Usage Guidelines

No specific guidelines impact the use of this command.

Usage Guidelines

No specific guidelines impact the use of this command.

Usage Guidelines

Use the additional-paths receive command to allow add-path receive capability to be negotiated for a specified address family. When the additional-paths receive command is configured, the receive capability is automatically enabled for all internal BGP neighbors for a specified address family. When this command is either not configured or explicitly disabled, none of the neighbors are allowed to negotiate receive capability for the address family.

After enabling the receive capability, the session needs to be reset for the configuration to take into effect.

Usage Guidelines

To configure additional paths selection mode for some or all prefixes, use the additional-paths selection command by specifying a route-policy.

Use the additional-path selection command with an appropriate route-policy to calculate backup paths and to enable Prefix Independent Convergence (PIC) functionality. Refer BGP Prefix Independent Convergence Unipath Primary/Backup section in BGP Configuration Guide for Cisco NCS 5000 Series Routers for details on the PIC functionality.

Usage Guidelines

Use the additional-paths send command to allow add-path send capability to be negotiated for a specified address family. When the additional-paths send command is configured, the send capability is automatically enabled for all internal BGP neighbors for the specified address family. When the command is either not configured or explicitly disabled, none of the neighbors are allowed to negotiate send capability for the address family.

After enabling the send capability, the session needs to be reset for the configuration to take into effect.

Usage Guidelines

Use the address-family command to enter various address family configuration modes while configuring BGP routing sessions. When you enter the address-family command from router configuration mode, you enable the address family and enter global address family configuration mode.

The IPv4 unicast address family must be configured in router configuration mode before configuring the IPv4 labeled-unicast address family for a neighbor in neighbor configuration mode.

The IPv4 unicast address family must be configured in router configuration mode before configuring the IPv4 labeled-unicast address family for a neighbor in neighbor configuration mode. The IPv6 unicast address family must be configured in router configuration mode before configuring the IPv6 labeled-unicast address family for a neighbor in neighbor configuration mode.

Usage Guidelines

No specific guidelines impact the use of this command.

Usage Guidelines

No specific guidelines impact the use of this command.

Usage Guidelines

If this command configures a neighbor group or session group, all neighbors using the group inherit the configuration. Values of commands configured specifically for a neighbor override inherited values.

Usage Guidelines

Use the af-group command to group address family-specific neighbor commands within an IPv4 or IPv6 address family. Neighbors that have address family configuration are able to use the address family group. Further, neighbors inherit the configuration parameters of the entire address family group.

You cannot define two address family groups with the same name in different address families.

Usage Guidelines

You can implement aggregate routing in BGP either by redistributing an aggregate route into BGP using the network command or the aggregate-address command.

Use the aggregate-address command without optional arguments to create an aggregate entry in the BGP routing table if any more-specific BGP routes are available that fall in the specified range. The aggregate route is advertised as coming from your autonomous system and has the atomic aggregate attribute set to show that information might be missing. (By default, the atomic aggregate attribute is set unless you specify the as-set keyword.)

Use of the as-set keyword creates an aggregate entry using the same rules that the command follows without this keyword. However, the advertised path for this route is an AS_SET, a set of all autonomous systems contained in all paths that are being summarized.

Do not use this form of the aggregate-address command when aggregating many paths because this route must be continually withdrawn and updated as autonomous system path reachability information for the summarized routes changes.

Use the as-confed-set keyword to create an AS_CONFED_SET in the autonomous system path of the aggregate from any confederation segments in the paths being summarized. This keyword takes effect only if the as-set keyword is also specified.

Use of the summary-only keyword creates an aggregate entry (for example, 10.0.0.0/8) but suppresses advertisements of more-specific routes to all neighbors. If you want to suppress only advertisements to certain neighbors, use the route-policy (BGP) command in neighbor address family configuration mode with caution. If a more-specific route leaks out, all BGP speakers (the local router) prefer that route over the less-specific aggregate you generate (using longest-match routing).

Use the route-policy keyword to specify a routing policy for the aggregate entry. The route-policy keyword is used to select which more-specific information to base the aggregate entry on and which more-specific information to suppress. You can also use the keyword to modify the attributes of the aggregate entry.

Usage Guidelines

No specific guidelines impact the use of this command.

Usage Guidelines

Cost community point of insertion can be configured either to be pre-bestpath or after igp cost. The transitive keyword is not required for iBGP sessions. However, the transitive keyword is required for eBGP sessions to convert AiGP metric into cost-community and advertise to the eBGP neighbors.

Usage Guidelines

Use the allocate-label command with a route policy to trigger BGP to allocate labels for all or a filtered set of global IPv4 routes (as dictated by the route policy). The command enables autonomous system border routers (ASBRs) that have labeled IPv4 unicast sessions to exchange Multiprotocol Label Switching (MPLS) labels with the IPv4 routes to the other autonomous system (AS) in Layer 3 Virtual Private Network (L3VPN) inter-AS deployments.

Note	The allocate-label all command is functionally equivalent to the allocate-label route-policy route-policy-name command when the route policy is a pass-all policy.

See MPLS Configuration Guide for Cisco NCS 5000 Series Routers for information on using the allocate-label command for L3VPN inter-AS deployments and carrier-supporting-carrier IPv4 BGP label distribution.

Usage Guidelines

Usage Guidelines

No specific guidelines impact the use of this command.

Usage Guidelines

Use the as-override command in conjunction with the site-of-origin (SoO) feature, identifying the site where a route originated, and preventing routing loops between routers within a VPN.

Usage Guidelines

Configure the as-path-loopcheck out disable command to disable the default behavior of PE router not announcing BGP routes to the CE router if the routes contain an AS number matching the AS number of the receiving CE router.

Usage Guidelines

Use the attribute-filter group command in neighbor configuration mode to configure a specific attribute filter group for a BGP neighbor.

Usage Guidelines

If the minimum interval is changed using the bfd minimum-interval command, the new parameter updates all affected BFD sessions under the command mode in which the minimum interval was changed.

If the multiplier is changed using the bfd multiplier command, the new parameter is used to update only the BFD sessions associated with the affected neighbor gets affected.

The assumption is that when BFD fast-detect is enabled under neighbor address family independent configuration, the values for the multiplier and minimum-interval values are always derived from the per-neighbor values if they are configured; otherwise, they are to be taken from the global BGP configuration mode. In the event that this has not been explicitly stated, then these values are taken to be the default values. Also, the bfd arguments can be configured under neighbor-group and session-group and the inheritance adheres to the standard way of BGP configuration inheritance.

Accordingly, there are four cases in which bfd-fast detect is enabled.

This is shown in table below where the BFD value is either multiplier or minimum-interval. Local indicates per NBR value, global is the BGP global value.

Usage Guidelines

No specific guidelines impact the use of this command.

Usage Guidelines

When BGP attribute download is enabled using the bgp attribute-download command, BGP reinstalls all routes whose attributes are not currently in the RIB. Likewise, if the user disables BGP attribute download using the no form of the command, BGP reinstalls previously installed routes with a null key, and removes the attributes from the RIB.

Use the bgp attribute-download command to enable the Netflow BGP data export function. When attribute download is enabled, BGP downloads the attribute information for prefixes (community, extended community, and as-path) to the Routing Information Base (RIB) and Forwarding Information Base (FIB). This enables FIB to associate the prefixes with attributes and send the Netflow statistics along with the associated attributes.

Usage Guidelines

If the inbound policy changes, it is not always possible to perform a soft reset. This is the case if the neighbor does not support route refresh and soft-reconfiguration inbound is not configured for the neighbor. In such instances, a message is logged in the system log indicating that a manual hard reset is needed.

Usage Guidelines

Use the bgp bestpath as-path ignore command to ignore the length of autonomous system paths when the software selects a preferred path. When the best path is selected, if this command is specified, all steps are performed as usual except comparison of the autonomous path length between candidate paths.

Usage Guidelines

Use the bgp bestpath compare-routerid command to affect how the software selects the best path, in the case where there are two paths of equal cost according to the BGP selection algorithm. This command is used to force the software to select the path with the lower router ID as the best path. If this command is not used, the software continues to use whichever path is currently the best path, regardless of which has the lower router ID.

Usage Guidelines

Use the bgp bestpath cost-community ignore command to disable the evaluation of the cost community attribute to help isolate problems and troubleshoot issues that relate to BGP path selection. This command can also be used to delay the activation of cost community attribute evaluation so that cost community filtering can be deployed in a large network at the same time.

Usage Guidelines

The MED is one of the parameters that is considered by the software when selecting the best path among many alternative paths. The software chooses the path with the lowest MED.

By default, during the best-path selection process, the software makes a MED comparison only among paths from the same autonomous system. This command changes the default behavior of the software by allowing comparison of MEDs among paths regardless of the autonomous system from which the paths are received.

When the bgp bestpath med always command is not enabled and distributed BGP is configured, speakers calculate partial best paths only (executes the best-path steps up to the MED comparison) and send them to BGP Routing Information Base (bRIB). bRIB calculates the final best path (executes all the steps in the best-path calculation). When the bgp bestpath med always command is enabled and distributed BGP is configured, speakers can compare the MED across all ASs, allowing the speaker to calculate a single best path to send it to bRIB. bRIB is the ultimate process that calculates the final best path, but when the bgp bestpath med always command is enabled, the speakers send a single best path instead of potentially sending multiple, partial best paths

Usage Guidelines

By default, the MED of the following paths is not compared with the MED of any other path:

• Paths with an empty autonomous system path
• Paths beginning with an AS_SET
• Paths containing only confederation segments
• Paths containing confederation segments followed by an AS_SET

Use the bgp bestpath med confed command to affect how the following types of paths are treated in the BGP best-path algorithm:

• Paths containing only confederation segments
• Paths containing confederation segments followed by an AS_SET

The MED for paths that start with an AS_SEQUENCE or that start with confederation segments followed by an AS_SEQUENCE only is compared with the MED of other paths that share the same first autonomous system number in the autonomous system sequence (the neighbor autonomous system number). This behavior is not affected by the bgp bestpath med confed command.

As an example, suppose that autonomous systems 65000, 65001, 65002, and 65004 are part of a confederation, but autonomous system 1 is not. Suppose that for a particular route, the following paths exist:

• Path 1: 65000 65004, med = 2, IGP metric = 20
• Path 2: 65001 65004, med = 3, IGP metric = 10
• Path 3: 65002 1, med = 1, IGP metric = 30

If the bgp bestpath med confed command is enabled, the software selects path 1 as the best path because it:

• Has a lower MED than path 2
• Has a lower IGP metric than path 3

The MED is not compared with path 3 because it has an external autonomous system number (that is, an AS_SEQUENCE) in the path. If the bgp bestpath med confed command is not enabled, then MED is not compared between any of these paths. Consequently, the software selects path 2 as the best path because it has the lowest IGP metric.

Usage Guidelines

No specific guidelines impact the use of this command.

Usage Guidelines

By default, the clients of a route reflector are not required to be fully meshed and the routes from a client are reflected to other clients. However, if the clients are fully meshed, route reflection is not required.

By default, the clients of a route reflector that are part of the same cluster are not required to be fully meshed and the routes from a client are reflected to other clients. However, if the clients are fully meshed, route reflection is not required. If the cluster-id is not specified, then this command disables intra-cluster route reflection for all clusters.

Usage Guidelines

Together, a route reflector and its clients form a cluster. A cluster of clients usually has a single route reflector. In such instances, the cluster is identified by the software as the router ID of the route reflector. To increase redundancy and avoid a single point of failure in the network, a cluster might have more than one route reflector. If it does, all route reflectors in the cluster must be configured with the same 4-byte cluster ID so that a route reflector can recognize updates from route reflectors in the same cluster.

A single route reflector can also support multiple clusters. Each cluster is identified by a unique cluster-id. The cluster-id configured by the bgp cluster-id command is taken as the default. If bgp cluster-id is not configured, the router ID for the default VRF identifies the default cluster. A neighbor can be associated with one cluster only, and the corresponding cluster-id is configured in neighbor configuration mode. If the cluster-id is not configured for a neighbor and the neighbor is a route reflector client, then the neighbor is assigned to the default cluster.

Usage Guidelines

One way to reduce the internal BGP (iBGP) mesh is to divide an autonomous system into multiple autonomous systems and group them into a single confederation. Each autonomous system is fully meshed within itself, and has a few connections to another autonomous system in the same confederation. Although the peers in different autonomous systems have external BGP (eBGP) sessions, they exchange routing information as if they are iBGP peers. Specifically, the confederation maintains the next hop and local preference information, and that allows you to retain a single Interior Gateway Protocol (IGP) for all autonomous systems. To the outside world, the confederation looks like a single autonomous system.

Use the bgp confederation identifier command to specify the autonomous system number for the confederation. This autonomous system number is used when BGP sessions are established with external peers in autonomous systems that are not part of the confederation.

Usage Guidelines

The autonomous systems specified in this command are visible internally to a confederation. Each autonomous system is fully meshed within itself. The bgp confederation identifier command specifies the confederation to which the autonomous systems belong.

To specify multiple autonomous systems, enter BGP confederation peer configuration mode then enter one autonomous-system-number for each command line.

Usage Guidelines

Use the bgp dampening command without arguments to enable BGP route dampening with the default parameters. The parameters can be changed by setting them on the command line or specifying them with a routing policy.

Usage Guidelines

Generally, the default value of 100 allows you to easily define a particular path as less preferable than paths with no local preference attribute. The preference is sent to all networking devices in the local autonomous system.

Usage Guidelines

By default, the software ignores any update received from an eBGP neighbor that does not have the autonomous system configured for that neighbor at the beginning of the AS path. When configured, the command applies to all eBGP peers of the router.

Usage Guidelines

By default, BGP sessions of any directly adjacent external peers are immediately reset, which allows the network to recover faster when links go down between BGP peers.

Usage Guidelines

Use the bgp graceful-restart command to enable graceful restart functionality on the router, and also to advertise graceful restart to neighboring routers.

Note	The bgp graceful-restart command with no options must be used to enable graceful restart before using the bgp graceful-restart purge-time , bgp graceful-restart restart-time , bgp graceful-restart stalepath-time , or bgp graceful-restart graceful-reset commands.

When graceful restart is enabled, the BGP graceful restart capability is negotiated with neighbors in the BGP OPEN message when the session is established. If the neighbor also advertises support for graceful restart, then graceful restart is activated for that neighbor session. If the neighbor does not advertise support for graceful restart, then graceful restart is not activated for that neighbor session even though it is enabled locally.

If you enter the bgp graceful-restart command after some BGP sessions are established, you must restart those sessions before graceful restart takes effect. Use the clear bgp command to restart sessions.

Usage Guidelines

BGP graceful restart must be enabled using the bgp graceful-restart command before enabling graceful reset using the bgp graceful-restart graceful-reset command.

Usage Guidelines

BGP graceful restart must be enabled using the bgp graceful-restart command before setting the purge time using the bgp graceful-restart purge-time command.

Usage Guidelines

BGP graceful restart must be enabled using the bgp graceful-restart command before setting the restart timer using the bgp graceful-restart restart-time command.

Usage Guidelines

BGP graceful restart must be enabled using the bgp graceful-restart command before setting the stalepath time using the bgp graceful-restart stalepath-time command.

If the stalepath time is exceeded before an End-of-RIB message is received from a neighbor, paths learned from the neighbor are purged from the BGP routing table.

Usage Guidelines

No specific guidelines impact the use of this command.

Usage Guidelines

No specific guidelines impact the use of this command.

Usage Guidelines

Logging of BGP neighbor status changes (up or down) and resets is used for troubleshooting network connectivity problems and measuring network stability. Unexpected neighbor resets might indicate high error rates or high packet loss in the network, and should be investigated.

Status change message logging does not substantially affect performance, unlike, for example, enabling per-BGP update debugging. If the UNIX syslog facility is enabled, messages are sent by the software to the UNIX host running the syslog daemon so that the messages can be stored and archived on disk. If the UNIX syslog facility is not enabled, the status change messages are kept in the internal buffer of the router, and are not stored to disk.

The neighbor status change messages are not tracked if the bgp log neighbor changes disable command is disabled, except for the last reset reason, which is always available as output of the show bgp neighbors command.

Up and down messages for BGP neighbors are logged by the software by default. Use the bgp log neighbor changes disable command to stop logging BGP neighbor changes.

Usage Guidelines

Any attempt to configure the neighbor limit below 1 or above 15000 fails. Similarly, attempting to configure the limit below the number of neighbors currently configured fails. For example, if there are 3250 neighbors configured, you cannot set the limit below 3250.

Usage Guidelines

When multiple connected routers start ignoring as-path onwards while computing multipath, it causes routing loops. Therefore, you should not configure the bgp multipath as-path ignore onwards command on routers that can form a loop.

Usage Guidelines

Use this command to apply the flow-tag to a specified interface. The packets are matched based on an incoming source, destination IP address or action (such as redirect, drop, PBTS) and redirected to a specific VRF.

Note	You will not be able to enable both QPPB and flow tag feature simultaneously on an interface.

Usage Guidelines

Use of the bgp redistribute-internal command requires the clear route * command to be issued to reinstall all BGP routes into the IP routing table.

Note	Redistributing iBGP routes into IGPs may cause routing loops to form within an autonomous system. Use this command with caution.

Usage Guidelines

If you do not use the bgp router-id command to configure a router ID, an IP address is not configured on any loopback interface, and no global router ID is configured, BGP neighbors remain down.

For more details on router IDs, see the BGP Configuration Guide for Cisco NCS 5000 Series Routers

Usage Guidelines

Use the bgp scan-time command to change how frequently the software processes scanner tasks, such as conditional advertisement, dynamic MED changes, and periodic maintenance tasks.

Usage Guidelines

When BGP is started, it waits a specified period of time for its neighbors to establish peering sessions and to complete sending their initial updates. After all neighbors complete their initial updates, or after the update delay timer expires, the best path is calculated for each route, and the software starts sending advertisements out to its peers. This behavior improves convergence time. If the software were to advertise a route as soon as it learned it, it would have to readvertise the route each time it learned a new path that was preferred over all previously learned paths.

Use the bgp update-delay command to tune the maximum time the software waits after the first neighbor is established until it starts calculating best paths and sending out advertisements.

Usage Guidelines

Use the bgp write-limit command to configure both a per-update group and a global limit on the number of messages the software queues when updating peers. Increasing these limits can result in faster Border Gateway Protocol (BGP) convergence, but also may result in higher memory use during convergence. In addition, this command can be used to enable desynchronization. Desynchronization can decrease memory use and speed up convergence for the fastest neighbors if one or more neighbors in an update group process updates significantly slower than other neighbors in the same group. However, enabling desynchronization can cause a significant degradation in overall convergence time, especially if the router is experiencing high CPU utilization. For this reason, enabling desynchronization is discouraged.

Usage Guidelines

No specific guidelines impact the use of this command.

Usage Guidelines

No specific guidelines impact the use of this command.

Usage Guidelines

Use the capability additional-paths receive command to selectively enable or disable additional paths receive capability negotiation for a particular neighbor or neighbor-group or session-group. Configuring additional-paths receive command in global address-family mode is a pre-requisite for negotiating additional paths receive capability with the peer.

If you enter the capability additional-paths receive command after some BGP sessions are established, you must restart those sessions for the new configuration to take effect. Use the clear bgp command to restart sessions.

Usage Guidelines

Use the capability additional paths send command to selectively enable or disable additional paths send capability negotiation for a particular neighbor or neighbor-group or session-group. Configuring the additional-paths send command in global address-family mode is a pre-requisite for negotiating additional paths send capability with the peer.

You must restart the BGP sessions for the new configuration to take effect. Use the clear bgp command to restart sessions.

Usage Guidelines

The advertisement of the prefix list ORF capability by a BGP speaker indicates whether the speaker can send prefix lists to the specified neighbor and whether it accepts prefix lists from the neighbor. The speaker sends a prefix list if it indicated the ability to send them, and if the neighbor indicated it was willing to accept them. Similarly, the neighbor sends a prefix list to the speaker if it indicated the ability to send them and the speaker indicated the willingness to accept them.

Note	The capability orf and prefix list filter specified by orf route-policy must be explicitly configured.

If the neighbor sends a prefix list and the speaker accepts it, the speaker applies the received prefix list, plus any locally configured outbound filters, to limit its outbound routing updates to the neighbor. Increased filtering prevents unwanted routing updates between neighbors and reduces resource requirements for routing update generation and processing.

Use the capability orf prefix command to set whether to advertise send and receive capabilities to the specified neighbor.

Note	Sending a receive capability can adversely affect performance, because updates sent to that neighbor cannot be replicated for any other neighbors.

If this command is configured for a neighbor group or neighbor address family group, all neighbors using the group inherit the configuration. Values of commands configured specifically for a neighbor override inherited values.

Usage Guidelines

By default, the software advertises the 4-byte AS capability to BGP peers. To override this default behavior, use the capability suppress 4-byte-as command under the command modes listed in the ''Command Modes'' section. If configured under the neighbor group or session group, all neighbors using the group inherit the configuration. Use the no option to remove the command.

Caution

The BGP session resets automatically, if the 4-byte AS capability of an existing BGP session is changed by configuring capability suppress 4-byte-as or capability suppress 4-byte-as inheritance-disable .