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FTOS Configuration Guide for the C-Series - Force10 Networks

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Area Border Router (ABR)<br />

Within an AS, an Area Border (ABR) connects one or more areas to <strong>the</strong> Backbone. The ABR keeps a copy<br />

of <strong>the</strong> link-state database <strong>for</strong> every area it connects to, so it may keep multiple copies of <strong>the</strong> link state<br />

database. 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.<br />

An ABR can connect to many areas in an AS, and is considered a member of each area it connects to.<br />

Autonomous System Border Router (ASBR)<br />

The Autonomous System Border Area Router (ASBR) connects to more than one AS, and exchanges<br />

in<strong>for</strong>mation with <strong>the</strong> routers in o<strong>the</strong>r ASs. Generally <strong>the</strong> ASBR connects to a non-Interior Gate Protocol<br />

(IGP) such as BGP or uses static routes.<br />

Internal Router (IR)<br />

The Internal Router (IR) has adjacencies with ONLY routers in <strong>the</strong> same area, as Router E, M and I are<br />

shown in Figure 376.<br />

Designated and Backup Designated Routers<br />

OSPF elects a Designated Router and a Backup Designated router. Among o<strong>the</strong>r things, <strong>the</strong><br />

designated router is responsible <strong>for</strong> generating LSAs <strong>for</strong> <strong>the</strong> entire multiaccess network.<br />

Designated routers allow a reduction in network traffic and in <strong>the</strong> size of <strong>the</strong> topological database.<br />

• The Designated Router (DR) maintains a complete topology table of <strong>the</strong> network and sends <strong>the</strong><br />

updates to <strong>the</strong> o<strong>the</strong>r routers via multicast. All routers in an area <strong>for</strong>m a slave/master relationship with<br />

<strong>the</strong> DR. Every time a router sends an update, it sends it to <strong>the</strong> Designated Router (DR) and Backup<br />

Designated Router (BDR). The DR sends <strong>the</strong> update out to all o<strong>the</strong>r routers in <strong>the</strong> area.<br />

• The Backup Designated Router (BDR) is <strong>the</strong> router that takes over if <strong>the</strong> DR fails.<br />

EaCH router exchanges in<strong>for</strong>mation with <strong>the</strong> DR and BDR. The DR and BDR relay <strong>the</strong> in<strong>for</strong>mation to <strong>the</strong><br />

o<strong>the</strong>r routers. On broadcast network segments <strong>the</strong> number of OSPF packets is fur<strong>the</strong>r reduced by <strong>the</strong> DR<br />

and BDR sending such OSPF updates to a multicast IP address that all OSPF routers on <strong>the</strong> network<br />

segment are listening on.<br />

These router designations are not <strong>the</strong> same ad <strong>the</strong> router IDs discussed earlier. The Designated and Backup<br />

Designated Routers are configurable in <strong>FTOS</strong>. If no DR or BDR is defined in <strong>FTOS</strong>, <strong>the</strong> system assigns<br />

<strong>the</strong>m. OSPF looks at <strong>the</strong> priority of <strong>the</strong> routers on <strong>the</strong> segment to determine which routers are <strong>the</strong> DR and<br />

BDR. The router with <strong>the</strong> highest priority is elected <strong>the</strong> DR. If <strong>the</strong>re is a tie, <strong>the</strong>n <strong>the</strong> router with <strong>the</strong> higher<br />

Router ID takes precedence. After <strong>the</strong> DR is elected, <strong>the</strong> BDR is elected <strong>the</strong> same way. A router with a<br />

router priority set to zero is cannot become <strong>the</strong> DR or BDR.<br />

<strong>FTOS</strong> <strong>Configuration</strong> <strong>Guide</strong>, version 7.7.1.0 533

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