TDC 363 Introduction to LANs

Transcription

1 TDC 363 Introduction to LANs OSPF Greg Brewster DePaul University TDC 363 Greg Brewster, DePaul University 1

2 OSPF Link State Routing Algorithms Open Shortest Path First (OSPF) Message Types Operations Configuration and Verification TDC 363 Greg Brewster, DePaul University 2

3 Link-State Routing Link state routing protocols -Also known as shortest path first algorithms -These protocols built around Dijkstra s SPF ITE PC v4.0 Chapter Cisco Systems, Inc. All rights reserved. Cisco Public 3

4 Link-State Routing Link-State Routing Process How routers using Link State Routing Protocols reach convergence -Each routers learns about its own directly connected networks -Link state routers exchange Hello packet to meet other directly connected link state routers. -Each router builds its own Link State Packet (LSP) which includes information about links and neighbors such as neighbor ID, link type, & bandwidth. -After the LSP is created the router floods it to all neighbors who then store the information and then forward it until all routers have the same information. -Once all the routers have received all the LSPs, the routers then construct a topological map of the entire network which is used to determine the best routes to all destinations. ITE PC v4.0 Chapter Cisco Systems, Inc. All rights reserved. Cisco Public 4

5 Link-State Routing Sending Hello Packets to Neighbors Connected interfaces that are using the same link state routing protocols will exchange hello packets. Once routers learn it has neighbors they form an adjacency -2 adjacent neighbors will exchange hello packets -These packets will serve as a keep alive function ITE PC v4.0 Chapter Cisco Systems, Inc. All rights reserved. Cisco Public 5

6 Link-State Routing Building the Link State Packet Each router builds its own Link State Packet (LSP) Contents of LSP: -State of each directly connected link -Includes information about neighbors such as neighbor ID, link type, & bandwidth. ITE PC v4.0 Chapter Cisco Systems, Inc. All rights reserved. Cisco Public 6

7 Link-State Routing LSPs are sent out under the following conditions -Initial router start up or routing process -When there is a change in topology ITE PC v4.0 Chapter Cisco Systems, Inc. All rights reserved. Cisco Public 7

8 Link-State Routing Constructing a link state data base Routers use a database to construct a topology map of the network ITE PC v4.0 Chapter Cisco Systems, Inc. All rights reserved. Cisco Public 8

9 Link-State Routing ITE PC v4.0 Chapter Cisco Systems, Inc. All rights reserved. Cisco Public 9

10 Link-State Routing Determining the shortest path The shortest path to a destination determined by adding the costs & finding the lowest total path cost ITE PC v4.0 Chapter Cisco Systems, Inc. All rights reserved. Cisco Public 10

11 Link-State Routing Once the SPF algorithm has determined the shortest path routes, these routes are placed in the routing table. ITE PC v4.0 Chapter Cisco Systems, Inc. All rights reserved. Cisco Public 11

12 Link-State Routing Protocols Advantages of a Link-State Routing Protocol Routing protocol Builds Topological map Router can independently determine the shortest path to every network. Convergence A periodic/ event driven routing updates Use of LSP Distance vector No No Slow Generally No No Link State Yes Yes Fast Generally Yes Yes ITE PC v4.0 Chapter Cisco Systems, Inc. All rights reserved. Cisco Public 12

13 Link-State Routing Protocols Requirements for using a link state routing protocol Memory requirements Typically link state routing protocols use more memory Processing Requirements More CPU processing is required of link state routing protocols Bandwidth Requirements Initial startup of link state routing protocols can consume lots of bandwidth ITE PC v4.0 Chapter Cisco Systems, Inc. All rights reserved. Cisco Public 13

14 Open Shortest Path First (OSPF) 1. A Link State Internal Gateway Protocol used within most Autonomous Systems on the Internet 2. Advantages over RIP (distance-vector protocol) TDC 363 a) Supports many distance metrics, including physical distance, transmission delay, queueing delay, etc. b) Adapts to changes in topology quickly. c) Supports multiple types of service such as least delay, highest throughput, etc. d) Does load balancing over multiple paths to the same destination. e) Supports hierarchical systems: i) Autonomous Systems ii) Areas iii) Networks iv) Subnets f) Improved security. Greg Brewster, DePaul University 14

15 OSPF Link State Database Each OSPF router maintains a Link State Database containing, for each link in the network: The Link Type Router IDs of routers connected to this link IP subnet address for this link IP subnet mask for this link Link cost Link status (up or down) TDC 363 Greg Brewster, DePaul University 15

16 Basic OSPF Configuration Router ID This is a single IP address used to identify a router 3 methods for determining the router ID Configure with OSPF router-id command -Takes precedence over other methods If router-id command not used then router chooses highest IP address of any loopback interfaces Commonly set up Loopback0 IP to become Router ID If no loopback interfaces are configured then the highest IP address on any active interface is used Using active interface IP address is not best practice because Router ID may change if interface goes down. ITE PC v4.0 Chapter Cisco Systems, Inc. All rights reserved. Cisco Public 16

17 Basic OSPF Configuration OSPF Router ID Commands used to verify current router ID Show ip protocols Show ip ospf Show ip ospf interface ITE PC v4.0 Chapter Cisco Systems, Inc. All rights reserved. Cisco Public 17

18 OSPF Hierarchical Networks Autonomous Systems Owned by single organization Areas Collection of subnets and routers within an AS. Each area has an Area Number. Routers inside an area flood information to all other routers in area Area border routers summarize area information to pass to another area All areas connect to backbone area (0). For TDC 363, we only study single-area OSPF all subnets and routers will be in Area 0. TDC 363 Greg Brewster, DePaul University 18

19 Figure OSPF Hierarchical Networks Backbone router: connect only to networks in the backbone area (Area 0) Area border router: attaches to multiple areas. Internal router: all interfaces connected to networks in the same area. AS boundary router (Also known as Border Router): connect to other ASes TDC 363 Greg Brewster, DePaul University 19

20 Introduction to OSPF OSPF Packet Types ITE PC v4.0 Chapter Cisco Systems, Inc. All rights reserved. Cisco Public 20

21 OSPF HELLO Packet When router first powers up or comes online, it will first discover neighbors Multicast HELLO messages on all its interfaces Listen for HELLO messages in response from neighbor routers Store neighbor router information in Adjacency Database. TDC 363 Greg Brewster, DePaul University 21

22 OSPF Router Tables / Databases OSPF maintains three databases which are used to create three tables. Database Table Description Adjacency Database Link-state Database Forwarding Database Neighbor Table Topology Table Routing Table List of all neighbors routers to which a router has established bidirectional communication. This table is unique for each router. Can be viewed using the show ip ospf neighbor command. List of information about all other routers in the network. The database shows the network topology. All routers within an area have identical link-state databases. Can be viewed using the show ip ospf database command. List of routes generated when an algorithm is run on the linkstate database. Each router s routing table is unique and contains information on how and where to send packets to other routers. Can be viewed using the show ip route command. Chapter , Cisco Systems, Inc. All rights reserved. Cisco Public 22

23 Introduction to OSPF Hello Packets Maintain adjacencies and set Designated Router OSPF Hello Intervals Usually multicast ( ) Sent every 10 seconds in general Sent every 30 seconds for NBMA OSPF Dead Intervals This is the time that must transpire before the neighbor is considered down Default time is 4 times the hello interval ITE PC v4.0 Chapter Cisco Systems, Inc. All rights reserved. Cisco Public 23

24 OSPF DATABASE DESCRIPTION Message When other routers receive the HELLO message: If this is the first time they have heard from this router, they will send DATABASE DESCRIPTION message This message gives a summary outline of all information in the Link State Database of the router TDC 363 Greg Brewster, DePaul University 24

25 OSPF LINK STATE REQUEST Message When a router wants more information about a particular set of links, it will send a LINK STATE REQUEST message to its neighbor. Neighbor will respond with LINK STATE UPDATE message containing requested information TDC 363 Greg Brewster, DePaul University 25

26 OSPF LINK STATE UPDATE Message When any link state changes (link goes up or down, cost changes, etc.) a router will flood a LINK STATE UPDATE message to all other routers in the Area. Link State Update contains one or more Link State Advertisements (LSAs) from the router. TDC 363 Greg Brewster, DePaul University 26

27 LSU Flooding When any router receives a Link State Update message, it checks whether this is a new update message If it is a New LSU message, then this router will flood the message (transmit a copy out all interfaces except the one it arrived on) If it is an Old LSU message (seen before), then this router will drop the message TDC 363 Greg Brewster, DePaul University 27

28 New or Old? A router determines whether an arriving LSU message is new or old using two header fields Sequence Number Each router remembers the Sequence Number of the last LSU seen from every other router Each router increments Sequence Number when sending out a new LSU Age Routers update Age field with estimated transit time. Messages dropped when Age gets too large TDC 363 Greg Brewster, DePaul University 28

29 OSPF Types of Links Point-to-Point Link Directly connects two routers with no other host or router inbetween. Transient Link A Link with multiple routers on it (example: a LAN) Stub Link A link with only one router on it Virtual Link A multi-hop connection between two routers (a tunnel) used for backup if direct link goes down. TDC 363 Greg Brewster, DePaul University 29

30 Point-to-Point Links Each of the two routers measures the metric from itself to its neighbor. Result: there are 2 metrics for each pointto-point link one for data sent in each direction. May be same value or different. TDC 363 Greg Brewster, DePaul University 30

31 Transient Links and DRs Many routers connect to the same link (multiaccess link), such as in a LAN. Problem: Its too much work to measure metrics between all possible pairs of routers Solution: Choose one router to be designated router (DR) Other routers only form adjacencies with DR. Cost metric is assigned from each router to designated router. No cost from designated router to others. Designated Router is elected based on Priority value. Backup Designated Router is also elected and takes over if DR fails. TDC 363 Greg Brewster, DePaul University 31

32 OSPF in Multiaccess Networks Solution to LSA flooding issue is the use of Designated router (DR) Backup designated router (BDR) DR & BDR selection Routers are elected to send & receive LSA Sending & Receiving LSA DRothers send LSAs via multicast to DR & BDR DR forward LSA via multicast address to all other routers ITE PC v4.0 Chapter Cisco Systems, Inc. All rights reserved. Cisco Public 32

33 OSPF in Multiaccess Networks Criteria for getting elected DR/BDR 1. DR: Router with the highest OSPF interface priority. 2. BDR: Router with the second highest OSPF interface priority. 3. If OSPF interface priorities are equal, the highest router ID is used to break the tie. ITE PC v4.0 Chapter Cisco Systems, Inc. All rights reserved. Cisco Public 33

34 OSPF in Multiaccess Networks Timing of DR/BDR Election Occurs as soon as 1 st router has its interface enabled on multiaccess network When a DR is elected it remains as the DR until one of the following occurs -The DR fails. -The OSPF process on the DR fails. -The multiaccess interface on the DR fails. ITE PC v4.0 Chapter Cisco Systems, Inc. All rights reserved. Cisco Public 34

35 OSPF in Multiaccess Networks OSPF Interface Priority Manipulating the DR/BDR election process continued Use the ip ospf priority interface command. Example:Router(config-if)#ip ospf priority {0-255} Priority number range 0 to means the router cannot become the DR or BDR 1 is the default priority value ITE PC v4.0 Chapter Cisco Systems, Inc. All rights reserved. Cisco Public 35

36 Stub Links A stub link is just a special case of a transient network where the number of routers on the network is 1 Stub links are advertised in LSAs sent by connected router. TDC 363 Greg Brewster, DePaul University 36

37 OSPF Protocol Overview When router first powers up, it will Determine who its neighbor routers are. Download a complete Link State Database from a neighbor router When something changes in the network (link goes up or down, cost metric changes, etc.) Discovering router will flood Link State Advertisements (LSAs) to all other routers in the Area to update their databases TDC 363 Greg Brewster, DePaul University 37

38 Link State Advertisement An LSA packet announces the state of all links connected to a router Four types of LSAs: Router Link LSA Network Link LSA Summary Link LSA (multi-area OSPF) External Link LSA (multi-area OSPF) TDC 363 Greg Brewster, DePaul University 38

39 Router Link LSA Advertises the state of the links connected to a true router Can be any of the 4 link types TDC 363 Greg Brewster, DePaul University 39

40 Network Link LSA Advertises the state of the links connected to a designated router TDC 363 Greg Brewster, DePaul University 40

41 Summary Link LSA Two Types (not required for TDC 363): Summary Link to Network LSA Used to provide a summary of all link information for one particular Area to all routers in another Area in the same Autonomous System Summary Link to AS Boundary Router LSA Advertises a path to an AS Boundary router so that Area routers can learn how to get outside of their own Autonomous System TDC 363 Greg Brewster, DePaul University 41

42 OSPF Metric OSPF uses path cost as metric for determining best route. Path Cost = sum of link costs on path to destination. -Link Cost is based on bandwidth of link interface Interface bandwidth parameter can be changed by admin. It is not affected by clock rate command. Link Cost is calculated using the formula Link Cost = 10 8 / (Interface Bandwidth) Default Link Costs Eth interface: 10 FastEth interface: 1 GigEth interface: 1 Serial Interface: 64 (regardless of clock rate) ITE PC v4.0 Chapter Cisco Systems, Inc. All rights reserved. Cisco Public 42

43 OSPF Metric COST of an OSPF route Is the accumulated value from one router to the next ITE PC v4.0 Chapter Cisco Systems, Inc. All rights reserved. Cisco Public 43

44 Basic OSPF Configuration Modifying the Cost of a link Both sides of a serial link should be configured with the same bandwidth Commands used to modify bandwidth value Bandwidth command Example: Router(config-if)#bandwidth bandwidth-kbps ip ospf cost command allows you to directly specify interface cost -Example:R1(config)#interface serial 0/0/0 R1(config-if)#ip ospf cost 1562 ITE PC v4.0 Chapter Cisco Systems, Inc. All rights reserved. Cisco Public 44

45 OSPF Configuration router ospf <proc-num> (global config) Creates OSPF route database, starts OSPF routing process number <proc-num> that manages OSPF messages, maintains database, and copies routes from database into routing table. network <a.b.c.d> netmask <w.x.y.z> area <num> TDC 363 Causes OSPF to advertise Subnet ID <a.b.c.d> with wildcard (inverse subnet mask) <w.x.y.z> in OSPF area number <num> and to send/receive OSPF Hellos and LSUs on all interfaces that directly connect to this subnet. The subnet specified in the network command should be directly connected to this router. Two routers connected to same link will only become OSPF neighbors if both of them execute network statements for the common link subnet in their OSPF configurations. Greg Brewster, DePaul University 45

46 Configure Single-area OSPFv2 The network Command This activates OSPF (sends out HELLOs) on interfaces connected to subnets /24, /30 and /30. It also includes these 3 subnets in OSPF database and OSPF Update messages. Presentation_ID 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 46

47 Specific Network Statements Type a separate specific OSPF network statement for each directly-connected subnet on the router. Do not type a network statement for any remote subnet that is not directly connected. Do not combine multiple subnets into a single network statement. Example: /24 & /24 are directly attached: Correct:» network area 0» network area 0 Wrong: (this will work in some cases, not others - you lose points in any case)» network area 0 It will always be wrong to type: network area 0 Sometimes this works, sometimes it messes things up. But it is always sloppy and not acceptable. TDC 363 Greg Brewster, DePaul University 47

48 Basic OSPF Configuration TDC 363 enable configure terminal interface loopback0! For OSPF ID ip address no shutdown interface ethernet0/0 ip address no shutdown interface serial0/0 ip address no shutdown router ospf 1 network area 0 network area 0 exit exit Greg Brewster, DePaul University 48

49 OSPF Configuration - Optional router ospf <proc-num> passive-interface <interface> (global config) Disables sending route updates out the specified interface router-id A.B.C.D Sets OSPF router-id to A.B.C.D ospf log-adjacency-changes Shows a message on router console any time an OSPF neighbor goes up or down. TDC 363 Greg Brewster, DePaul University 49

50 Configure Single-Area OSPFv2 Passive Interface By default, OSPF messages are forwarded out all OSPF-enabled interfaces. However, these messages really only need to be sent out interfaces connecting to other OSPF-enabled routers. Sending out unneeded messages on a LAN affects the network in three ways: Inefficient Use of Bandwidth Inefficient Use of Resources Increased Security Risk OSPF messages are not sent out a Passive Interface, but its IP subnet is included in OSPF database and in Update messages sent out other interfaces. The Passive Interface feature helps limit the scope of routing update advertisements. Presentation_ID 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 50

51 Configure Single-area OSPFv2 Configuring Passive Interfaces Use the passive-interface router configuration mode command to prevent the transmission of routing messages through a router interface, but still allow that network to be advertised to other routers. Presentation_ID 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 51

52 OSPF Interface Configuration ip ospf cost <cost> (optional) Sets the cost for the link connected to this interface to <cost> ip ospf retransmit-interval <time> Sets the time between Link State Advertisement retransmissions to <time> seconds. ip ospf priority <num> Sets the OSPF priority of this interface to <num>. Highest priority interface on transient network will be designated router. ip ospf hello-interval <time> Sets the time between HELLO packets to <time> seconds. ip ospf dead-interval <time> If no HELLO packets seen for <time> seconds, then neighbor router is declared down. TDC 363 Greg Brewster, DePaul University 52

53 TDC 363 OSPF Interface Configuration enable configure terminal interface ethernet0/0 ip address ip ospf cost 10 ip ospf priority 5 no shutdown interface serial0/0 ip address no shutdown router ospf 1 network area 0 network area 0 exit exit Greg Brewster, DePaul University 53

54 OSPF Verification/Debug show ip protocols show ip ospf interface show ip ospf database show ip ospf router debug ip ospf? adj - OSPF adjacency events events - OSPF events flood - OSPF flooding lsa-generation - OSPF lsa generation packet - OSPF packets retransmission - OSPF retransmission events tree - OSPF database tree TDC Greg Brewster, DePaul University 54