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For the latest feature information and caveats. However, a BGP routing process and autonomous system can support multiple concurrent BGP address family and subaddress family configurations. The destination TCP port is assigned , and the local port assigned a random port number. BGP is mainly used to connect a local network to an external network to gain access to the Internet or to connect to other organizations.

Although BGP is referred to as an exterior gateway protocol EGP many networks within an organization are becoming so complex that BGP can be used to simplify the internal network used within the organization. BGP uses a path-vector routing algorithm to exchange network reachability information with other BGP speaking networking devices. Network reachability information is exchanged between BGP peers in routing updates.

Network reachability information contains the network number, path specific attributes, and the list of autonomous system numbers that a route must transit through to reach a destination network.

This list is contained in the AS-path attribute. BGP prevents routing loops by rejecting any routing update that contains the local autonomous system number because this indicates that the route has already travelled through that autonomous system and a loop would therefore be created. The BGP path-vector routing algorithm is a combination of the distance-vector routing algorithm and the AS-path loop detection.

BGP selects a single path, by default, as the best path to a destination host or network. The best path selection algorithm analyzes path attributes to determine which route is installed as the best path in the BGP routing table.

Each path carries well-known mandatory, well-know discretionary, and optional transitive attributes that are used in BGP best path analysis. BGP uses the best-path selection algorithm to find a set of equally good routes. These routes are the potential multipaths.

Internal BGP can help with issues such as scaling the existing IGPs to match the traffic demands while maintaining network efficiency. BGP Autonomous Systems An autonomous system is a network controlled by a single technical administration entity. BGP autonomous systems are used to divide global external networks into individual routing domains where local routing policies are applied.

This organization simplifies routing domain administration and simplifies consistent policy configuration. Consistent policy configuration is important to allow BGP to efficiently process routes to destination networks. Each routing domain can support multiple routing protocols. However, each routing protocol is administrated separately. Other routing protocols can dynamically exchange routing information with BGP through redistribution.

BGP peers within the same autonomous system exchange routing information through iBGP peering sessions. The figure below illustrates two routers in separate autonomous systems that can be connected using BGP. These routers carry traffic across the Internet.

Figure 1. Each public autonomous system that directly connects to the Internet is assigned a unique number that identifies both the BGP routing process and the autonomous system. Due to increased demand for autonomous system numbers, the Internet Assigned Number Authority IANA will start in January to allocate four-octet autonomous system numbers in the range from to Cisco has implemented the following two methods: Asplain--Decimal value notation where both 2-byte and 4-byte autonomous system numbers are represented by their decimal value.

For example, is a 2-byte autonomous system number and is a 4-byte autonomous system number. Asdot--Autonomous system dot notation where 2-byte autonomous system numbers are represented by their decimal value and 4-byte autonomous system numbers are represented by a dot notation.

For example, is a 2-byte autonomous system number and 1. For details about the third method of representing autonomous system numbers, see RFC When using regular expressions to match 4-byte autonomous system numbers the asdot format includes a period which is a special character in regular expressions.

The table below shows the format in which 2-byte.

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Table 1. In addition, the default format for matching 4-byte autonomous system numbers in regular expressions is asplain, so you must ensure that any regular expressions to match 4-byte autonomous system numbers are written in the asplain format. If you want to change the default show command output to display 4-byte autonomous system numbers in the asdot format, use the bgp asnotation dot command under router configuration mode.

When the asdot format is enabled as the default, any regular expressions to match 4-byte autonomous system numbers must be written using the asdot format, or the regular expression match will fail. The tables below show that although you can configure 4-byte autonomous system numbers in either asplain or asdot format, only one format is used to display show command output and control 4-byte autonomous system number matching for regular expressions, and the default is asplain format.

To display 4-byte autonomous system numbers in show command output and to control matching for regular expressions in the asdot format, you must configure the bgp asnotation dot command.

If you are upgrading to an image that supports 4-byte autonomous system numbers, you can still use 2-byte autonomous system numbers. The show command output and regular expression match are not changed and remain in asplain decimal value format for 2-byte autonomous system numbers regardless of the format configured for 4-byte autonomous system numbers.

Table 2. RFC was developed to allow BGP to support a gradual transition from 2-byte autonomous system numbers to 4-byte autonomous system numbers. Use of the reserved numbers allow configuration examples to be accurately documented and avoids conflict with production networks if these configurations are literally copied. The reserved numbers are documented in the IANA autonomous system number registry. Reserved 2-byte autonomous system numbers are in the contiguous block, to and reserved 4-byte autonomous system numbers are from to inclusive.

Private 2-byte autonomous system numbers are still valid in the range from to with being reserved for special use. Private autonomous system numbers can be used for internal routing domains but must be translated for traffic that is routed out to the Internet.

BGP should not be configured to advertise private autonomous system numbers to external networks. Cisco IOS software does not remove private autonomous system numbers from routing updates by default. We recommend that ISPs filter private autonomous system numbers. Autonomous system number assignment for public and private networks is governed by the IANA.

For information about autonomous-system numbers, including reserved number assignment, or to apply to register an autonomous system number, see the following URL: CIDR eliminates classful network boundaries, providing more efficient usage of the IPv4 address space. CIDR provides a method to reduce the size of routing tables by configuring aggregate routes or supernets.

CIDR processes a prefix as an IP address and bit mask bits are processed from left to right to define each network. A prefix can represent a network, subnetwork, supernet, or single host route. For example, using classful IP addressing, the IP address These extensions are backward-compatible to enable routers that do not support multiprotocol extensions to communicate with those routers that do support multiprotocol extensions. Multiprotocol BGP carries routing information for multiple network-layer protocols and IP multicast routes.

BGP carries different sets of routes depending on the protocol. A multiprotocol BGP network is backward-compatible with a BGP network, but BGP peers that do not support multiprotocol extensions cannot forward routing information, such as address family identifier information, that the multiprotocol extensions carry. In less complex networks we recommend using multiprotocol BGP because it offers the following benefits: A network can support incongruent unicast and multicast topologies.

A multiprotocol BGP network is backward compatible because the routers that support the multiprotocol extensions can interoperate with routers that do not support the extensions. In summary, multiprotocol BGP support for multiple network layer protocol address families provides a flexible and scalable infrastructure that allows you to define independent policy and peering configurations on a per-address family basis.

Multiprotocol BGP is useful when you want a link dedicated to multicast traffic, perhaps to limit which resources are used for which traffic. For example, you want all multicast traffic exchanged at one network access point NAP.

Multiprotocol BGP allows you to have a unicast routing topology different from a multicast routing topology that allows you more control over your network and resources. In BGP, the only way to perform interdomain multicast routing is to use the BGP infrastructure that is in place for unicast routing. If the routers are not multicast-capable, or there are differing policies about where multicast traffic should flow, multicast routing cannot be supported without multiprotocol BGP.

The multicast table is the primary source for the router, but if the route is not found in the multicast table then the unicast table is searched. Multiprotocol extensions, however, will be ignored by any peers that do not support multiprotocol BGP. If the unicast network runs multiprotocol BGP, peering can be configured using the appropriate multicast address family. The multicast address family configuration enables multiprotocol BGP to carry the multicast information and the RPF lookup will succeed.

The figure below illustrates a simple example of unicast and multicast topologies that are incongruent; these topologies cannot exchange information without implementing multiprotocol BGP. One is used for unicast peering and therefore the exchanging of unicast traffic.

The Multicast Friendly Interconnect MFI ring is used for multicast peering and therefore the exchanging of multicast traffic. Each router is unicast- and multicast-capable. Figure 2. The figure below is a topology of unicast-only routers and multicast-only routers. The two routers on the left are unicast-only routers that is, they do not support or are not configured to perform multicast routing.

The two routers on the right are multicast-only routers. Routers A and B support both unicast and multicast routing. The unicast-only and multicast-only routers are connected to a single NAP.

In the figure below, only unicast traffic can travel from Router A to the unicast routers to Router B and back. Multicast traffic could not flow on that path, because multicast routing is not configured on the unicast routers and therefore the BGP routing table does not contain any multicast routes. On the multicast routers, multicast routes are enabled and BGP builds a separate routing table to hold the multicast routes. Multicast traffic uses the path from Router A to the multicast routers to Router B and back.

If you want to use other address family configurations such as IPv4 unicast or multicast. The NLRI format offers only limited support for multicast routing information and does not support multiple network layer protocols. Both of the autonomous systems must be configured for internal multiprotocol BGP in the figure. Multiprotocol BGP allows these routes to be noncongruent.

IP Routing: Networks that are configured in the NLRI format have the following limitations: Each of the separate autonomous systems shown in the figure below may be running several routing protocols such as Multiprotocol Label Switching MPLS and IPv6 and require both unicast and multicast routes to be transported via BGP. Networks are increasing in complexity and many companies are now using BGP to connect to many autonomous systems.

No support for IPv6. In the NLRI format. Multiprotocol BGP carries routing information for multiple network layer protocols and IP multicast routes. Each address family maintains a separate BGP database. If the routers in the infrastructure do not have multicast capabilities. Limited support for multicast interdomain routing and incongruent multicast and unicast topologies.

Congruent unicast and multicast topologies that have different policies BGP filtering configurations are supported. A router that is configured using the AFI model can carry routing information for multiple network layer protocol address families for example. To configure BGP commands and functionality for other address family prefixes for example. AFI configuration is similar in all address families. IPv4 and IPv6. All BGP routing policy capabilities and commands are supported.

A router that is configured using the AFI model has the following features: CLNS is supported. The BGP commands supported in address family configuration mode configure the same functionality as the BGP commands supported in router configuration mode. Within a specific address family configuration mode. Unicast or multicast address prefixes can be specified within the IPv6 address family. Peering is established using IP addresses. Routing information for address family IPv4 unicast is advertised by default when a BGP peer is configured unless the advertisement of unicast IPv4 information is explicitly turned off.

Unicast or multicast address prefixes can be specified within the IPv4 address family. Note Routing information for address family IPv4 unicast is advertised by default when you configure a BGP peer unless you explicitly turn off the advertisement of unicast IPv4 information. Unicast address prefixes are the default when NSAP address prefixes are configured. Unicast address prefixes are the default when VPNv4 address prefixes are configured.

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This prevents information from being sent outside the VPN and allows the same subnet to be used in several VPNs without causing duplicate IP address problems. In private LANs. VPNv4 routes implicitly have a label associated with each route.

Companies are moving their business applications to their intranets to extend over a WAN. Within a VPN. With extranets. To take advantage of this business opportunity. Prefix and path information is stored in the L2VPN database.

Each VPN needs its own set of prefixes. Companies use an IP VPN as the foundation for deploying or administering value-added services including applications and data hosting network commerce.

Companies are also addressing the needs of their customers. Each of these appears to its users as a private network. IP-based intranets have fundamentally changed the way companies conduct their business. The VPN address space is isolated from the global address space by design. VPNv4 routes are the same as IPv4 routes. For more details about VPLS. If you need to remove any CLI configuration. All other route map commands are supported.

For example. For route maps used within BGP. Analyze the current running configuration to determine the current BGP neighbor relationships. BGP Based" feature. The following configuration will remove both the route map and the redistribution: Almost every configuration command has a no form. Redistribution without the route map may cause unexpected results in your network.

When you remove an access list or route map. Cisco software releases. Access to most tools on the Cisco Support website requires a Cisco. Unless noted otherwise. This table lists only the software release that introduced support for a given feature in a given software release train.

Feature Information for Cisco BGP Overview The following table provides release information about the feature or features described in this module. To receive security and technical information about your products. Because of increased demand for autonomous system numbers. The following commands were introduced or modified by this feature: To change the default regular expression match and output display of 4-byte autonomous system numbers to asdot format.

These extensions are backward compatible to enable routers that do not support multiprotocol extensions to communicate with those routers that do support multiprotocol extensions. Any examples.

To view a list of Cisco trademarks. Multiprotocol Extensions for BGP We recommend the use of route reflectors to address the issue of a large internal BGP mesh.

BGP extended communities are not supported by this feature. The following BGP commands are not supported by this feature: Implementing BGP in routers at the edge of each internal network means that the existing interior protocols need not be changed. Using the BGP address-family support. This will continue to be the case unless you enter the no bgp default ipv4-unicast command as the first command under the router bgp command. This feature was developed to solve a scaling issue with a data communications network DCN where large numbers of network elements are managed remotely.

Network administrators can control the BGP routing information because BGP neighbor relationships peering are manually configured and routing updates use incremental broadcasts. In CLNS. To avoid IP Routing: The no bgp default ipv4-unicast command is configured on the router to disable the default behavior of the BGP routing process exchanging IPv4 addressing information with BGP neighbor routers.

Fewer connections between routers simplifies the network design and the amount of traffic in the network. Connectivity to the rest of the network is provided by R2.

IS-IS is used as the intradomain routing protocol. To facilitate this feature. R2 and R3. Although the links are not shown in the figure. Between autonomous systems. In AS there are two routers. The configuration tasks and examples are based on the generic network design shown in the figure above. Configurations for all the routers in the figure above are listed in. Each autonomous system in this example is configured to demonstrate various BGP features and how these features work with CLNS to provide a scalable interdomain routing solution.

BGP and its multiprotocol extensions are used as the interdomain routing protocol. In the figure above. To be consistent with the BGP terminology. SONET is typically used by telecommunications companies to send data over fiber-optic networks. The figure below shows some components of a DCN network.

Routers that link a Level 1 area with a Level 2 area are defined as Level routers. Smaller Cisco routers such as the Cisco series were selected to run as the Level routers because shelf space in the central office CO of a service provider is very expensive. A network element that connects to the Level 2 routers that provide a path to the DCN core is represented by a gateway network element--GNE in Figure 2.

The network topology here is a point-to-point link between each network element router. Routing within an area is referred to as Level 1 routing. The network is divided into areas defined as a collection of routing nodes. A Cisco series router IP Routing: Each routing node networking device is called an intermediate system IS.

Routing between areas involves Level 2 routing. In this example. Using the default routes.

Static routing does not scale because the growth in the network can exceed the ability of a network administrator to maintain the static routes. Multiplying the linked autonomous systems by the nodes within each autonomous system could allow up to 6 million network elements. Assuming that each autonomous system--for example. To provide connectivity between NE routers. AS and AS in Figure remains the same size with up to nodes. Due to the limited signaling bandwidth between network elements and the limited amount of processing power and memory in the NE routers running IS-IS.

With the hub-and-spoke design where each autonomous system is directly linked to the core autonomous system. Each address prefix can have two paths associated with it to provide redundancy because there are two links between each autonomous system and the core autonomous system. The benefits of using this feature are not confined to DCN networks. We can assume that the core autonomous system can support about directly linked autonomous systems.

With a maximum of areas containing 10 to 15 network elements per area. BGP has been shown to scale to over On average. Service providers are looking to implement over Each autonomous system advertises one address prefix to the core autonomous system.

The number of Level 1 areas under this configuration is limited to about The entire Level 2 network is also limited by the speed of the slowest Level 2 router. BGP has been shown to support In the autonomous systems AS and AS You must perform the steps in the required procedures.

It may not be necessary to go through each procedure for your particular network. Note If you have configured a peer group as a BGP neighbor. By default. Router config-router neighbor Router config-router address-family nsap Step 7 neighbor ip-address activate Example: Router config-router-af Specifies the NSAP address family and enters address family configuration mode.

Valid values are from 0 to Private autonomous system numbers that can be used in internal networks range from to Router config router bgp Step 4 no bgp default ipv4-unicast The as-number argument identifies the autonomous system in which the router resides. Router configure terminal Step 3 router bgp as-number Configures a BGP routing process and enters router configuration mode.

Router configure terminal IP Routing: Router config router isis osi-as Step 4 net network-entity-title The area-tag argument is a meaningful name for a routing process. When a neighboring OSI system is found. If both the preceding conditions are met.

If you are configuring multiarea IS-IS. Router config-router net Exits router configuration mode and returns to privileged EXEC mode. Configures the router to act as a Level 1 intra-area router. Configures a network entity title NET for the routing process. These interfaces will normally be directly connected to their eBGP neighbor. Router config-if clns enable IP Routing: Router config-if ip address Router configure terminal Step 3 interface type number Specifies the interface type and number and enters interface configuration mode.

Note This step is required only when the interface needs to communicate with an iBGP neighbor. Note This step is required only when the interface needs to Router config-if ip router isis osi-as Step 7 no shutdown communicate with an iBGP neighbor.

Turns on the interface. To configure advertisement of networking prefixes. Router config-if clns router isis osi-as Step 6 ip router isis area-tag Example: Specifies that the interface is actively routing IS-IS when the network protocol is IP and identifies the area associated with this routing process.

Router config-router no bgp default ipv4unicast IP Routing: Router config router bgp Step 4 no bgp default ipv4-unicast Disables the default behavior of the BGP routing process exchanging IPv4 addressing information with BGP neighbor routers. Router configure terminal Step 3 router bgp as-number Configures a BGP routing process and enters router configuration mode for the specified routing process. Router config-router-af network Router config-router-af end IP Routing: Router config-router-af neighbor If no route map is specified.

Exits address family configuration mode and returns to privileged EXEC mode. Note See the description of the neighbor command in the documents listed in the "Additional References" for more details on the use of this command. Note It is possible to advertise a single prefix.

Specifies the NSAP address family and enters address family configuration mode. Route maps can be used to control which dynamic routes are redistributed. Router config router isis osi-as Step 4 net network-entity-title Configures a network entity title NET for the routing process.

Router configure terminal Step 3 router isis area-tag Example: Configures an IS-IS routing process and enters router configuration mode for the specified routing process. Router config-router end Redistributing Routes from IS-IS into BGP Route redistribution must be approached with caution because redistributed route information is stored in the routing tables.

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Large routing tables may make the routing process slower. The redistribution of routes can be controlled by using the optional route-map keyword. Router config-router no bgp default ipv4-unicast Step 5 address-family nsap [unicast] Specifies the NSAP address family and enters address family configuration mode. Router config-router address-family nsap Step 6 redistribute protocol [process-id] [route-type] [route-map map-tag] Example: Router configure terminal Step 3 router bgp as-number Example: Router config router bgp IP Routing: Without a peer group.

Repeat Step 9 to configure other BGP neighbors as members of the peer group. Using a BGP peer group with a local router configured as a BGP route reflector allows BGP routing information received from one member of the group to be replicated to all other group members.

In this task. This is an optional task and is used with internal BGP neighbors. Router config-router address-family nsap Step 8 neighbor peer-group-name route-reflector-client Configures the router as a BGP route reflector and configures the specified peer group as its client.

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Router config-router no bgp default ipv4-unicast Step 5 neighbor peer-group-name peer-group Creates a BGP peer group. Router config-router neighbor ibgp-peers remoteas Step 7 address-family nsap [unicast] Specifies the NSAP address family and enters address family configuration mode.

Router config-router-af neighbor ibgp-peers route-reflector-client Step 9 neighbor ip-address peer-group peer-group Assigns a BGP neighbor to a BGP peer group. Router config-router neighbor ibgp-peers peergroup Step 6 neighbor peer-group-name remote-as as-number Adds the peer group name of the BGP neighbor in the specified autonomous system to the BGP neighbor table of the local router.

The neighbor prefix-list in command is configured in address family configuration mode to filter inbound routes. See descriptions for the clns filter-expr and clns filter-set commands for more information. The clns-filter-set-name argument is defined with the clns filter-set configuration command. A BGP peer group is created and the filter is applied to outbound BGP updates for the neighbor that is a member of the peer group. Router config-router no bgp default ipv4unicast Step 5 address-family nsap [unicast] Specifies the address family and enters address family configuration mode.

Router config clns filter-set routes deny Router config router bgp Step 6 no bgp default ipv4-unicast Disables the default behavior of the BGP routing process exchanging IPv4 addressing information with BGP neighbor routers. Router config-router address-family nsap IP Routing: Step 5 router bgp as-number Configures a BGP routing process and enters router configuration mode for the specified routing process.

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Router config-router no bgp default ipv4-unicast Step 7 neighbor peer-group-name peer-group Creates a BGP peer group. Router config-router neighbor ebgppeers remote-as Step 9 address-family nsap [unicast] Specifies the NSAP address family and enters address family configuration mode. Router config clns filter-set routes permit This is an optional task and is normally used only with external BGP neighbors. R2 has three CLNS neighbors.

Router show clns neighbors Tag osi-as In the following example. In the following example of output from router R2. Enter the show bgp nsap unicast command to display all the NSAP prefix routes that the local router has discovered. If the local router has any directly connected external BGP peers. DecnetIV I. In the following example output.

ES-IS B. Router show bgp nsap unicast BGP table version is 3. Router show clns route Codes: Router show bgp nsap unicast summary BGP router identifier S Stale Origin codes: Use Telnet to access a router port. These commands are intended only for troubleshooting purposes because the volume of output generated by the software when they are used can result in severe performance degradation on the router. Step 8 no debug bgp nsap unicast [neighbor-address dampening keepalives updates] Enter the specific no debug bgp nsap unicastcommand when you are finished.

Use appropriate arguments and keywords to generate more detailed debug information on specified subcomponents. If you cannot connect to a console directly. Step 6 debug bgp nsap unicast [neighbor-address dampening keepalives updates] Enter only specific debug bgp nsap unicastcommands to isolate the output to a certain subcomponent and minimize the load on the processor.

If you must break the Telnet connection. Step 9 logging console This command reenables logging to the console. Step 5 terminal monitor This command enables logging on the virtual terminal.

Step 2 Step 3 Step 4 no logging console This command disables all logging to the console terminal. Step 7 no terminal monitor This command disables logging on the virtual terminal. Configuring Interfaces Example In the following example. BGP Configuration Guide.. The NSAP prefix unique to autonomous system AS is advertised to allow the other autonomous systems to discover the existence of autonomous system AS in the network: After the NSAP address family configuration mode is enabled with the address-family nsap command.

Without a route map being specified. Router R7 router isis osi-as net The peer group is automatically activated under the address-family nsap command by configuring the peer group as a route reflector client allowing it to exchange NSAP routing information between group members. A route map is used to permit only routes from within the local autonomous system to be redistributed into BGP.

To reduce the number of configuration commands.. The no bgp default ipv4unicast command is configured on the router to disable the default behavior of the BGP routing process exchanging IPv4 addressing information with BGP neighbor routers. Without a route map being specified..

Router R1 forwards the packets to router R2 using the default route. This example is configured at Router 7 in the figure below. Router R2 in AS provides the connectivity to the rest of the network for autonomous system AS by sending a default route to R1. This section contains complete configurations for all routers shown in the figure below. Autonomous System AS Router 6 router isis osi-as net Autonomous System AS Router 7 clns filter-set external-routes deny Router 8 router isis osi-as net Router 9 clns filter-set external-routes deny Related Documents Related Topic.

Protocol for providing the connectionless-mode network service. End system to Intermediate system routing exchange protocol for use in conjunction with the protocol for providing the connectionless-mode network service ISO Intermediate system to Intermediate system intradomain routing information exchange protocol for use in conjunction with the protocol for providing the connectionless-mode network service ISO The Cisco Support website provides extensive http: If you have a valid service contract but do not have a user ID or password, you can register on Cisco.

Unless noted otherwise, subsequent releases of that software release train also support that feature. To access Cisco Feature Navigator, go to www. In Release Feature Information flap-statistics, show bgp nsap inconsistent-as, show bgp nsap neighbors, show bgp nsap paths, show bgp nsap quoteregexp, show bgp nsap regexp, show bgp nsap summary.

Glossary address family --A group of network protocols that share a common format of network address. Address families are defined by RFC AS --autonomous system. An IP term to describe a routing domain that has its own independent routing policy and is administered by a single authority.

Equivalent to the OSI term "routing domain. Interdomain routing protocol that exchanges reachability information with other BGP systems. An OSI network-layer protocol. In OSI, a network management protocol created and standardized by ISO for the monitoring and control of heterogeneous networks. DCC --data communications channel. DCN --data communications network. OSI protocol that defines how end systems hosts announce themselves to intermediate systems routers.

In OSI, an application-layer protocol developed for network file exchange and management between diverse types of computers. Internet protocol used to exchange routing information within an autonomous system. A proprietary Cisco protocol, developed to address the issues associated with routing in large, heterogeneous networks. IS --intermediate system.

Routing node in an OSI network. OSI link-state hierarchical routing protocol based on DECnet Phase V routing, where routers exchange routing information based on a single metric, to determine network topology.

International organization that is responsible for a wide range of standards, including those relevant to networking. NSAP address --network service access point address. The network address format used by OSI networks. International standardization program created by ISO and ITU-T to develop standards for data networking that facilitate multivendor equipment interoperability. Standard that defines a set of rate and format standards that are sent using optical signals over fiber.

High-speed synchronous network specification designed to run on optical fiber. BGP is an interdomain routing protocol that is designed to provide loop-free routing between organizations. The Cisco IOS implementation of the neighbor and address family commands is explained. Finding Feature Information Your software release may not support all the features documented in this module.

For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the Feature Information Table at the end of this document. However, a BGP routing process and autonomous system can support multiple address family configurations. BGP Version 4 Border Gateway Protocol BGP is an interdomain routing protocol designed to provide loop-free routing between separate routing domains that contain independent routing policies autonomous systems.

BGP requires more configuration than other routing protocols, and the effects of any configuration changes must be fully understood. Incorrect configuration can create routing loops and negatively impact normal network operation. If no loopback interface is configured on the router, then the software chooses the highest IPv4 address configured to a physical interface on the router to represent the BGP router ID. A network administrator usually manually configures the relationships between BGP-speaking routers.

This relationship between BGP devices is often referred to as a neighbor but, as this can imply the idea that the BGP devices are directly connected with no other router in between, the term neighbor will be avoided whenever possible in this document. After this initial exchange only incremental updates are sent when there has been a topology change in the network, or when a routing policy has been implemented or modified. In the periods of inactivity between these updates, peers exchange special messages called keepalives.

A BGP autonomous system is a network controlled by a single technical administration entity. Peer routers are called external peers when they are in different autonomous systems and internal peers when they are in the same autonomous system. Usually, external peers are adjacent and share a subnet; internal peers may be anywhere in the same autonomous system. The table below shows the format in which 2byte and 4-byte autonomous system numbers are configured, matched in regular expressions, and displayed in show command output in Cisco IOS images where only asdot formatting is available.

Table 7. For an example of BGP peers in two autonomous systems using 4byte numbers, see the figure below. To view a configuration example of the configuration between three neighbor peers in separate 4-byte autonomous systems configured using asdot notation, see the Examples Configuring a BGP Routing Process and Peers Using 4-Byte Autonomous System Numbers, page Cisco also supports RFC , which was developed to allow BGP to support a gradual transition from 2byte autonomous system numbers to 4-byte autonomous system numbers.

To ensure a smooth transition, we recommend that all BGP speakers within an autonomous system that is identified using a 4-byte autonomous system number be upgraded to support 4-byte autonomous system numbers.

Idle--Initial state the BGP routing process enters when the routing process is enabled or when the router is reset. In this state, the router waits for a start event, such as a peering configuration with a remote peer. After the router receives a TCP connection request from a remote peer, the router initiates another start event to wait for a timer before starting a TCP connection to a remote peer. If the router is reset then the peer is reset and the BGP routing process returns to the Idle state.

Start events are ignored while the BGP routing process is in the Active state. If the BGP routing process is reconfigured or if an error occurs, the BGP routing process will release system resources and return to an Idle state. If the connection fails, the BGP routing process transitions to the Active state. When a keepalive message is received, the BGP routing process transitions to the Established state. If a notification message is received, the BGP routing process transitions to the Idle state.

If an error or configuration change occurs that affects the peering session, the BGP routing process sends a notification message with the Finite State Machine FSM error code and then transitions to the Idle state. Established--The initial keepalive is received from the remote peer. Peering is now established with the remote neighbor and the BGP routing process starts exchanging update message with the remote peer.

The hold timer restarts when an update or keepalive message is received. If the BGP process receives an error notification, it will transition to the Idle state. All commands that are independent of the address family are grouped together at the beginning highest level of the configuration, and these are followed by separate submodes for commands specific to each address family with the exception that commands relating to IPv4 unicast can also be entered at the beginning of the configuration.

When a network operator configures BGP, the flow of BGP configuration categories is represented by the following bullets in order: Global configuration--Configuration that is applied to BGP in general, rather than to specific neighbors. For example, the network, redistribute, and bgp bestpath commands. Address family-dependent configuration--Configuration that applies to a specific address family such as policy on an individual neighbor.

The relationship between BGP global and BGP address family-dependent configuration categories is shown in the table below. Table VRF specific AS independent commands! Session config. The following is an example of BGP configuration statements showing the grouping of global address family-independent and address family-dependent commands. AF independent address-family ipv4 unicast!

Policy config. AF dependant exit-address-family address-family ipv4 multicast! For complete support of AFI commands and features. Network operators can configure commands in the address family identifier AFI format and save these command configurations to existing NLRI formatted configurations.

A route that is not installed into the RIB is an inactive route. The use of route aggregation reduces the amount of information involved. Soft reconfiguration uses stored update information.

Peers that support the route refresh capability are unaffected by the configuration of this command. Inactive route advertisement can occur. Soft reset--A soft reset uses stored prefix information to reconfigure and activate BGP routing tables without tearing down existing peering sessions. BGP peering sessions must be reset using the clear ip bgp command. Route refresh must first be advertised through BGP capability negotiation between peers.

Conditional aggregation involves creating an aggregate route and then advertising or suppressing the advertising of certain routes on the basis of route maps. Basic route redistribution involves creating an aggregate route and then redistributing the routes into BGP. The bgp soft-reconfig-backup command was introduced to configure BGP to perform inbound soft reconfiguration for peers that do not support the route refresh capability.

It instead relies on dynamic exchange with supporting peers. The following message is displayed in the output when the router supports the route refresh capability: Received route refresh capability from peer. Aggregation is the process of combining the attributes of several different routes so that only a single route is advertised.

You can redistribute an aggregated route into BGP or you can use a form of conditional aggregation. All BGP routers must support the route refresh capability. The route refresh capability does not store update information locally for non disruptive policy changes.

Inactive route advertisements can be suppressed to provide more consistent data forwarding. To determine if a BGP router supports this capability. Two methods are available in BGP to implement route aggregation.

Aggregate prefixes use the classless interdomain routing CIDR principle to combine contiguous networks into one classless set of IP addresses that can be summarized in routing tables.

Dynamic inbound soft reset--The route refresh capability. The configuration of this command allows you to configure BGP to store updates soft reconfiguration only as necessary. Fewer routes now need to be advertised. Soft reconfiguration can be configured for inbound or outbound sessions. The prefixes in the BGP. Performing outbound reset causes the new local outbound policy configured on the router to take effect without resetting the BGP session.

Performing inbound reset enables the new inbound policy configured on the router to take effect. There are two types of reset: Note Does not reset outbound routing table updates. Does not reset inbound routing table updates. The table below lists their advantages and disadvantages.

Dynamic inbound soft reset Does not clear the BGP session and cache. Not recommended. Does not require storing of routing table updates. Outbound soft reset No configuration.

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Outbound policy changes require an outbound reset on the local router or an inbound reset on the peer router. As a new set of updates is sent during outbound policy reset. Routing Policy Change Management Routing policies for a peer include all the configurations for elements such as route map. This means that after changing inbound policy you must do an inbound reset on the local router or an outbound reset on the peer router. Whenever there is a change in the routing policy.

Clearing the BGP session in this way will have a negative impact upon network operations and should be used only as a last resort. If you subsequently change a BGP filter. A soft reset updates the routing table for inbound and outbound routing updates. Once you have defined two routers to be BGP neighbors. Stores all received inbound routing policy updates without modification.

The existing methods include redistribution and using the network or aggregate-address command. Routing accuracy is obscured by common route aggregation because a prefix that represents multiple addresses or hosts over a large topological area cannot be accurately reflected in a single route.

When soft reset is used to send a new set of updates to a neighbor. This feature allows more specific routes to be generated based on administrative policy or traffic engineering information in order to provide more specific control over the forwarding of packets to these more specific routes. Conditional BGP Route Injection Routes that are advertised through the BGP are commonly aggregated to minimize the number of routes that are used and reduce the size of global routing tables.

To use soft reset without preconfiguration. BGP conditional route injection allows you to originate a prefix into a BGP routing table without the corresponding match. Only IP Routing: Requires preconfiguration. Recommended only when absolutely necessary. There are two types of soft reset: These methods assume the existence of more specific routing information matching the route to be originated in either the routing table or the BGP table.

Enabling this feature will allow you to improve the accuracy of common route aggregation by conditionally injecting or replacing less specific prefixes with more specific prefixes.

This soft reset allows the dynamic exchange of route refresh requests and routing information between BGP routers. Changing the default administrative distances is not recommended because changing the administrative distance may lead to routing loops. The inject map defines the prefixes that will be created and installed into the local BGP table. In the figure below.

The exist map specifies the prefixes that the BGP speaker will track. BGP treats the network specified by the network backdoor command as a locally assigned network. When you have many peers. EIGRP routes. BGP conditional route injection is enabled with the bgp inject-map exist-mapcommand and uses two route maps inject map and exist map to install one or more more specific prefixes into a BGP routing table.

Neighbors with the same update policies can be grouped into BGP peer groups to simplify configuration and.

Peer templates also allow the IP Routing: A peer template is a configuration pattern that can be applied to neighbors that share policies. For the best optimization of BGP update group generation. These limitations existed to balance optimal update generation and replication against peer group configuration.