306
C
HAPTER
34: OSPF C
ONFIGURATION
■
The DR on a network segment is not necessarily the router with the highest
priority. Likewise, the BDR is not necessarily the router with the second-highest
priority.
OSPF Packets
OSPF uses five types of packets:
Hello packet
Hello packets are most commonly used OSPF packets, which are periodically sent
by a router to its neighbors. A Hello packet contains the values of some timers, the
DR, the BDR and the known peers.
DD packet
When two routers synchronize their databases, they use database description (DD)
packets to describe their own LSDBs, including the digest of each LSA. The digest
refers to the HEAD of an LSA which uniquely identifies the LSA. This reduces the
size of traffic transmitted between the routers because the HEAD of an LSA only
occupies a small portion of the LSA. With the HEAD, the peer router can judge
whether it has the LSA or not.
LSR packet
After exchanging DD packets, the two routers know which LSAs of the peer router
are lacked in the local LSDB, and send link state request (LSR) packets requesting
for the lacked LSAs to the peer. These LSR packets contain the digest of the
needed LSAs.
LSU packet
Link state update (LSU) packets are used to transmit the needed LSAs to the peer
router. An LSU packet is a collection of multiple LSAs (complete LSAs, not LSA
digest).
LSAck packet
Link state acknowledgment (LSAck) packets are used to acknowledge received
LSU packets. An LSAck contains the HEAD(s) of LSA(s) to be acknowledged (one
LSAck packet can acknowledge multiple LSAs).
LSA Types
Five basic LSA types
As described in the preceding sections, LSAs are the primary source for OSPF to
calculate and maintain routes. RFC 2328 defines five types of LSAs:
■
Router-LSA: Type-1 LSAs, generated by every router to describe the router’s link
states and costs and advertised only in the area where the router resides.
■
Network-LSA: Type-2 LSAs, generated by the DRs of broadcast or NBMA
network to describe the link states of the current network segment and are
advertised only in the area where the DRs reside.
■
Summary-LSA: Type-3 and Type-4 LSAs, generated by ABRs and advertised in
the areas associated with the LSAs. Each Summary-LSA describes a route to a
destination in another area of the AS (also called inter-area route).Type-3
Summary-LSAs are for routes to networks (that is, their destinations are
segments), while Type-4 Summary-LSAs are for routes to ASBRs.
Summary of Contents for Switch 7757
Page 32: ...32 CHAPTER 1 CLI OVERVIEW...
Page 70: ...70 CHAPTER 5 LOGGING IN USING MODEM...
Page 76: ...76 CHAPTER 7 LOGGING IN THROUGH NMS...
Page 86: ...86 CHAPTER 9 CONFIGURATION FILE MANAGEMENT...
Page 120: ...120 CHAPTER 13 ISOLATE USER VLAN CONFIGURATION...
Page 126: ...126 CHAPTER 14 SUPER VLAN...
Page 136: ...136 CHAPTER 16 IP PERFORMANCE CONFIGURATION...
Page 152: ...152 CHAPTER 17 IPX CONFIGURATION...
Page 164: ...164 CHAPTER 19 QINQ CONFIGURATION...
Page 172: ...172 CHAPTER 21 SHARED VLAN CONFIGURATION...
Page 182: ...182 CHAPTER 22 PORT BASIC CONFIGURATION...
Page 198: ...198 CHAPTER 24 PORT ISOLATION CONFIGURATION...
Page 208: ...208 CHAPTER 25 PORT SECURITY CONFIGURATION...
Page 224: ...224 CHAPTER 27 DLDP CONFIGURATION...
Page 232: ...232 CHAPTER 28 MAC ADDRESS TABLE MANAGEMENT...
Page 240: ...240 CHAPTER 29 CENTRALIZED MAC ADDRESS AUTHENTICATION CONFIGURATION...
Page 280: ...280 CHAPTER 30 MSTP CONFIGURATION...
Page 348: ...348 CHAPTER 35 IS IS CONFIGURATION...
Page 408: ...408 CHAPTER 39 802 1X CONFIGURATION...
Page 412: ...412 CHAPTER 40 HABP CONFIGURATION...
Page 422: ...422 CHAPTER 41 MULTICAST OVERVIEW...
Page 426: ...426 CHAPTER 42 GMRP CONFIGURATION...
Page 480: ...480 CHAPTER 47 PIM CONFIGURATION...
Page 506: ...506 CHAPTER 48 MSDP CONFIGURATION...
Page 552: ...552 CHAPTER 51 TRAFFIC ACCOUNTING CONFIGURATION...
Page 570: ...570 CHAPTER 53 HA CONFIGURATION...
Page 582: ...582 CHAPTER 54 ARP CONFIGURATION SwitchA arp protective down recover interval 200...
Page 622: ...622 CHAPTER 58 DHCP RELAY AGENT CONFIGURATION...
Page 684: ...684 CHAPTER 61 QOS CONFIGURATION...
Page 718: ...718 CHAPTER 63 CLUSTER...
Page 738: ...738 CHAPTER 67 UDP HELPER CONFIGURATION...
Page 752: ...752 CHAPTER 69 RMON CONFIGURATION...
Page 772: ...772 CHAPTER 70 NTP CONFIGURATION...
Page 796: ...796 CHAPTER 72 FILE SYSTEM MANAGEMENT...
Page 802: ...802 CHAPTER 73 BIMS CONFIGURATION...
Page 814: ...814 CHAPTER 74 FTP AND TFTP CONFIGURATION...
Page 830: ...830 CHAPTER 75 INFORMATION CENTER...
Page 836: ...836 CHAPTER 76 DNS CONFIGURATION...
Page 852: ...852 CHAPTER 77 BOOTROM AND HOST SOFTWARE LOADING...
Page 858: ...858 CHAPTER 78 BASIC SYSTEM CONFIGURATION DEBUGGING...