Introduction to OSPF
281
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NBMA (Non-Broadcast Multi-Access): When the link layer protocol is Frame
Relay, ATM or X.25, OSPF considers the network type as NBMA by default.
Packets on these networks are sent to unicast addresses.
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P2MP (point-to-multipoint): By default, OSPF considers no link layer protocol as
P2MP, which is a conversion from other network types such as NBMA in
general. On P2MP networks, packets are sent to multicast addresses
(224.0.0.5).
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P2P (point-to-point): When the link layer protocol is PPP or HDLC, OSPF
considers the network type as P2P. On P2P networks, packets are sent to
multicast addresses (224.0.0.5).
NBMA network configuration principle
Typical NBMA networks are ATM and Frame Relay networks.
You need to perform some special configuration on NBMA interfaces. Since these
interfaces cannot broadcast hello packets for neighbor location, you need to
specify neighbors manually and configure whether the neighbors have the DR
election right.
An NBMA network is fully meshed, which means any two routers in the NBMA
network have a direct virtual link for communication. If direct connections are not
available between some routers, the type of interfaces associated should be
configured as P2MP, or as P2P for interfaces with only one neighbor.
Differences between NBMA and P2MP networks:
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NBMA networks are fully meshed, non-broadcast and multi access. P2MP
networks are not required to be fully meshed.
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It is required to elect the DR and BDR on NBMA networks, while DR and BDR
are not available on P2MP networks.
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NBMA is the default network type, while P2MP is a conversion from other
network types, such as NBMA in general.
■
On NBMA networks, packets are unicast, and neighbors are configured
manually on routers. On P2MP networks, packets are multicast.
DR and BDR
DR/BDR introduction
On broadcast or NBMA networks, any two routers exchange routing information
with each other. If n routers are present on a network, n(n-1)/2 adjacencies are
required. Any change on a router in the network generates traffic for routing
information synchronization, consuming network resources. The Designated
Router is defined to solve the problem. All other routers on the network send
routing information to the DR, which is responsible for advertising link state
information.
If the DR fails to work, routers on the network have to elect another DR and
synchronize information with the new DR. It is time-consuming and prone to
routing calculation errors. The Backup Designated Router (BDR) is introduced to
reduce the synchronization period.
The BDR is elected along with the DR and establishes adjacencies for routing
information exchange with all other routers. When the DR fails, the BDR will
Summary of Contents for 4800G Series
Page 26: ...26 CHAPTER NETWORKING APPLICATIONS ...
Page 30: ...30 CHAPTER 1 LOGGING IN TO AN ETHERNET SWITCH ...
Page 62: ...62 CHAPTER 3 LOGGING IN THROUGH TELNET ...
Page 70: ...70 CHAPTER 5 LOGGING IN THROUGH WEB BASED NETWORK MANAGEMENT SYSTEM ...
Page 72: ...72 CHAPTER 6 LOGGING IN THROUGH NMS ...
Page 82: ...82 CHAPTER 8 CONTROLLING LOGIN USERS ...
Page 98: ...98 CHAPTER 9 VLAN CONFIGURATION ...
Page 108: ...108 CHAPTER 10 VOICE VLAN CONFIGURATION ...
Page 119: ...GVRP Configuration Examples 119 DeviceB display vlan dynamic No dynamic vlans exist ...
Page 120: ...120 CHAPTER 11 GVRP CONFIGURATION ...
Page 160: ...160 CHAPTER 17 PORT ISOLATION CONFIGURATION ...
Page 172: ...172 CHAPTER 19 LINK AGGREGATION CONFIGURATION ...
Page 196: ...196 CHAPTER 22 DLDP CONFIGURATION ...
Page 240: ...240 CHAPTER 23 MSTP CONFIGURATION ...
Page 272: ...272 CHAPTER 27 RIP CONFIGURATION ...
Page 364: ...364 CHAPTER 29 IS IS CONFIGURATION ...
Page 426: ...426 CHAPTER 31 ROUTING POLICY CONFIGURATION ...
Page 442: ...442 CHAPTER 33 IPV6 RIPNG CONFIGURATION ...
Page 466: ...466 CHAPTER 35 IPV6 IS IS CONFIGURATION ...
Page 488: ...488 CHAPTER 36 IPV6 BGP CONFIGURATION ...
Page 498: ...498 CHAPTER 37 ROUTING POLICY CONFIGURATION ...
Page 540: ...540 CHAPTER 40 TUNNELING CONFIGURATION ...
Page 552: ...552 CHAPTER 41 MULTICAST OVERVIEW ...
Page 604: ...604 CHAPTER 43 MLD SNOOPING CONFIGURATION ...
Page 628: ...628 CHAPTER 46 IGMP CONFIGURATION ...
Page 700: ...700 CHAPTER 48 MSDP CONFIGURATION ...
Page 812: ...812 CHAPTER 57 DHCP SERVER CONFIGURATION ...
Page 822: ...822 CHAPTER 58 DHCP RELAY AGENT CONFIGURATION ...
Page 834: ...834 CHAPTER 61 BOOTP CLIENT CONFIGURATION ...
Page 850: ...850 CHAPTER 63 IPV4 ACL CONFIGURATION ...
Page 856: ...856 CHAPTER 64 IPV6 ACL CONFIGURATION ...
Page 860: ...860 CHAPTER 65 QOS OVERVIEW ...
Page 868: ...868 CHAPTER 66 TRAFFIC CLASSIFICATION TP AND LR CONFIGURATION ...
Page 888: ...888 CHAPTER 69 PRIORITY MAPPING ...
Page 894: ...894 CHAPTER 71 TRAFFIC MIRRORING CONFIGURATION ...
Page 904: ...904 CHAPTER 72 PORT MIRRORING CONFIGURATION ...
Page 930: ...930 CHAPTER 74 UDP HELPER CONFIGURATION ...
Page 990: ...990 CHAPTER 79 FILE SYSTEM MANAGEMENT CONFIGURATION ...
Page 1000: ...1000 CHAPTER 80 FTP CONFIGURATION ...
Page 1020: ...1020 CHAPTER 82 INFORMATION CENTER CONFIGURATION ...
Page 1038: ...1038 CHAPTER 84 SYSTEM MAINTAINING AND DEBUGGING ...
Page 1046: ...1046 CHAPTER 85 DEVICE MANAGEMENT ...
Page 1129: ...SSH Client Configuration Examples 1129 SwitchB ...
Page 1130: ...1130 CHAPTER 88 SSH CONFIGURATION ...
Page 1160: ...1160 CHAPTER 90 RRPP CONFIGURATION ...
Page 1180: ...1180 CHAPTER 91 PORT SECURITY CONFIGURATION ...
Page 1192: ...1192 CHAPTER 92 LLDP CONFIGURATION ...
Page 1202: ...1202 CHAPTER 93 POE CONFIGURATION ...
Page 1218: ...1218 CHAPTER 96 HTTPS CONFIGURATION ...