Hello padding

• Topology

• Task

Configure XR2 and XE7 so that Hellos are only padded the first six times. All the routers should use the configuration under IS-IS process, excep XE7, which should have the configuration under the interfaces.

• Answer

XR1-XR4
router isis
 interface g0/0/0/x   hello-padding sometimes [level 1,2]



XE5-XE6,XE8
router isis
 no hello padding [multi-point|point-to-point]


XE7
interface GigabitEthernet1
 no isis hello padding
interface GigabitEthernet2
 no isis hello padding
interface GigabitEthernet3
 no isis hello padding

• Explanation

By default, the router will always pad Hellos. This can be considered an unnecessary waste of bandwidth, as once you’ve verified MTU via the padded Hellos, you no longer really need to continue to pad Hellos.

When using the above configuration, the router will only pad the first 6 Hellos. After this, all subsequent Hellos are not padded, which saves some bandwidth.

If for some reason you wanted to never pad Hellos, not even at the beginning of the adjacency, you can use no isis hello padding always under the interface on IOS-XE, or hello-padding disable on IOS-XR. This is similar to OSPF’s ip ospf mtu-ignore setting.

Verify that XE7 is not padding Hellos on Gi1

When you use no isis hello padding always, you will instead see “Never pad hello PDU”: