Advanced OSPF Design Concepts

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To redistribute routes from one routing domain into another routing domain, use the redistribute router configuration command. To disable redistribution, use the no form of this command. The syntax for this command (and the no form) is as follows:

    redistribute protocol [process-id] {level-1 | level-1-2 | level-2}    [metric metric-value]    [metric-type type-value] [match {internal | external 1 | external 2}]    [tag tag-value] [route-map map-tag] [weight weight] [subnets]    no redistribute protocol [process-id] {level-1 | level-1-2 | level-2}    [metric metric-value] [metric-type type-value] [match {internal |    external 1 | external 2}]    [tag tag-value] [route-map map-tag] [weight weight] [subnets] 

Syntax Description:

protocol. Source protocol from which routes are being redistributed. It can be one of the following keywords: bgp, egp, igrp, isis, ospf, static [ip], connected, and rip. The keyword static [ip] is used to redistribute IP static routes. The optional ip keyword is used when redistributing into IS-IS. The keyword connected refers to routes that are established automatically by the virtue of having enabled IP on an interface. For routing protocols such as OSPF and IS-IS, these routes will be redistributed as external to the autonomous system.
process-id (optional). For bgp, egp, or igrp, this is an autonomous system number, which is a 16-bit decimal number. For isis, this is an optional tag that defines a meaningful name for a routing process. You can specify only one IS-IS process per router. Creating a name for a routing process means that you use names when configuring routing or ospf, this is an appropriate OSPF process ID from which routes are to be redistributed. This identifies the routing process. This value takes the form of a nonzero decimal number. For rip, no process-id value is needed.
level-1. For IS-IS, Level 1 routes are redistributed into other IP routing protocols independently.
level-1-2. For IS-IS, both Level 1 and Level 2 routes are redistributed into other IP routing protocols.
level-2. For IS-IS, Level 2 routes are redistributed into other IP routing protocols independently.
metric metric-value (optional). Metric used for the redistributed route. If a value is not specified for this option, and no value is specified using the default-metric command, the default metric value is 0. Use a value consistent with the destination protocol.
metric-type type-value (optional). For OSPF, the external link type associated with the default route advertised into the OSPF routing domain. It can be either of the following two values: 1 (Type 1 external route) or 2 (Type 2 external route). If a metric-type is not specified, the Cisco IOS software adopts a Type 2 external route. For IS-IS, it can be one of two values: internal (IS-IS metric which is < 63) or external (IS-IS metric which is > 64 < 128). The default is internal.
match {internal | external 1 | external 2} (optional). For OPSF, the criteria by which OSPF routes are redistributed into other routing domains. It can be one of the following: internal (routes that are internal to a specific autonomous system), external 1 (routes that are external to the autonomous system, but are imported into OSPF as type 1 external route), external 2 (routes that are external to the autonomous system but are imported into OSPF as type 2 external route).
tag tag-value (optional). 32-bit decimal value attached to each external route. This is not used by the OSPF protocol itself. It may be used to communicate information between ASBRs. If none is specified, then the remote autonomous system number is used for routes from BGP and EGP; for other protocols, zero is used.
route-map (optional). Route map should be interrogated to filter the importation of routes from this source routing protocol to the current routing protocol. If not specified, all routes are redistributed. If this keyword is specified, but no route map tags are listed, no routes will be imported.
map-tag (optional). Identifier of a configured route map.
weight weight (optional). Network weight when redistributing into BGP. An integer from 0 to 65,535.
subnets (optional). For redistributing routes into OSPF, the scope of redistribution for the specified protocol.

Command Mode: Router configuration.

Usage Guidelines: Changing or disabling any keyword will not affect the state of other keywords.

A router receiving a LSP with an internal metric will consider the cost of the route from itself to the redistributing router plus the advertised cost to reach the destination. An external metric only considers the advertised metric to reach the destination. Routes learned from IP routing protocols can be redistributed at level-1 into an attached area or at level-2. The keyword level-1-2 allows both in a single command.

Redistributed routing information should always be filtered by the distribute-list out router configuration command. This ensures that only those routes intended by the administrator are passed along to the receiving routing protocol.

Whenever you use the redistribute or the default-information router configuration commands to redistribute routes into an OSPF routing domain, the router automatically becomes an ASBR. However, an ASBR does not, by default, generate a default route into the OSPF routing domain. When routes are redistributed between OSPF processes, no OSPF metrics are preserved.

When routes are redistributed into OSPF and no metric is specified in the metric keyword, the default metric that OSPF uses is 20 for routes from all protocols except BGP route, which gets a metric of 1. When redistributing routes into OSPF, only routes that are not subnetted are redistributed if the subnets keyword is not specified.

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OSPF Network Design Solutions
OSPF Network Design Solutions
ISBN: 1578700469
EAN: 2147483647
Year: 1998
Pages: 200
Authors: Tom Thomas

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