— Exhibit —
[edit]
user@router# run show ospf database external lsa-id 71.23.48.0 extensive
OSPF AS SCOPE link state database
Type ID Adv Rtr Seq Age Opt Cksum Len
Extern 71.23.48.0 67.176.255.5 0x80000001 114 0x22 0x171b 36
mask 255.255.248.0
Topology default (ID 0)
TypE. 2, MetriC. 0, Fwd addr: 0.0.0.0, TaG. 0.0.0.0
Aging timer 00:58:06
Installed 00:01:53 ago, expires in 00:58:06, sent 00:01:53 ago
Last changed 00:01:53 ago, Change count: 1
Extern 71.23.48.0 67.176.255.7 0x8000005a 487 0x22 0x587e 36
mask 255.255.248.0
Topology default (ID 0)
TypE. 2, MetriC. 0, Fwd addr: 0.0.0.0, TaG. 0.0.0.0
Aging timer 00:51:52
Installed 00:08:01 ago, expires in 00:51:53, sent 00:07:59 ago
Last changed 2d 19:33:58 ago, Change count: 1
Extern 71.23.48.0 67.176.255.8 0x8000005c 540 0x22 0xf73e 36
mask 255.255.248.0
Topology default (ID 0)
TypE. 1, MetriC. 30, Fwd addr: 0.0.0.0, TaG. 0.0.0.0
Aging timer 00:51:00
Installed 00:08:59 ago, expires in 00:51:00, sent 00:08:59 ago
Last changed 00:08:59 ago, Change count: 3
— Exhibit —
Click the Exhibit button.
As shown in the exhibit, a router is receiving three external LSAs for the prefix 71.23.48.0.
Which path is preferred?
A.
The path through 67.176.255.5 is preferred.
B.
The path through 67.176.255.7 is preferred.
C.
The path through 67.176.255.8 is preferred.
D.
The paths through 67.176.255.7 and 67.176.255.8 become active to allow load-balancing.
Type 1 external metrics always take precedence
Extern 71.23.48.0 67.176.255.8 0x8000005c 540 0x22 0xf73e 36
mask 255.255.248.0
Topology default (ID 0)
TypE. 1, MetriC. 30, Fwd addr: 0.0.0.0, TaG. 0.0.0.0
Aging timer 00:51:00
========================================================================
http://www.ietf.org/rfc/rfc2328.txt
OSPF supports two types of external metrics. Type 1 external
metrics are expressed in the same units as OSPF interface cost
(i.e., in terms of the link state metric). Type 2 external
metrics are an order of magnitude larger; any Type 2 metric is
considered greater than the cost of any path internal to the AS.
Use of Type 2 external metrics assumes that routing between
AS’es is the major cost of routing a packet, and eliminates the
need for conversion of external costs to internal link state
metrics.
========================================================================
OSPF External Metrics Overview
When OSPF exports route information from external autonomous systems (ASs), it includes a cost, or external metric, in the route. OSPF supports two types of external metrics: Type 1 and Type 2. The difference between the two metrics is how OSPF calculates the cost of the route.
Type 1 external metrics are equivalent to the link-state metric, where the cost is equal to the sum of the internal costs plus the external cost. This means that Type 1 external metrics include the external cost to the destination as well as the cost (metric) to reach the AS boundary router.
Type 2 external metrics are greater than the cost of any path internal to the AS. Type 2 external metrics use only the external cost to the destination and ignore the cost (metric) to reach the AS boundary router.
By default, OSPF uses the Type 2 external metric.
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 takes the precedence.
Type 1 external paths are always preferred over Type 2 external paths. When all paths are Type 2 external paths, the paths with the smallest advertised Type 2 metric are always preferred.