IP Routing: BGP Configuration Guide, Cisco IOS XE Release 3S
BGP Route-Map Continue Support for Outbound Policy
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BGP Route-Map Continue Support for Outbound Policy

BGP Route-Map Continue Support for Outbound Policy

The BGP Route-Map Continue Support for an Outbound Policy feature introduces support for continue clauses to be applied to outbound route maps.

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Iinformation About BGP Route-Map Continue Support for Outbound Policy

BGP Route Map with a Continue Clause

Subsequent to the Cisco implementation of route maps, the continue clause was introduced into BGP route map configuration. The continue clause allows for more programmable policy configuration and route filtering. The continue clause introduces the ability to execute additional entries in a route map after an entry is executed with successful match and set clauses. Continue clauses allow you to configure and organize more modular policy definitions so that specific policy configurations need not be repeated within the same route map. Before the continue clause was introduced, route map configuration was linear and did not allow any control over the flow of a route map.

Route Map Operation Without Continue Clauses

A route map evaluates match clauses until a successful match occurs. After the match occurs, the route map stops evaluating match clauses and starts executing set clauses, in the order in which they were configured. If a successful match does not occur, the route map “falls through” and evaluates the next sequence number of the route map until all configured route map entries have been evaluated or a successful match occurs. Each route map sequence is tagged with a sequence number to identify the entry. Route map entries are evaluated in order starting with the lowest sequence number and ending with the highest sequence number. If the route map contains only set clauses, the set clauses will be executed automatically, and the route map will not evaluate any other route map entries.

Route Map Operation with Continue Clauses

When a continue clause is configured, the route map will continue to evaluate and execute match clauses in the specified route map entry after a successful match occurs. The continue clause can be configured to go to (jump to) a specific route map entry by specifying the sequence number, or if a sequence number is not specified, the continue clause will go to the next sequence number. This behavior is called an “implied continue.” If a match clause exists, the continue clause is executed only if a match occurs. If no successful matches occur, the continue clause is ignored.

Match Operations with Continue Clauses

If a match clause does not exist in the route map entry but a continue clause does, the continue clause will be automatically executed and go to the specified route map entry. If a match clause exists in a route map entry, the continue clause is executed only when a successful match occurs. When a successful match occurs and a continue clause exists, the route map executes the set clauses and then goes to the specified route map entry. If the next route map entry contains a continue clause, the route map will execute the continue clause if a successful match occurs. If a continue clause does not exist in the next route map entry, the route map will be evaluated normally. If a continue clause exists in the next route map entry but a match does not occur, the route map will not continue and will “fall through” to the next sequence number if one exists.


Note


If the number of community lists in a match community clause within a route map exceed 256 characters in a line, you must nvgen multiple match community statements in a new line.


Set Operations with Continue Clauses

Set clauses are saved during the match clause evaluation process and are executed after the route-map evaluation is completed. The set clauses are evaluated and executed in the order in which they were configured. Set clauses are executed only after a successful match occurs, unless the route map does not contain a match clause. The continue statement proceeds to the specified route map entry only after configured set actions are performed. If a set action occurs in the first route map and then the same set action occurs again, with a different value, in a subsequent route map entry, the last set action may override any previous set actions that were configured with the same set command unless the set command permits more than one value. For example, the set as-path prepend command permits more than one autonomous system number to be configured.


Note


A continue clause can be executed, without a successful match, if a route map entry does not contain a match clause.



Note


Route maps have a linear behavior, not a nested behavior. Once a route is matched in a route map permit entry with a continue command clause, it will not be processed by the implicit deny at the end of the route-map. For an example, see the “Examples: Filtering Traffic Using Continue Clauses in a BGP Route Map” section.


How to Filter Traffic Using Continue Clauses in a BGP Route Map

Filtering Traffic Using Continue Clauses in a BGP Route Map

SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    router bgp autonomous-system-number

    4.    neighbor {ip-address | peer-group-name} remote-as autonomous-system-number

    5.    address-family ipv4 [unicast | multicast | vrf vrf-name]

    6.    neighbor {ip-address | peer-group-name} route-map map-name {in | out}

    7.    exit

    8.    exit

    9.    route-map map-name {permit | deny} [sequence-number]

    10.    match ip address {access-list-number | access-list-name} [... access-list-number | ... access-list-name]

    11.    set community { { [community-number] [well-known-community] [additive]} | none}

    12.    continue [sequence-number]

    13.    end

    14.    show route-map [map-name]


DETAILED STEPS
     Command or ActionPurpose
    Step 1 enable


    Example:
    Device> enable
     

    Enables privileged EXEC mode.

    • Enter your password if prompted.

     
    Step 2 configure terminal


    Example:
    Device# configure terminal
     

    Enters global configuration mode.

     
    Step 3 router bgp autonomous-system-number


    Example:
    Device(config)# router bgp 50000
     

    Enters router configuration mode, and creates a BGP routing process.

     
    Step 4 neighbor {ip-address | peer-group-name} remote-as autonomous-system-number


    Example:
    Device(config-router)# neighbor 10.0.0.1 remote-as 50000
     

    Adds the IP address or peer group name of the neighbor in the specified autonomous system to the IPv4 multiprotocol BGP neighbor table of the local device.

     
    Step 5 address-family ipv4 [unicast | multicast | vrf vrf-name]


    Example:
    Device(config-router)# address-family ipv4 unicast
     

    Specifies the IPv4 address family and enters address family configuration mode.

    • The unicast keyword specifies the IPv4 unicast address family. By default, the device is placed in address family configuration mode for the IPv4 unicast address family if the unicast keyword is not specified.

    • The multicast keyword specifies IPv4 multicast address prefixes.

    • The vrf keyword and vrf-name argument specify the name of the VRF instance to associate with subsequent IPv4 address family configuration mode commands.

     
    Step 6 neighbor {ip-address | peer-group-name} route-map map-name {in | out}


    Example:
    Device(config-router-af)# neighbor 10.0.0.1 route-map ROUTE-MAP-NAME in
     

    Applies the inbound route map to routes received from the specified neighbor, or applies an outbound route map to routes advertised to the specified neighbor.

     
    Step 7 exit


    Example:
    Device(config-router-af)# exit
     

    Exits address family configuration mode and enters router configuration mode.

     
    Step 8 exit


    Example:
    Device(config-router)# exit
     

    Exits router configuration mode and enters global configuration mode.

     
    Step 9 route-map map-name {permit | deny} [sequence-number]


    Example:
    Device(config)# route-map ROUTE-MAP-NAME permit 10
     

    Enters route-map configuration mode to create or configure a route map.

     
    Step 10 match ip address {access-list-number | access-list-name} [... access-list-number | ... access-list-name]


    Example:
    Device(config-route-map)# match ip address 1
     

    Configures a match command that specifies the conditions under which policy routing and route filtering occur.

    • Multiple match commands can be configured. If a match command is configured, a match must occur in order for the continue statement to be executed. If a match command is not configured, set and continue clauses will be executed.

    Note   

    The match and set commands used in this task are examples that are used to help describe the operation of the continue command. For a list of specific match and set commands, see the continue command in the Cisco IOS IP Routing: BGP Command Reference.

     
    Step 11 set community { { [community-number] [well-known-community] [additive]} | none}


    Example:
    Device(config-route-map)# set community 10:1
     

    Configures a set command that specifies the routing action to perform if the criteria enforced by the match commands are met.

    • Multiple set commands can be configured.

    • In this example, a clause is created to set the specified community number in aa:nn format.

     
    Step 12 continue [sequence-number]


    Example:
    Device(config-route-map)# continue
     

    Configures a route map to continue to evaluate and execute match statements after a successful match occurs.

    • If a sequence number is configured, the continue clause will go to the route map with the specified sequence number.

    • If no sequence number is specified, the continue clause will go to the route map with the next sequence number. This behavior is called an “implied continue.”

     
    Step 13 end


    Example:
    Device(config-route-map)# end
     

    Exits route-map configuration mode and enters privileged EXEC mode.

     
    Step 14 show route-map [map-name]


    Example:
    Device# show route-map
     

    (Optional) Displays locally configured route maps. The name of the route map can be specified in the syntax of this command to filter the output.

     

    Examples

    The following sample output shows how to verify the configuration of continue clauses using the show route-map command. The output displays configured route maps including the match, set, and continue clauses.

    Device# show route-map
    
    route-map MARKETING, permit, sequence 10
      Match clauses:
        ip address (access-lists): 1 
        metric 10 
      Continue: sequence 40
      Set clauses:
        as-path prepend 10
      Policy routing matches: 0 packets, 0 bytes
    route-map MARKETING, permit, sequence 20
      Match clauses:
        ip address (access-lists): 2 
        metric 20 
      Set clauses:
        as-path prepend 10 10
      Policy routing matches: 0 packets, 0 bytes
    route-map MARKETING, permit, sequence 30
      Match clauses:
      Continue: to next entry 40
      Set clauses:
        as-path prepend 10 10 10
      Policy routing matches: 0 packets, 0 bytes
    route-map MARKETING, permit, sequence 40
      Match clauses:
        community (community-list filter): 10:1 
      Set clauses:
        local-preference 104
      Policy routing matches: 0 packets, 0 bytes
    route-map MKTG-POLICY-MAP, permit, sequence 10
      Match clauses:
      Set clauses:
        community 655370
      Policy routing matches: 0 packets, 0 bytes

    Configuration Examples for BGP Route-Map Continue Support for Outbound Policy

    Examples: Filtering Traffic Using Continue Clauses in a BGP Route Map

    The following example shows continue clause configuration in a route map sequence.

    The first continue clause in route map entry 10 indicates that the route map will go to route map entry 30 if a successful matches occurs. If a match does not occur, the route map will “fall through” to route map entry 20. If a successful match occurs in route map entry 20, the set action will be executed and the route map will not evaluate any additional route map entries. Only the first successful match ip address clause is supported.

    If a successful match does not occur in route map entry 20, the route map will fall through to route map entry 30. This sequence does not contain a match clause, so the set clause will be automatically executed and the continue clause will go to the next route map entry because a sequence number is not specified.

    If there are no successful matches, the route map will fall through to route map entry 30 and execute the set clause. A sequence number is not specified for the continue clause, so route map entry 40 will be evaluated.

    There are two behaviors that can occur when the same set command is repeated in subsequent continue clause entries. For set commands that configure an additive or accumulative value (for example, set community additive, set extended community additive, and set as-path prepend), subsequent values are added by subsequent entries. The following example illustrates this behavior. After each set of match clauses, a set as-path prepend command is configured to add an autonomous system number to the as-path. After a match occurs, the route map stops evaluating match clauses and starts executing the set clauses, in the order in which they were configured. Depending on the number of successful match clauses, the as-path is prepended by one, two, or three autonomous system numbers.

    route-map ROUTE-MAP-NAME permit 10
     match ip address 1
     match metric 10
     set as-path prepend 10
     continue 30
     !
    route-map ROUTE-MAP-NAME permit 20
     match ip address 2
     match metric 20
     set as-path prepend 10 10
     !
    route-map ROUTE-MAP-NAME permit 30
     set as-path prepend 10 10 10
     continue
     !
    route-map ROUTE-MAP-NAME permit 40
     match community 10:1
     set local-preference 104
    

    In this example, the same set command is repeated in subsequent continue clause entries but the behavior is different from the first example. For set commands that configure an absolute value, the value from the last instance will overwrite the previous value(s). The following example illustrates this behavior. The set clause value in sequence 20 overwrites the set clause value from sequence 10. The next hop for prefixes from the 172.16/16 network is set to 10.2.2.2 and not 10.1.1.1.

    ip prefix-list 1 permit 172.16.0.0/16 
    ip prefix-list 2 permit 192.168.1.0/24 
    route-map RED permit 10 
     match ip address prefix-list 1 
     set ip next hop 10.1.1.1 
     continue 20 
     exit 
    route-map RED permit 20 
     match ip address prefix-list 2 
     set ip next hop 10.2.2.2 
     end 
    

    Note


    Route maps have a linear behavior, not a nested behavior. Once a route is matched in a route map permit entry with a continue command clause, it will not be processed by the implicit deny at the end of the route map. The following example illustrates this case.


    In the following example, when routes match an AS-path of 10, 20, or 30, the routes are permitted and the continue clause jumps over the explicit deny clause to process the match ip address prefix-list command. If a match occurs here, the route metric is set to 100. Only routes that do not match an AS-path of 10, 20, or 30 and do match a community number of 30 are denied. To deny other routes, you must configure an explicit deny statement.

    route-map test permit 10
     match as-path 10 20 30
     continue 30
     exit
    route-map test deny 20
     match community 30
     exit
    route-map test permit 30
     match ip address prefix-list 1
     set metric 100
     exit

    Additional References

    Related Documents

    Related Topic

    Document Title

    Cisco IOS commands

    Cisco IOS Master Command List, All Releases

    BGP commands

    Cisco IOS IP Routing: BGP Command Reference

    Technical Assistance

    Description

    Link

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    http:/​/​www.cisco.com/​cisco/​web/​support/​index.html

    Feature Information for BGP Route-Map Continue Support for Outbound Policy

    The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

    Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/​go/​cfn. An account on Cisco.com is not required.

    Table 1 Feature Information for BGP Route-Map Continue Support for Outbound Policy

    Feature Name

    Releases

    Feature Information

    BGP Route-Map Continue Support for Outbound Policy

    12.2(33)SB

    12.2(33)SRB

    Cisco IOS XE Release 2.1

    The BGP Route-Map Continue Support for an Outbound Policy feature introduces support for continue clauses to be applied to outbound route maps.

    This feature was introduced on the Cisco ASR 1000 Series Routers.