Brocade FastIron Ethernet Switch Layer 3 Routing Configuration Guide - Manual (Page 454)

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Syntax: show ip bgp neighbor ip-addr received prefix-filter
Four-byte Autonomous System Numbers (AS4)
This section describes the reasons for enabling four-byte autonomous system numbers (AS4s). AS4s
are supported by default. You can specify and view AS4s by default and using the enable facility
described in this section. However, not all devices in a network are always capable of utilizing AS4s.
The act of enabling them on the local device initiates a facility for announcing the capability and
negotiating its use with neighbors. If you do not enable AS4s on a device, other devices do not know
that this device is sending them.
The system uses a hierarchy to prioritize the utilization of the AS4 capability. The prioritization
depends on the CLI configuration commands. AS4s can be enabled and configured 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 configuration for AS4s but it belongs to a peer group, the neighbor uses the
configuration from the peer group. For example, if you configure a neighbor but do not include a
specification for AS4s, one of the following applies:
‐ The neighbor uses the AS4 configuration for a peer group if it belongs to a peer group.
‐ The neighbor uses the device configuration if it does not belong to a peer group or the
peer group has no AS4 configuration.
• If a peer group has no configuration for AS4s, it can use the global configuration of the device. If the
device has no configuration for AS4s, then a neighbor or peer group without a configuration for
AS4s use the device default--no announcement or negotiation of AS4s.
• If a neighbor belongs to peer group with an AS4 configuration but you want that neighbor to be
disabled or have a different AS4 configuration, the neighbor AS4 configuration overrides the peer
group configuration. 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 configuration for AS4 can be enabled, disabled, or can have no explicit configuration.
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 configured for AS4s.
If a AS4 is configured for a local-autonomous systemS, the system signals this configuration by
sending AS_TRANS in the My Autonomous System field 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
different types of notation that you can use to represent a AS4.
Four-byte Autonomous System Numbers (AS4)
454 FastIron Ethernet Switch Layer 3 Routing Configuration Guide
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