Network Protocols Configuration Guide, Part 1 - Overview [Cisco IOS Software Releases 11.1]

Table Of Contents

AppleTalk, IP, and Novell IPX Overview

AppleTalk

Standard AppleTalk Services

Enhancements to Standard AppleTalk

IP

IP Routing Protocols

Interior Gateway Protocols

Exterior Gateway Protocols

Router Discovery Protocols

Multiple Routing Protocols

Novell IPX

AppleTalk, IP, and Novell IPX Overview

The Cisco IOS software supports a variety of routing protocols. The Network Protocols Configuration Guide, Part 1 discusses the following network protocols:

•AppleTalk

•IP

•IP Routing Protocols

•Novell IPX

The Network Protocols Configuration Guide, Part 2 discusses the following network protocols:

•Apollo Domain

•Banyan VINES

•DECNet

•ISO CLNS

•XNS

This overview chapter gives a high-level description of AppleTalk, IP, and Novell IPX. For configuration information, refer to the appropriate chapter in this publication.

AppleTalk

AppleTalk is a LAN system designed and developed by Apple Computer, Inc. It can run over Ethernet, Token Ring, and Fiber Data Distributed Interface (FDDI) networks, and over Apple's proprietary twisted-pair media access system (LocalTalk). AppleTalk specifies a protocol stack comprising several protocols that direct the flow of traffic over the network.

Apple Computer uses the name AppleTalk to refer to the Apple network protocol architecture. Apple Computer refers to the actual transmission media used in an AppleTalk network as LocalTalk, TokenTalk (AppleTalk over Token Ring), EtherTalk (AppleTalk over Ethernet), and FDDITalk (AppleTalk over FDDI).

Cisco IOS software supports AppleTalk Phase 1 and AppleTalk Phase 2. For AppleTalk Phase 2, Cisco devices support both extended and nonextended networks. Cisco's implementation of AppleTalk routes packets over Ethernet, Token Ring, and FDDI LANs, and over X.25, High-Level Data Link Control (HDLC), Frame Relay, and Switched Multimegabit Data Service (SMDS) WANs.

Cisco routers and access servers also support AppleTalk Enhanced IGRP. AppleTalk Enhanced IGRP provides the following features:

•Automatic redistribution. By default, AppleTalk Routing Table Maintenance Protocol (RTMP) routes are automatically redistributed into Enhanced IGRP, and AppleTalk Enhanced IGRP routes are automatically redistributed into RTMP. If desired, you can turn off redistribution. You can also completely turn off AppleTalk Enhanced IGRP and AppleTalk RTMP on the device or on individual interfaces.

•Configuration of routing protocols on individual interfaces. You can configure interfaces that are configured for AppleTalk to use either RTMP, Enhanced IGRP, or both routing protocols. If two neighboring routers are configured to use both RTMP and Enhanced IGRP, the Enhanced IGRP routing information supersedes the RTMP information. However, both routers continue to send RTMP routing updates. This feature allows you to control the excessive bandwidth usage of RTMP on WAN links. Because a WAN link is a point-to-point link (there are no other devices on the link), there is no need to run RTMP to perform end-node router discovery. Using Enhanced IGRP on WAN links allows you to save bandwidth and, in the case of packet switched data networks, traffic charges.

Standard AppleTalk Services

The Cisco implementation of AppleTalk supports the following standard AppleTalk protocols:

•AppleTalk Address Resolution Protocol (AARP)

•AppleTalk Port Group

•Datagram Delivery Protocol (DDP)

•Routing Table Maintenance Protocol (RTMP)

•Name Binding Protocol (NBP)

•Zone Information Protocol (ZIP)

•AppleTalk Echo Protocol (AEP)

•AppleTalk Transaction Protocol (ATP)

AARP, DDP, and RTMP provide end-to-end connectivity between internetworked nodes. AARP maps AppleTalk node addresses to the addresses of the underlying data link, thus making it possible for AppleTalk to run on several data links. DDP provides socket-to-socket delivery of packets. RTMP establishes and maintains routing tables.

NBP and ZIP maintain node name and zone information. NBP maps network names to AppleTalk addresses. ZIP tracks which networks are in which zones.

AEP is an echo, or ping-type, protocol. It generates packets that test the reachability of network nodes.

ATP is a reliable transport protocol that provides data acknowledgment and retransmission for transaction-based applications, such as file services provided by the AppleTalk Filing Protocol (AFP) and print services provided by the Printer Access Protocol (PAP).

Our software provides support for the AppleTalk Management Information Base (MIB) variables as described in RFC 1243.

Enhancements to Standard AppleTalk

The Cisco AppleTalk implementation includes the following enhancements to standard AppleTalk:

•Support for EtherTalk 1.2 and EtherTalk 2.0 without the need for translation or transition routers.

•Support for WAN protocols, including SMDS, Frame Relay, X.25, and HDLC.

•Configurable protocol constants (examples include controlling the aging of entries in the routing table and controlling the AARP interval and number of retransmissions).

•No software limits on the number of zones or routes.

•MacTCP support via a MacIP server.

•Support of IPTalk, which provides Internet Protocol (IP) encapsulation of AppleTalk, IPTalk, and the Columbia AppleTalk Package (CAP).

•Access control for filtering network traffic by network number, by NBP entity names, filtering routing table updates, and filtering GetZoneList (GZL) responses.

•Integrated node name support to simplify AppleTalk network management.

•Interactive access to AEP and NBP provided by the ping command.

•Configured (seed) and discovered interface configuration.

•Support for the AppleTalk Responder, which is used by network monitoring packages such as Inter·Poll.

•SNMP over AppleTalk.

•Encapsulation (tunneling) of AppleTalk RTMP packets over an IP backbone.

•Support for AppleTalk static routes.

•SMRP over AppleTalk

AppleTalk, like many network protocols, makes no provisions for network security. The design of the AppleTalk protocol architecture requires that security measures be implemented at higher application levels. Cisco supports AppleTalk distribution lists, allowing control of routing updates on a per-interface basis. This security feature is similar to those that Cisco provides for other protocols.

Note that Cisco's implementation of AppleTalk does not forward packets with local source and destination network addresses. This behavior does not conform with the definition of AppleTalk in Apple Computer's Inside AppleTalk publication. However, this behavior is designed to prevent any possible corruption of the AARP table in any AppleTalk node that is performing Media Access Control (MAC)-address gleaning.

A Cisco router or access server may receive equivalent routes advertised by neighboring routers with one router giving an AppleTalk Phase 1 form of the route, for example, 101, and another giving an AppleTalk Phase 2 form of the route, for example, 101-101. When neighboring routers advertise equivalent overlapping routes to a router, the router always uses the AppleTalk Phase 2 form of the route and discards the AppleTalk Phase 1 route.

IP

The Internet Protocol (IP) is a packet-based protocol used to exchange data over computer networks. IP handles addressing, fragmentation, reassembly, and protocol demultiplexing. It is the foundation on which all other IP protocols, collectively referred to as the IP Protocol suite, are built. IP is a network-layer protocol that contains addressing and control information that allows data packets to be routed.

The Transmission Control Protocol (TCP) is built upon the IP layer. TCP is a connection-oriented protocol that specifies the format of data and acknowledgments used in the transfer of data. TCP also specifies the procedures that the computers use to ensure that the data arrives correctly. TCP allows multiple applications on a system to communicate concurrently because it handles all demultiplexing of the incoming traffic among the application programs.

Cisco's implementation of IP provides most of the major services contained in the various protocol specifications. Cisco IOS software also provides the TCP and User Datagram Protocol (UDP) services called Echo and Discard, which are described in RFCs 862 and 863, respectively.

Cisco supports both TCP and UDP at the transport layer, for maximum flexibility in services. Cisco also supports all standards for IP broadcasts.

IP Routing Protocols

Cisco's implementation of each of the IP routing protocols is discussed in detail at the beginning of the individual protocol sections throughout the IP Routing chapter in this publication.

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.

Note Many routing protocol specifications refer to routers as gateways, so the word gateway often appears as part of routing protocol names. However, a router usually is defined as a Layer 3 internetworking device, whereas a protocol translation gateway usually is defined as a Layer 7 internetworking device. The reader should understand that whether a routing protocol name contains the word "gateway" or not, routing protocol activities occur at Layer 3 of the OSI reference model.

Interior Gateway Protocols

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. Cisco IOS software supports the interior routing protocols:

•Internet Gateway Routing Protocol (IGRP)

•Enhanced Internet Gateway Routing Protocol (Enhanced IGRP)

•Open Shortest Path First (OSPF)

•Routing Information Protocol (RIP)

•Intermediate System-to-Intermediate System (IS-IS)

Exterior Gateway Protocols

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:

•A list of neighbor (or peer) routers or access servers with which to exchange routing information.

•A list of networks to advertise as directly reachable.

•The autonomous system number of the local router.

The supported exterior gateway protocols are:

•Border Gateway Protocol (BGP)

•Exterior Gateway Protocol (EGP)

Router Discovery Protocols

Our routers and access servers also support two router discovery protocols, Gateway Discovery Protocol (GDP) and Internet Control Message Protocol (ICMP) Router Discovery Protocol (IRDP), which allow hosts to locate routers and access servers.

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.

Multiple Routing Protocols

You can configure multiple routing protocols in a single router or access 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, 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.

The Cisco IOS software can handle simultaneous operation of up to 30 dynamic IP routing processes. The combination of routing processes on a router or access server consists of the following protocols (with the limits noted):

•Up to 30 IGRP routing processes

•Up to 30 OSPF routing processes

•One RIP routing process

•One IS-IS process

•One BGP routing process

•Up to 30 EGP routing processes

Novell IPX

Novell Internet Packet Exchange (IPX) is derived from the Xerox Network Systems (XNS) Internet Datagram Protocol (IDP). IPX and XNS have the following differences:

•IPX and XNS do not always use the same Ethernet encapsulation format.

•IPX uses Novell's proprietary Service Advertisement Protocol (SAP) to advertise special network services. File servers and print servers are examples of services that are typically advertised.

•IPX uses delay, measured in ticks, while XNS uses hop count as the primary metric in determining the best path to a destination.

Cisco's implementation of Novell's IPX protocol is certified to provide full IPX routing functionality.

Cisco supports the IPX MIB (currently, read-only access is supported). The IPX Accounting group represents one of the local Cisco-specific IPX variables we support. This group provides access to the active database that is created and maintained if IPX accounting is enabled on a router or access server.

Cisco IOS software also supports IPX Enhanced IGRP, which provides the following features:

•Automatic redistribution. IPX RIP routes are automatically redistributed into Enhanced IGRP, and Enhanced IGRP routes are automatically redistributed into RIP. If desired, you can turn off redistribution. You also can completely turn off Enhanced IGRP and IPX RIP on the device or on individual interfaces.

•Increased network width. With IPX RIP, the largest possible width of your network is 15 hops. When Enhanced IGRP is enabled, the largest possible width is 224 hops. Because the Enhanced IGRP metric is large enough to support thousands of hops, the only barrier to expanding the network is the transport layer hop counter. Cisco works around this problem by incrementing the transport control field only when an IPX packet has traversed 15 routers and the next hop to the destination was learned via Enhanced IGRP. When a RIP route is being used as the next hop to the destination, the transport control field is incremented as usual.

•Incremental SAP updates. Complete SAP updates are sent periodically on each interface until an Enhanced IGRP neighbor is found and thereafter only when there are changes to the SAP table. This procedure works by taking advantage of Enhanced IGRP's reliable transport mechanism, which means that an Enhanced IGRP peer must be present for incremental SAPs to be sent. If no peer exists on a particular interface, periodic SAPs will be sent on that interface until a peer is found. This functionality is automatic on serial interfaces and can be configured on LAN media.

	Network Protocols Configuration Guide, Part 1
	Overview
Network Protocols Configuration Guide, Part 1 About The Cisco IOS Software Documentation Overview Configuring AppleTalk Configuring IP Configuring IP Routing Protocols Configuring Novell IPX	Download this chapter Overview Table Of Contents AppleTalk, IP, and Novell IPX Overview AppleTalk Standard AppleTalk Services Enhancements to Standard AppleTalk IP IP Routing Protocols Interior Gateway Protocols Exterior Gateway Protocols Router Discovery Protocols Multiple Routing Protocols Novell IPX AppleTalk, IP, and Novell IPX Overview The Cisco IOS software supports a variety of routing protocols. The Network Protocols Configuration Guide, Part 1 discusses the following network protocols: •AppleTalk •IP •IP Routing Protocols •Novell IPX The Network Protocols Configuration Guide, Part 2 discusses the following network protocols: •Apollo Domain •Banyan VINES •DECNet •ISO CLNS •XNS This overview chapter gives a high-level description of AppleTalk, IP, and Novell IPX. For configuration information, refer to the appropriate chapter in this publication. AppleTalk AppleTalk is a LAN system designed and developed by Apple Computer, Inc. It can run over Ethernet, Token Ring, and Fiber Data Distributed Interface (FDDI) networks, and over Apple's proprietary twisted-pair media access system (LocalTalk). AppleTalk specifies a protocol stack comprising several protocols that direct the flow of traffic over the network. Apple Computer uses the name AppleTalk to refer to the Apple network protocol architecture. Apple Computer refers to the actual transmission media used in an AppleTalk network as LocalTalk, TokenTalk (AppleTalk over Token Ring), EtherTalk (AppleTalk over Ethernet), and FDDITalk (AppleTalk over FDDI). Cisco IOS software supports AppleTalk Phase 1 and AppleTalk Phase 2. For AppleTalk Phase 2, Cisco devices support both extended and nonextended networks. Cisco's implementation of AppleTalk routes packets over Ethernet, Token Ring, and FDDI LANs, and over X.25, High-Level Data Link Control (HDLC), Frame Relay, and Switched Multimegabit Data Service (SMDS) WANs. Cisco routers and access servers also support AppleTalk Enhanced IGRP. AppleTalk Enhanced IGRP provides the following features: •Automatic redistribution. By default, AppleTalk Routing Table Maintenance Protocol (RTMP) routes are automatically redistributed into Enhanced IGRP, and AppleTalk Enhanced IGRP routes are automatically redistributed into RTMP. If desired, you can turn off redistribution. You can also completely turn off AppleTalk Enhanced IGRP and AppleTalk RTMP on the device or on individual interfaces. •Configuration of routing protocols on individual interfaces. You can configure interfaces that are configured for AppleTalk to use either RTMP, Enhanced IGRP, or both routing protocols. If two neighboring routers are configured to use both RTMP and Enhanced IGRP, the Enhanced IGRP routing information supersedes the RTMP information. However, both routers continue to send RTMP routing updates. This feature allows you to control the excessive bandwidth usage of RTMP on WAN links. Because a WAN link is a point-to-point link (there are no other devices on the link), there is no need to run RTMP to perform end-node router discovery. Using Enhanced IGRP on WAN links allows you to save bandwidth and, in the case of packet switched data networks, traffic charges. Standard AppleTalk Services The Cisco implementation of AppleTalk supports the following standard AppleTalk protocols: •AppleTalk Address Resolution Protocol (AARP) •AppleTalk Port Group •Datagram Delivery Protocol (DDP) •Routing Table Maintenance Protocol (RTMP) •Name Binding Protocol (NBP) •Zone Information Protocol (ZIP) •AppleTalk Echo Protocol (AEP) •AppleTalk Transaction Protocol (ATP) AARP, DDP, and RTMP provide end-to-end connectivity between internetworked nodes. AARP maps AppleTalk node addresses to the addresses of the underlying data link, thus making it possible for AppleTalk to run on several data links. DDP provides socket-to-socket delivery of packets. RTMP establishes and maintains routing tables. NBP and ZIP maintain node name and zone information. NBP maps network names to AppleTalk addresses. ZIP tracks which networks are in which zones. AEP is an echo, or ping-type, protocol. It generates packets that test the reachability of network nodes. ATP is a reliable transport protocol that provides data acknowledgment and retransmission for transaction-based applications, such as file services provided by the AppleTalk Filing Protocol (AFP) and print services provided by the Printer Access Protocol (PAP). Our software provides support for the AppleTalk Management Information Base (MIB) variables as described in RFC 1243. Enhancements to Standard AppleTalk The Cisco AppleTalk implementation includes the following enhancements to standard AppleTalk: •Support for EtherTalk 1.2 and EtherTalk 2.0 without the need for translation or transition routers. •Support for WAN protocols, including SMDS, Frame Relay, X.25, and HDLC. •Configurable protocol constants (examples include controlling the aging of entries in the routing table and controlling the AARP interval and number of retransmissions). •No software limits on the number of zones or routes. •MacTCP support via a MacIP server. •Support of IPTalk, which provides Internet Protocol (IP) encapsulation of AppleTalk, IPTalk, and the Columbia AppleTalk Package (CAP). •Access control for filtering network traffic by network number, by NBP entity names, filtering routing table updates, and filtering GetZoneList (GZL) responses. •Integrated node name support to simplify AppleTalk network management. •Interactive access to AEP and NBP provided by the ping command. •Configured (seed) and discovered interface configuration. •Support for the AppleTalk Responder, which is used by network monitoring packages such as Inter·Poll. •SNMP over AppleTalk. •Encapsulation (tunneling) of AppleTalk RTMP packets over an IP backbone. •Support for AppleTalk static routes. •SMRP over AppleTalk AppleTalk, like many network protocols, makes no provisions for network security. The design of the AppleTalk protocol architecture requires that security measures be implemented at higher application levels. Cisco supports AppleTalk distribution lists, allowing control of routing updates on a per-interface basis. This security feature is similar to those that Cisco provides for other protocols. Note that Cisco's implementation of AppleTalk does not forward packets with local source and destination network addresses. This behavior does not conform with the definition of AppleTalk in Apple Computer's Inside AppleTalk publication. However, this behavior is designed to prevent any possible corruption of the AARP table in any AppleTalk node that is performing Media Access Control (MAC)-address gleaning. A Cisco router or access server may receive equivalent routes advertised by neighboring routers with one router giving an AppleTalk Phase 1 form of the route, for example, 101, and another giving an AppleTalk Phase 2 form of the route, for example, 101-101. When neighboring routers advertise equivalent overlapping routes to a router, the router always uses the AppleTalk Phase 2 form of the route and discards the AppleTalk Phase 1 route. IP The Internet Protocol (IP) is a packet-based protocol used to exchange data over computer networks. IP handles addressing, fragmentation, reassembly, and protocol demultiplexing. It is the foundation on which all other IP protocols, collectively referred to as the IP Protocol suite, are built. IP is a network-layer protocol that contains addressing and control information that allows data packets to be routed. The Transmission Control Protocol (TCP) is built upon the IP layer. TCP is a connection-oriented protocol that specifies the format of data and acknowledgments used in the transfer of data. TCP also specifies the procedures that the computers use to ensure that the data arrives correctly. TCP allows multiple applications on a system to communicate concurrently because it handles all demultiplexing of the incoming traffic among the application programs. Cisco's implementation of IP provides most of the major services contained in the various protocol specifications. Cisco IOS software also provides the TCP and User Datagram Protocol (UDP) services called Echo and Discard, which are described in RFCs 862 and 863, respectively. Cisco supports both TCP and UDP at the transport layer, for maximum flexibility in services. Cisco also supports all standards for IP broadcasts. IP Routing Protocols Cisco's implementation of each of the IP routing protocols is discussed in detail at the beginning of the individual protocol sections throughout the IP Routing chapter in this publication. 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. Note Many routing protocol specifications refer to routers as gateways, so the word gateway often appears as part of routing protocol names. However, a router usually is defined as a Layer 3 internetworking device, whereas a protocol translation gateway usually is defined as a Layer 7 internetworking device. The reader should understand that whether a routing protocol name contains the word "gateway" or not, routing protocol activities occur at Layer 3 of the OSI reference model. Interior Gateway Protocols 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. Cisco IOS software supports the interior routing protocols: •Internet Gateway Routing Protocol (IGRP) •Enhanced Internet Gateway Routing Protocol (Enhanced IGRP) •Open Shortest Path First (OSPF) •Routing Information Protocol (RIP) •Intermediate System-to-Intermediate System (IS-IS) Exterior Gateway Protocols 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: •A list of neighbor (or peer) routers or access servers with which to exchange routing information. •A list of networks to advertise as directly reachable. •The autonomous system number of the local router. The supported exterior gateway protocols are: •Border Gateway Protocol (BGP) •Exterior Gateway Protocol (EGP) Router Discovery Protocols Our routers and access servers also support two router discovery protocols, Gateway Discovery Protocol (GDP) and Internet Control Message Protocol (ICMP) Router Discovery Protocol (IRDP), which allow hosts to locate routers and access servers. 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. Multiple Routing Protocols You can configure multiple routing protocols in a single router or access 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, 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. The Cisco IOS software can handle simultaneous operation of up to 30 dynamic IP routing processes. The combination of routing processes on a router or access server consists of the following protocols (with the limits noted): •Up to 30 IGRP routing processes •Up to 30 OSPF routing processes •One RIP routing process •One IS-IS process •One BGP routing process •Up to 30 EGP routing processes Novell IPX Novell Internet Packet Exchange (IPX) is derived from the Xerox Network Systems (XNS) Internet Datagram Protocol (IDP). IPX and XNS have the following differences: •IPX and XNS do not always use the same Ethernet encapsulation format. •IPX uses Novell's proprietary Service Advertisement Protocol (SAP) to advertise special network services. File servers and print servers are examples of services that are typically advertised. •IPX uses delay, measured in ticks, while XNS uses hop count as the primary metric in determining the best path to a destination. Cisco's implementation of Novell's IPX protocol is certified to provide full IPX routing functionality. Cisco supports the IPX MIB (currently, read-only access is supported). The IPX Accounting group represents one of the local Cisco-specific IPX variables we support. This group provides access to the active database that is created and maintained if IPX accounting is enabled on a router or access server. Cisco IOS software also supports IPX Enhanced IGRP, which provides the following features: •Automatic redistribution. IPX RIP routes are automatically redistributed into Enhanced IGRP, and Enhanced IGRP routes are automatically redistributed into RIP. If desired, you can turn off redistribution. You also can completely turn off Enhanced IGRP and IPX RIP on the device or on individual interfaces. •Increased network width. With IPX RIP, the largest possible width of your network is 15 hops. When Enhanced IGRP is enabled, the largest possible width is 224 hops. Because the Enhanced IGRP metric is large enough to support thousands of hops, the only barrier to expanding the network is the transport layer hop counter. Cisco works around this problem by incrementing the transport control field only when an IPX packet has traversed 15 routers and the next hop to the destination was learned via Enhanced IGRP. When a RIP route is being used as the next hop to the destination, the transport control field is incremented as usual. •Incremental SAP updates. Complete SAP updates are sent periodically on each interface until an Enhanced IGRP neighbor is found and thereafter only when there are changes to the SAP table. This procedure works by taking advantage of Enhanced IGRP's reliable transport mechanism, which means that an Enhanced IGRP peer must be present for incremental SAPs to be sent. If no peer exists on a particular interface, periodic SAPs will be sent on that interface until a peer is found. This functionality is automatic on serial interfaces and can be configured on LAN media.
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