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Configuring OSPF Services

4-13

Deﬁning a Range for a Summary Route

Border routers generate summary advertisements for their attached areas. Each

summary advertisement speciﬁes a range of destinations in an area. An area range

speciﬁcation consists of a network address and a variable-length mask. For

example, a summary advertisement for the destination 140.191.0.0 with a mask of

255.255.0.0 describes a single route to the collection of destinations 140.191.0.0

to 140.191.255.255. When a packet is forwarded, it is always forwarded to the

network that is the best (longest or most speciﬁc) match for the packet’s

destination.

Conﬁguring Border Services for a Stub

A stub area does not import ASEs and may or may not import internal route

summaries.

In place of routes to destinations outside the stub, a border router connected to a

stub injects a default route advertisement. When an internal router encounters a

datagram addressed to a destination outside the stub, the router forwards it to the

border router speciﬁed in the default route advertisement.

Assume, for example, that the stub area in Figure 4-2 has been conﬁgured to

import no internal or external routing information. BR8 receives ASEs and

internal summaries from its interface to the backbone. However, BR8 does not

forward the ASEs or summaries to the stub. Instead, R8 injects a default route that

internal routers use to forward datagrams to destinations beyond the stub.

For instructions on using Site Manager to deﬁne a range for a summary route, conﬁgure a

cost for the summary, and suppress any advertisement for the range, see “Adding a

Range to an Area” on page 4-49.

Site Manager allows you to specify whether an area is a stub and, if so, whether it imports

internal routing summaries. Site Manager also lets you supply a metric for the default

route that the border router injects into the stub. For instructions, see “Editing OSPF Area

Parameters” on page 4-44.

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Avaya Configuring IP Services Manual do Utilizador Página 151