Foundry Configuration Guide for the FESX, FSX, and FWSX
21 - 26
© Foundry Networks, Inc.
December 2005
Advertising the Default Route
By default, the Layer 3 Switch does not originate and advertise a default route using BGP4. A BGP4 default route
is the IP address 0.0.0.0 and the route prefix 0 or network mask 0.0.0.0. For example, 0.0.0.0/0 is a default route.
You can enable the router to advertise a default BGP4 route using either of the following methods.
NOTE:
The Foundry Layer 3 Switch checks for the existence of an IGP route for 0.0.0.0/0 in the IP route table
before creating a local BGP route for 0.0.0.0/0.
To enable the router to originate and advertise a default BGP4 route, enter the following command:
FESX424 Router(config-bgp-router)# default-information-originate
Syntax:
[no] default-information-originate
Changing the Default MED (Metric) Used for Route Redistribution
The Foundry Layer 3 Switch can redistribute directly connected routes, static IP routes, RIP routes, and OSPF
routes into BGP4. The MED (metric) is a global parameter that specifies the cost that will be applied to all routes
by default when they are redistributed into BGP4. When routes are selected, lower metric values are preferred
over higher metric values. The default BGP4 MED value is 0 and can be assigned a value from 0 – 4294967295.
NOTE:
RIP and OSPF also have default metric parameters. The parameters are set independently for each
protocol and have different ranges.
To change the default metric to 40, enter the following command:
FastIron SuperX Router(config-bgp-router)# default-metric 40
Syntax:
default-metric <num>
The <num> indicates the metric and can be a value from 0 – 4294967295.
Enabling Next-Hop Recursion
For each BGP4 route a Layer 3 Switch learns, the Layer 3 Switch performs a route lookup to obtain the IP address
of the route’s next hop. A BGP4 route becomes eligible for installation into the IP route table only if the following
conditions are true:
•
The lookup succeeds in obtaining a valid next-hop IP address for the route.
•
The path to the next-hop IP address is an Interior Gateway Protocol (IGP) path or a static route path.
By default, the software performs only one lookup for a BGP route’s next-hop IP address. If the next-hop lookup
does not result in a valid next-hop IP address or the path to the next-hop IP address is a BGP path, the software
considers the BGP route’s destination to be unreachable. The route is not eligible to be installed in the IP route
table.
It is possible for the BGP route table to contain a route whose next-hop IP address is not reachable through an
IGP route, even though a hop farther away can be reached by the Layer 3 Switch through an IGP route. This can
occur when the IGPs do not learn a complete set of IGP routes, resulting in the Layer 3 Switch learning about an
internal route through IBGP instead of through an IGP. In this case, the IP route table does not contain a route that
can be used to reach the BGP route’s destination.
To enable the Layer 3 Switch to find the IGP route to a BGP route’s next-hop gateway, enable recursive next-hop
lookups. When you enable recursive next-hop lookup, if the first lookup for a BGP route results in an IBGP path
originated within the same Autonomous System (AS), rather than an IGP path or static route path, the Layer 3
Switch performs a lookup on the next-hop gateway’s next-hop IP address. If this second lookup results in an IGP
path, the software considers the BGP route to be valid and thus eligible for installation in the IP route table.
Otherwise, the Layer 3 Switch performs a lookup on the next-hop IP address of the next-hop gateway’s next hop,
and so on, until one of the lookups results in an IGP route.
Содержание FastIron Edge Switch X424
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