The system uses a hierarchy to prioritize the utilization of the AS4 capability. The prioritization depends on the CLI conguration
commands. AS4s can be enabled and congured at the level of a neighbor, a peer group, or globally for the entire device, according to
the following bottom-up hierarchy:
• If a neighbor has no conguration for AS4s but it belongs to a peer group, the neighbor uses the conguration from the peer
group. For example, if you congure a neighbor but do not include a specication for AS4s, one of the following applies:
– The neighbor uses the AS4 conguration for a peer group if it belongs to a peer group.
– The neighbor uses the device conguration if it does not belong to a peer group or the peer group has no AS4
conguration.
• If a peer group has no conguration for AS4s, it can use the global conguration of the device. If the device has no conguration
for AS4s, then a neighbor or peer group without a conguration for AS4s use the device default--no announcement or
negotiation of AS4s.
• If a neighbor belongs to peer group with an AS4 conguration but you want that neighbor to be disabled or have a dierent AS4
conguration, the neighbor AS4 conguration overrides the peer group conguration. For example, you can ensure that
neighbor has no AS4 announcement and negotiation activity even though the peer group is enabled for AS4 capability.
NOTE
The conguration for AS4 can be enabled, disabled, or can have no explicit
conguration.
CLI commands allow you to disable AS4s on an entity whose larger context has AS4s enabled. For example, you can use a CLI
command to disable AS4s on a neighbor that is a member of a peer group that is enabled for AS4s.
Normally, AS4s are sent only to a device, peer group, or neighbor that is similarly congured for AS4s. If a AS4 is congured for a local-
autonomous systemS, the system signals this conguration by sending AS_TRANS in the My Autonomous System eld of the OPEN
message. However, if the AS4 capability for a neighbor is disabled, the local device does not send the four-byte Autonomous System
number capability to the neighbor.
Enabling AS4 numbers
This section describes how to enable the announcement and negotiation of AS4s and describes the dierent types of notation that you
can use to represent a AS4.
You can enable AS4s on a device, a peer group, and a neighbor. For global conguration, the capability command in the BGP4
conguration context enables or disables AS4 support. For a peer group or a neighbor, capability is a keyword for the neighbor
command. In addition to enabling AS4s for a neighbor or a peer group, you can also use the combination of the capability keyword and
the optional enable or disable keyword to disable this feature in a specic case where the AS4s are enabled for a larger context. The
Neighbor conguration of AS4s section illustrates this capability.
Global AS4
conguration
To enable AS4s globally, use the capability command in the BGP4 conguration context as shown.
device(config-bgp)# capability as4 enable
Syntax: [no] capability as4 enable | disable
The no form of the capability command deletes the announcement and negotiation conguration of AS4s (if it has been enabled) at the
global level. Using the regular form of the command with the disable keyword has the same eect on the global conguration. Disabling
or using the no form of the command does not aect the conguration at the level of a peer or neighbor.
Four-byte Autonomous System Numbers (AS4)
FastIron Ethernet Switch Layer 3 Routing
53-1003627-04 419
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