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isisIS-IS Config

address-family ipv4 unicast

Configures IPv4 unicast address family parameters within IS-IS routing process on Cisco IOS-XR.

Overview

The 'address-family ipv4 unicast' command is a fundamental configuration step in IS-IS on Cisco IOS-XR. IS-IS (Intermediate System to Intermediate System) is a link-state routing protocol originally designed for OSI CLNS networks. To support IPv4 routing, the address family must be explicitly enabled. This command enters the IPv4 unicast address family configuration submode, where you can set parameters such as metric style, default metric, redistribution policies, and route policies. Without this command, IS-IS will only operate for CLNS and will not exchange IPv4 routes. In IOS-XR, the IS-IS configuration hierarchy is strict: after entering 'router isis <process-name>', you must use 'address-family ipv4 unicast' to configure IPv4-specific settings. This is different from Cisco IOS, where IPv4 is enabled by default. The command is typically used in service provider and large enterprise networks where IS-IS is the IGP. It fits into troubleshooting workflows when verifying that IPv4 routing is enabled and correctly configured. Common tasks under this mode include setting the metric style to 'wide' (required for large networks with high bandwidth), configuring default metrics, and redistributing routes from other protocols like BGP or OSPF.

Syntax·IS-IS Config
address-family ipv4 unicast

When to Use This Command

  • Enabling IPv4 routing in IS-IS for a service provider core network.
  • Configuring route redistribution from BGP into IS-IS for IPv4 prefixes.
  • Setting metric style and default metric for IPv4 routes in IS-IS.
  • Applying route policy to filter IPv4 routes in IS-IS.

Parameters

ParameterSyntaxDescription
ipv4ipv4Specifies the IPv4 address family. This is the only address family supported for unicast routing in IS-IS on IOS-XR (IPv6 is configured separately with 'address-family ipv6 unicast').
unicastunicastSpecifies unicast routing. Multicast is not supported under IS-IS; only unicast is available.

Command Examples

Enter IPv4 unicast address family configuration mode

router isis 1 address-family ipv4 unicast
RP/0/RP0/CPU0:router(config-isis)# address-family ipv4 unicast
RP/0/RP0/CPU0:router(config-isis-af)#

The prompt changes to indicate IS-IS address-family configuration mode for IPv4 unicast.

Configure default metric and redistribute connected routes

router isis 1 address-family ipv4 unicast metric-style wide default-metric 10 redistribute connected
RP/0/RP0/CPU0:router(config-isis-af)# metric-style wide
RP/0/RP0/CPU0:router(config-isis-af)# default-metric 10
RP/0/RP0/CPU0:router(config-isis-af)# redistribute connected

Commands are entered under the address-family mode. 'metric-style wide' enables wide metrics, 'default-metric 10' sets default metric, 'redistribute connected' redistributes directly connected routes.

Understanding the Output

The command itself does not produce output; it enters a configuration submode. The prompt changes from 'router(config-isis)#' to 'router(config-isis-af)#', indicating you are now configuring IPv4 unicast address family parameters. All subsequent commands apply specifically to IPv4 unicast routing within IS-IS. To verify configuration, use 'show running-config router isis' or 'show isis protocol'. In the running config, you will see the 'address-family ipv4 unicast' block containing all subcommands. Healthy configuration shows correct metric style, redistribution policies, and interface assignments. Problem values include missing address-family block (IPv4 not enabled) or mismatched metric styles between routers.

Configuration Scenarios

Basic IPv4 IS-IS Configuration

A small enterprise network with two routers running IS-IS as the IGP for IPv4.

Topology

R1 --- R2

Steps

  1. 1.Enter global configuration mode.
  2. 2.Configure IS-IS process and enter router configuration.
  3. 3.Enter address-family ipv4 unicast mode.
  4. 4.Set metric-style wide.
  5. 5.Configure network entity title (NET) and interfaces.
Configuration
!
router isis 1
 net 49.0001.0000.0000.0001.00
 address-family ipv4 unicast
  metric-style wide
!
interface GigabitEthernet0/0/0/0
 ipv4 address 10.1.1.1 255.255.255.0
 router isis 1
!

Verify: Use 'show isis neighbors' to verify adjacency, 'show isis database' to check LSDB, and 'show route isis' to see IPv4 routes.

Watch out: Forgetting to assign the NET address; IS-IS requires a NET to operate.

Redistribute BGP into IS-IS

A service provider redistributing BGP routes into IS-IS for internal reachability.

Topology

PE1 --- P --- PE2

Steps

  1. 1.Enter IS-IS router configuration.
  2. 2.Enter address-family ipv4 unicast mode.
  3. 3.Configure redistribution from BGP with a route policy.
  4. 4.Apply route policy to filter prefixes.
Configuration
!
router isis 1
 address-family ipv4 unicast
  redistribute bgp 65000 route-policy BGP_TO_ISIS
!
route-policy BGP_TO_ISIS
  if destination in (10.0.0.0/8 ge 16) then
    pass
  else
    drop
  endif
end-policy
!

Verify: Check 'show isis protocol' to see redistribution settings, 'show route isis' to verify redistributed routes.

Watch out: Route policy must be defined before applying redistribution; otherwise, the command is rejected.

Troubleshooting with This Command

When troubleshooting IS-IS IPv4 routing issues on IOS-XR, the 'address-family ipv4 unicast' configuration is a critical starting point. First, verify that the address family is enabled using 'show running-config router isis' or 'show isis protocol'. If the address-family block is missing, IS-IS will not exchange IPv4 routes. Check that the metric style is consistent across all routers; mismatch can cause routes to be ignored. Use 'show isis neighbors' to confirm adjacency; if neighbors are not forming, ensure that interfaces are configured under IS-IS and that the NET is correct. For redistribution issues, verify route policies and use 'show isis route' to see which routes are being advertised. The 'debug isis' commands can help, but be cautious in production. Common problems include missing 'address-family ipv4 unicast' after upgrading from IOS to IOS-XR, as the default behavior differs. Also, ensure that the IS-IS process name matches on all routers if using multiple processes.

CCNA Exam Tips

1.

Remember that 'address-family ipv4 unicast' is required to enable IPv4 routing in IS-IS; without it, IS-IS only runs CLNS.

2.

On IOS-XR, the address-family command is entered under the IS-IS router configuration, not under interface configuration.

3.

Be aware that metric-style must be consistent across all routers in the same area; 'wide' is required for large networks.

Common Mistakes

Forgetting to enter address-family mode before configuring IPv4-specific parameters, causing commands to be rejected.

Assuming IS-IS automatically supports IPv4; it does not until 'address-family ipv4 unicast' is configured.

Misconfiguring metric-style as 'narrow' when the network requires 'wide' metrics, leading to route incompatibility.

Platform Notes

On Cisco IOS-XR, the 'address-family ipv4 unicast' command is mandatory to enable IPv4 routing in IS-IS, unlike Cisco IOS where IPv4 is implicitly enabled. In IOS, the command is 'address-family ipv4' (without 'unicast') and is entered under router configuration, but the behavior is similar. On IOS-XR, the address family must be explicitly configured; otherwise, IS-IS only runs CLNS. Additionally, IOS-XR supports multiple IS-IS processes, each with its own address family configuration. The command syntax is consistent across IOS-XR versions, but some parameters like 'metric-style' may have different defaults. For equivalent commands on other platforms, Juniper JunOS uses 'family inet' under IS-IS, and Nokia SR OS uses 'ipv4 unicast' under 'router isis'. Understanding these differences is crucial for multi-vendor environments.

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