Brocade Communications Systems ICX 7250 series, Руководство по настройке

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When selecting a route from among different sources (BGP4, OSPF, RIP, static routes, and so on), the software compares the routes on
the basis of the administrative distance for each route. If the administrative distance of the paths is lower than the administrative distance
of paths from other sources (such as static IP routes, RIP, or OSPF), the BGP4 paths are installed in the IP route table.
The default administrative distances on the device are:
• Directly connected - 0 (this value is not configurable)
• Static - 1 is the default and applies to all static routes, including default routes. This can be assigned a different value.
• EBGP - 20
• OSPF - 110
• RIP - 120
• IBGP - 200
• Local BGP4 - 200
• Unknown - 255 (the device will not use this route)
Lower administrative distances are preferred over higher distances. For example, if the device receives routes for the same network from
OSPF and from RIP, the device will prefer the OSPF route by default. The administrative distances are configured in different places in
the software. The device re-advertises a learned best BGP4 route to neighbors by default, regardless of whether the administrative
distance for the route is lower than the administrative distances of other routes from different route sources to the same destination:
• To change the EBGP, IBGP, and Local BGP4 default administrative distances, refer to the instructions in this section.
• To change the default administrative distance for OSPF, RIP, refer to Configuring a static BGP4 network on page 356.
• To change the administrative distance for static routes, refer to the instructions in this section.
To change the default administrative distances for EBGP, IBGP, and Local BGP4, enter a command such as the following.
device(config-bgp-router)# distance 200 200 200
Syntax: [no] distance external-distance internal-distance local-distance
The external-distance sets the EBGP distance and can be a value from 1 through 255.
The internal-distance sets the IBGP distance and can be a value from 1 through 255.
The local-distance sets the Local BGP4 distance and can be a value from 1 through 255.
Requiring the first AS to be the neighbor AS
By default, the Brocade device does not require the first AS listed in the AS_SEQUENCE field of an AS path update message from
EBGP neighbors to be the AS of the neighbor that sent the update. However, you can enable the Brocade device to have this
requirement. You can enable this requirement globally for the device, or for a specific neighbor or peer group. This section describes how
to enable this requirement.
When you configure the device to require that the AS an EBGP neighbor is in be the same as the first AS in the AS_SEQUENCE field of
an update from the neighbor, the device accepts the update only if the AS numbers match. If the AS numbers do not match, the Brocade
device sends a notification message to the neighbor and closes the session. The requirement applies to all updates received from EBGP
neighbors.
The hierarchy for enforcement of this feature is: a neighbor will try to use the enforce-first-as value if one is configured; if none is
configured, the neighbor will try to use the configured value for a peer group. If neither configuration exists, enforcement is simply that of
the global configuration (which is disabled by default).
To enable this feature globally, enter the enforce-first-as command at the BGP4 configuration level of the CLI.
device(config-bgp-router)# enforce-first-as
Optional BGP4 configuration tasks
Brocade FastIron Layer 3 Routing Configuration Guide
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