What is the default IS-IS metric, and why do you configure wide metrics?
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Say this first
Cisco IS-IS gives every interface a metric of 10 regardless of bandwidth, so out of the box it behaves
like hop count until you set metrics. The original narrow metrics allow 6 bits per link (max 63) and
10 bits per path (max 1023). Wide metrics (RFC 5305) allow 24 bits per link and 32 bits per path, and
they are required for MPLS traffic engineering and segment routing extensions. Every router must agree
on the style, so you migrate through metric-style transition.
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Configuring
router isis CORE
metric-style wide
!
interface TenGigabitEthernet0/1/0
isis metric 100 level-2
!
interface GigabitEthernet0/0/1
isis metric 1000 level-2A simple plan is a reference value divided by the link speed, set by hand, so that 10 Gbps is 100 and 1 Gbps is 1000. Cisco IOS does not derive the IS-IS metric from bandwidth the way OSPF does, so an unplanned network ends up with 10 on every link.
Migrating from narrow to wide
- Set
metric-style transitionon every router: it sends and accepts both. - When all routers run transition, set
metric-style wideeverywhere. A router left on narrow ignores wide TLVs. The adjacency stays up but routes are missing, which is a confusing failure to troubleshoot.
They will ask next
Why does MPLS-TE need wide metrics?
The TE attributes (bandwidth, admin groups, TE metric) are sub-TLVs of the extended IS reachability TLV defined with wide metrics. Narrow TLVs have no room for them.
Traps
- Assuming IS-IS derives cost from bandwidth like OSPF. On Cisco every link is 10 until you change it.