IS-IS supports four different metric values:
- Default Metric: every interface has a default metric of 10.
- Delay: similar to how EIGRP uses delay.
- Expense: the actual monetary cost of a link.
- Error: similar to how EIGRP uses reliability.
Cisco IOS routers, however, only support the default metric. The other metric values are not used. The default metric is always set to 10, no matter the interface. A Ten Gigabit interface gets the same metric as a slow serial link.
In practice, this means that IS-IS will act similar to RIP, the path with the least amount of hops will be used. This is something you might want to change.
In this lesson, I’ll show you how we can configure the metric and some other tricks.
Here’s the topology I will use:
We have four routers in a single area, these routers are configured as level 1 routers.
Want to take a look for yourself? Here you will find the startup configuration of each device.
hostname R1 ! ip cef ! interface GigabitEthernet0/1 ip address 192.168.12.1 255.255.255.0 ip router isis ! interface GigabitEthernet0/2 ip address 192.168.13.1 255.255.255.0 ip router isis ! router isis net 49.1234.0000.0000.0001.00 is-type level-1 log-adjacency-changes ! end
hostname R2 ! ip cef ! interface GigabitEthernet0/1 ip address 192.168.12.2 255.255.255.0 ip router isis ! interface GigabitEthernet0/2 ip address 192.168.24.2 255.255.255.0 ip router isis ! router isis net 49.1234.0000.0000.0002.00 is-type level-1 log-adjacency-changes ! end
hostname R3 ! ip cef ! interface GigabitEthernet0/1 ip address 192.168.34.3 255.255.255.0 ip router isis ! interface GigabitEthernet0/2 ip address 192.168.13.3 255.255.255.0 ip router isis ! router isis net 49.1234.0000.0000.0003.00 is-type level-1 log-adjacency-changes ! end
hostname R4 ! ip cef ! interface Loopback0 ip address 188.8.131.52 255.255.255.255 ip router isis ! interface GigabitEthernet0/1 ip address 192.168.34.4 255.255.255.0 ip router isis ! interface GigabitEthernet0/2 ip address 192.168.24.4 255.255.255.0 ip router isis ! router isis net 49.1234.0000.0000.0004.00 is-type level-1 log-adjacency-changes ! end
Let’s take a look the routing table of R1:
R1#show ip route isis 184.108.40.206/32 is subnetted, 1 subnets i L1 220.127.116.11 [115/30] via 192.168.13.3, 00:06:43, GigabitEthernet0/2 [115/30] via 192.168.12.2, 00:06:43, GigabitEthernet0/1 i L1 192.168.24.0/24 [115/20] via 192.168.12.2, 00:06:43, GigabitEthernet0/1 i L1 192.168.34.0/24 [115/20] via 192.168.13.3, 00:06:43, GigabitEthernet0/2
Above we see that R1 has two equal metric paths for 18.104.22.168/32. Both have a metric of 30 in total. We have to cross two GigabitEthernet interfaces and the loopback interface is added as well. What if we want to change this? We can do so by setting the metric manually. This is done on the interface level:
R1(config)#interface GigabitEthernet 0/2 R1(config-if)#isis metric ? <1-16777214> Default metric maximum Maximum metric. All routers will exclude this link from their SPF
We can select a different metric or use the maximum command. I’ll show you what this command does is in a minute, let’s start with a custom metric first:
R1(config-if)#isis metric 50 ? <1-16777214> Delay metric level-1 Apply metric to level-1 links level-2 Apply metric to level-2 links <cr>
We can make one more change. You can choose if this metric should apply to level 1, level 2 links or both. If you don’t specify this then it will apply to both. Let’s set the metric of this interface to 50:
R1(config-if)#isis metric 50
Let’s have another look at the routing table:
R1#show ip route 22.214.171.124 255.255.255.255 Routing entry for 126.96.36.199/32 Known via "isis", distance 115, metric 30, type level-1 Redistributing via isis Last update from 192.168.12.2 on GigabitEthernet0/1, 00:00:10 ago Routing Descriptor Blocks: * 192.168.12.2, from 188.8.131.52, 00:00:10 ago, via GigabitEthernet0/1 Route metric is 30, traffic share count is 1
Since the path through R2 now has the lowest metric, this is the path that IS-IS will use. We can see the metric that was set to 50 in the database though:
R1#show isis database level-1 R1.00-00 detail IS-IS Level-1 LSP R1.00-00 LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL R1.00-00 * 0x00000016 0x2D65 1071 0/0/0 Area Address: 49.1234 NLPID: 0xCC Hostname: R1 Metric: 10 IS-Extended R1.01 Metric: 50 IS-Extended R3.02 IP Address: 192.168.13.1 Metric: 10 IP 192.168.12.0/24 Metric: 50 IP 192.168.13.0/24
There is a limit to the metric you can set on an interface. For example, if I try to change it 100 this will happen:
R1(config-if)#isis metric 100 Warning: for metrics greater than 63, 'metric-style wide' should be configured on level-1-2, or it will be capped at 63.
By default, the maximum metric that IS-IS supports to reach any destination is 1023. The maximum metric for an interface is 63. We can change this behavior by using “wide” metrics. We should do this on all routers: