Table Of Contents
Configuring IP Routing Protocol-Independent Features
Use Variable-Length Subnet Masks
Understand Gateway of Last Resort
Change the Maximum Number of Paths
Redistribute Routing Information
Understand Supported Metric Translations
Prevent Routing Updates through an Interface
Control the Advertising of Routes in Routing Updates
Control the Processing of Routing Updates
Filter Sources of Routing Information
Enable Fast-Switched Policy Routing
Monitor and Maintain the IP Network
Clear Routes from the IP Routing Table
Display System and Network Statistics
IP Routing Protocol-Independent Configuration Examples
Variable-Length Subnet Mask Example
Overriding Static Routes with Dynamic Protocols Example
Administrative Distance Examples
Static Routing Redistribution Example
RIP and IGRP Redistribution Example
RIP and EIGRP Redistribution Examples
OSPF Routing and Route Redistribution Examples
Basic OSPF Configuration Examples
Internal Router, ABR, and ASBRs Configuration Example
Complex OSPF Configuration Example
Default Metric Values Redistribution Example
Configuring IP Routing Protocol-Independent Features
This chapter describes how to configure IP routing protocol-independent features. For a complete description of the IP routing protocol-independent commands in this chapter, refer to the "IP Routing Protocol-Independent Commands" chapter of the Network Protocols Command Reference, Part 1. To locate documentation of other commands in this chapter, use the command reference master index or search online.
Previous chapters addressed configurations of specific routing protocols. The following sections describe optional features that are protocol-independent:
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Use Variable-Length Subnet Masks
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See the section "IP Routing Protocol-Independent Configuration Examples" at end of this chapter for configuration examples.
Use Variable-Length Subnet Masks
Enhanced Interior Gateway Routing Protocol (EIGRP), IS-IS, OSPF, RIP Version 2, and static routes support variable-length subnet masks (VLSMs). With VLSMs, you can use different masks for the same network number on different interfaces, which allows you to conserve IP addresses and more efficiently use available address space. However, using VLSMs also presents address assignment challenges for the network administrator and ongoing administrative challenges.
Refer to RFC 1219 for detailed information about VLSMs and how to correctly assign addresses.
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The best way to implement VLSMs is to keep your existing numbering plan in place and gradually migrate some networks to VLSMs to recover address space. See the "Variable-Length Subnet Mask Example" section at the end of this chapter for an example of using VLSMs.
Configure Static Routes
Static routes are user-defined routes that cause packets moving between a source and a destination to take a specified path. Static routes can be important if the Cisco IOS software cannot build a route to a particular destination. They are also useful for specifying a gateway of last resort to which all unroutable packets will be sent.
To configure a static route, use the following command in global configuration mode:
ip route prefix mask {address | interface} [distance] [tag tag] [permanent]
Establish a static route.
See the "Overriding Static Routes with Dynamic Protocols Example" section at the end of this chapter for an example of configuring static routes.
The software remembers static routes until you remove them (using the no form of the ip route global configuration command). However, you can override static routes with dynamic routing information through prudent assignment of administrative distance values. Each dynamic routing protocol has a default administrative distance, as listed in . If you would like a static route to be overridden by information from a dynamic routing protocol, simply ensure that the administrative distance of the static route is higher than that of the dynamic protocol.
Static routes that point to an interface will be advertised via RIP, IGRP, and other dynamic routing protocols, regardless of whether redistribute static commands were specified for those routing protocols. This is because static routes that point to an interface are considered in the routing table to be connected and hence lose their static nature. However, if you define a static route to an interface that is not one of the networks defined in a network command, no dynamic routing protocols will advertise the route unless a redistribute static command is specified for these protocols.
When an interface goes down, all static routes through that interface are removed from the IP routing table. Also, when the software can no longer find a valid next hop for the address specified as the forwarding router's address in a static route, the static route is removed from the IP routing table.
Specify Default Routes
A router might not be able to determine the routes to all other networks. To provide complete routing capability, the common practice is to use some routers as smart routers and give the remaining routers default routes to the smart router. (Smart routers have routing table information for the entire internetwork.) These default routes can be passed along dynamically, or can be configured into the individual routers.
Most dynamic interior routing protocols include a mechanism for causing a smart router to generate dynamic default information that is then passed along to other routers.
Specify a Default Network
If a router has a directly connected interface onto the specified default network, the dynamic routing protocols running on that device will generate or source a default route. In the case of RIP, it will advertise the pseudonetwork 0.0.0.0. In the case of IGRP, the network itself is advertised and flagged as an exterior route.
A router that is generating the default for a network also may need a default of its own. One way of doing this is to specify a static route to the network 0.0.0.0 through the appropriate device.
To define a static route to a network as the static default route, use the following command in global configuration mode:
Understand Gateway of Last Resort
When default information is being passed along through a dynamic routing protocol, no further configuration is required. The system periodically scans its routing table to choose the optimal default network as its default route. In the case of RIP, there is only one choice, network 0.0.0.0. In the case of IGRP, there might be several networks that can be candidates for the system default. The Cisco IOS software uses both administrative distance and metric information to determine the default route (gateway of last resort). The selected default route appears in the gateway of last resort display of the show ip route EXEC command.
If dynamic default information is not being passed to the software, candidates for the default route are specified with the ip default-network command. In this usage, ip default-network takes an unconnected network as an argument. If this network appears in the routing table from any source (dynamic or static), it is flagged as a candidate default route and is a possible choice as the default route.
If the router has no interface on the default network, but does have a route to it, it considers this network as a candidate default path. The route candidates are examined and the best one is chosen, based on administrative distance and metric. The gateway to the best default path becomes the gateway of last resort.
Change the Maximum Number of Paths
By default, most IP routing protocols install a maximum of four parallel routes in a routing table. The exception is BGP, which by default allows only one path to a destination.
The range of maximum paths is 1 to 6 paths. To change the maximum number of parallel paths allowed, use the following command in router configuration mode:
maximum-paths maximum
Configure the maximum number of parallel paths allowed in a routing table.
Redistribute Routing Information
In addition to running multiple routing protocols simultaneously, the Cisco IOS software can redistribute information from one routing protocol to another. For example, you can instruct the software to readvertise IGRP-derived routes using the RIP protocol, or to readvertise static routes using the IGRP protocol. This applies to all of the IP-based routing protocols.
You also can conditionally control the redistribution of routes between routing domains by defining a method known as route maps between the two domains.
The following five tables list tasks associated with route redistribution. Although redistribution is a protocol-independent feature, some of the match and set commands are specific to a particular protocol.
To define a route map for redistribution, use the following command in global configuration mode:
route-map map-tag [permit | deny] [sequence-number]
Define any route maps needed to control redistribution.
One or more match commands and one or more set commands typically follow a route-map command. If there are no match commands, then everything matches. If there are no set commands, nothing is done (other than the match). Therefore, you need at least one match or set command. To define conditions for redistributing routes from one routing protocol into another, use at least one of the following commands in route-map configuration mode:
One or more match commands and one or more set commands should follow a route-map command. To define conditions for redistributing routes from one routing protocol into another, perform at least one of the following tasks in route-map configuration mode:
See the "BGP Route Map Examples" section in the "Configuring BGP" chapter for examples of BGP route maps. See the "BGP Community with Route Maps Examples" section in the "Configuring BGP" chapter for examples of BGP communities and route maps.
To distribute routes from one routing domain into another and to control route redistribution, use the following commands in router configuration mode:
The metrics of one routing protocol do not necessarily translate into the metrics of another. For example, the RIP metric is a hop count and the IGRP metric is a combination of five quantities. In such situations, an artificial metric is assigned to the redistributed route. Because of this unavoidable tampering with dynamic information, carelessly exchanging routing information between different routing protocols can create routing loops, which can seriously degrade network operation.
Understand Supported Metric Translations
This section describes supported automatic metric translations between the routing protocols. The following descriptions assume that you have not defined a default redistribution metric that replaces metric conversions:
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Filter Routing Information
You can filter routing protocol information by performing the following tasks, each of which is described in this section:
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Prevent Routing Updates through an Interface
To prevent other routers on a local network from learning about routes dynamically, you can keep routing update messages from being sent through a router interface. This is done to prevent other systems on an interface from learning about routes dynamically. This feature applies to all IP-based routing protocols except BGP.
OSPF and IS-IS behave somewhat differently. In OSPF, the interface address you specify as passive appears as a stub network in the OSPF domain. OSPF routing information is neither sent nor received through the specified router interface. In IS-IS, the specified IP addresses are advertised without actually running IS-IS on those interfaces.
To prevent routing updates through a specified interface, use the following command in router configuration mode:
passive-interface type number
Suppress the sending of routing updates through the specified interface.
See the "Passive Interface Examples" section at the end of this chapter for examples of configuring passive interfaces.
Control the Advertising of Routes in Routing Updates
To prevent other routers from learning one or more routes, you can suppress routes from being advertised in routing updates. This is done to prevent other routers from learning a particular device's interpretation of one or more routes. You cannot specify an interface name in OSPF. When used for OSPF, this feature applies only to external routes.
To suppress routes from being advertised in routing updates, use the following command in router configuration mode:
distribute-list {access-list-number | name} out [interface-name]
Permit or deny routes from being advertised in routing updates depending upon the action listed in the access list.
Control the Processing of Routing Updates
You might want to avoid processing certain routes listed in incoming updates. This feature does not apply to OSPF or IS-IS. Use the following command in router configuration mode:
distribute-list {access-list-number | name} in [interface-name]
Suppress routes listed in updates from being processed.
Filter Sources of Routing Information
This is done to prioritize routing information from different sources, because some pieces of routing information may be more accurate than others. An administrative distance is a rating of the trustworthiness of a routing information source, such as an individual router or a group of routers. In a large network, some routing protocols and some routers can be more reliable than others as sources of routing information. Also, when multiple routing processes are running in the same router for IP, it is possible for the same route to be advertised by more than one routing process. By specifying administrative distance values, you enable the router to intelligently discriminate between sources of routing information. The router will always pick the route whose routing protocol has the lowest administrative distance.
To filter sources of routing information, use the following command in router configuration mode:
distance {ip-address {ip-address mask}} [ip standard list] [ip extended list]
Filter on routing information sources.
There are no general guidelines for assigning administrative distances, because each network has its own requirements. You must determine a reasonable matrix of administrative distances for the network as a whole. shows the default administrative distance for various routing information sources.
For example, consider a router using IGRP and RIP. Suppose you trust the IGRP-derived routing information more than the RIP-derived routing information. In this example, because the default IGRP administrative distance is lower than the default RIP administrative distance, the router uses the IGRP-derived information and ignores the RIP-derived information. However, if you lose the source of the IGRP-derived information (because of a power shutdown in another building, for example), the router uses the RIP-derived information until the IGRP-derived information reappears.
For an example of filtering on sources of routing information, see the section "Administrative Distance Examples" at the end of this chapter.
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Enable Policy Routing
Policy routing is a more flexible mechanism for routing packets than destination routing. It is a process whereby the router puts packets through a route map before routing them. The route map determines which packets are routed to which router next. You might enable policy routing if you want certain packets to be routed some way other than the obvious shortest path. Some possible applications for policy routing are to provide equal access, protocol-sensitive routing, source-sensitive routing, routing based on interactive versus batch traffic, or routing based on dedicated links.
To enable policy routing, you must identify which route map to use for policy routing and create the route map. The route map itself specifies the match criteria and the resulting action if all of the match clauses are met. These steps are described in the following three task tables.
To enable policy routing on an interface, indicate which route map the router should use by performing the following task in interface configuration mode. All packets arriving on the specified interface will be subject to policy routing. This command disables fast switching of all packets arriving on this interface.
You must also define the route map to be used for policy routing. Use the following command in global configuration mode:
route-map map-tag [permit | deny] [sequence-number]
Define a route map to control where packets are output.
The next step is to define the criteria by which packets are examined to see if they will be policy-routed. No match clause in the route map indicates all packets. Use one or more of the following commands in route-map configuration mode:
The last step is to set the precedence and specify where the packets that pass the match criteria are output. To do so, use the following commands in route-map configuration mode:
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The precedence setting in the IP header determines whether, during times of high traffic, the packets will be treated with more or less precedence than other packets. By default, the Cisco IOS software leaves this value untouched; the header remains with the precedence value it had.
The precedence bits in the IP header can be set in the router when policy routing is enabled. When the packets containing those headers arrive at another router, the packets are ordered for transmission according to the precedence set, if the queuing feature is enabled. The router does not honor the precedence bits if queuing is not enabled; the packets are sent in first in, first out order.
You can change the precedence setting, using either a number or name. The names came from RFC 791, but are evolving. You can enable other features that use the values in the set ip precedence command to determine precedence. lists the possible numbers and their corresponding name, from least important to most important.
Table 6 IP Precedence Values
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routine
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priority
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immediate
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flash
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flash-override
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critical
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internet
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network
The set commands can be used in conjunction with each other. They are evaluated in the order shown in the previous task table. A usable next hop implies an interface. Once the local router finds a next hop and a usable interface, it routes the packet.
To display the cache entries in the policy route-cache, use the show ip cache policy command.
If you want policy routing to be fast-switched, see the section "Enable Fast-Switched Policy Routing," which follows.
See the "Policy Routing Example" section at the end of this chapter for an example of policy routing.
Enable Fast-Switched Policy Routing
IP policy routing can now be fast-switched. Prior to this feature, policy routing could only be process switched, which meant that on most platforms, the switching rate was approximately 1,000 to 10,000 packets per second. This was not fast enough for many applications. Users who need policy routing to occur at faster speeds can now implement policy routing without slowing down the router.
Fast-switched policy routing supports all of the match commands and most of the set commands, except for the following restrictions:
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Policy routing must be configured before you configure fast-switched policy routing. Fast switching of policy routing is disabled by default. To have policy routing be fast-switched, use the following command in interface configuration mode:
Enable Local Policy Routing
Packets that are generated by the router are not normally policy-routed. To enable local policy routing for such packets, indicate which route map the router should use by using the following command in global configuration mode. All packets originating on the router will then be subject to local policy routing.
ip local policy route-map map-tag
Identify the route map to use for local policy routing.
Use the show ip local policy command to display the route map used for local policy routing, if one exists.
Manage Authentication Keys
Key management is a method of controlling authentication keys used by routing protocols. Not all protocols can use key management. Authentication keys are available for Director Response Protocol (DRP) Agent, EIGRP, and RIP Version 2.
Before you manage authentication keys, authentication must be enabled. See the appropriate protocol chapter to see how to enable authentication for that protocol.
To manage authentication keys, define a key chain, identify the keys that belong to the key chain, and specify how long each key is valid. Each key has its own key identifier (specified with the key number command), which is stored locally. The combination of the key identifier and the interface associated with the message uniquely identifies the authentication algorithm and MD5 authentication key in use.
You can configure multiple keys with lifetimes. Only one authentication packet is sent, regardless of how many valid keys exist. The software examines the key numbers in order from lowest to highest, and uses the first valid key it encounters. The lifetimes allow for overlap during key changes. Note that the router must know the time. Refer to the NTP and calendar commands in the "Performing Basic System Management" chapter of the Configuration Fundamentals Configuration Guide.
To manage authentication keys, use the following commands beginning in global configuration mode:
Use the show key chain command to display key chain information. For examples of key management, see the "Manage Authentication Keys" section at the end of this chapter.
Monitor and Maintain the IP Network
You can remove all contents of a particular cache, table, or database. You also can display specific statistics. The following sections describe each of these tasks.
Clear Routes from the IP Routing Table
You can remove all contents of a particular table. Clearing a table can become necessary when the contents of the particular structure have become, or are suspected to be, invalid.
To clear one or more routes from the IP routing table, use the following command in EXEC mode:
clear ip route {network [mask] | *}
Clear one or more routes from the IP routing table.
Display System and Network Statistics
You can display specific statistics such as the contents of IP routing tables, caches, and databases. Information provided can be used to determine resource utilization and solve network problems. You can also display information about node reachability and discover the routing path your device's packets are taking through the network.
To display various routing statistics, use the following commands in EXEC mode:
IP Routing Protocol-Independent Configuration Examples
The following sections provide routing protocol-independent configuration examples:
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Variable-Length Subnet Mask Example
In the following example, a 14-bit subnet mask is used, leaving two bits of address space reserved for serial line host addresses. There is sufficient host address space for two host endpoints on a point-to-point serial link.
interface ethernet 0ip address 131.107.1.1 255.255.255.0! 8 bits of host address space reserved for ethernetsinterface serial 0ip address 131.107.254.1 255.255.255.252! 2 bits of address space reserved for serial lines! Router is configured for OSPF and assigned AS 107router ospf 107! Specifies network directly connected to the routernetwork 131.107.0.0 0.0.255.255 area 0.0.0.0Overriding Static Routes with Dynamic Protocols Example
In the following example, packets for network 10.0.0.0 from Router B (where the static route is installed) will be routed through 131.108.3.4 if a route with an administrative distance less than 110 is not available. illustrates this point. The route learned by a protocol with an administrative distance of less than 110 might cause Router B to send traffic destined for network 10.0.0.0 via the alternate path—through Router D.
ip route 10.0.0.0 255.0.0.0 131.108.3.4 110Figure 35 Overriding Static Routes
Administrative Distance Examples
In the following example, the router igrp global configuration command sets up IGRP routing in autonomous system 109. The network router configuration commands specify IGRP routing on networks 192.31.7.0 and 128.88.0.0. The first distance router configuration command sets the default administrative distance to 255, which instructs the router to ignore all routing updates from routers for which an explicit distance has not been set. The second distance command sets the administrative distance to 90 for all routers on the Class C network 192.31.7.0. The third distance command sets the administrative distance to 120 for the router with the address 128.88.1.3.
router igrp 109network 192.31.7.0network 128.88.0.0distance 255distance 90 192.31.7.0 0.0.0.255distance 120 128.88.1.3 0.0.0.0The following example assigns the router with the address 192.31.7.18 an administrative distance of 100, and all other routers on subnet 192.31.7.0 an administrative distance of 200:
distance 100 192.31.7.18 0.0.0.0distance 200 192.31.7.0 0.0.0.255However, if you reverse the order of these commands, all routers on subnet 192.31.7.0 are assigned an administrative distance of 200, including the router at address 192.31.7.18:
distance 200 192.31.7.0 0.0.0.255distance 100 192.31.7.18 0.0.0.0Assigning administrative distances is a problem unique to each network and is done in response to the greatest perceived threats to the connected network. Even when general guidelines exist, the network manager must ultimately determine a reasonable matrix of administrative distances for the network as a whole.
In the following example, the distance value for IP routes learned is 90. Preference is given to these IP routes rather than routes with the default administrative distance value of 110.
router isisdistance 90 ipStatic Routing Redistribution Example
In the example that follows, three static routes are specified, two of which are to be advertised. Do this by specifying the redistribute static router configuration command, then specifying an access list that allows only those two networks to be passed to the IGRP process. Any redistributed static routes should be sourced by a single router to minimize the likelihood of creating a routing loop.
ip route 192.1.2.0 255.255.255.0 192.31.7.65ip route 193.62.5.0 255.255.255.0 192.31.7.65ip route 131.108.0.0 255.255.255.0 192.31.7.65access-list 3 permit 192.1.2.0access-list 3 permit 193.62.5.0!router igrp 109network 192.31.7.0default-metric 10000 100 255 1 1500redistribute staticdistribute-list 3 out staticIGRP Redistribution Example
Each IGRP routing process can provide routing information to only one autonomous system; the Cisco IOS software must run a separate IGRP process and maintain a separate routing database for each autonomous system it services. However, you can transfer routing information between these routing databases.
Suppose the router has one IGRP routing process for network 15.0.0.0 in autonomous system 71 and another for network 192.31.7.0 in autonomous system 109, as the following commands specify:
router igrp 71network 15.0.0.0router igrp 109network 192.31.7.0To transfer a route to 192.31.7.0 into autonomous system 71 (without passing any other information about autonomous system 109), use the command in the following example:
router igrp 71redistribute igrp 109distribute-list 3 out igrp 109access-list 3 permit 192.31.7.0RIP and IGRP Redistribution Example
Consider a WAN at a university that uses RIP as an interior routing protocol. Assume that the university wants to connect its WAN to a regional network, 128.1.0.0, which uses IGRP as the routing protocol. The goal in this case is to advertise the networks in the university network to the routers on the regional network. The commands for the interconnecting router are listed in the example that follows:
router igrp 109network 128.1.0.0redistribute ripdefault-metric 10000 100 255 1 1500distribute-list 10 out ripIn this example, the router global configuration command starts an IGRP routing process. The network router configuration command specifies that network 128.1.0.0 (the regional network) is to receive IGRP routing information. The redistribute router configuration command specifies that RIP-derived routing information be advertised in the routing updates. The default-metric router configuration command assigns an IGRP metric to all RIP-derived routes.
The distribute-list router configuration command instructs the Cisco IOS software to use access list 10 (not defined in this example) to limit the entries in each outgoing update. The access list prevents unauthorized advertising of university routes to the regional network.
EIGRP Redistribution Examples
Each EIGRP process provides routing information to only one autonomous system. The Cisco IOS software must run a separate EIGRP process and maintain a separate routing database for each autonomous system it services. However, you can transfer routing information between these routing databases.
Suppose the software has one EIGRP routing process for network 15.0.0.0 in autonomous system 71 and another for network 192.31.7.0 in autonomous system 109, as the following commands specify:
router eigrp 71network 15.0.0.0router eigrp 109network 192.31.7.0To transfer a route from 192.31.7.0 into autonomous system 71 (without passing any other information about autonomous system 109), use the command in the following example:
router eigrp 71redistribute eigrp 109 route-map 109-to-71route-map 109-to-71 permitmatch ip address 3set metric 10000 100 1 255 1500access-list 3 permit 192.31.7.0The following example is an alternative way to transfer a route to 192.31.7.0 into autonomous system 71. Unlike the previous configuration, this one does not allow you to arbitrarily set the metric.
router eigrp 71redistribute eigrp 109distribute-list 3 out eigrp 109access-list 3 permit 192.31.7.0RIP and EIGRP Redistribution Examples
This section provides a simple RIP redistribution example and a complex redistribution example between EIGRP and BGP.
Example 1: Simple Redistribution
Consider a WAN at a university that uses RIP as an interior routing protocol. Assume that the university wants to connect its WAN to a regional network, 128.1.0.0, which uses EIGRP as the routing protocol. The goal in this case is to advertise the networks in the university network to the routers on the regional network. The commands for the interconnecting router are listed in the example that follows:
router eigrp 109network 128.1.0.0redistribute ripdefault-metric 10000 100 255 1 1500distribute-list 10 out ripIn this example, the router global configuration command starts an EIGRP routing process. The network router configuration command specifies that network 128.1.0.0 (the regional network) is to send and receive EIGRP routing information. The redistribute router configuration command specifies that RIP-derived routing information be advertised in the routing updates. The default-metric router configuration command assigns an EIGRP metric to all RIP-derived routes.
The distribute-list router configuration command instructs the Cisco IOS software to use access list 10 (not defined in this example) to limit the entries in each outgoing update. The access list prevents unauthorized advertising of university routes to the regional network.
Example 2: Complex Redistribution
The most complex redistribution case is one in which mutual redistribution is required between an IGP (in this case EIGRP) and BGP.
Suppose that BGP is running on a router somewhere else in autonomous system 1, and that the BGP routes are injected into EIGRP routing process 1. You must use filters to ensure that the proper routes are advertised. The example configuration for router R1 illustrates use of access filters and a distribution list to filter routes advertised to BGP neighbors. This example also illustrates configuration commands for redistribution between BGP and EIGRP.
! Configuration for router R1:router bgp 1network 131.108.0.0neighbor 192.5.10.1 remote-as 2neighbor 192.5.10.15 remote-as 1neighbor 192.5.10.24 remote-as 3redistribute eigrp 1distribute-list 1 out eigrp 1!! All networks that should be advertised from R1 are controlled with access lists:!access-list 1 permit 131.108.0.0access-list 1 permit 150.136.0.0access-list 1 permit 128.125.0.0!router eigrp 1network 131.108.0.0network 192.5.10.0redistribute bgp 1OSPF Routing and Route Redistribution Examples
OSPF typically requires coordination among many internal routers, area border routers, and autonomous system boundary routers. At a minimum, OSPF-based routers can be configured with all default parameter values, with no authentication, and with interfaces assigned to areas.
Three types of examples follow:
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Basic OSPF Configuration Examples
The following example illustrates a simple OSPF configuration that enables OSPF routing process 9000, attaches Ethernet 0 to area 0.0.0.0, and redistributes RIP into OSPF, and OSPF into RIP:
interface ethernet 0ip address 130.93.1.1 255.255.255.0ip ospf cost 1!interface ethernet 1ip address 130.94.1.1 255.255.255.0!router ospf 9000network 130.93.0.0 0.0.255.255 area 0.0.0.0redistribute rip metric 1 subnets!router ripnetwork 130.94.0.0redistribute ospf 9000default-metric 1The following example illustrates the assignment of four area IDs to four IP address ranges. In the example, OSPF routing process 109 is initialized, and four OSPF areas are defined: 10.9.50.0, 2, 3, and 0. Areas 10.9.50.0, 2, and 3 mask specific address ranges, while Area 0 enables OSPF for all other networks.
router ospf 109network 131.108.20.0 0.0.0.255 area 10.9.50.0network 131.108.0.0 0.0.255.255 area 2network 131.109.10.0 0.0.0.255 area 3network 0.0.0.0 255.255.255.255 area 0!! Interface Ethernet0 is in area 10.9.50.0:interface ethernet 0ip address 131.108.20.5 255.255.255.0!! Interface Ethernet1 is in area 2:interface ethernet 1ip address 131.108.1.5 255.255.255.0!! Interface Ethernet2 is in area 2:interface ethernet 2ip address 131.108.2.5 255.255.255.0!! Interface Ethernet3 is in area 3:interface ethernet 3ip address 131.109.10.5 255.255.255.0!! Interface Ethernet4 is in area 0:interface ethernet 4ip address 131.109.1.1 255.255.255.0!! Interface Ethernet5 is in area 0:interface ethernet 5ip address 10.1.0.1 255.255.0.0Each network router configuration command is evaluated sequentially, so the specific order of these commands in the configuration is important. The Cisco IOS software sequentially evaluates the address/wildcard-mask pair for each interface. See the "IP Routing Protocols Commands" chapter of the Network Protocols Command Reference, Part 1 for more information.
Consider the first network command. Area ID 10.9.50.0 is configured for the interface on which subnet 131.108.20.0 is located. Assume that a match is determined for interface Ethernet 0. Interface Ethernet 0 is attached to Area 10.9.50.0 only.
The second network command is evaluated next. For Area 2, the same process is then applied to all interfaces (except interface Ethernet 0). Assume that a match is determined for interface Ethernet 1. OSPF is then enabled for that interface and Ethernet 1 is attached to Area 2.
This process of attaching interfaces to OSPF areas continues for all network commands. Note that the last network command in this example is a special case. With this command, all available interfaces (not explicitly attached to another area) are attached to Area 0.
Internal Router, ABR, and ASBRs Configuration Example
The following example outlines a configuration for several routers within a single OSPF autonomous system. provides a general network map that illustrates this example configuration.
Figure 36 Sample OSPF Autonomous System Network Map
In this configuration, five routers are configured in OSPF autonomous system 109:
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Autonomous system 109 is connected to the outside world via the BGP link to the external peer at IP address 11.0.0.6.
Router A—Internal Router
interface ethernet 1ip address 131.108.1.1 255.255.255.0router ospf 109network 131.108.0.0 0.0.255.255 area 1Router B—Internal Router
interface ethernet 2ip address 131.108.1.2 255.255.255.0router ospf 109network 131.108.0.0 0.0.255.255 area 1Router C—ABR
interface ethernet 3ip address 131.108.1.3 255.255.255.0interface serial 0ip address 131.108.2.3 255.255.255.0router ospf 109network 131.108.1.0 0.0.0.255 area 1network 131.108.2.0 0.0.0.255 area 0Router D—Internal Router
interface ethernet 4ip address 10.0.0.4 255.0.0.0interface serial 1ip address 131.108.2.4 255.255.255.0router ospf 109network 131.108.2.0 0.0.0.255 area 0network 10.0.0.0 0.255.255.255 area 0Router E—ASBR
interface ethernet 5ip address 10.0.0.5 255.0.0.0interface serial 2ip address 11.0.0.5 255.0.0.0router ospf 109network 10.0.0.0 0.255.255.255 area 0redistribute bgp 109 metric 1 metric-type 1router bgp 109network 131.108.0.0network 10.0.0.0neighbor 11.0.0.6 remote-as 110Complex OSPF Configuration Example
The following example configuration accomplishes several tasks in setting up an ABR. These tasks can be split into two general categories:
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The specific tasks outlined in this configuration are detailed briefly in the following descriptions. illustrates the network address ranges and area assignments for the interfaces.
Figure 37 Interface and Area Specifications for OSPF Example Configuration
The basic configuration tasks in this example are as follows:
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Configuration tasks associated with redistribution are as follows:
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The following is an example OSPF configuration:
interface ethernet 0ip address 192.42.110.201 255.255.255.0ip ospf authentication-key abcdefghip ospf cost 10!interface ethernet 1ip address 131.119.251.201 255.255.255.0ip ospf authentication-key ijklmnopip ospf cost 20ip ospf retransmit-interval 10ip ospf transmit-delay 2ip ospf priority 4!interface ethernet 2ip address 131.119.254.201 255.255.255.0ip ospf authentication-key abcdefghip ospf cost 10!interface ethernet 3ip address 36.56.0.201 255.255.0.0ip ospf authentication-key ijklmnopip ospf cost 20ip ospf dead-interval 80OSPF is on network 131.119.0.0:
router ospf 201network 36.0.0.0 0.255.255.255 area 36.0.0.0network 192.42.110.0 0.0.0.255 area 192.42.110.0network 131.119.0.0 0.0.255.255 area 0area 0 authenticationarea 36.0.0.0 stubarea 36.0.0.0 authenticationarea 36.0.0.0 default-cost 20area 192.42.110.0 authenticationarea 36.0.0.0 range 36.0.0.0 255.0.0.0area 192.42.110.0 range 192.42.110.0 255.255.255.0area 0 range 131.119.251.0 255.255.255.0area 0 range 131.119.254.0 255.255.255.0redistribute igrp 200 metric-type 2 metric 1 tag 200 subnetsredistribute rip metric-type 2 metric 1 tag 200IGRP autonomous system 200 is on 131.119.0.0:
router igrp 200network 131.119.0.0!! RIP for 192.42.110!router ripnetwork 192.42.110.0redistribute igrp 200 metric 1redistribute ospf 201 metric 1Default Metric Values Redistribution Example
The following example shows a router in autonomous system 109 using both RIP and IGRP. The example advertises IGRP-derived routes using the RIP protocol and assigns the IGRP-derived routes a RIP metric of 10.
router ripdefault-metric 10redistribute igrp 109Route Map Examples
The examples in this section illustrate the use of redistribution, with and without route maps. Examples from both the IP and CLNS routing protocols are given.
The following example redistributes all OSPF routes into IGRP:
router igrp 109redistribute ospf 110The following example redistributes RIP routes with a hop count equal to 1 into OSPF. These routes will be redistributed into OSPF as external link state advertisements with a metric of 5, metric type of Type 1, and a tag equal to 1.
router ospf 109redistribute rip route-map rip-to-ospf!
