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FTOS Configuration Guide for the C-Series - Force10 Networks

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Link-State Advertisements (LSAs)<br />

A Link-State Advertisement (LSA) communicates <strong>the</strong> router's local routing topology to all o<strong>the</strong>r local<br />

routers in <strong>the</strong> same area.<br />

The LSA types supported by Force 10 are defined as follows:<br />

• Type 1 - Router LSA<br />

• The router lists links to o<strong>the</strong>r routers or networks in <strong>the</strong> same area. Type 1 LSAs are flooded<br />

across <strong>the</strong>ir own area only. The Link-State ID of <strong>the</strong> Type 1 LSA is <strong>the</strong> originating router ID.<br />

• Type 2 - Network LSA<br />

• The Designated Router (DR) in an area lists which routers are joined toge<strong>the</strong>r within <strong>the</strong> area.<br />

Type 2 LSAs are flooded across <strong>the</strong>ir own area only. The Link-State ID of <strong>the</strong> Type 2 LSA is<br />

<strong>the</strong> IP interface address of <strong>the</strong> DR.<br />

• Type 3 - Summary LSA<br />

• An Area Border Router (ABR) takes in<strong>for</strong>mation it has learned on one of its attached areas and<br />

can summarize it be<strong>for</strong>e sending it out on o<strong>the</strong>r areas it is connected to. The Link-State ID of<br />

<strong>the</strong> Type 3 LSA is <strong>the</strong> destination network number.<br />

• Type 4 - Autonomous System Border Router Summary LSA<br />

• In some cases, Type 5 External LSAs are flooded to areas where <strong>the</strong> detailed next-hop<br />

in<strong>for</strong>mation may not be available. An Area Border Router will (ABR) flood <strong>the</strong> in<strong>for</strong>mation<br />

<strong>for</strong> <strong>the</strong> router (i.e. <strong>the</strong> Autonomous System Border Router (ASBR) where <strong>the</strong> Type 5<br />

advertisement originated. The Link-State ID <strong>for</strong> Type 4 LSAs is <strong>the</strong> router ID of <strong>the</strong> described<br />

ASBR.<br />

• Type 5 - External LSA<br />

• These LSAs contain in<strong>for</strong>mation imported into OSPF from o<strong>the</strong>r routing processes. They are<br />

flooded to all areas, except stub areas. The Link-State ID of <strong>the</strong> Type 5 LSA is <strong>the</strong> external<br />

network number.<br />

• Type 7<br />

• Routers in a Not-So-Stubby-Area (NSSA) do not receive external LSAs from Area Border<br />

Routers (ABRs), but are allowed to send external routing in<strong>for</strong>mation <strong>for</strong> redistribution. They<br />

use Type 7 LSAs to tell <strong>the</strong> ABRs about <strong>the</strong>se external routes, which <strong>the</strong> Area Border Router<br />

<strong>the</strong>n translates to Type 5 external LSAs and floods as normal to <strong>the</strong> rest of <strong>the</strong> OSPF network.<br />

• Type 9<br />

• This is a link-local "opaque" LSA (defined by RFC2370) in OSPFv2, and <strong>the</strong><br />

Intra-Area-Prefix LSA in OSPFv3.<br />

For all LSA types, <strong>the</strong>re are 20-byte LSA headers. One of <strong>the</strong> fields of <strong>the</strong> LSA header is <strong>the</strong> Link-State ID.<br />

Each router link is defined as one of four types: type 1, 2, 3, or 4. The LSA includes a link ID field that<br />

identifies, by <strong>the</strong> network number and mask, <strong>the</strong> object this link connects to.<br />

Depending on <strong>the</strong> type, <strong>the</strong> link ID has different meanings.<br />

• 1: point-to-point connection to ano<strong>the</strong>r router neighboring router<br />

• 2: connection to a transit network IP address of Designated Router<br />

534 Open Shortest Path First

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