3Com S7902E, Configuration Manual

Page: 641 / 2621

1-8
Route types
OSPF prioritize routes into four levels:
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Intra-area route
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Inter-area route
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Type-1 external route
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Type-2 external route
The intra-area and inter-area routes describe the network topology of the AS, while external routes
describe routes to destinations outside the AS.
OSPF classifies external routes into two types: Type-1 and Type-2. A Type-1 external route is an IGP
route, such as a RIP or static route, which has high credibility and whose cost is comparable with the
cost of an OSPF internal route. The cost from a router to the destination of the Type-1 external route=
the cost from the router to the corresponding ASBR+ the cost from the ASBR to the destination of the
external route.
A Type-2 external route is an EGP route, which has low credibility, so OSPF considers the cost from the
ASBR to the destination of the Type-2 external route is much greater than the cost from the ASBR to an
OSPF internal router. Therefore, the cost from the internal router to the destination of the Type-2
external route= the cost from the ASBR to the destination of the Type-2 external route. If two routes to
the same destination have the same cost, then take the cost from the router to the ASBR into
consideration.
Classification of OSPF Networks
OSPF network types
OSPF classifies networks into four types upon the link layer protocol:
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Broadcast: When the link layer protocol is Ethernet or FDDI, OSPF considers the network type
broadcast by default. On Broadcast networks, hello packets, LSU packets, and LSAck packets are
generally sent to multicast addresses 224.0.0.5 (reserved for OSPF routers) and 224.0.0.6
(reserved for OSPF DRs), while DD packets and LSR packets are unicast.
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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.