Disregard OSPF path

[Chriztoffer Hansen]

Robert,

Robert Sander wrote:
> On 17.02.20 09:03, Kees Meijs wrote:
>> The path I'm trying to avoid has the lowest OSPF cost but in the
>> physical world the most... :-)
> I always thought the OSPF cost represents the "physical" cost of a link.

Not necessarily.

From RFC 2328, p. 18 [0]:
> A cost is associated with the output side of each router
> interface.  This cost is configurable by the system
> administrator.  The lower the cost, the more likely the
> interface is to be used to forward data traffic.  Costs are
> also associated with the externally derived routing data
> (e.g., the BGP-learned routes).

From RFC 2328, p. 66 [0]:
> Interface output cost(s)
> The cost of sending a data packet on the interface, expressed in
> the link state metric.  This is advertised as the link cost for
> this interface in the router-LSA. The cost of an interface must
> be greater than zero.

In essence. The interface metric used by OSPF is just an arbitrarily
decided value. With every Network Operating System having some built-in
values used [3]. But as well, often available to be re-configured by the
Systems/Network Administrator if the Network design of choice requires so.

Footnotes:
------------------------------------------------------------
[0]: https://www.rfc-archive.org/getrfc?rfc=2328
[1]: https://packetlife.net/media/library/10/OSPF.pdf
[2]:
https://packetlife.net/media/library/40/IOS_Interior_Routing_Protocols.pdf
[3]:
https://www.omnisecu.com/cisco-certified-network-associate-ccna/what-is-ospf-metric-value-cost-and-ospf-default-cost-reference-bandwidth.php
[4]: http://docs.frrouting.org/en/latest/ospfd.html#configuring-ospf
[5]: http://docs.frrouting.org/en/latest/ospf6d.html#ospf6-router
[6]:
https://unix.stackexchange.com/questions/381905/ospf-route-costs-in-bird
[7]: https://gitlab.labs.nic.cz/labs/bird/wikis/OSPF_example

-- 

Best regards,

Chriztoffer

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 228 bytes
Desc: OpenPGP digital signature
URL: <http://trubka.network.cz/pipermail/bird-users/attachments/20200217/aef2340c/attachment.sig>