Alteon Application Switch Operating System Application Guide
Border Gateway Protocol
130
Document
ID:
RDWR-ALOS-V2900_AG1302
Redistributing Routes
In addition to running multiple routing protocols simultaneously, Alteon can redistribute information
from one routing protocol to another. For example, you can instruct Alteon to use BGP to readvertise
static routes. This applies to all of the IP-based routing protocols.
You can also conditionally control the redistribution of routes between routing domains by defining a
method known as route maps between the two domains. For more information on route maps, see
. Redistributing routes is another way of providing policy control over whether
to export OSPF routes, fixed routes, static routes, and virtual IP address routes. For an example
configuration, see
Default Redistribution and Route Aggregation Example, page 134
Default routes can be configured using the following methods:
•
Import
•
Originate—The router sends a default route to peers even though it does not have any default
routes in its routing table.
•
Redistribute—Default routes are either configured through the default gateway or learned via
other protocols and redistributed to peer routers. If the default routes are from the default
gateway, enable the static routes because default routes from the default gateway are static
routes. Similarly, if the routes are learned from another routing protocol, enable that protocol
for redistribution.
•
None
BGP Attributes
The following two BGP attributes are discussed in this section:
•
Local Preference Attribute, page 130
•
Metric (Multi-Exit Discriminator) Attribute, page 130
Local Preference Attribute
When there are multiple paths to the same destination, the local preference attribute indicates the
preferred path. The path with the higher preference is preferred (the default value of the local
preference attribute is 100). Unlike the weight attribute, which is only relevant to the local router,
the local preference attribute is part of the routing update and is exchanged among routers in the
same AS.
The local preference attribute can be set in one of two ways:
•
/cfg/l3/bgp/pref
—This command uses the BGP default local preference method, affecting
the outbound direction only.
•
/cfg/l3/rmap/lp
—This command uses the route map local preference method, which affects
both inbound and outbound directions.
Metric (Multi-Exit Discriminator) Attribute
This attribute is a hint to external neighbors about the preferred path into an AS when there are
multiple entry points. A lower metric value is preferred over a higher metric value. The default value
of the metric attribute is 0.
Unlike local preference, the metric attribute is exchanged between ASs. However, a metric attribute
that comes into an AS does not leave the AS.
When an update enters the AS with a certain metric value, that value is used for decision making
within the AS. When BGP sends that update to another AS, the metric is reset to 0.
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