Dell SmartFabric OS10 User Guide Release 10.5.3

Link-local-only-nexthop command at ROUTER BGP level

This section provides information about the behavior of the link-local-only-nexthop command at the ROUTER BGP level.

The following behavior occurs when you configure this feature at the ROUTER BGP level:

• When you configure the link-local-only-nexthop command at the ROUTER BGP level, the incoming route updates are accepted when the Next Hop fields have link local addresses in both address fields.
• All outgoing route updates are sent with the link-local address that is entered in both fields of the Next Hop.
• When enabled globally at the ROUTER BGP level, this behavior is applicable for all unnumbered peers present in all VRF instances.
• Configuration and removal of this feature does not impact route exchange between numbered peers that are formed with other vendors based on the FRR stack.
• Route updates exchanged before you configure this feature follow the existing behavior.
• All other updates after you configure this feature follows the new behavior for interop with the FRR vendors.
• All unnumbered neighbors in all VRFs display the global configuration status in the show ip bgp neighbors command output.
• Before removing the BGP unnumbered with FRR interop feature by running the no link-local-only-nexthop command, clear all unnumbered neighbors to restore default behavior for route exchange.
• During configuration flow, you need not clear all unnumbered neighbors as the session is already in IDLE state after receiving updates with the link-local address in both fields of Next Hop.
• The new configuration at the global level impacts all unnumbered sessions. Use the neighbor or template level link-local-only-nexthop command to disable this feature and avoid this impact.

Consider the following topology where the BGP unnumbered with FRR vendor feature interoperates with FRR-based vendors:

Configuration steps on SmartFabric OS10 switches are as follows:

OS10-B configuration:

To configure OS10-B:

1. Create a BGP instance with ASN 100.

   OS10-B(config)# router bgp 100

2. Enable the support of link local Next Hop in both fields for unnumbered peers to internetwork with a FRR-based switch.

   OS10-B(config-router-bgp-100)#link-local-only-nexthop

3. Create a BGP unnumbered neighbor with the FRR switch.

   OS10-B(config-router-bgp-100)# neighbor interface ethernet 1/1/5
   OS10-B(config-router-neighbor)# no shutdown

4. Create a BGP unnumbered neighbor with OS10-A. The neighbor level command link-local-only-nexthop disable disables the link-local address in both Next Hop fields for the unnumbered peer on interface Ethernet 1/1/6. This ensures legacy behavior for the route exchange.

   OS10-B(config-router-bgp-100)# neighbor interface ethernet 1/1/6
   OS10-B(config-router-neighbor)# no shutdown
   OS10-B(config-router-neighbor)link-local-only-nexthop disable

OS10-A configuration:

To configure OS10-A:

1. Create a BGP instance with ASN 200.

   OS10-A(config)# router bgp 200

2. Create a BGP unnumbered neighbor with OS10-B.

   OS10-A(config-router-bgp-200)# neighbor interface ethernet 1/1/6
   OS10-A(config-router-neighbor)# no shutdown

Run the link-local-only-nexthop command in OS10-B connecting to the FRR-based switch at the ROUTER BGP level. After you configure this feature in OS10-B, all outgoing route updates are sent with two Next hops as link-local addresses for all unnumbered peers in all VRFs.

To restore the legacy (default) behavior for unnumbered sessions in OS10-B connecting to other SmartFabric OS10 switches, where the BGP unnumbered feature is not supported, use the link-local-only-nexthop disable command at the BGP neighbor or template level to restore the legacy behavior. Otherwise, route updates with two link-local Next hops are dropped in OS10-A and the session terminates.