link%state%&%ospf% - university of california, … protocol% • a set of algorithms and...

Link State & OSPF

Spring 2015 CE 151 -‐ Advanced Networks 1

Stuff •  VMs -‐ Resume-‐Session…

–  Don’t change “port” field to your login… –  But may need to change #... from 5910 to 5911, 5912, …

•  Palo Alto Networks summer internship hUps://www.paloaltonetworks.com/company/careers/open-‐posiXons-‐north-‐america.html?p=job%2Fo1KI0fwd

•  Guest lectures –  Thursday 4/30 – Cruzio CEO and CTO discuss running a regional ISP –  Thursday 5/21 – Jim Warner (Senior Network Engineer) on cable plant –  Tuesday 5/26 -‐ Prof Garcia-‐Luna on DUAL/EIGRP and current research.

•  Dates –  IPv6 quiz Thursday –  IPv6 lab Sunday –  Dordal readings

•  SecXons 9.5-‐7 for rouXng intro and OSPF •  SecXons 9.1-‐4 for Distance Vector and RIP


Summary of VM Guidelines •  Before logging out or disconnecXng

–  Halt VB VMs •  This may take some Xme… wait for couple of lines aoer “login:” prompt

–  Stop GNS3 (the red buUon) –  Quit GNS3 –  Logout/disconnect

•  To “Resume-‐Session” (aoer disconnecXon) –  Don’t change port # field to login name –  May need to increase port # by small amount

•  From 5910 to 5911, 5912, … •  Caused by other X11 sessions on host… e.g. SSH


Projects •  Proposal

–  Due Tuesday, 4/28 (next week) –  A document (pdf) including

•  Describe topic you will develop a lab for •  Drao outline of what you plan to include in the lab •  What you need to invesXgate

•  Deliverables –  Paper describing

•  Technology covered in the lab •  Lessons learned

–  Lab –  Answer key –  Netref content


Overall Scores


Per-‐QuesXon Averages


Routing Introduction •  Remember… delivery of IP packets implemented by two processes

–  Forwarding –  Routing

•  Dynamic routing process is a distributed computation –  Triggered by topology changes –  Processing defined by a routing protocol –  Output of the computation is forwarding state

•  Goal of computation is convergence –  After finite sequence of topology changes –  Process should terminate updates to forwarding state –  Forwarding state should be correct

•  Loop-free (after convergence vs. instantaneous) •  With desired characteristics: performance, possibly conform to policy


“Fate-sharing” Principle •  “The fate-sharing model suggests that it is acceptable to lose the state

information associated with an entity if, at the same time, the entity itself is lost.” Dave Clark, “Design Philosophy of the DARPA Internet Protocols”, SIGCOMM ‘88.

•  Benefits –  Ensures the failure of any single component of an internet does not invalidate

state located elsewhere in the internet –  Localizing the effects of any failures –  More robust system

•  Internet routing architecture achives fate-sharing by co-locating –  Forwarding state –  Routing process that computes the state

•  Compared with virtual-circuit routing?


Routing Protocol •  A set of algorithms and messages that are used to exchange topology

information and populate the forwarding table with the routing protocol’s choice of best paths.

•  Purpose: –  Discover remote subnets –  Maintain up-to-date forwarding tables –  Choose the best path to destination subnets

•  Components of a routing protocol: –  Algorithm:

•  Event-driven procedures for –  Processing routing information –  Selecting best-paths

•  The data structures needed for these steps. –  Routing protocol messages:

•  Discover neighboring routers •  Exchange topology information


Classifying Routing Protocols •  Function:

–  Intra-domain/Interior Gateway Protocol (IGP) –  Inter-domain/Exterior Gateway Protocol (EGP)

•  Algorithm… distinguished by information exchanged: –  Distance-Vector –  Link-State –  Path-Vector


Functional Classification •  An autonomous system (AS) or routing domain is a region of the

Internet that is administered by a single entity –  UCSC’s network –  IBM’s corporate network –  AT&T’s ISP network

•  Routing inside an AS –  Focus is on performance –  Popular protocols: RIP, OSPF, IS-IS –  Called intra-domain or internal gateway (IGP) routing

•  Routing between ASs –  Focus is on policy –  Popular protocol: BGP –  Called inter-domain or external gateway (EGP) routing


Ethernet

Router

Ethernet

Ethernet

RouterRouter

Ethernet

Ethernet

EthernetRouterRouter

Router

AutonomousSystem 2

AutonomousSystem 1

Spring 2015 12

How ensure correct routes? •  Requirement for correctness of routing protocol

–  Loop-free –  Desired path characteristics

•  Two strategies for ensuring correctness –  Use identical algorithm for selecting paths

•  Share minimal topology information •  Use identical path selection algorithm at all nodes •  Used for IGP/Intra-domain routing •  Use shortest-path routing algorithm

–  Use custom (private) algorithm for selecting paths •  Share full path information •  Use policy-specific path selection algorithm at each node •  Used for EGP/Inter-domain routing •  Use path-vector protocol

CE 151 -‐ Advanced Networks

Shortest-‐Path RouXng •  Shortest-‐path spanning tree is computed for each router.

•  Internet rouXng is based on two SP algorithms –  Dijkstra – used in link-‐state protocols –  Bellman-‐Ford – used in distance-‐vector protocols


Algorithm Classification •  Distance-Vector – “sign-‐post”

–  Vectors of destination and distance sent to neighbors •  “Tell your neighbors about the rest of the network”

–  Destination in terms of a network prefix –  Distance in terms of a metric: hop count, delay, bandwidth –  Use Distributed Bellman-Ford path selection algorithm –  Popular protocol: Routing Information Protocol (RIP)

•  Link-State – “map” –  Flood description of your links (link state) to whole network

•  “Tell the rest of the network about your neighbors” –  Links described by

•  End-point routers of subnet in internet •  Cost of subnet: delay, bandwidth

–  Use Dijkstra path selection algorithm –  Popular protocol: Open Shortest Path First (OSPF)

•  Path-Vector –  Routes advertised as full-paths –  Paths described by sequence of ASs –  Popular protocol is Border Gateway Routing Protocol (BGP)


Destination-Based Forwarding •  Internet rouXng uses a single path per desXnaXon

•  Des/na/on-‐based forwarding is a restricted version of single-‐path

•  A path through a node to a desXnaXon…

•  …must be an extension of the path from the node to the des3na3on.

•  This is coming back to haunt us…


Review •  The Internet implements a distributed routing architecture that is triggered

by topology change events.

•  Convergence of routing in the Internet depends on the stability of the network topology for a sufficient period of time.

•  The Fate-Sharing Principle – “It is acceptable to lose the state information associated with an entity if, at the same time, the entity itself is lost.”

–  Ensures the failure of any single component of an internet does not invalidate state located elsewhere in the internet

–  Localizing the effects of any failures –  Results in a more robust system –  Achieved in Internet by co-locating

•  Forwarding state •  Routing process that computes the state


Review •  An Autonomous System (AS) is a region of the Internet that is administered

by a single entity and follows a single routing policy.

•  Functional classification of routing protocols –  IGP - routing inside an AS

•  Share minimal topology information •  Use identical path selection algorithm at all nodes

•  Optimize performance

–  EGP - routing between ASs •  Share full path information

•  Use custom path selection algorithm at each node to implement desired policies •  Enforce policies


Review •  Algorithmic classification of routing protocols

–  Distance-vector •  Send vectors of distances to destinations to neighbors

•  “Tell your neighbors about the rest of the network” •  “Distributed computation”

–  Link-state •  Flood description of your links to all routers •  “Tell the rest of the network about your neighbors”

•  “Distributed database”

–  Path-vector •  Distance-vector with full paths


Review •  Destination-based forwarding

–  A path through a node to a destination… must be an extension of the path from the node to the destination

–  Tends concentrates traffic on a subset of the network topology.


Link-‐State


RouXng Algorithms •  Distance-‐Vector – “sign-‐post”

–  Vectors of desXnaXon and distance sent to neighbors •  “Tell your neighbors about the rest of the network”

–  DesXnaXon in terms of a network prefix –  Distance in terms of a metric: hop count, delay, bandwidth –  Use Distributed Bellman-‐Ford path selecXon algorithm –  Popular protocol: RouXng InformaXon Protocol (RIP)

•  Link-‐State – “map” –  Flood descripXon of your links (link state)

•  “Tell the rest of the network about your neighbors”

–  Links described by •  End-‐point routers of subnet in internet •  Cost of subnet: delay, bandwidth

–  Use Dijkstra path selecXon algorithm –  Popular protocol: Open Shortest Path First (OSPF)

•  Path-‐Vector –  Routes adverXsed as full-‐paths –  Paths described by sequence of ASs –  Popular protocol is Border Gateway RouXng Protocol (BGP)


How ensure correct routes? •  Requirement for correctness of rouXng protocol

–  Loop-‐free –  Desired path characterisXcs

•  Two strategies for ensuring correctness –  Use idenXcal algorithm for selecXng paths

•  Share minimal topology informaXon •  Use idenXcal path selecXon algorithm at all nodes •  Used for IGP/Intra-‐domain rouXng •  Use link-‐state or distance vector protocol

–  Use custom (private) algorithm for selecXng paths •  Share full path informaXon •  Use policy-‐specific path selecXon algorithm at each node •  Used for EGP/Inter-‐domain rouXng •  Use path-‐vector protocol


Link-‐State Protocols •  Are Interior-‐Gateway Protocols (IGPs)

•  Exchange link-‐state informaXon –  Pair of routers connected by a subnet –  Cost of subnet (hop count, delay, etc.)

•  Conceptually, very simple…


Link-‐State Protocols •  Maintains a topology database of all the links it has heard of

–  IniXalize with the subnets it is connected to.

•  Floods link-‐state updates describing its directly connected subnets, including any changes to these links. –  “Tell the rest of the network about your neighbors”

•  ParXcipates in the flooding of link-‐state updates from other routers.

•  On update of its topology database –  Runs a shortest-‐path algorithm on the database to compute routes

•  Dijkstra is most efficient

–  Updates its forwarding table with any changes.


Characterizing Link State Link-‐State •  # updates per link change?

–  One.

•  How far propagate updates? –  Flooded to all nodes.

•  One update, global distribuXon.

•  Scaling problems due to flooding

•  As we’ll see next lecture, the characteris3cs of distance vector… –  …are very different –  …hint at a much beGer solu3on


Dijstra Shortest-‐Path Algorithm •  Breadth-‐first search of paths, by increasing path cost, for best paths to all

desXnaXons. Terminate when path has been found for all desXnaXons.

•  Maintain two sets –  DesXnaXons for which shortest paths have been found.

•  Permanently labeled desXnaXons P •  IniXalize with self

–  DesXnaXons for which candidate shortest paths have been found. •  Temporarily labeled desXnaXons T •  IniXalize with my neighbors.

•  Iterate –  Move shortest path in T, say for desXnaXon D, to P –  Add routes for D’s neighbors, that are extensions of the path to T, to T if they are

shorter than the current path in T for each neighbor. The “relaxa3on” step. –  Repeat unXl a route has been added to P for all desXnaXons


More formally… •  E is the set of edges.

•  wij is the weight of the link between nodes i and j.

•  P and T… see previous slide.

•  P and T entries are triples, <d, p, w>: –  d is the desXnaXon –  p is the predecessor –  w is the link weight


2 BRAD

algorithm Dijkstrabegin

1 Push(<i, i, 0>, P );2 for each {(i, j) ⇥ E}3 Insert(<j, i, �ij >, T );4 while (|T | > 0)

begin5 <x, px, dx > � Min(T );6 DeleteMin(T );7 Push(<x, px, dx >, P );8 for each {(x, j) ⇥ E}9 if (Tj = ⇤)10 then Insert(<j, x, dx + �xj >, T )11 else if (dx + �xj < Tj .dj)12 then DecreaseKey(<j, x, dx + �xj >, T );

endend

Figure 2. Traditional Dijkstra Shortest-Path Algorithm.

The Dijkstra algorithm in Figure 2 works by maintaining a set T of temporarily assignedroutes, and a set P of permanently assigned routes. Each routes is specified by a 3-tuple< x, p

x

, dx

>. dx

is the path weight currently assigned to node x. For nodes in P , dx

isthe final weight assignment specifying the shortest distance to x. For nodes in T , d

x

is thecurrent best estimate of the shortest distance to x based on routes currently contained inP . p

x

is the predecessor to x on the currently selected route with weight �x

. Tj

is theentry in T for node j.

The Distributed Bellman-Ford protocols in Figures 3 and 4 implement the DBF al-gorithm with and without neighbor tables (respectively). These protocols compute theshortest path from the current (source) node s in a network with a set of nodes V . d

i

isthe current distance from s to i with n

i

is the next hop, and �ij

is the weight of a linkbetween nodes i and j. N is the set of neighbors of node s.

Dijkstra… increasing path cost C,9 B,2

G,6 H,∞ 6

2 2

2 F,∞ E,4 1 A,0 D,∞

7 3

2

3 2

4

C,9

F,6 E,4 A,0

B,2

D,∞

G,5 H,∞ C,9

F,6 E,4 A,0

B,2

D,∞

G,5 H,9

C,9

F,6 E,4 A,0

B,2

D,∞

G,5 H,8 C,9

F,6 E,4 A,0

B,2

D,10

G,5 H,8

C,9

F,6 E,4 A,0

B,2

D,10

G,5 H,8 C,9

F,6 E,4 A,0

B,2

D,10

G,5 H,8 Spring 2015 CE 151 -‐ Advanced Networks 28

Permanent Temporary

TranslaXng to a Protocol •  Dijkstra requires a “centralized” implementaXon

–  Maintain a full graph of the network, on an event-‐driven basis –  Re-‐compute routes as the graph changes –  Flood changes to your links

•  “Brute-‐force” protocol. –  Straight-‐forward, easy to understand –  Inefficient… lots of overhead


Review •  Dijkstra

–  Iterates on “next shortest path” –  Requires centralized computaXon

•  LS protocols –  Is an IGP –  Implements a centralized rouXng model –  Floods link-‐state updates describing current state of its links

•  “Tell the rest of the network about your neighbors” –  Use Dijkstra algorithm because it is most efficient shortest-‐path algorithm


The Challenge of Internet RouXng •  Independent rouXng computaXons at each router…

•  Need to compute paths that… –  Support desXnaXon-‐based forwarding –  Are shortest –  Are loop-‐free

•  This is trickier than it looks…


Herding Packets… •  DesXnaXon-‐based, hop-‐by-‐hop rouXng technology depends

on “tame” metrics

•  As we move away from basic integers, things fall apart…


Shortest Forwarding paths •  C→D: C – D = 1 •  B→D: B – C – D = 2 •  A→D: A – B – C – D = 3

Forwarding Path ProperXes •  Shortest •  Loop-‐free


A

B

D

C

1

3

3

1

1

Widest-‐Shortest Metrics: (delay, bandwidth) •  “+”: (d1,b1) + (d2,b2) = (d1+d2, Min(b1,b2)) •  “≤”: (d1,b1) ≤ (d2,b2) = (d1 < d2) or

((d1 = d2) and (b1 ≥ b2))

Forwarding paths •  C→D: C – D = (1,10) •  B→D: B – C – D = (2,10) •  A→D: A – B – C – D = (3,5)

A – B – D = (3,5)

Forwarding Path ProperXes •  Shortest •  Loop-‐free


A

B

D

C

(1,5)

(3,5)

(2,5)

(1,10)

(1,10)

Shortest-‐Widest Metrics: (bandwidth, delay) •  “+”: (b1,d1) + (b2,d2) = (Min(b1,b2), d1+d2) •  “≤”: (b1,d1) ≤ (b2,d2) = (b1 > b2) or

((b1 = b2) and (d1 ≤ d2))

What is the problem here?

Forwarding paths •  C→D: C – B – D = (10,4) •  B→D: B – D = (10,3) •  A→D: A – B – C – D = (5,3)

Forwarding Path ProperXes •  Not Shortest •  Loop-‐free


A

B

D

C

(5,1)

(5,4)

(10,3)

(5,1)

(10,1)

“Slope” Metrics: (cost, distance) •  “+”: (c1,d1) + (c2,d2) = (c1+c2, d1+d2) •  “≤”: (c1,d1) ≤ (c2,d2) = (c1 /d1) ≤ (c2 /d2)

Forwarding paths •  C→D: C – B – D = 3/2 •  B→D: B – C – D = 3/2 •  A→D: A – B – C – D = 4/3

Forwarding Path ProperXes •  Not Shortest •  Not Loop-‐free


A

B

D

C

(1,1)

(5,1)

(2,1)

(2,1)

(1,1)

Challenges

•  …algorithms depend on properXes of metric algebra to behave correctly.

–  Strictly Bounded –  Monotonic


a ≤ b⇒ (a+ c) ≤ (b+ c)a < a+ b

OSPF


OSPF •  OSPF = Open Shortest Path First •  The most widely used rouXng protocol •  The complexity of OSPF is significant

•  History: –  1989: RFC 1131 OSPF Version 1 –  1991: RFC1247 OSPF Version 2 –  1994: RFC 1583 OSPF Version 2 (revised) –  1997: RFC 2178 OSPF Version 2 (revised) –  1998: RFC 2328 OSPF Version 2 (current version)


What We Cover… •  Messages

•  Router IDs

•  Flooding process

•  Metrics

•  Designated routers

•  Areas


OSPF Messages •  An OSPF message can contain one of five packet types.

•  OSPF is embedded directly in an IP frame (doesn’t use UDP) –  Protocol field is set to 89 (OSPF) –  DesXnaXon address is typically set to one of two mulXcast addresses:

•  224.0.0.5 (“AllSPFRouters”… Hello messages) or •  224.0.0.6 (“AllDRouters”… rouXng info to “Designated Routers”).

•  If the OSPF packet is encapsulated in an Ethernet frame, the desXnaXon MAC address is also a mulXcast address: –  01-00-5E-00-00-05 or 01-00-5E-00-00-06!


OSPF Packet Types •  Hello: Used to establish and maintain adjacency with other OSPF routers.

•  DBD: The database descripXon (DBD) packet contains an abbreviated list of the sending router’s link-‐state database and is used by receiving routers to check against the local link-‐state database.

•  LSR: Receiving routers can then request more informaXon about any entry in the DBD by sending a link-‐state request (LSR).

•  LSU: Link-‐state update (LSU) packets are used to reply to LSRs and to announce new informaXon. LSUs contain seven different types of link-‐state adverXsements (LSA).

•  LSAck: When an LSU is received, the router sends a link-‐state acknowledgment (LSAck) to confirm receipt of the LSU.


OSPF Message Header

source router IP address

authentication

authentication32 bits

version type message length

Area ID

checksum authentication type

Body of OSPF MessageOSPF MessageHeader

2: current version is OSPF V2

Message types: 1: Hello (tests reachability) 2: Database description 3: Link Status request 4: Link state update 5: Link state acknowledgement

ID of the Area from which the packet originated

Standard IP checksum taken over entire packet

0: no authentication 1: Cleartext password 2: MD5 checksum (added to end packet)

Authentication passwd = 1: 64 cleartext password Authentication passwd = 2: 0x0000 (16 bits)

KeyID (8 bits) Length of MD5 checksum (8 bits) Nondecreasing sequence number (32 bits)

Prevents replay attacks


RouterIDs

Router ID

Router ID

Router ID

Router ID

Router ID

Router ID

•  Router ID plays an important role in OSPF –  Uniquely idenXfies each router in a rouXng domain –  Used in Designated Router elecXon process

(explained later)

•  Router ID is an IP address of a router

•  Cisco routers use following algorithm to determine Router ID

–  IP address configured with OSPF router-id command

–  If not configured, use highest IP address of a loopback interface

–  If no loopback interfaces, use highest acXve IP address of physical interface

•  Advantage of loopback interface is it cannot fail


OSPF Hello Packets •  Used to

–  Discover OSPF neighbors and establish neighbor adjacencies –  Elect the Designated Router and Backup Designated Router on

multiaccess networks such as Ethernet and Frame Relay –  Negotiate

•  Hello interval (e.g. 10 sec on Ethernet segments) •  Dead interval: time to declare neighbor down (4x Hello interval) •  Network type

•  Five network types –  Point-to-point –  Point-to-multipoint –  Broadcast multiaccess (Ethernet) –  Nonbroadcast multiaccess (Frame Relay) –  Virtual links


OSPF LSU Packets •  Link State Update (LSU) packets

–  Used for OSPF routing updates –  Contain one or more LSAs

•  Link State Advertisements (LSAs) –  Contain route information for destination networks –  There are 11 types of LSAs


Link State AdverXsement (LSA) •  The LSA of router 10.10.10.1 is as follows:

•  Link State ID: 10.10.10.1 = can be Router ID •  AdverXsing Router: 10.10.10.1 = Router ID •  Number of links: 3 = 2 links plus router itself

•  DescripXon of Link 1: Link ID = 10.1.1.1, Metric = 4 •  DescripXon of Link 2: Link ID = 10.1.2.1, Metric = 3 •  DescripXon of Link 3: Link ID = 10.10.10.1, Metric = 0


10.1.1.0 / 24.1 .2 .2

10.10.10.1

10.1.4.0 / 24

10.1.2.0 / 24

.1

.4

10.1.7.0 / 24

10

.1.6

.0 /

24

10

.1.3

.0 /

24

10.1.5.0/24

10.1.

8.0 / 2

4

.3

.3 .5

.2

.3

.5

.5

.4

.4

.6

.6

10.10.10.2 10.10.10.4 10.10.10.6

10.10.10.3 10.10.10.5

Each router sends its LSA to all routers in the network (using a method called reliable flooding)

LSA Format


Link State ID

link sequence number

advertising router

Link Age Link Type

checksum length

Link ID

Link Data

Link Type Metric#TOS metrics

LSA

LSAHeader

LSAData

Link ID

Link Data

Link Type Metric#TOS metrics

LSA Header

Link 1

Link 2

OSPF Metrics •  The OSPF metric is called cost. The following passage is from RFC 2328:

–  A cost is associated with the output side of each router interface. This cost is configurable

•  RFC 2328 does not specify which values should be used to determine the cost.

•  In Cisco IOS –  Link cost is 108 ÷ link bandwidth. –  Reference bandwidth (108) can be changed with auto-cost referencebandwidth!–  Modify link bandwidth value with bandwidth!


Link State Database •  The collecXon of all LSAs is called the link-‐state database

•  Each router has an idenXcal link-‐state database –  Useful for debugging: Each router has a complete descripXon of the network

•  If neighboring routers discover each other for the first Xme, they will exchange their link-‐state databases

•  The link-‐state databases are synchronized using reliable flooding


Link State Database


10.1.1.0 / 24.1 .2 .2

10.10.10.1

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.1

.4

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.1.6

.0 /

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.1.3

.0 /

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8.0 / 2

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.4

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.6

10.10.10.2 10.10.10.4 10.10.10.6

10.10.10.2 10.10.10.5

Each router has a database which contains the LSAs from all other routers

LS Type Link StateID Adv. Router Checksum LS SeqNo LS Age

Router-LSA 10.1.10.1 10.1.10.1 0x9b47 0x80000006 0

Router-LSA 10.1.10.2 10.1.10.2 0x219e 0x80000007 1618

Router-LSA 10.1.10.3 10.1.10.3 0x6b53 0x80000003 1712

Router-LSA 10.1.10.4 10.1.10.4 0xe39a 0x8000003a 20

Router-LSA 10.1.10.5 10.1.10.5 0xd2a6 0x80000038 18

Router-LSA 10.1.10.6 10.1.10.6 0x05c3 0x80000005 1680

Neighbor Discovery •  Router mulXcasts OSPF Hello packets on all OSPF-‐enabled interfaces. •  If two routers share a link, they can become neighbors, and establish an

adjacency

•  Aoer becoming a neighbor, routers exchange their link state databases


OSPF Hello

OSPF Hello: I heard 10.1.10.2

10.1.10.1 10.1.10.2

Scenario: Router 10.1.10.2 restarts

Neighbor discovery and database synchronizaXon


Scenario: Router 10.1.10.2 restarts

OSPF Hello

OSPF Hello: I heard 10.1.10.2

Database Description: Sequence = X

10.1.10.1 10.1.10.2

Database Description: Sequence = X, 5 LSA headers = Router-LSA, 10.1.10.1, 0x80000006 Router-LSA, 10.1.10.2, 0x80000007 Router-LSA, 10.1.10.3, 0x80000003 Router-LSA, 10.1.10.4, 0x8000003a Router-LSA, 10.1.10.5, 0x80000038 Router-LSA, 10.1.10.6, 0x80000005

Database Description: Sequence = X+1, 1 LSA header= Router-LSA, 10.1.10.2, 0x80000005

Database Description: Sequence = X+1

Sends empty database description

Discovery of adjacency

Sends database description. (description only contains LSA headers)

Database description of 10.1.10.2 Acknowledges

receipt of description

After neighbors are discovered the nodes exchange their databases

Regular LSA exchanges


10.1.10.1 10.1.10.2

Link State Request packets, LSAs = Router-LSA, 10.1.10.1, Router-LSA, 10.1.10.2, Router-LSA, 10.1.10.3, Router-LSA, 10.1.10.4, Router-LSA, 10.1.10.5, Router-LSA, 10.1.10.6,

Link State Update Packet, LSA = Router-LSA, 10.1.1.6, 0x80000006

Link State Update Packet, LSAs = Router-LSA, 10.1.10.1, 0x80000006 Router-LSA, 10.1.10.2, 0x80000007 Router-LSA, 10.1.10.3, 0x80000003 Router-LSA, 10.1.10.4, 0x8000003a Router-LSA, 10.1.10.5, 0x80000038 Router-LSA, 10.1.10.6, 0x80000005

10.1.10.2 explicitly requests each LSA from 10.1.10.1

10.1.10.1 sends requested LSAs 10.1.10.2 has more

recent value for 10.0.1.6 and sends it to 10.1.10.1 (with higher sequence number)

DisseminaXon of LSA-‐Update •  A router sends and refloods LSA-‐Updates, whenever the topology or link

cost changes. (If a received LSA does not contain new informaXon, the router will not flood the packet)

•  ExcepXon: Infrequently (every 30 minutes), a router will flood LSAs even if there are no new changes.

•  Acknowledgements of LSA-‐updates: –  explicit ACK, or –  implicit via recepXon of an LSA-‐Update


Why Designated Routers? •  Large number of adjacencies

–  Full mesh of adjacencies –  n(n -‐ 1)/2 adjacencies

•  Excessive load from flooding LSAs


ElecXng Designated Router •  OSPF elects a Designated Router (DR) on mulXaccess networks •  DR is collecXon and distribuXon point for LSAs on network •  Backup Designated Router (BDR) also elected for case where DR fails


224.0.0.6

224.0.0.5

ElecXng Designated Router •  ElecXon held through exchange of Hello messages

•  DR/BDR elecXon criteria –  DR is router with highest interface priority –  BDR is router with second-‐highest interface priority –  If interface prioriXes are equal, use Router ID


ElecXng Designated Router •  Further elecXons only occur on failure of DR/BDR

•  On DR failure –  BDR promoted to DR –  New BDR elected

•  On BDR failure –  New BDR elected

•  Live (B)DR not replaced by elecXon. –  If DR dies

•  BDR replaces DR •  ElecXon held to replace BDR

–  If BDR dies •  ElecXon held to replace BDR


OSPF Areas •  An OSPF Area is a set of routers that share link state informaXon

•  Goal is to address scalability problem with OSPF flooding

•  Area 0 is the backbone area –  All areas must be connected to he backbone area –  Best pracXce to use area 0 in single-‐area OSPF –  Eases conversion to mulX-‐area


Review •  OSPF message types

–  Hello -‐ establish and maintain adjacency with OSPF routers –  DataBase DescripXon (DBD) -‐ summary of my database –  Link-‐State Request (LSR) -‐ request for detailed entry –  Link-‐State Update (LSU) -‐ response to LSR –  Link-‐State Ack (LSAck) -‐ acknowledge receipt of LSU

•  Router IDs… –  Uniquely idenXfies each router in a rouXng domain –  Used in Designated Router elecXon process –  Best pracXce is to use loopback interface... it can’t fail


Review •  OSPF metrics…

–  “Cost is associated with the output side of each router interface.” –  In IOS is 108 / link bandwidth (inversely proporXonal to bandwidth)

•  Designated routers… –  MiXgate impact of “full-‐mesh” on # of adjacencies in topology (n(n-‐1)/2) –  DR is collecXon and distribuXon point for LSAs on network

•  OSPF deals with scaling problem by using “areas,” area 0 is the backbone.


The End


link%state%&%ospf% - university of california, … protocol% • a set of algorithms and...

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