This document explains the behaviour of ISIS protocol where if MAX metric value (16777215) is configured on a router, the route will become unreachable towards the immediate neighbour on that link.
In Junos OS, IS-IS metrics are configurable and can be adjusted to influence path selection. By default, IS-IS metrics are set to 10 for all links. However, Junos allows for wider metric ranges, up to 16,777,215, through the use of wide metrics.
This wider range is particularly useful for traffic engineering and when route leaking is employed.
In case of wide metrics, value 16,777,215 represents the MAX LINK metric, which is also treated as unreachable metric towards the immediate neighbour on that link. So, if we configure anything < 16,777,215 (i.e <= 16,777,214) route will still appear in the routing table but not with 16,777,215.
This is expected behaviour and RFC5305 can be referred for the same, which states "If a link is advertised with the maximum link metric (2^24 - 1), this link MUST NOT be considered during the normal SPF computation. This will allow advertisement of a link for purposes other than building the normal Shortest Path Tree."
Example:
mx-re0# run show route 192.18.0.5
inet.0: 10 destinations, 10 routes (10 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both
192.18.0.0/24 *[IS-IS/18] 00:00:06, metric 33554429
> to 192.18.1.2 via et-0/0/0.0
[edit]
mx-re0# set protocols isis interface et-0/0/0.0 level 2 metric 16777215
mx-re0# commit
[edit protocols]
commit complete
inet.0: 8 destinations, 8 routes (8 active, 0 holddown, 0 hidden)
0.0.0.0/0 *[Static/5] 1d 04:59:52
> to 198.19.1.1 via fxp0.0
mx-re0# set protocols isis interface et-0/0/0.0 level 2 metric 16777214
192.18.0.0/24 *[IS-IS/18] 00:00:02, metric 33554429