5 - Link State Routing Flashcards
What do link state protocols atempt to achieve?
Minimise convergence times.
Eliminate non-transient packet looping
Allows multiple metrics to be used
What do routers broadcast the state of?
The state of its locally attached links and IP subnets
What do routers construct?
in terms of toplogy
A complete topology view of the net, based on link state updates.
Its next-hop routing table
Topology Database
Router IDs
Link IDs
- From Router ID
- To Router ID
Metric(s)
Sequence Number
List of shortest paths to destinations
How are removals announced?
Link State Operation
By metric set to infinity
What process occurs when adjacencies refresh?
Neighbours use “hello” protocol
If a router loses a neighbour then recompute routes via there.
Send announcemens with metric set to infinity
What type of algorithm is Djikstra’s Algorithm?
Shortest Path First graph algorithm
Summary of Djikstra’s Algo
Search outward and add router IDs as you expand the front
OSPF
Open Shortest Path First
OSPF two level hierarchy
2 items
Local area and backbone
OSPF: Link state announcements only where?
Link state announcements only in respective areas
OSPF: What knowledge do nodes in each area have?
Nodes in each area have detailed topology and only know direction (shortest path) to networks in other other areas
Area border routers
Summarise distances to networks in the area and advertise them.
What algorithm do Backbone routers run and where is it limited to?
Run an OSPF routing algorithm limited to the backbone
Boundary routers
Connect to other ASes
IS:IS Routing Protocol
Intermediate System to Intermediate System Routing
Intermediate System
Router
End System
Network host or workstation
Network Service Access Point
logical point between network and transport layers where services are delivered to transport
NSAP address is location
Subnetwork point of attachment
Datalink interface (address used in IS-IS to reach neighbor on broadcast media)
What is Packet data unit (PDU) analogous to?
Analogous to IP Packet
Link state PDU
Routing information packet
Which L2 protocols form Connectionless Network Service?
CLNP
IS-IS
ES-IS
ES-IS
End System to Intermediate System
What does each router contain?
Database of entire … what?
Database of entire topology - links + state (inc cost)
The same information as ny other router
The best path to every destination
What type of communication occurs when link state changes occur?
They are flooded across the network
What layers does ISIS operate over?
(Hint:2 different layers)
Layer 1 (backbone)
Layer 2 (Areas)
Does ISIS have a backbone area (like OSPF)?
No
What is the backbone on ISIS?
Contiguous collection of Level 2 capable routers
Where do IS-IS area borders reside?
Links
Network Entity Title
Identifies each router in ISIS
CLNS
Connection-less mode Network Service
NSAP
Network Service Access Point address
CLNS Addressing: NSAP 3 main components
Area ID (1 byte AFI + 1-12bytes area address)
System ID (6 bytes)
N-Selector (1byte)
CLNS Addressing cons
- At least one NSAP per node
- All routers in same area must share Area ID
- Each node in area must have unique System ID
- All level 2 routers in domain must have unique SYstem IDs relative to each other
SPF
Shortest Path First
By default, SPF runs no more frequently than…?
SPF = Shortest Path First
5 seconds
When is a full SPF run?
When topology changes
PRC
Partial Route Calculation
When is PRC used?
When leaf elements such as IP prefixes change. (SPF)
IS-IS Basic Config
Each router/interface needs…
Each router needs a routeable loopback address.
Each router needs an ID (NET)
Each interface needs IS-IS on.
IS-IS Loopback Address
Is it unique? What is the subnet?
Unique for each router.
/32 subnet
IS-IS NSAP address format
Start with AFI 49
Then area ID 0001
System ID
- ABC.DEF.GHI.JKL goes to
- ABCD.EFGH.IJKL
End with 00
A NSAP address has AFI 49, Area ID 0001 and system id 7.7.7.1. What is the address?
49.0001.0070.0700.7001.00
