Dell PowerConnect B-FCXs, Configuration Manual

Page: 1057 / 1494

PowerConnect B-Series FCX Configuration Guide 1015
53-1002266-01
Optional configuration tasks
30
Lower administrative distances are preferred over higher distances. For example, if the router
receives routes for the same network from OSPF and from RIP, the router will prefer the OSPF route
by default. The administrative distances are configured in different places in the software. The
Layer 3 Switch re-advertises a learned best BGP4 route to neighbors by default, regardless of
whether the route administrative distance is lower than other routes from different route sources to
the same destination.
•
To change the EBGP, IBGP, and Local BGP default administrative distances, see the
instructions in this section.
•
To change the default administrative distance for OSPF, refer to “Modifying the administrative
distance” on page 959.
•
To change the default administrative distance for RIP, refer to “Changing the administrative
distance” on page 911.
•
To change the default administrative distance for static routes, refer to “Configuring static
routes” on page 819.
You can change the default EBGP, IBGP, and Local BGP administrative distances using either of the
following methods.
To change the default administrative distances for EBGP, IBGP, and Local BGP, enter a command
such as the following.
PowerConnect(config-bgp-router)#distance 180 160 40
Syntax: 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 BGP distance and can be a value from 1 through 255.
Requiring the first AS to be the neighbor AS
By default, the Dell PowerConnect device does not require the first AS listed in the AS_SEQUENCE
field of an AS path Update from an EBGP neighbor to be the AS that the neighbor who sent the
Update is in. You can enable the Dell PowerConnect device for this requirement.
When you enable the Dell PowerConnect device to require the AS that an EBGP neighbor is in to be
the same as the first AS in the AS_SEQUENCE field of an Update from the neighbor, the Dell
PowerConnect device accepts the Update only if the autonomous systems match. If the
autonomous systems do not match, the Dell PowerConnect device sends a Notification message to
the neighbor and closes the session. The requirement applies to all Updates received from EBGP
neighbors.
To enable this feature, enter the following command at the BGP configuration level of the CLI.
PowerConnect(config-bgp-router)#enforce-first-as
Syntax: [no] enforce-first-as
Disabling or re-enabling comparison of the AS-Path length
AS-Path comparison is Step 5 in the algorithm BGP4 uses to select the next path for a route.
Comparison of the AS-Path length is enabled by default. To disable it, enter the following command
at the BGP configuration level of the CLI.