3com S7906E Manuel De Montage

 

1-8 

OSPF prioritize routes into four levels: 

z 

Intra-area route 

z 

Inter-area route 

z 

Type-1 external route 

z 

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. 

OSPF classifies networks into four types upon the link layer protocol: 

z 

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. 

z 

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. 

z 

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). 

z 

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). 

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.