— Exhibit –
— Exhibit —
Click the Exhibit button.
As shown in the exhibit, the 10.10/16 prefix is redistributed into OSPF through R2 and R5. R2 is
advertising the prefix with a Type 1 metric of 100 and R5 is advertising the prefix with a Type 2
metric of 10.
What is the preferred path to reach 10.10/16 from R6?
A.
R6-R5
B.
R6-R4-R5
C.
R6-R4-R5-R2
D.
R6-R4-R3-R2
http://kb.juniper.net/InfoCenter/index?page=content&id=KB2258&actp=search&viewlocale=en_US&searchid=1356874440836&smlogin=true
+
http://www.ietf.org/rfc/rfc2328.txt
SUMMARY:
OSPF External Metrics – Type 1 externals are always preferred over Type 2, regardless of metric
PROBLEM OR GOAL:
Why are OSPF External Type 1 routes always preferred over External Type 2 routes, even when the External Type 2 metric is less than or equal to the External Type 1 metric?
SOLUTION:
Per RFC 2328, section 2.3:
Both Type 1 and Type 2 external metrics can be present in the AS
at the same time. In that event, Type 1 external metrics always
take precedence.
External Type 1 metrics are similar to internal OSPF metrics in that the cost to reach an External Type 1 route is the sum of the link costs traversed to reach the router advertising the prefix, plus a user-defined initial cost (if configured).
External Type 2 metrics are considered to be much larger than the cost of routing across the OSPF AS; they do not accumulate the cost of the links they traverse. As a result, External Type 1 metrics can be considered more representative of the cost of routing the packet and are therefore preferred over External Type 2 metrics.
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