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Giao thức định tuyến - Chapter 11

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In this chapter, you will learn basic, single-area OSPF implementations and configurations. More complex OSPF configurations and concepts (multi-areas OSPF) are reserved for CCNP-level courses.

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  1. OSPF (Single Area OSPF) Routing Protocols and Concepts – Chapter 11 ITE PC v4.0 1 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
  2. Introduction •In this chapter, you will learn basic, single-area OSPF implementations and configurations and configurations. •More complex OSPF configurations and concepts (multi-areas OSPF) are reserved for CCNP-level courses. ITE PC v4.0 2 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
  3. Introduction to OSPF Introduction to OSPF Background of OSPF Began in 1987 1989 OSPFv1 released in RFC 1131 This version was experimental & never deployed 1991 OSPFv2 released in RFC 1247 1998 OSPFv2 updated in RFC 2328 1999 OSPFv3 published in RFC 2740 ITE PC v4.0 3 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
  4. Introduction to OSPF Introduction to OSPF OSPF Message Encapsulation OSPF packet type OSPF packet type – There exist 5 types (next slide) OSPF packet header –Contains - Router ID an area ID and Type code for OSPF packet type IP packet header – Contains - Source IP address, Destination IP address, & Protocol es add field set to 89. the destination address is set to one of two multicast addresses: 224.0.0.5 or 224 224.0.0.6. Data Link Frame Header –Contains - destination MAC address is also a multicast address: 01-00-5E-00- 00-05 or 01-00-5E-00-00-06. ITE PC v4.0 4 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
  5. Introduction to OSPF OSPF 5 OSPF Packet Types: 1. Hello - Hello packets are used to establish and maintain adjacency with other OSPF routers. 2. DBD - The Database Description (DBD) packet contains an abbre contains an abbreviated list of the sending router's list of the sending ro link-state database and is used by receiving routers to check against the local link-state database. 3. LSR - Receiving routers can then request more information about any entry in the DBD by sending a Link-State Request (LSR). 4. LSU - Link-State Update (LSU) packets are used to reply to LSRs as well as to announce new information. –LSUs contain 7 different types of Link-State Advertisements (LSAs). –LSUs and LSAs are discussed in a later topic. 5. LSAck - When an LSU is received, the router LSA LSU th sends a Link-State Acknowledgement (LSAck) to confirm receipt of the LSU. ITE PC v4.0 5 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
  6. OSPF: Hello Protocol OSPF: Hello Protocol Purpose of Hello Packet Discover OSPF neighbors & establish adjacencies OSPF neighbors establish adjacencies Advertise parameters on which routers must agree to become neighbors Used by multi-access networks to elect a Designated Router and a Backup Designated Router Type: OSPF Packet Type: Hello (1), DD (2), LS Request (3), LS Update (4), LS ACK (5) Router ID: ID of the originating router Area ID: area from which the packet originated Network Mask: Subnet mask associated with the sending interface sending interface Hello Interval: number of seconds between the sending router's hellos Router Priority: Used in DR/BDR election (discussed later) Designated Router (DR): Router ID of the DR, if any Backup Designated Router (BDR): Router ID of the BDR, if any List of Neighbors: lists the OSPF Router ID of the neighboring router(s) ITE PC v4.0 6 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
  7. OSPF: Hello Protocol OSPF: Hello Protocol Also need to have the same Area ID. Establish adjacencies: – They must agree on three values: Hello Why 10 second hello interval Why 10 second hello interval interval, Dead interval, and network type. communications consider better than OSPF Hello Intervals the 30 second routing update for RIP? –Hello interval indicates how often an OSPF router transmits its Hello packets router transmits its Hello packets –Usually multicast (224.0.0.5) for ALLSPFRouters –sent every 10 seconds on multiaccess and point-to-point segments –Sent every 30 seconds for NBMA segments OSPF Dead Intervals –This is the time that must transpire before the neighbor is considered down –Default time is 4 times the hello interval –For multiaccess and point-to-point segments, multiaccess and point segments this period is 40 seconds. –For NBMA networks, the Dead interval is 120 seconds. –If the Dead interval expires before the routers If the Dead interval expires before the routers receive a Hello packet, OSPF will remove that neighbor from its link-state database. ITE PC v4.0 7 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
  8. OSPF: Hello Protocol To reduce the amount of OSPF traffic on To reduce the amount of OSPF traffic on multiaccess networks, OSPF elects a Designated Router (DR) and Backup Designated Router (BDR). Hello protocol packets contain information that is used in electing DR and BDR –The DR is responsible for updating all other DR is responsible for updating all other OSPF routers (called DROthers) when a change occurs in the multiaccess network. –The BDR monitors the DR and takes over as BDR monitors the DR and takes over as DR if the current DR fails. In the figure, R1, R2, and R3 are connected through point through point-to-point links. Therefore, no links Therefore no More detail discussion detail discussion DR/BDR election occurs. on the DR, BDR, DROther later. You –The DR/BDR election and processes will be discussed in later topic and the topology will discussed in a later topic and the topology will need to know this for need to know this for be changed to a multiaccess network. CCNA exam. ITE PC v4.0 8 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
  9. Introduction to OSPF Introduction to OSPF OSPF Link-state Updates Purpose of Link State Update (LSU) Purpose of a Link State Update (LSU) –Used to deliver link state advertisements Purpose of a Link State Advertisement (LSA) –Contains information about neighbors & path costs –An LSU packet can contain 11 different types of LSAs, ITE PC v4.0 9 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
  10. Introduction to OSPF Introduction to OSPF OSPF Algorithm OSPF routers build & maintain link-state database containing LSA LSA received from other routers routers 1. Information found in database is utilized upon execution of Dijkstra SPF of Dijkstra SPF algorithm 2. SPF algorithm used to create SPF tree 3. SPF tree used to populate routing table routing table ITE PC v4.0 10 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
  11. Introduction to OSPF Introduction to OSPF Administrative Distance Distance Default Administrative Distance for OSPF is 110 ITE PC v4.0 11 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
  12. Introduction to OSPF Introduction to OSPF OSPF Authentication –It is good practice to authenticate transmitted is good practice to authenticate transmitted routing information. –This is an interface specific configuration –This practice ensures that routers will only accept routing information from other routers that have been configured with the same password or authentication configured with the same password or authentication information MD5 authentication uses a key ID that ID th allows the router to Note: Authentication reference multiple does not encrypt the passwords, making router routing table router's routing table. password migration easier and more ? secure. ITE PC v4.0 12 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
  13. Basic OSPF Configuration Lab Topology Topology Topology used for this chapter –Discontiguous IP addressing scheme –Since OSPF is a classless routing protocol the subnet mask is will be configured as part of our is will be configured as part of our OSPF configuration. ITE PC v4.0 13 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
  14. Basic OSPF Configuration Basic OSPF Configuration The router ospf command router ospf command To enable OSPF on a router use the following command R1(config)#router ospf process-id ID cannot be 0 Process id A locally significant number between 1 and 65535 ITE PC v4.0 14 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
  15. Basic OSPF Configuration Basic OSPF Configuration OSPF network command –Requires entering: •network address •wildcard mask - the inverse of the subnet mask •area-id - area-id refers to the OSPF area. OSPF area refers to the OSPF area OSPF area is a group of routers that share link state information Router(config-router)#network network-address wildcard-ask area area-id wildcard area 255.255.255.255 Subtract the subnet mask - 255.255.255.240 -------------------- Wildcard mask 0. 0. 0. 15 15 255.255.255.255 Subtract the subnet mask - 255.255.255.252 -------------------- Wildcard mask 0. 0. 0. 03 ITE PC v4.0 15 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
  16. Basic OSPF Configuration Basic OSPF Configuration Cisco IOS now properly handles overlapping network ... area configuration commands. Consider the following example: fw#conf t Enter configuration commands, one per line. End with CNTL/Z. fw(config)#router ospf 100 fw(config-router)#network 0.0.0.0 255.255.255.255 area 0 255 area fw(config-router)#network 10.0.0.0 0.0.3.255 area 1 13:06:57: %OSPF-6-AREACHG: 10.0.0.0 255.255.252.0 changed from area 0 to area 1 fw(config-router)#network 10.0.0.0 0.0.0.7 area 2 13:07:10: %OSPF-6-AREACHG: 10.0.0.0 255.255.255.248 changed from area 1 to area 2 fw(config-router)#^Z I've entered overlapping network statements, each one with a smaller address range. Not only does IOS detect that they overlap, it also prints nice syslog messages and reorders IOS th th it the commands in the running configuration. Well done ! fw#show run | begin router ospf router ospf 100 log-adjacency-changes http://blog.ioshints.info/2006/11/ network 10.0.0.0 0.0.0.7 area 2 network-statements-in-ospf- network 10.0.0.0 0.0.3.255 area 1 process-are.html network 0.0.0.0 255.255.255.255 area 0 255 area ITE PC v4.0 16 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
  17. Basic OSPF Configuration Basic OSPF Configuration ospf network definition for adding all interfaces / default route What’s the difference? router ospf 1 network 0.0.0.0 0.0.0.0 area 0 vs. router ospf 1 network 0.0.0.0 255.255.255.255 area 0 Both add all existing interfaces into area 0 and all later added interfaces also. Both statements are valid. http://blog.sazza.de/?p=427 ITE PC v4.0 17 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
  18. Basic OSPF Configuration OSPF Area area-id area An OSPF area is a group of routers that share link-state information. In this chapter, we will configure all of the OSPF routers within a single area. This is known as single-area OSPF. Multi-area OSPF is covered in CCNP. ITE PC v4.0 18 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
  19. Basic OSPF Configuration OSPF Router ID – This is an IP address used to identify a router – 3 criteria for deriving the router ID 1. Use IP address configured with OSPF router-id command IP OSPF -Takes precedence over loopback and physical interface addresses 2. If router-id command not used then router chooses highest IP address of any loopback interfaces 3. If no loopback interfaces are configured then the highest IP no loopback interfaces are configured then the highest IP address on any active physical interface is used The interface does not need to be enabled for OSPF, meaning that it does not need to be included in one of the meaning that it does not need to be included in one of the OSPF network commands. However, the interface must be active - it must be in the up state up state. However!!!!!! ITE PC v4.0 19 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
  20. Basic OSPF Configuration OSPF Router ID ID – If you are the king when the th ki th kingdom is built, you are the kingdom is built, you are the KING for life – It means when ID is elected, it is the ID for the router, unless th ID th ………….. ITE PC v4.0 20 Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
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