Understanding route metric 0

is-is

I have the below output from an IOS-XR box:

RP/0/RSP0/CPU0:RTR1#show route 10.10.10.10
Wed Mar 27 13:34:52.553 IST

Routing entry for 10.10.10.10/32
  Known via "isis IGP", distance 115, metric 261, type level-2
  Installed Mar 27 01:21:50.992 for 12:13:01
  Routing Descriptor Blocks
    172.16.185.6, from 10.10.10.10, via TenGigE0/0/0/1, Backup
      Route metric is 0
    172.16.184.253, from 10.10.10.10, via Bundle-Ether7, Protected
      Route metric is 261
  No advertising protos.

Q1:what is the meaning of 'Route metric is 0' in the above output?

Q2: Is fast-reroute in any way related to Q1 ? I see that fast-reroute is configured under BE7 and not under Te0/0/0/1 in the running config.

Best Answer

Route metrics are used by routing protocols to determine the best paths through your network. The actual metric isn't used to determine the path unless the Administrative Distance is the same between two or more paths. Cisco uses a default Administrative Distance of 0 for directly connected paths. If you want to learn more about Administrative Distance, here's a link to Cisco's support doc on it. As far as actual routing metrics, that depends on which routing protocols you're using. Here's a short explanation of a few metrics.

As for the fast-reroute command, that's used on a Label Switched Path in order to continue passing traffic even in the event of a link or node failure. A good explanation of Label Switched Paths can be found here, and the Cisco support doc on Fast Reroute links can be found here.

Related Solutions

ISIS Max Metric – Understanding and Configuration Guide

I just labbed it in VIRL. For the PC subnet, I get a metric of 50331642 on Router D:

Routing entry for 10.0.0.4/30
  Known via "isis 1", distance 115, metric 50331642, type level-2
  Installed Feb 28 17:46:19.166 for 00:00:07
  Routing Descriptor Blocks
    10.0.0.17, from 192.168.0.2, via GigabitEthernet0/0/0/0
      Route metric is 50331642
  No advertising protos.

Note that I had to manually set the IS-IS link metric to 16777214 on all applicable interfaces under the ISIS config. Otherwise it defaults to a link metric of 10 for a total metric of 30 on Router D.

MPLS ISIS LDP – Does LDP Generate a Label for Default Route in ISIS Level 2?

Some implementations set the ATT bit automatically and provide you a command or knob under a configuration stanza to disable it. i.e on Juniper: ignore-attached-bit

Ignore the attached bit on IS-IS Level 1 routers. Configuring this statement enables the routing device to ignore the attached bit on incoming Level 1 link-state PDUs. If the attached bit is ignored, no default route, which points to the routing device which has set the attached bit, is installed.

Others, require you to actually manually configure the ATT bit. i.e Cisco: attached-bit send

To configure an Intermediate System-to-Intermediate System (IS-IS) instance with an attached bit in the Level 1 link-state packet (LSP), use the attached-bit send command in address family configuration mode.

So whichever it is, you certainly have power over what you want to achieve, in this case you have decided to perform leaking to have specific routes advertised to your internal area. In this example you have given, leaving the attached bit and thinking about a worst case scenario where there are no failsafe mechanisms, you could end up with a blackhole if you loose connectivity to the specific routes because you still have the default one.

On the other hand, I'm not sure which parameters have been taken into consideration for the leaking, so if it were to be something dynamic where the routes are only leaked if certain conditions you specified are met, then if one of those evaluates to false you will stop seeing those routes and this could potentially result in connectivity loss if there is no other way to work around, hence default route.

In regards to LDP not creating a FEC for the default route introduced by the attached bit in ISIS.

Personally I haven't tested this yet, as soon my hands get on some equipment will do, but for what Google has showed me:

From Microtik:

/mpls ldp set distribute-for-default-route=yes does. It seems to add an entry in the MPLS forwarding table for the default route

Reason being:

MPLS switching does not have "matching longest prefix" feature of IP routing.