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This chapter describes how to configure the various Internet Protocol (IP) routing protocols. For a complete description of the commands listed in this chapter, refer to the "IP Routing Protocols Commands" chapter of the Router Products Command Reference publication. For information on configuring the IP protocol, refer to the "Configuring IP" chapter of this manual. For historical background and a technical overview of IP routing protocols, see the Internetworking Technology Overview publication.
Cisco's implementation of each of the IP routing protocols is discussed in detail at the beginning of the individual protocol sections throughout this chapter.
IP routing protocols are divided into two classes: interior gateway protocols (IGPs) and exterior gateway protocols (EGPs). The IGPs and EGPs that Cisco supports are listed in the following sections.
Interior protocols are used for routing networks that are under a common network administration. All IP interior gateway protocols must be specified with a list of associated networks before routing activities can begin. A routing process listens to updates from other routers on these networks and broadcasts its own routing information on those same networks. The interior routing protocols supported are as follows:
Exterior protocols are used to exchange routing information between networks that do not share a common administration. IP exterior gateway protocols require three sets of information before routing can begin:
The supported exterior gateway protocols are as follows:
Our routers also support two router discovery protocols, Gateway Discovery Protocol (GDP) and ICMP Router Discovery Protocol (IRDP), which allow hosts to locate routers.
GDP was developed by Cisco and is not an industry standard. Unsupported example GDP clients can be obtained upon request from Cisco. Our IRDP implementation fully conforms to the router discovery protocol outlined in RFC 1256.
You can configure multiple routing protocols in a single router to connect networks that use different routing protocols. You can, for example, run RIP on one subnetted network, IGRP on another subnetted network, and exchange routing information between them in a controlled fashion. The available routing protocols were not designed to interoperate with one another, so each protocol collects different types of information and reacts to topology changes in its own way. For example, RIP uses a hop-count metric and IGRP uses a five-element vector of metric information. In the case where routing information is being exchanged between different networks that use different routing protocols, there are many configuration options that allow you to filter the exchange of routing information.
Our routers can handle simultaneous operation of up to 30 dynamic IP routing processes. The combination of routing processes on a router can consist of the following protocols (with the limits noted):
With any of the IP routing protocols, you need to create the routing process, associate networks with the routing process, and customize the routing protocol for your particular network.
You will need to perform some combination of the tasks in the following sections to configure IP routing protocols:
See the end of this chapter for IP routing protocol configuration examples.
Choosing a routing protocol is a complex task. When choosing a routing protocol, consider (at least) the following:
The following sections describe the configuration tasks associated with each supported routing protocol. This publication does not provide in-depth information on how to choose routing protocols; you must choose routing protocols that best suit your needs. For detailed information on the technology behind the major routing protocols, see the Internetworking Technology Overview manual or other internetworking publications.
The Interior Gateway Routing Protocol (IGRP) is a dynamic distance-vector routing protocol designed by Cisco Systems in the mid-1980s for routing in an autonomous system that contains large, arbitrarily complex networks with diverse bandwidth and delay characteristics.
IGRP uses a combination of user-configurable metrics including internetwork delay, bandwidth, reliability, and load.
IGRP also advertises three types of routes: interior, system, and exterior, as shown in Figure 18-1. Interior routes are routes between subnets in the network attached to a router interface. If the network attached to a router is not subnetted, IGRP does not advertise interior routes.
System routes are routes to networks within an autonomous system. The router derives system routes from directly connected network interfaces and system route information provided by other IGRP-speaking routers. System routes do not include subnet information.
Exterior routes are routes to networks outside the autonomous system that are considered when identifying a gateway of last resort. The router chooses a gateway of last resort from the list of exterior routes that IGRP provides. The router uses the gateway (router) of last resort if it does not have a better route for a packet and the destination is not a connected network. If the autonomous system has more than one connection to an external network, different routers can choose different exterior routers as the gateway of last resort.
By default, a router running IGRP sends an update broadcast every 90 seconds. It declares a route inaccessible if it does not receive an update from the first router in the route within three update periods (270 seconds). After seven update periods (630 seconds), the router removes the route from the routing table.
IGRP uses flash update and poison reverse updates to speed up the convergence of the routing algorithm. Flash update is the sending of an update sooner than the standard periodic update interval of notifying other routers of a metric change. Poison reverse updates are intended to defeat larger routing loops caused by increases in routing metrics. The poison reverse updates are sent to remove a route and place it in holddown, which keeps new routing information from being used for a certain period of time.
To configure IGRP, perform the tasks in the following sections. It is only mandatory to create the IGRP routing process; the other tasks described are optional.
To create the IGRP routing process, perform the following required tasks:
IGRP sends updates to the interfaces in the specified networks. If an interface's network is not specified, it will not be advertised in any IGRP update.
It is not necessary to have a registered autonomous system number to use IGRP. If you do not have a registered number, you are free to create your own. We recommend that if you do have a registered number, you use it to identify the IGRP process.
Because IGRP is normally a broadcast protocol, in order for IGRP routing updates to reach point-to-point or nonbroadcast networks, you must configure the router to permit this exchange of routing information.
To permit information exchange, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Define a neighboring router with which to exchange point-to-point routing information. | neighbor ip-address |
To control the set of interfaces that you want to exchange routing updates with, you can disable the sending of routing updates on specified interfaces by configuring the passive-interface command. See the discussion on filtering in the section in this chapter titled "Filter Routing Information."
IGRP can simultaneously use an asymmetric set of paths for a given destination. This feature is known as unequal-cost load balancing. Unequal-cost load balancing allows traffic to be distributed among multiple (up to four) unequal-cost paths to provide greater overall throughput and reliability. Alternate path variance (that is, the difference in desirability between the primary and alternate paths) is used to determine the feasibility of a potential route. An alternate route is feasible if the next router in the path is closer to the destination (has a lower metric value) than the current router and if the metric for the entire alternate path is within the variance. Only paths that are feasible can be used for load balancing and included in the routing table. These conditions limit the number of cases in which load balancing can occur, but ensure that the dynamics of the network will remain stable.
The following general rules apply to IGRP unequal-cost load balancing:
If these conditions are met, the route is deemed feasible and can be added to the routing table.
By default, the amount of variance is set to one (equal-cost load balancing). You can define how much worse an alternate path can be before that path is disallowed by performing the following task in router configuration mode:
| Task | Command |
|---|---|
| Define the variance associated with a particular path. | variance multiplier |
See the "IP Routing Protocol Configuration Examples" section at the end of this chapter for an example of configuring IGRP feasible successor.
By default, if IGRP or Enhanced IGRP have multiple routes of unequal cost to the same destination, the router will distribute traffic among the different routes by giving each route a share of the traffic in inverse proportion to its metric. If you want to have faster convergence to alternate routes but you do not want to send traffic across inferior routes in the normal case, you might prefer to have no traffic flow along routes with higher metrics.
To control how traffic is distributed among multiple routes of unequal cost, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Distribute traffic proportionately to the ratios of metrics, or by the minimum-cost route. | traffic-share {balanced | min} |
You have the option of altering the default behavior of IGRP routing and metric computations. This allows, for example, tuning system behavior to allow for transmissions via satellite. Although IGRP metric defaults were carefully selected to provide excellent operation in most networks, you can adjust the IGRP metric. Adjusting IGRP metric weights can dramatically affect network performance, however, so ensure you make all metric adjustments carefully.
To adjust the IGRP metric weights, perform the following task in router configuration mode. Due to the complexity of this task, we recommend that you only perform it with guidance from an experienced system designer.
| Task | Command |
|---|---|
| Adjust the IGRP metric. | metric weights tos k1 k2 k3 k4 k5 |
By default, the IGRP composite metric is a 24-bit quantity that is a sum of the segment delays and the lowest segment bandwidth (scaled and inverted) for a given route. For a network of homogeneous media, this metric reduces to a hop count. For a network of mixed media (FDDI, Ethernet, and serial lines running from 9600 bps to T1 rates), the route with the lowest metric reflects the most desirable path to a destination.
When a router learns that a network is at a greater distance than was previously known, or it learns the network is down, the route to that network is placed into holddown. During the holddown period, the route is advertised, but incoming advertisements about that network from any router other than the one that originally advertised the network's new metric will be ignored. This mechanism is often used to help avoid routing loops in the network, but has the effect of increasing the topology convergence time. To disable holddowns with IGRP, perform the following task in router configuration mode. All routers in an IGRP autonomous system must be consistent in their use of holddowns.
| Task | Command |
|---|---|
| Disable the IGRP holddown period. | no metric holddown |
The router enforces a maximum diameter to the IGRP network. Routes whose hop counts exceed this diameter will not be advertised. The default maximum diameter is 100 hops. The maximum diameter is 255 hops.
To configure the maximum diameter, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Configure the maximum network diameter. | metric maximum-hops hops |
To disable the default function that validates the source IP addresses of incoming routing updates, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Disable the checking and validation of the source IP address of incoming routing updates. | no validate-update-source |
Enhanced IGRP is an enhanced version of the Interior Gateway Routing Protocol (IGRP) developed by Cisco Systems, Inc. Enhanced IGRP uses the same distance vector algorithm and distance information as IGRP. However, the convergence properties and the operating efficiency of Enhanced IGRP have improved significantly over IGRP.
The convergence technology is based on research conducted at SRI International and employs an algorithm referred to as the Diffusing Update Algorithm (DUAL). This algorithm guarantees loop-free operation at every instant throughout a route computation and allows all routers involved in a topology change to synchronize at the same time. Routers that are not affected by topology changes are not involved in recomputations. The convergence time with DUAL rivals that of any other existing routing protocol.
IP Enhanced IGRP provides the following features:
Enhanced IGRP offers the following features:
Enhanced IGRP has four basic components:
Neighbor discovery/recovery is the process that routers use to dynamically learn of other routers on their directly attached networks. Routers must also discover when their neighbors become unreachable or inoperative. Neighbor discovery/recovery is achieved with low overhead by periodically sending small hello packets. As long as hello packets are received, a router can determine that a neighbor is alive and functioning. Once this status is determined, the neighboring routers can exchange routing information.
The reliable transport protocol is responsible for guaranteed, ordered delivery of Enhanced IGRP packets to all neighbors. It supports intermixed transmission of multicast and unicast packets. Some Enhanced IGRP packets must be transmitted reliably and others need not be. For efficiency, reliability is provided only when necessary. For example, on a multiaccess network that has multicast capabilities, such as Ethernet, it is not necessary to send hellos reliably to all neighbors individually. Therefore, Enhanced IGRP sends a single multicast hello with an indication in the packet informing the receivers that the packet need not be acknowledged. Other types of packets, such as updates, require acknowledgment, and this is indicated in the packet. The reliable transport has a provision to send multicast packets quickly when there are unacknowledged packets pending. Doing so helps ensure that convergence time remains low in the presence of varying speed links.
The DUAL finite state machine embodies the decision process for all route computations. It tracks all routes advertised by all neighbors. DUAL uses the distance information, known as a metric, to select efficient, loop-free paths. DUAL selects routes to be inserted into a routing table based on feasible successors. A successor is a neighboring router used for packet forwarding that has a least-cost path to a destination that is guaranteed not to be part of a routing loop. When there are no feasible successors but there are neighbors advertising the destination, a recomputation must occur. This is the process whereby a new successor is determined. The amount of time it takes to recompute the route affects the convergence time. Even though the recomputation is not processor intensive, it is advantageous to avoid recomputation if it is not necessary. When a topology change occurs, DUAL will test for feasible successors. If there are feasible successors, it will use any it finds in order to avoid unnecessary recomputation.
The protocol-dependent modules are responsible for network layer protocol-specific tasks. An example is the IP Enhanced IGRP module, which is responsible for sending and receiving Enhanced IGRP packets that are encapsulated in IP. It is also responsible for parsing Enhanced IGRP packets and informing DUAL of the new information received. IP Enhanced IGRP asks DUAL to make routing decisions, but the results are stored in the IP routing table. Also, IP Enhanced IGRP is responsible for redistributing routes learned by other IP routing protocols.
To configure IP Enhanced IGRP, complete the tasks in the following sections. At a minimum, you must enable IP Enhanced IGRP. The remaining tasks are optional.
See the end of this chapter for configuraiton examples.
To create an IP Enhanced IGRP routing process, perform the following tasks:
IP Enhanced IGRP sends updates to the interfaces in the specified network(s). If you do not specify an interface's network, it will not be advertised in any IP Enhanced IGRP update.
If you have routers on your network that are configured for IGRP and you want to make a transition to routing Enhanced IGRP, you need to designate transition routers that have both IGRP and Enhanced IGRP configured. In these cases, perform the tasks as noted in the previous section, "Enable IP Enhanced IGRP," and also read the section on configuring IGRP in this chapter. You must use the same autonomous system number in order for routes to be redistributed automatically.
To configure IP Enhanced IGRP-specific parameters, perform one or more of the following tasks:
IP Enhanced IGRP can simultaneously use an asymmetric set of paths for a given destination. This feature is known as unequal-cost load balancing. Unequal-cost load balancing allows traffic to be distributed among up to four unequal-cost paths to provide greater overall throughput and reliability. Alternate path variance (the difference in desirability between the primary and alternate paths) is used to determine the feasibility of a potential route. An alternate route is feasible if the next router in the path is closer to the destination (has a lower metric value) than the current router and if the metric for the entire alternate path is within the variance. Only paths that are feasible can be used for load balancing and included in the routing table. These conditions limit the number of cases in which load balancing can occur, but ensure that the dynamics of the network will remain stable.
The following general rules apply to IP Enhanced IGRP unequal-cost load balancing:
If these conditions are met, the route is deemed feasible and can be added to the routing table.
By default, the amount of variance is set to one (equal-cost load balancing). To change the variance to define how much worse an alternate path can be before that path is disallowed, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Define the variance associated with a particular path. | variance multiplier |
You can adjust the default behavior of IP Enhanced IGRP routing and metric computations. For example, this allows you to tune system behavior to allow for satellite transmission. Although IP Enhanced IGRP metric defaults have been carefully selected to provide excellent operation in most networks, you can adjust the IP Enhanced IGRP metric. Adjusting IP Enhanced IGRP metric weights can dramatically affect network performance, so be careful if you adjust them.
To adjust the IP Enhanced IGRP metric weights, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Adjust the IP Enhanced IGRP metric. | metric weights tos k1 k2 k3 k4 k5 |
By default, the IP Enhanced IGRP composite metric is a 32-bit quantity that is a sum of the segment delays and the lowest segment bandwidth (scaled and inverted) for a given route. For a network of homogeneous media, this metric reduces to a hop count. For a network of mixed media (FDDI, Ethernet, and serial lines running from 9600 bps to T1 rates), the route with the lowest metric reflects the most desirable path to a destination.
You can configure IP Enhanced IGRP to perform automatic summarization of subnet routes into network-level routes. For example, you can configure subnet 131.108.1.0 to be advertised as 131.108.0.0 over interfaces that have subnets of 192.31.7.0 configured. Automatic summarization is performed when there are two or more network router configuration commands configured for the IP Enhanced IGRP process. By default, this feature is enabled.
To disable automatic summarization, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Disable automatic summarization. | no auto-summary |
Route summarization works in conjunction with the ip summary-address eigrp interface configuration command, in which additional summarization can be performed. If auto-summary is in effect, there usually is no need to configure network level summaries using the ip summary-address eigrp command.
You can configure a summary aggregate address for a specified interface. If there are any more-specific routes in the routing table, IP Enhanced IGRP will advertise the summary address out the interface with a metric equal to the minimum of all more-specific routes.
To configure a summary aggregate address, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Configure a summary aggregate address. | ip summary-address eigrp autonomous-system-number address mask |
To configure protocol-independent parameters, perform one or more of the following tasks:
In addition to running multiple routing protocols simultaneously, the router can redistribute information from one routing protocol to another. For example, you can instruct the router to readvertise IP Enhanced IGRP-derived routes using the RIP protocol, or to readvertise static routes using the IP Enhanced IGRP protocol. This capability applies to all the IP-based routing protocols.
You may also conditionally control the redistribution of routes between routing domains by defining a method known as route maps between the two domains.
To redistribute routes from one protocol into another, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Redistribute routes from one routing protocol into another. | redistribute protocol autonomous-system-number [route-map map-tag] |
To define route maps, perform the following task in global configuration mode:
| Task | Command |
|---|---|
| Define any route maps needed to control redistribution. | route-map map-tag {permit | deny} sequence-number |
By default, the redistribution of default information between IP Enhanced IGRP processes is enabled. To disable the redistribution, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Disable the redistribution of default information between IP Enhanced IGRP processes. | no default-information allowed {in | out} |
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 IP Enhanced 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.
To set metrics for redistributed routes, perform the first task when redistributing from IP Enhanced IGRP, and perform the second task when redistributing into IP Enhanced IGRP. Each task is done in router configuration mode.
You can filter routing protocol information by performing the following tasks:
Use the information in the following sections to perform these tasks.
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 feature applies to all IP-based routing protocols except BGP and EGP.
To prevent routing updates through a specified interface, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Suppress the sending of routing updates through a router interface. | passive-interface type unit |
To control which routers learn about routes, you can control the advertising of routesin routing updates. To do this, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Control the advertising of routes in routing updates. | distribute-list access-list-number out [interface-name | routing-process | autonomous-system-number] |
To control the processing of routes listed in incoming updates, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Control which incoming route updates are processes. | distribute-list access-list-number in [interface-name] |
To provide a local mechanism for increasing the value of routing metrics, you can apply an offset to routing metrics. To do so, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Apply an offset to routing metrics. | offset-list [access-list-number] {in | out} offset |
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, the same route may be advertised by more than one routing process. Specifying administrative distance values enables the router to discriminate between sources of routing information. The router always picks the route whose routing protocol has the lowest administrative distance.
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. Table 18-1 shows the default administrative distance for various routing information sources.
| Route Source | Default Distance |
|---|---|
| Connected interface | 0 |
| Static route | 1 |
| Enhanced IGRP summary route | 5 |
| External BGP | 20 |
| Internal Enhanced IGRP | 90 |
| IGRP | 100 |
| OSPF | 110 |
| IS-IS | 115 |
| RIP | 120 |
| EGP | 140 |
| External Enhanced IGRP | 170 |
| Internal BGP | 200 |
| Unknown | 255 |
For example, consider a router using IP Enhanced IGRP and RIP. Suppose you trust the IP Enhanced IGRP-derived routing information more than the RIP-derived routing information. Because the default IP Enhanced IGRP administrative distance is lower than that for RIP, the router uses the IP Enhanced IGRP-derived information and ignores the RIP-derived information. However, if you lose the source of the IP Enhanced IGRP-derived information (for example, because of a power shutdown), the router uses the RIP-derived information until the IP Enhanced IGRP-derived information reappears.
To filter sources of routing information, perform the following tasks in router configuration mode:
| Task | Command |
|---|---|
| Filter on routing information sources. | distance eigrp internal-distance external-distance |
You can adjust the interval between hello packets and the hold time.
Routers periodically send hello packets to each other to dynamically learn of other routers on their directly attached networks. The routers use this information to discover who their neighbors are and to learn when their neighbors become unreachable or inoperative. By default, hello packets are sent every 5 seconds.
You can configure the hold time on a specified interface for the IP Enhanced IGRP routing process designated by the autonomous system number. The hold time is advertised in hello packets and indicates to neighbors the length of time they should consider the sender valid. The default hold time is three times the hello interval, or 15 seconds.
To change the interval between hello packets, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Configure the hello interval for an IP Enhanced IGRP routing process. | ip hello-interval eigrp autonomous-system-number seconds |
On very congested and large networks, 15 seconds may not be sufficient time for all routers to receive hello packets from their neighbors. In this case, you may want to increase the hold time.
To change the hold time, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Configure the hold time for an IP Enhanced IGRP routing process. | ip hold-time eigrp autonomous-system-number seconds |
Split horizon controls the sending of IP Enhanced IGRP update and query packets. When split horizon is enabled on an interface, these packets are not sent for destinations for which this interface is the next hop. This reduces the possibility of routing loops.
By default, split horizon is enabled on all interfaces.
Split horizon blocks information about routes from being advertised by a router out any interface from which that information originated. This behavior usually optimizes communications among multiple routers, particularly when links are broken. However, with nonbroadcast networks, such as Frame Relay and SMDS, situations can arise for which this behavior is less than ideal. For these situations, you may wish to disable split horizon.
To disable split horizon, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Disable split horizon. | no ip split-horizon eigrp autonomous-system-number |
Open Shortest Path First (OSPF) is an IGP developed by the OSPF working group of the Internet Engineering Task Force (IETF). Designed expressly for IP networks, OSPF supports IP subnetting and tagging of externally derived routing information. OSPF also allows packet authentication and uses IP multicast when sending/receiving packets.
We support RFC 1253, Open Shortest Path First (OSPF) MIB, August 1991. The OSPF MIB defines an IP routing protocol that provides management information related to OSPF and is supported by Cisco routers.
Cisco's implementation conforms to the OSPF Version 2 specifications detailed in the Internet RFC 1583. The list that follows outlines key features supported in Cisco's OSPF implementation:
OSPF typically requires coordination among many internal routers, area border routers (routers connected to multiple areas), and autonomous system boundary routers. At a minimum, OSPF-based routers can be configured with all default parameter values, no authentication, and interfaces assigned to areas. If you intend to customize your environment, you must ensure coordinated configurations of all routers.
To configure OSPF, complete the tasks in the following sections. Enabling OSPF is mandatory; the other tasks are optional but might be required for your application.
In addition, you can specify route redistribution; see the task "Redistribute Routing Information" later in this chapter for information on how to configure route redistribution.
As with other routing protocols, enabling OSPF requires that you create an OSPF routing process, specify the range of IP addresses to be associated with the routing process, and assign area IDs to be associated with that range of IP addresses. Perform the following tasks, starting in global configuration mode:
Our OSPF implementation allows you to alter certain interface-specific OSPF parameters, as needed.You are not required to alter any of these parameters, but some interface parameters must be consistent across all routers in an attached network. Therefore, be sure that if you do configure any of these parameters, the configurations for all routers on your network have compatible values.
In interface configuration mode, specify any of the following interface parameters as needed for your network:
OSPF classifies different media into three types of networks by default:
You can configure your network as either a broadcast or a nonbroadcast multiaccess network.
X.25 and Frame Relay provide an optional broadcast capability that can be configured in the map to allow OSPF to run as a broadcast network. See the x25 map and frame-relay map command descriptions in the Router Products Command Reference publication for more detail.
You have the choice of configuring your OSPF network type to either broadcast or nonbroadcast multiaccess, regardless of the default media type. Using this feature, you can configure broadcast networks as nonbroadcast multiaccess networks when, for example, you have routers in your network that do not support multicast addressing. You also can configure nonbroadcast multiaccess networks, such as X.25, Frame Relay, and SMDS, as broadcast networks. This feature saves you from having to configure neighbors, as described in the section "Configure OSPF for Nonbroadcast Networks."
Configuring nonbroadcast multiaccess networks as either broadcast or nonbroadcast assumes that there are virtual circuits from every router to every router or fully-meshed network. This is not true for some cases, for example, due to cost constraints or when you have only a partially-meshed network. In these cases, you can configure the OSPF network type as a point-to-multipoint network. Routing between two routers that are not directly connected will go through the router that has virtual circuits to both routers. Note that you do not need to configure neighbors when using this feature.
To configure your OSPF network type, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Configure the OSPF network type for a specified interface. | ip ospf network {broadcast | non-broadcast | point-to-multipoint} |
Because there might be many routers attached to an OSPF network, a designated router is selected for the network. It is necessary to use special configuration parameters in the designated router selection if broadcast capability is not configured.
These parameters need only be configured in those routers that are themselves eligible to become the designated router or backup designated router (in other words, routers with a nonzero router priority value).
To configure routers that interconnect to nonbroadcast networks, perform the following task in router configuration mode
| Task | Command |
|---|---|
| Configure routers interconnecting to nonbroadcast networks. | neighbor ip-address [priority number] [poll-interval seconds] |
You can specify the following neighbor parameters, as required:
Our OSPF software allows you to configure several area parameters. These area parameters, shown in the following table, include authentication, defining stub areas, and assigning specific costs to the default summary route. Authentication allows password-based protection against unauthorized access to an area. Stub areas are areas into which information on external routes is not sent. Instead, there is a default external route generated by the area border router into the stub area for destinations outside the autonomous system.
To further reduce the number of link state advertisements sent into a stub area, you can configure no-summary on the Area Border Router (ABR) to prevent it from sending summary link advertisements (link state advertisements type 3) into the stub area.
In router configuration mode, specify any of the following area parameters as needed for your network:
Route summarization is the consolidation of advertised addresses. This feature causes a single summary route to be advertised to other areas by an area border router. In OSPF, an area border router will advertise networks in one area into another area. If the network numbers in an area are assigned in a way such that they are contiguous, you can configure the area border router to advertise a summary route that covers all the individual networks within the area that fall into the specified range.
To specify an address range, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Specify an address range for which a single route will be advertised. | area area-id range address mask |
When redistributing routes from other protocols into OSPF (as described in the section "Configure Routing Protocol-Independent Features" later in this chapter, each route is advertised individually in an external link state advertisement (LSA). However, you can configure the router to advertise a single route for all the redistributed routes that are covered by a specified network address and mask. Doing so helps decrease the size of the OSPF link state database.
To have the router advertise one summary route for all redistributed routes covered by a network address and mask, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Specifiy an address and mask that covers redistributed routes, so only one summary route is advertised. | summary-address address mask |
In OSPF, all areas must be connected to a backbone area. If there is a break in backbone continuity, or the backbone is purposefully partitioned, you can establish a virtual link. The two end points of a virtual link are Area Border Routers. The virtual link must be configured in both routers. The configuration information in each router consists of the other virtual endpoint (the other Area Border Router), and the nonbackbone area that the two routers have in common (called the transit area). Note that virtual links cannot be configured through stub areas.
To establish a virtual link, perform the following task in router configuration mode:
To display information about virtual links, use the show ip ospf virtual-links EXEC command. To display the router ID of an OSPF router, use the show ip ospf EXEC command.
You can force an autonomous system boundary router to generate a default route into an OSPF routing domain. Whenever you specifically configure redistribution of routes into an OSPF routing domain, the router automatically becomes an autonomous system boundary router. However, an autonomous system boundary router does not, by default, generate a default route into the OSPF routing domain.
To force the autonomous system boundary router to generate a default route, perform the following task in router configuration mode:
See also the discussion of redistribution of routes in the "Configure Routing Protocol-Independent Features" section later in this chapter.
You can configure OSPF to look up Domain Name System (DNS) names for use in all OSPF show command displays. This feature makes it easier to identify a router, because it is displayed by name rather than by its router ID or neighbor ID.
To configure DNS name lookup, perform the following task in global configuration mode:
| Task | Command |
|---|---|
| Configure DNS name lookup. | ip ospf-name-lookup |
OSPF uses the largest IP address configured on the router's interfaces as its router ID. If the interface associated with this IP address is ever brought down, or if the address is removed, the OSPF process must recalculate a new router ID and resend all of its routing information out its interfaces.
If a loopback interface is configured with an IP address, the router will use this IP address as its router ID, even if other interfaces have larger IP addresses. Since loopback interfaces never go down, greater stability in the routing table is achieved.
OSPF automatically prefers a loopback interface over any other kind, and it chooses the first loopback interface found. If no loopback interfaces are present, the highest IP address in the router is chosen. You cannot tell OSPF to use any particular interface.
To configure an IP address on a loopback interface, perform the following tasks, starting in global configuration mode:
| Task | Command |
|---|---|
| Step 1 Create a loopback interface, which places you in interface configuration mode. | interface loopback 01 |
| Step 2 Assign an IP address to this interface. | ip address address mask |
In Cisco IOS Release 10.2 and earlier, OSPF assigned default OSPF metrics to interfaces regardless of the interface bandwidth. It gave both 64K and T1 links the same metric (1562), and thus required an explicit ip ospf cost command in order to take advantage of the faster link.
In Cisco IOS Release 10.3, by default, OSPF calculates the OSPF metric for an interface according to the bandwidth of the interface. For example, a 64K link gets a metric of 1562, while a T1 link gets a metric of 64. To disable this feature, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Disable default OSPF metric calculations based on interface bandwidth, resulting in a fixed default metric assignment. | no ospf auto-cost-determination |
Because simplex interfaces between two routers on an Ethernet represent only one network segment, for OSPF you have to configure the transmitting interface to be a passive interface. This prevents OSPF from sending hello packets for the transmitting interface. Both routers are able to see each other via the hello packet generated for the receiving interface.
To configure OSPF on simplex Ethernet interfaces, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Suppress the sending of hello packets through the specified interface. | passive-interface interface |
You can configure the delay time between when OSPF receives a topology change and when it starts a Shortest Path First (SPF) calculation. You can also configure the hold time between two consecutive SPF calculations. To do this, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Configure route calculation timers. | timers spf spf-delay spf-holdtime |
The Routing Information Protocol (RIP) is a relatively old but still commonly used IGP created for use in small, homogeneous networks. It is a classical distance-vector routing protocol.
RIP uses broadcast User Datagram Protocol (UDP) data packets to exchange routing information. Each router sends routing information updates every 30 seconds; this process is termed advertising. If a router does not receive an update from another router for 180 seconds or more, it marks the routes served by the nonupdating router as being unusable. If there is still no update after 240 seconds, the router removes all routing table entries for the nonupdating router.
The measure, or metric, that RIP uses to rate the value of different routes is the hop count. The hop count is the number of routers that can be traversed in a route. A directly connected network has a metric of zero; an unreachable network has a metric of 16. This small range of metrics makes RIP unsuitable as a routing protocol for large networks. If the router has a default network path, RIP advertises a route that links the router to the pseudonetwork 0.0.0.0. The network 0.0.0.0 does not exist; RIP treats 0.0.0.0 as a network to implement the default routing feature. Our routers will advertise the default network if a default was learned by RIP, or if the router has a gateway of last resort and RIP is configured with a default metric.
RIP sends updates to the interfaces in the specified networks. If an interface's network is not specified, it will not be advertised in any RIP update.
For information about filtering RIP information, see the "Filter Routing Information" section later in this chapter. RIP is documented in RFC 1058.
To configure RIP, perform the following tasks, starting in global configuration mode:
| Task | Command |
|---|---|
| Step 1 Enable a RIP routing process, which places you in router configuration mode. | router rip |
| Step 2 Associate a network with a RIP routing process. | network network-number |
It is possible to run IGRP and RIP concurrently. The IGRP information will override the RIP information by default because of IGRP's administrative distance.
However, running IGRP and RIP concurrently does not work well when the network topology changes. Because IGRP and RIP have different update timers and because they require different amounts of time to propagate routing updates, one part of the network will end up believing IGRP routes and another part will end up believing RIP routes. This will result in routing loops. Even though these loops do not exist for very long, the time to live (TTL) will quickly reach zero, and ICMP will send a "TTL exceeded" message. This message will cause most applications to stop attempting network connections.
To disable the default function that validates the source IP addresses of incoming routing updates, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Disable the checking and validation of the source IP address of incoming routing updates. | no validate-update-source |
Because RIP is normally a broadcast protocol, in order for RIP routing updates to reach point-to-point or nonbroadcast networks, you must configure the router to permit this exchange of routing information.
You configure the router to permit this exchange of routing information by performing the following task in router configuration mode:
| Task | Command |
|---|---|
| Define a neighboring router with which to exchange point-to-point routing information. | neighbor ip-address |
To control the set of interfaces that you want to exchange routing updates with, you can disable the sending of routing updates on specified interfaces by configuring the passive-interface command. See the discussion on filtering in the section in this chapter titled "Filter Routing Information."
IS-IS, which stands for Intermediate System-to-Intermediate System, is an International Organization for Standardization (ISO) dynamic routing specification. IS-IS is described in ISO 10589. Cisco's implementation of IS-IS allows you to configure IS-IS as an IP routing protocol on your router.
To configure IS-IS, complete the tasks in the following sections. Only enabling IS-IS is required; the remainder of the tasks are optional although you might be required to perform them depending upon your specific application.
In addition, you can filter routing information (see the task "Filter Routing Information" later in this chapter for information on how to do this), and specify route redistribution (see the task "Redistribute Routing Information" later in this chapter for information on how to do this).
As with other routing protocols, enabling IS-IS requires that you create an IS-IS routing process and assign it to specific networks. You can specify only one IS-IS process per router. Only one IS-IS process is allowed whether you run it in integrated mode, ISO CLNS only, or IP only.
Network Entity Titles (NETs) define the area addresses for the IS-IS area. Multiple NETs per router are allowed, up to a maximum of three. Refer to the "Configuring ISO CLNS" chapter for a more detailed discussion of NETs.
Perform the following tasks to enable IS-IS on the router:
See the "IP Routing Protocol Configuration Examples" section at the end of this chapter for an example of configuring IS-IS as an IP routing protocol.
Our IS-IS implementation allows you to alter certain interface-specific IS-IS parameters. You can do the following:
You are not required to alter any of these parameters, but some interface parameters must be consistent across all routers in an attached network. Therefore, be sure that if you do configure any of these parameters, the configurations for all routers on the network have compatible values.
You can configure a cost for a specified interface. The only metric that is supported by the router and that you can configure is the default-metric, which you can configure for Level 1 and/or Level 2 routing. The other metrics currently are not supported.
To configure the metric for the specified interface, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Configure the metric (or cost) for the specified interface. | isis metric default-metric [delay-metric [expense-metric [error-metric]]] {level-1 | level-2} |
You can specify the length of time, in seconds, between hello packets that the router sends on the interface.
To specify the length of time between hello packets for the specified interface, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Specify the length of time, in seconds, between hello packets the router sends on the specified interface. | isis hello-interval seconds {level-1 | level-2} |
The hello interval can be configured independently for Level 1 and Level 2, except on serial
point-to-point interfaces. (Because there is only a single type of hello packet sent on serial links, it is independent of Level 1 or Level 2.) Specify an optional level for X.25, SMDS, and Frame Relay multiaccess networks.
Complete Sequence Number PDUs (CSNPs) are sent by the designated router to maintain database synchronization. You can configure the IS-IS CSNP interval for the interface.
To configure the CSNP interval for the specified interface, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Configure the IS-IS CSNP interval for the specified interface. | isis csnp-interval seconds {level-1 | level-2} |
This feature does not apply to serial point-to-point interfaces. It applies to WAN connections if the WAN is viewed as a multiaccess meshed network.
You can configure the number of seconds between retransmission of IS-IS link state PDUs (LSPs) for point-to-point links.
To set the retransmission level, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Configure the number of seconds between retransmission of IS-IS LSPs for point-to-point links. | isis retransmit-interval seconds |
The value you specify should be an integer greater than the expected round-trip delay between any two routers on the attached network. The setting of this parameter should be conservative, or needless retransmission will result. The value should be larger for serial lines and virtual links.
You can configure the priority to use for designated router election. Priorities can be configured for Level 1 and Level 2 individually.
To specify the designated router election, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Configure the priority to use for designated router election. | isis priority value {level-1 | level-2} |
You can specify adjacency levels on a specified interface. This parameter is also referred to as the interface circuit type.
To specify the interface circuit type, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Configure the type of adjacency desired for neighbors on the specified interface (the interface circuit type). | isis circuit-type {level-1 | level-1-2 | level-2-only} |
You can assign different passwords for different routing levels. Specifying Level 1 or Level 2 configures the password for only Level 1 or Level 2 routing, respectively. If you do not specify a level, the default is Level 1. By default, authentication is disabled.
To configure a password for the specified level, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Configure the authentication password for a specified interface. | isis password password {level-1 | level-2} |
You can configure the following miscellaneous, optional IS-IS parameters:
You can force a default route into an IS-IS routing domain. Whenever you specifically configure redistribution of routes into an IS-IS routing domain, the router does not, by default, generate a default route into the IS-IS routing domain. The following feature allows you to force the boundary router do this.
To generate a default route, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Force a default route into the IS-IS routing domain. | default-information originate [metric metric-value] [metric-type type-value] {level-1 | level-1-2 | level-2} [route-map map-name] |
See also the discussion of redistribution of routes in the "Configure Routing Protocol-Independent Features" section later in this chapter.
You can configure the router to act as a Level 1 (intra-area) router, as both a Level 1 router and a Level 2 (interarea) router, or as an interarea router only.
To specify router level support, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Configure the level at which the router should operate. | is-type {level-1 | level-1-2 | level-2-only} |
You can assign passwords to areas and domains.
The area authentication password is inserted in Level 1 (station router level) LSPs, CSNPs, and Partial Sequence Number PDUs (PSNPs). The routing domain authentication password is inserted in Level 2 (the area router level) LSP, CSNP, and PSNPs.
To configure either area or domain authentication passwords, perform the following tasks in router configuration mode:
| Task | Command |
|---|---|
| Configure the area authentication password. | area-password password |
| Configure the routing domain authentication password. | domain-password password |
You can create aggregate addresses that are represented in the routing table by a summary address. This process is called route summarization. One summary address can include multiple groups of addresses for a given level. Routes learned from other routing protocols also can be summarized. The metric used to advertise the summary is the smallest metric of all the more specific routes.
To create a summary of addresses for a given level, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Create a summary of addresses for a given level. | summary-address address mask {level-1 | level-1-2 | level-2} |
The Border Gateway Protocol (BGP), as defined in RFCs 1163 and 1267, allows you to set up an interdomain routing system that automatically guarantees the loop-free exchange of routing information between autonomous systems.
In BGP, each route consists of a network number, a list of autonomous systems that information has passed through (called the autonomous system path), and a list of other path attributes. We support BGP Versions 2, 3, and 4, as defined in RFCs 1163, 1267, and 1654, respectively.
The primary function of a BGP system is to exchange network reachability information with other BGP systems, including information about the list of autonomous system paths. This information can be used to construct a graph of autonomous system connectivity from which routing loops can be pruned and with which autonomous system-level policy decisions can be enforced.
You can configure the value for the multiple exit discriminator (MULTI_EXIT_DISC, or MED) metric attribute using route maps. (The name of this metric for BGP Versions 2 and 3 is INTER_AS.) When an update is sent to an IBGP peer, the MED will be passed along without any change. This will enable all the peers in the same autonomous system to make a consistent path selection.
A third-party next-hop router address is used in the NEXT_HOP attribute, regardless of the autonomous system of that third-party router. The router automatically calculates the value for this attribute.
Transitive, optional path attributes are passed along to other BGP-speaking routers. The current BGP implementation does not generate such attributes.
BGP Version 4 (BGP4) supports classless interdomain routing (CIDR), which lets you reduce the size of your routing tables by creating aggregate routes, resulting in supernets. CIDR eliminates the concept of network classes within BGP and supports the advertising of IP prefixes. CIDR routes can be carried by OSPF and ISIS-IP.
See the "Using Route Maps with BGP" section for examples of how to use route maps to redistribute BGP4 routes.
The BGP process selects a single autonomous system path to use and to pass along to other BGP-speaking routers. Cisco's BGP implementation has a reasonable set of factory defaults that can be overridden by administrative weights. The algorithm for path selection is as follows:
To configure BGP, complete the tasks in the following sections:
The tasks in the following sections are optional:
To enable BGP routing, establish a BGP routing process on the router and specify those networks within the router's autonomous system to be advertised. Perform the following steps. There is a limit of 200 networks that can be advertised from one autonomous system.
Like other exterior gateway protocols (EGPs), BGP must completely understand the relationships it has with its neighbors. BGP supports two kinds of neighbors: internal and external. Internal neighbors are in the same autonomous system; external neighbors are in different ASs. Normally, external neighbors are adjacent to each other and share a subnet, while internal neighbors may be anywhere in the same autonomous system.
To configure BGP neighbors, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Specify a BGP neighbor. | neighbor ip-address remote-as number |
You also can configure neighbor templates that use a word argument rather than an IP address to configure BGP neighbors. This is an advanced feature requiring a well-thought-out network architecture. Do not use this feature without thoroughly understanding its application.
Perform the following tasks in router configuration mode to configure BGP neighbor templates:
Once you have defined two routers to be BGP neighbors, they will form a BGP connection and exchange routing information. If you subsequently change a BGP filter, weight, distance, version, or timer, or make a similar configuration change, you need to reset BGP connections for the configuration change to take effect. Perform either of the following tasks in EXEC mode to reset BGP connections:
| Task | Command |
| Reset a particular BGP connection. | clear ip bgp address |
| Reset all BGP connections. | clear ip bgp * |
To automatically reset BGP sessions, perform the following task in router configuration mode:
| Task | Command |
| Automatically reset BGP sessions of any directly adjacent external peer if the link used to reach it goes down. | bgp fast-external-fallover |
If you want to restrict the routing information that the router learns or advertises, you can filter BGP routing updates to and from particular neighbors. To do this, define an access list and apply it to the updates. Distribute-list filters are applied to network numbers and not autonomous system paths.
To filter BGP routing updates, perform the following task in router configuration mode:
| Task | Command |
| Filter BGP routing updates to/from neighbors as specified in an access list. | neighbor ip-address distribute-list access-list-number {in | out} |
In addition to filtering routing updates based on network numbers, you can specify an access list filter on both incoming and outbound updates based on the BGP autonomous system paths. Each filter is an access list based on regular expressions. To do this, define an autonomous system path access list and apply it to updates to and from particular neighbors. See the "Regular Expressions" appendix in the Router Products Command Reference publication for more information on forming regular expressions.
Perform the following tasks to configure BGP path filtering:
BGP supports transit policies via controlled distribution of routing information. The distribution of routing information is based on one of three values:
The COMMUNITIES attribute is a way to group destinations into communities and apply routing decisions based on the communities. This method simplifies a BGP speaker's configuration that controls distribution of routing information.
A community is a group of destinations that share some common attribute. Each destination can belong to multiple communities. Autonomous system administrators can define which communities a destination belongs to. By default, all destinations belong to the general Internet community. The community is carried as the COMMUNITIES attribute.
The COMMUNITIES attribute is an optional, transitive, global attribute in the numerical range from 1 to 4,294,967,200 or set to one of these values: