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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 Access and Communication Servers Command Reference publication. For information on configuring the IP protocol, refer to the "Configuring IP" chapter of this publication. For historical background and a technical overview of IP routing protocols, see the Internetworking Technology Overview publication.
Cisco's implementation of each IP routing protocol is discussed in detail at the beginning of the individual protocol configuration 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 next two 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 communication servers on these networks and broadcasts its own routing information on those same networks. The interior routing protocols supported are as follows:
Exterior protocols exchange routing information between networks that do not share a common administration. The supported exterior routing protocols are as follows:
IP Exterior Gateway Protocols require three pieces of information before routing can begin:
Our communication servers also support two communication server discovery protocols, Gateway Discovery Protocol (GDP) and ICMP Router Discovery Protocol (IRDP), which allow hosts to locate communication servers.
GDP was developed by Cisco and is not an industry standard. Examples of unsupported GDP clients can be obtained upon request from Cisco. Our IRDP implementation fully conforms to the communication server discovery protocol outlined in RFC 1256.
You can configure multiple routing protocols in a single communication server 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 communication servers can handle simultaneous operation of up to 30 dynamic IP routing processes.The combination of routing processes on a communication server 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 communication server interface. If the network attached to a communication server is not subnetted, IGRP does not advertise interior routes.
System routes are routes to networks within an autonomous system. The communication server derives system routes from directly connected network interfaces and system route information provided by other IGRP-speaking communication servers. 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 communication server chooses a gateway of last resort from the list of exterior routes that IGRP provides. The communication server uses the gateway (communication server) 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 communication servers can choose different exterior communication servers as the gateway of last resort.
By default, a communication server running IGRP sends an update broadcast every 90 seconds. It declares a route inaccessible if it does not receive an update from the first communication server in the route within three update periods (270 seconds). After seven update periods (630 seconds), the communication server 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 communication servers 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. Creating the IGRP routing process is mandatory; the other tasks described are optional.
To create the IGRP routing process, perform the following required tasks:
| Task | Command |
|---|---|
| Step 1 Enter global configuration mode. | See Table 2-1 earlier in this manual. |
| Step 2 Enable an IGRP routing process, which places you in router configuration mode. | router igrp process-number |
| Step 3 Associate networks with an IGRP routing process. | network network-number |
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.
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 communication server to permit this exchange of routing information.
To permit information exchange, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Define a neighboring communication server 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 "Filter Routing Information" section later in this chapter.
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 communication server in the path is closer to the destination (has a lower metric value) than the current communication server 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 communication server 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. For example, you can tune system behavior to enable 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 communication server 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 communication server 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 communication servers in an IGRP autonomous system must be consistent in their use of holddowns.
| Task | Command |
|---|---|
| Disable the IGRP holddown period. | no metric holddown |
The communication server 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 communication servers involved in a topology change to synchronize at the same time. Communication servers 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 "IP Routing Protocol Configuration Examples" section at the end of this chapter for configuration 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 communication servers on your network that are configured for IGRP and you want to make a transition to routing Enhanced IGRP, you need to designate transition communication servers that have both IGRP and Enhanced IGRP configured. In these cases, perform the tasks as noted in the previous section, "Create the IP Enhanced IGRP Routing Process," and also read the section "Configure IGRP" earlier 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 communication server in the path is closer to the destination (has a lower metric value) than the current communication server 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 |
See the "IP Routing Protocol Configuration Examples" section at the end of this chapter for an example of configuring an IP Enhanced IGRP feasible successor.
You can adjust the default behavior of IP Enhanced IGRP routing and metric computations. For example, you can 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 |
You can display communication server statistics such as the contents of IP routing tables, caches, and databases. You can use the information displayed to determine resource utilization and solve network problems. You can also display information about node reachability and discover the routing path that your communication server's packets are taking through the network.
To display various statistics, perform one or more of the following tasks at the EXEC prompt:
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.
Cisco's implementation of OSPF conforms to the OSPF Version 2 specifications detailed in Internet RFC 1583. The list that follows outlines key features supported in Cisco's OSPF implementation:
OSPF typically requires coordination among many internal communication servers, communication servers acting as area border routers (routers connected to multiple areas), and communication servers acting as autonomous system boundary routers. At a minimum, OSPF-based communication servers 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 communication servers.
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 "Redistribute Routing Information" section 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 communication servers in an attached network. Therefore, be sure that if you do configure any of these parameters, the configurations for all communication servers 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 Access and Communication Servers 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 communication servers 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 "Configure OSPF for Nonbroadcast Networks," following this section.
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} |
Because there might be many communication servers attached to an OSPF network, a designated communication server is selected for the network. It is necessary to use special configuration parameters in the designated communication server selection if broadcast capability is not configured.
These parameters need only be configured in those communication servers that are themselves eligible to become the designated communication server or backup designated communication server (in other words, communication servers with a nonzero communication server priority value).
To configure communication servers that interconnect to nonbroadcast networks, perform the following task in router configuration mode
| Task | Command |
|---|---|
| Configure communication servers 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.
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 a communication server acting as an area border router. In OSPF, a communication server acting as 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 communication server acting as an 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 |
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 communication servers acting as area border routers. The virtual link must be configured in both communication servers. The configuration information in each communication server consists of the other virtual endpoint (the other Area Border Router), and the nonbackbone area that the two communication servers 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 communication server to generate a default route into an OSPF routing domain. Whenever you specifically configure redistribution of routes into an OSPF routing domain, the communication server automatically becomes an autonomous system boundary communication server. However, an autonomous system boundary communication server does not, by default, generate a default route into the OSPF routing domain.
To force the communication server acting as an 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 communication server, because it is displayed by name rather than by its communication server 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 communication server'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 communication server will use this IP address as its router ID, even if other interfaces have larger IP addresses. Because 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 communication server 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 |
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 communication server sends routing information updates every 30 seconds; this process is termed advertising. If a communication server does not receive an update from another communication server for 180 seconds or more, it marks the routes served by the nonupdating communication server as being unusable. If there is still no update after 240 seconds, the communication server removes all routing table entries for the nonupdating communication server.
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 communication servers 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 communication server has a default network path, RIP advertises a route that links the communication server 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 systems will advertise the default network if a default was learned by RIP or if the communication server 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 communication server to permit this exchange of routing information.
You configure the communication server to permit this exchange of routing information by performing the following task in router configuration mode:
| Task | Command |
|---|---|
| Define a neighboring communication server 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 "Filter Routing Information" section later in this chapter.
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 or AS path), and a list of other path attributes. We support BGP Versions 2, 3, and 4. This section describes our implementation of BGP.
The primary function of a BGP system is to exchange network reachability information with other BGP systems, including information about the list of AS 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 communication server address is used in the NEXT_HOP attribute, regardless of the autonomous system path of that third-party communication server. The communication server automatically calculates the value for this attribute.
Transitive, optional path attributes are passed along to other BGP-speaking communication servers. 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.
See the "Using Route Maps with BGP" section later in this chapter 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 communication servers. 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 communication server and specify those networks within the communication server'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 path; external neighbors are in different autonomous system paths. 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 communication servers 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 communication server 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 AS 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 AS paths. Each filter is an access list based on regular expressions. To do this, define an AS path access list and apply it to updates to and from particular neighbors. See the "Regular Expressions" appendix in the Access and Communication Servers Command Reference publication for more information on forming regular expressions.
Perform the following tasks to configure BGP path filtering:
| Task | Command |
|---|---|
| Step 1 Enter global configuration mode. | See Table 2-1 earlier in this chapter. |
| Step 2 Define a BGP-related access list. | ip as-path access-list access-list-number {permit | deny} as-regular-expression |
| Step 3 Enter router configuration mode. | See Table 2-1 earlier in this manual. |
| Step 4 Establish a BGP filter. | neighbor ip-address filter-list access-list-number {in | out | weight weight} |
You can configure the communication server to disable next-hop processing for BGP updates to a neighbor. This is useful in non-meshed networks such as Frame Relay or X.25 where BGP neighbors might not have direct access to all other neighbors on the same IP subnet.
To disable next-hop processing, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Disable next-hop processing on BGP updates to a neighbor. | neighbor ip-address next-hop-self |
An administrative weight is a number that you can assign to a path so that you can control the path selection process.The administrative weight is local to the communication server. A weight can be a number from 0 to 65535. Paths that the communication server originates have weight 32768 by default, other paths have weight zero. If you have particular neighbors that you want to prefer for most of your traffic, you can assign a weight to all paths learned from a neighbor.
Perform the following task in router configuration mode to configure BGP administrative weights:
| Task | Command |
| Specify a weight for all paths from a neighbor. | neighbor ip-address weight weight |
In addition, you can assign weights based on autonomous system path access lists. A given weight becomes the weight of the path if the AS path is accepted by the access list. Any number of weight filters are allowed.
Perform the following tasks to assign weights based on AS path access lists:
| Task | Command |
|---|---|
| Step 1 Enter global configuration mode. | See Table 2-1 earlier in this manual. |
| Step 2 Define a BGP-related access list. | ip as-path access-list access-list-number {permit | deny} as-regular-expression |
| Step 3 Enter router configuration mode. | See Table 2-1 earlier in this manual. |
| Step 4 Configure set administrative weight on all incoming routes matching an autonomous system path filter. | neighbor ip-address filter-list access-list-number weight weight |
If your autonomous system will be passing traffic through it from another autonomous system to a third autonomous system, it is very important that your autonomous system be consistent about the routes that it advertises. For example, if your BGP were to advertise a route before all communication servers in your network had learned about the route through your IGP, your autonomous system could receive traffic that some communication servers cannot yet route. To prevent this from happening, BGP must wait until the IGP has propagated routing information across your autonomous system. This causes BGP to be synchronized with the IGP. Synchronization is enabled by default.
In some cases, you do not need synchronization. If you will not be passing traffic from a different autonomous system through your autonomous system, or if all communication servers in your autonomous system will be running BGP, you can disable synchronization. Disabling this feature can allow you to carry fewer routes in your IGP, increase the number of paths that BGP can select, and allow BGP to converge more quickly, however you must run BGP on all communication servers in your autonomous system and there must be a full IBGP connectivity mesh between these communication servers. To disable synchronization, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Disable synchronization between BGP and an IGP. | no synchronization |
When you disable synchronization, you should also clear BGP routes using the clear ip bgp command.
In general, you will not want to redistribute most BGP routes into your IGP. A common design is to redistribute one or two routes and to make them exterior routes in IGRP or have your BGP speakers generate a default route for your autonomous system. When redistributing from BGP into IGP, only the routes learned using EBGP get redistributed.
In most circumstances, you also will not want to redistribute your IGP into BGP. Just list the networks in your autonomous system with network router configuration commands and your networks will be advertised. Networks that are listed this way are referred to as local networks and have a BGP origin attribute of "IGP." They must appear in the main IP routing table and can have any source; for example, they can be directly connected or learned via an IGP. The BGP routing process periodically scans the main IP routing table to detect the presence or absence of local networks, updating the BGP routing table as appropriate.
If you do perform redistribution into BGP, you must be very careful about the routes that can be in your IGP, especially if the routes were redistributed from BGP into the IGP elsewhere. This creates a situation where BGP is potentially injecting information into the IGP and then sending such information back into BGP and vice versa.
Networks that are redistributed into BGP from the EGP protocol will be given the BGP origin attribute "EGP." Other networks that are redistributed into BGP will have the BGP origin attribute of "incomplete." The origin attribute in our implementation is only used in the path selection process.
See the "IP Routing Protocol Configuration Examples" section at the end of this chapter for an example of synchronization.
Classless Interdomain Routing (CIDR) lets you create aggregate routes, or supernets, to minimize the size of routing tables. You can configure aggregate routes in BGP either by redistributing an aggregate route into BGP or by using the conditional aggregation feature described in the next task table.
To create an aggregate address in the routing table, perform one or more of the following tasks in router configuration mode:
To disable automatic network number summarization when redistributing to BGP from IGPs, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Disable automatic network summarization. | no auto-summary |
You can adjust several miscellaneous BGP parameters, as indicated in the following subsections.
If you would like to provide BGP routing information to a large number of neighbors, you can configure BGP to accept neighbors based on an access list. If a neighbor attempts to initiate a BGP connection, its address must be accepted by the access list for the connection to be accepted. If you do this, the communication server will not attempt to initiate a BGP connection to these neighbors, so the neighbors must be explicitly configured to initiate the BGP connection. If no access list is specified, all connections are accepted.
If a neighbor is running a different version of BGP, you should configure the version of BGP that the neighbor is speaking.
External BGP peers normally must reside on a directly connected network. Sometimes it is useful to relax this restriction in order to test BGP; do so by specifying the neighbor ebgp-multihop command
For internal BGP, you might want to allow your BGP connections to stay up regardless of which interfaces are available on the communication server. To do this, you first configure a loopback interface and assign it an IP address. Next, configure the BGP update source to be the loopback interface. Finally, configure your neighbor to use the address on the loopback interface.
You can also set the minimum interval of time between BGP routing updates and apply a route map to incoming and outgoing routes.
Configure any of the following neighbor options in router configuration mode:
See the "IP Routing Protocol Configuration Examples" section at the end of this chapter for examples of configuring BGP neighbor options.
To set the absolute weight for a network, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Set the weight for a network. | network address weight weight |
You can indicate which networks are reachable using a backdoor route that the border communication server should use. A backdoor network is treated as a local network, except that it is not advertised. To configure backdoor routes, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Indicate reachable networks through backdoor routes. | network address backdoor |
To modify metric and tag information when the IP routing table is updated with BGP learned routes, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Apply route-map to routes when updating the IP routing table. | table-map route-map-name |
Administrative distance is a measure of the ability of a routing protocol to provide optimal routes. BGP uses three different administrative distances--external, internal, and local. Routes learned through external BGP are given the external distance, routes learned with internal BGP are given the internal distance, and routes that are part of this autonomous system are given the local distance. To assign a BGP administrative distance, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Assign a BGP administrative distance. | distance bgp external-distance internal-distance local-distance |
Changing the administrative distance of BGP routes is considered dangerous and generally is not recommended. The external distance should be lower than any other dynamic routing protocol, and the internal and local distances should be higher than any other dynamic routing protocol.
BGP uses certain timers to control periodic activities such as the sending of keepalive messages and the interval after not receiving a keepalive message after which the communication server declares a peer dead. You can adjust these timers. When a connection is started, BGP will negotiate the hold time with the neighbor. The smaller of the two hold times will be chosen. The keepalive timer is then set based on the negotiated holdtime and the configured keepalive time. To adjust BGP timers, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Adjust BGP timers. | timers bgp keepalive holdtime |
BGP uses the MULTI_EXIT_DISC METRIC as a hint to external neighbors about preferred paths. (The name of this metric for BGP Versions 2 and 3 is INTER_AS.) If you have a communication server that traffic should avoid, you can configure that communication server with a higher MULTI_EXIT_DISC METRIC. Doing this sets the MULTI_EXIT_DISC METRIC on all paths that the communication server advertises. Perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Set a MULTI_EXIT_DISC METRIC. | default-metric number |
You can define a particular path as more or less preferable than other paths by changing the default local preference value of 100. To assign a different default local preference value, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Change the default local preference value. | bgp default local-preference value |
You can use route maps to change the default local preference of specific paths. See the "Using Route Maps with BGP" section for examples.
To redistribute network 0.0.0.0, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Allow the redistribution of network 0.0.0.0 into BGP. | default-information originate |
The Exterior Gateway Protocol (EGP), specified in RFC 904, is an older EGP used for communicating with certain communication servers in the Defense Data Network (DDN) that the U.S. Department of Defense designates as core communication servers. EGP also was used extensively when attaching to the National Science Foundation Network (NSFnet) and other large backbone networks.
An exterior communication server uses EGP to advertise its knowledge of routes to networks within its autonomous system. It sends these advertisements to the core communication servers, which then readvertise their collected routing information to the exterior communication server. A neighbor or peer communication server is any communication server with which the communication server communicates using EGP.
Cisco's implementation of EGP supports three primary functions, as specified in RFC 904:
To enable EGP routing on your communication server, complete the tasks in the following sections. The tasks in the first two sections are mandatory; the tasks in the other sections are optional.
To enable EGP routing, you must specify an autonomous system number, generate an EGP routing process, and indicate the networks for which the EGP process will operate.
Perform these required tasks in the order given as shown in the following table:
| Task | Command |
|---|---|
| Step 1 Enter global configuration mode. | See Table 2-1 earlier in this manual. |
| Step 2 Specify the autonomous system that the communication server resides in for EGP. | autonomous-system local-as |
| Step 3 Enable an EGP routing process, which places you in router configuration mode. | router egp remote-as |
| Step 4 Specify a network to be advertised to the EGP peers of an EGP routing process. | network network-number |
A communication server using EGP cannot dynamically determine its neighbor or peer communication servers. You must therefore provide a list of neighbor communication servers.
To specify an EGP neighbor, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Specify an EGP neighbor. | neighbor ip-address |
The EGP timers consist of a hello timer and a poll time interval timer. The hello timer determines the frequency in seconds with which the communication server sends hello messages to its peer. The poll time specifies how frequently to exchange updates. Our implementation of EGP allows these timers to be adjusted by the user.
To adjust EGP timers, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Adjust EGP timers. | timers egp hello polltime |
EGP supports a third-party mechanism in which EGP tells an EGP peer that another communication server (the third party) on the shared network is the appropriate communication server for some set of destinations.
To specify third-party communication servers in updates, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Specify a third-party through which certain destinations can be achieved. | neighbor ip-address third-party third-party-ip-address [internal | external] |
See the "IP Routing Protocol Configuration Examples" section later in this chapter for an example of configuring third-party EGP support.
You might want to provide backup in the event of site failure by having a second communication server belonging to a different autonomous system act as a backup to the EGP communication server for your autonomous system. To differentiate between the primary and secondary EGP communication servers, the two communication servers will advertise network routes with differing EGP distances or metrics. A network with a low metric is generally favored over a network with a high metric.
Networks declared as local are always announced with a metric of zero. Networks that are redistributed will be announced with a metric specified by the user. If no metric is specified, redistributed routes will be advertised with a metric of three. All redistributed networks will be advertised with the same metric. The redistributed networks can be learned from static or dynamic routes. See also the "Redistribute Routing Information" section later in this chapter.
See the "IP Routing Protocol Configuration Examples" section later in this chapter for an example of configuring backup communication servers.
You also can designate network 0.0.0.0 as a default route. If the next hop for the default route can be advertised as a third party, it will be included as a third party.
To enable the use of default EGP routes, perform the following task in router configuration mode:
| Task | Command |
|---|---|
| Configure EGP to generate a default route. | default-information originate |
Normally, an EGP process expects to communicate with neighbors from a single autonomous system. Because all neighbors are in the same autonomous system, the EGP process assumes that these neighbors all have consistent internal information. Therefore, if the EGP process is informed about a route from one of its neighbors, it will not send it out to other neighbors.
With core EGP, the assumption is that all neighbors are from different autonomous systems, and all have inconsistent information. In this case, the EGP process distributes routes from one neighbor to all others (but not back to the originator). This allows the EGP process to be a central clearinghouse for information with a single, central manager of routing information (sometimes called a core gateway). To this end, one core gateway process can be configured for each communication server.
To define a core gateway process, perform the following steps in the order in which they appear:
| Task | Command |
|---|---|
| Step 1 Enter global configuration mode. | See Table 2-1 earlier in this manual. |
| Step 2 Allow a specific communication server to act as a peer with any reachable autonomous system. | router egp 0 |
| Step 3 Define how an EGP process determines which neighbors will be treated as peers. or Allow the specified address to be used as the next hop in EGP advertisements. | neighbor any [access-list-number]
|
The EGP process defined in this way can act as a peer with any autonomous system, and information is interchanged freely between autonomous systems.
See the "IP Routing Protocol Configuration Examples" section at the end of this chapter for an example of configuring an EGP core gateway.
The Gateway Discovery Protocol (GDP), designed by Cisco to address customer needs, allows hosts to dynamically detect the arrival of new communication servers, as well as determine when a communication server goes down. You must have host software to take advantage of this protocol.
For ease of implementation on a variety of host software, GDP is based on the User Datagram Protocol (UDP). The UDP source and destination ports of GDP datagrams are both set to 1997 (decimal).
There are two types of GDP messages: report and query. On broadcast media, report message packets are periodically sent to the IP broadcast address announcing that the communication server is present and functioning. By listening for these report packets, a host can detect a vanishing or appearing communication server. If a host issues a query packet to the broadcast address, the communication servers each respond with a report sent to the host's IP address. On nonbroadcast media, communication servers send report message packets only in response to query message packets. The protocol provides a mechanism for limiting the rate at which query messages are sent on nonbroadcast media.
Figure 18-2 shows the format of the GDP report message packet format. A GDP query message packet has a similar format, except that the count field is always zero and no address information is present.
The fields in the report and query messages are as follows:
Numerous actions can be taken by the host software listening to GDP packets. One possibility is to flush the host's ARP cache whenever a communication server appears or disappears. A more complex possibility is to update a host routing table based on the coming and going of communication servers. The particular course of action taken depends on the host software and your network requirements.
To enable GDP routing and other optional GDP tasks as required for your network, perform the following tasks in interface configuration mode:
Like GDP, the ICMP Router Discovery Protocol (IRDP) allows hosts to locate communication servers. When operating as a client, communication server discovery packets are generated, and when operating as a host, communication server discovery packets are received.
The only required task for configuring IRDP routing on a specified interface is to enable IRDP processing on an