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| 1.2.-
External Routes |
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| OSPF management of external routes
(external to the AS) is some like different. Once the tree is created
the external routing information is examined. External route may originate
from exterior routing protocols like BGP, or be statically
configured. Nevertheless, external routing information is flooded unlatered
throughout the AS. |
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OSPF supports two types of external metric. Type 1 metrics are
expressed in the same unit as OSPF metric. Type 2 metrics are
one order of magnitude larger. Differences between them is better explained
with an example: |
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| Routers RT9 and RT10 are both
advertising to the AS, an external route to network N20. For
calculating cost to access the network N20 from router RT1,
for example,
following operation should be done: |
| Throughout router RT9: because
this router is advertising an external Type 1 route, we have to add
interior costs from RT1 to RT9 to the external cost advertised
by the RT9 router; then cost to network N20 throughout router
RT9 will be = 13 + 4 = 17. |
| Throughout router RT10: because
this router is advertising an external Type 2 route, interior costs
should not be considered; then cost to network N20 throughout router
RT10 will be = 12. |
| Given this example, the preferred route for
router RT1 to go to network N20 will be throughout router
RT10. |
| Also: when several equal-cost Type 2
router exist, the internal distance to the advertising router is used
to break the tie. |
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When dealing with the advertisement of external
routes, some very useful and common utilized concept is what is called
forwarding address. When an AS administrator recognizes
that there exist a better route to an external network than the router
itself that is advertising this, he/she can setup this router using a
forwarding address. This way, the AS Boundary Router (ABR)
advertises the external route indicating also the forwarding address
to be used to reach that route. |
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