Link State Announcement
Floods whenever there is a change or every 30s
When is a network said to be converged?
When none of the forwarding tables are changing
Why are areas used in OSPF?
They are used to reduce the routing traffic, processing overhead and routing table calculation
What does the Spanning Tree Protocol ensure?
a loop-free topology
Why are VLANs (Virtual LANs) used?
used to isolate traffic, cheaper then buying two switches
How do routers build a network topology? How do routers learn the network topology?
Routers flood LSA’s containing adjacency (list of neighbours) information
