IP Addressing: NAT Configuration Guide, Cisco IOS XE Release 3S
Interchassis Asymmetric Routing Support for Zone-Based Firewall and NAT
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Interchassis Asymmetric Routing Support for Zone-Based Firewall and NAT

The Interchassis Asymmetric Routing Support for Zone-Based Firewall and NAT feature supports the forwarding of packets from a standby redundancy group to the active redundancy group for packet handling. If this feature is not enabled, the return TCP packets forwarded to the router that did not receive the initial synchronization (SYN) message are dropped because they do not belong to any known existing session.

This module provides an overview of asymmetric routing and describes how to configure asymmetric routing

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.

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.

Restrictions for Interchassis Asymmetric Routing Support for Zone-Based Firewall and NAT

  • Asymmetric routing over Multiprotocol Label Switching (MPLS) and VPN is not supported.
  • LANs that use virtual IP addresses and virtual MAC (VMAC) addresses do not support asymmetric routing.
  • VPN routing and forwarding (VRF) is not supported.

Information About Interchassis Asymmetric Routing Support for Zone-Based Firewall and NAT

Asymmetric Routing Overview

Asymmetric routing occurs when packets from TCP or UDP connections flow in different directions through different routes. In asymmetric routing, packets that belong to a single TCP or UDP connection are forwarded through one interface in a redundancy group (RG), but returned through another interface in the same RG. In asymmetric routing, the packet flow remains in the same RG. When you configure asymmetric routing, packets received on the standby RG are redirected to the active RG for processing. If asymmetric routing is not configured, the packets received on the standby RG may be dropped.

Asymmetric routing determines the RG for a particular traffic flow. The state of the RG is critical in determining the handling of packets. If an RG is active, normal packet processing is performed. In case the RG is in a standby state and you have configured asymmetric routing and the asymmetric-routing always-divert enable command, packets are diverted to the active RG. Use the asymmetric-routing always-divert enable command to always divert packets received from the standby RG to the active RG.

The figure below shows an asymmetric routing scenario with a separate asymmetric-routing interlink interface to divert packets to the active RG.

Figure 1. Asymmetric Routing Scenario

The following rules apply to asymmetric routing:

  • 1:1 mapping exists between the redundancy interface identifier (RII) and the interface.
  • 1:n mapping exists between the interface and an RG. (An interface can have multiple RGs.)
  • 1:n mapping exists between an RG and applications that use it. (Multiple applications can use the same RG).
  • 1:1 mapping exists between an RG and the traffic flow. The traffic flow must map only to a single RG. If a traffic flow maps to multiple RGs, an error occurs.
  • 1:1 or 1:n mapping can exist between an RG and an asymmetric-routing interlink as long as the interlink has sufficient bandwidth to support all the RG interlink traffic.

Asymmetric routing consists of an interlink interface that handles all traffic that is to be diverted. The bandwidth of the asymmetric-routing interlink interface must be large enough to handle all expected traffic that is to be diverted. An IPv4 address must be configured on the asymmetric-routing interlink interface, and the IP address of the asymmetric routing interface must be reachable from this interface.


Note

We recommend that the asymmetric-routing interlink interface be used for interlink traffic only and not be shared with high availability (HA) control or data interfaces because the amount of traffic on the asymmetric-routing interlink interface could be quite high.

Asymmetric Routing Support in Firewalls

For intrabox asymmetric routing support, the firewall does a stateful Layer 3 and Layer 4 inspection of Internet Control Message Protocol (ICMP), TCP, and UDP packets. The firewall does a stateful inspection of TCP packets by verifying the window size and order of packets. The firewall also requires the state information from both directions of the traffic for stateful inspection. The firewall does a limited inspection of ICMP information flows. It verifies the sequence number associated with the ICMP echo request and response. The firewall does not synchronize any packet flows to the standby redundancy group (RG) until a session is established for that packet. An established session is a three-way handshake for TCP, the second packet for UDP, and informational messages for ICMP. All ICMP flows are sent to the active RG.

The firewall does a stateless verification of policies for packets that do not belong to the ICMP, TCP, and UDP protocols.

The firewall depends on bidirectional traffic to determine when a packet flow should be aged out and diverts all inspected packet flows to the active RG. Packet flows that have a pass policy and that include the same zone with no policy or a drop policy are not diverted.


Note

The firewall does not support the asymmetric-routing always-divert enable command that diverts packets received on the standby RG to the active RG. By default, the firewall forces all packet flows to be diverted to the active RG.

Asymmetric Routing in NAT

By default, when asymmetric routing is configured, Network Address Translation (NAT) processes non-ALG packets on the standby RG, instead of forwarding them to the active. The NAT-only configuration (that is when the firewall is not configured) can use both the active and standby RGs for processing packets. If you have a NAT-only configuration and you have configured asymmetric routing, the default asymmetric routing rule is that NAT will selectively process packets on the standby RG. You can configure the asymmetric-routing always-divert enable command to divert packets received on the standby RG to the active RG. Alternatively, if you have configured the firewall along with NAT, the default asymmetric routing rule is to always divert the packets to the active RG.

When NAT receives a packet on the standby RG and if you have not configured the diverting of packets, NAT does a lookup to see if a session exists for that packet. If a session exists and there is no ALG associated for that session, NAT processes the packet on the standby RG. The processing of packets on the standby RG when a session exists significantly increases the bandwidth of the NAT traffic.

ALGs are used by NAT to identify and translate payload and to create child flows. ALGs require a two-way traffic to function correctly. NAT must divert all traffic to the active RG for any packet flow that is associated with an ALG. This is accomplished by checking if ALG data that is associated with the session is found on the standby RG. If ALG data exits, the packet is diverted for asymmetric routing.

Asymmetric Routing in a WAN-LAN Topology

Asymmetric routing supports only a WAN-LAN topology. In a WAN-LAN topology, devices are connected through LAN interfaces on the inside and WAN interfaces on the outside. There is no control on the routing of return traffic received through WAN links. Asymmetric routing controls the routing of return traffic received through WAN links in a WAN-LAN topology. The figure below shows a WAN-LAN topology.
Figure 2. Asymmetric Routing in a WAN-LAN Topology

How to Configure Interchassis Asymmetric Routing Support for Zone-Based Firewall and NAT

Configuring a Redundancy Application Group and a Redundancy Group Protocol

Redundancy groups consist of the following configuration elements:
  • The amount by which the priority will be decremented for each object.
  • Faults (objects) that decrement the priority
  • Failover priority
  • Failover threshold
  • Group instance
  • Group name
  • Initialization delay timer
SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    redundancy

    4.    application redundancy

    5.    group id

    6.    name group-name

    7.    priority value [failover threshold value]

    8.    preempt

    9.    track object-number decrement number

    10.    exit

    11.    protocol id

    12.    timers hellotime {seconds | msec msec} holdtime {seconds | msec msec}

    13.    authentication {text string | md5 key-string [0 | 7] key [timeout seconds] | key-chain key-chain-name}

    14.    bfd

    15.    end


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 redundancy


    Example:
    Device(config)# redundancy
     

    Enters redundancy configuration mode.

     
    Step 4 application redundancy


    Example:
    Device(config-red)# application redundancy
     

    Configures application redundancy and enters redundancy application configuration mode.

     
    Step 5 group id


    Example:
    Device(config-red-app)# group 1
     

    Configures a redundancy group and enters redundancy application group configuration mode.

     
    Step 6 name group-name


    Example:
    Device(config-red-app-grp)# name group1
     

    Specifies an optional alias for the protocol instance.

     
    Step 7 priority value [failover threshold value]


    Example:
    Device(config-red-app-grp)# priority 100 failover threshold 50
     

    Specifies the initial priority and failover threshold for a redundancy group.

     
    Step 8 preempt


    Example:
    Device(config-red-app-grp)# preempt
     
    Enables preemption on the redundancy group and enables the standby device to preempt the active device.
    • The standby device preempts only when its priority is higher than that of the active device.
     
    Step 9 track object-number decrement number


    Example:
    Device(config-red-app-grp)# track 50 decrement 50
     

    Specifies the priority value of a redundancy group that will be decremented if an event occurs on the tracked object.

     
    Step 10 exit


    Example:
    Device(config-red-app-grp)# exit
     

    Exits redundancy application group configuration mode and enters redundancy application configuration mode.

     
    Step 11 protocol id


    Example:
    Device(config-red-app)# protocol 1
     

    Specifies the protocol instance that will be attached to a control interface and enters redundancy application protocol configuration mode.

     
    Step 12 timers hellotime {seconds | msec msec} holdtime {seconds | msec msec}


    Example:
    Device(config-red-app-prtcl)# timers hellotime 3 holdtime 10
     
    Specifies the interval between hello messages sent and the time period before which a device is declared to be down.
    • Holdtime should be at least three times the hellotime.
     
    Step 13 authentication {text string | md5 key-string [0 | 7] key [timeout seconds] | key-chain key-chain-name}


    Example:
    Device(config-red-app-prtcl)# authentication md5 key-string 0 n1 timeout 100
     

    Specifies authentication information.

     
    Step 14 bfd


    Example:
    Device(config-red-app-prtcl)# bfd
     
    Enables the integration of the failover protocol running on the control interface with the Bidirectional Forwarding Detection (BFD) protocol to achieve failure detection in milliseconds.
    • BFD is enabled by default.
     
    Step 15 end


    Example:
    Device(config-red-app-prtcl)# end
     

    Exits redundancy application protocol configuration mode and enters privileged EXEC mode.

     

    Configuring Data, Control, and Asymmetric Routing Interfaces

    In this task, you configure the following redundancy group (RG) elements:
    • The interface that is used as the control interface.
    • The interface that is used as the data interface.
    • The interface that is used for asymmetric routing. This is an optional task. Perform this task only if you are configuring asymmetric routing for Network Address Translation (NAT).

    Note

    Asymmetric routing, data, and control must be configured on separate interfaces for zone-based firewall. However, for Network Address Translation (NAT), asymmetric routing, data, and control can be configured on the same interface.

    SUMMARY STEPS

      1.    enable

      2.    configure terminal

      3.    redundancy

      4.    application redundancy

      5.    group id

      6.    data interface-type interface-number

      7.    control interface-type interface-number protocol id

      8.    timers delay seconds [reload seconds]

      9.    asymmetric-routing interface type number

      10.    asymmetric-routing always-divert enable

      11.    end


    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 redundancy


      Example:
      Device(config)# redundancy
       

      Enters redundancy configuration mode.

       
      Step 4 application redundancy


      Example:
      Device(config-red)# application redundancy
       

      Configures application redundancy and enters redundancy application configuration mode.

       
      Step 5 group id


      Example:
      Device(config-red-app)# group 1
       

      Configures a redundancy group (RG) and enters redundancy application group configuration mode.

       
      Step 6 data interface-type interface-number


      Example:
      Device(config-red-app-grp)# data GigabitEthernet 0/0/1
       

      Specifies the data interface that is used by the RG.

       
      Step 7 control interface-type interface-number protocol id


      Example:
      Device(config-red-app-grp)# control GigabitEthernet 1/0/0 protocol 1
       
      Specifies the control interface that is used by the RG.
      • The control interface is also associated with an instance of the control interface protocol.
       
      Step 8 timers delay seconds [reload seconds]


      Example:
      Device(config-red-app-grp)# timers delay 100 reload 400
       

      Specifies the time required for an RG to delay role negotiations that start after a fault occurs or the system is reloaded.

       
      Step 9 asymmetric-routing interface type number


      Example:
      Device(config-red-app-grp)# asymmetric-routing interface GigabitEthernet 0/1/1
       

      Specifies the asymmetric routing interface that is used by the RG.

       
      Step 10 asymmetric-routing always-divert enable


      Example:
      Device(config-red-app-grp)# asymmetric-routing always-divert enable
       

      Always diverts packets received from the standby RG to the active RG.

       
      Step 11 end


      Example:
      Device(config-red-app-grp)# end
       

      Exits redundancy application group configuration mode and enters privileged EXEC mode.

       

      Configuring a Redundant Interface Identifier and Asymmetric Routing on an Interface


      Note
      • You must not configure a redundant interface identifier (RII) on an interface that is configured either as a data interface or as a control interface.
      • You must configure the RII and asymmetric routing on both active and standby devices.
      • You cannot enable asymmetric routing on the interface that has a virtual IP address configured.
      SUMMARY STEPS

        1.    enable

        2.    configure terminal

        3.    interface type number

        4.    redundancy rii id

        5.    redundancy group id [decrement number]

        6.    redundancy asymmetric-routing enable

        7.    end


      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 interface type number


        Example:
        Device(config)# interface GigabitEthernet 0/1/3
         

        Selects an interface to be associated with the redundancy group (RG) and enters interface configuration mode.

         
        Step 4 redundancy rii id


        Example:
        Device(config-if)# redundancy rii 600
         

        Configures the redundancy interface identifier (RII).

         
        Step 5 redundancy group id [decrement number]


        Example:
        Device(config-if)# redundancy group 1 decrement 20
         

        Enables the RG redundancy traffic interface configuration and specifies the amount to be decremented from the priority when the interface goes down.

        Note   

        You need not configure an RG on the traffic interface on which asymmetric routing is enabled.

         
        Step 6 redundancy asymmetric-routing enable


        Example:
        Device(config-if)# redundancy asymmetric-routing enable
         

        Establishes an asymmetric flow diversion tunnel for each RG.

         
        Step 7 end


        Example:
        Device(config-if)# end
         

        Exits interface configuration mode and enters privileged EXEC mode.

         

        Configuring Dynamic Inside Source Translation with Asymmetric Routing

        The following configuration is a sample dynamic inside source translation with asymmetric routing. You can configure asymmetric routing with the following types of NAT configurations—dynamic outside source, static inside and outside source, and Port Address Translation (PAT) inside and outside source translations. For more information on different types of NAT configurations, see the “Configuring NAT for IP Address Conservation” chapter.

        SUMMARY STEPS

          1.    enable

          2.    configure terminal

          3.    interface type number

          4.    ip address ip-address mask

          5.    ip nat outside

          6.    exit

          7.    redundancy

          8.    application redundancy

          9.    group id

          10.    asymmetric-routing always-divert enable

          11.    end

          12.    configure terminal

          13.    ip nat pool name start-ip end-ip {mask | prefix-length prefix-length}

          14.    exit

          15.    ip nat inside source list acl-number pool name redundancy redundancy-id mapping-id map-id

          16.    access-list standard-acl-number permit source-address wildcard-bits

          17.    end


        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 interface type number


          Example:
          Device(config)# interface gigabitethernet 0/1/3
           

          Configures an interface and enters interface configuration mode.

           
          Step 4 ip address ip-address mask


          Example:
          Device(config-if)# ip address 10.1.1.1 255.255.255.0
           

          Sets a primary IP address for an interface.

           
          Step 5 ip nat outside


          Example:
          Device(config-if)# ip nat outside
           

          Marks the interface as connected to the outside.

           
          Step 6 exit


          Example:
          Device(config-if)# exit
           

          Exits interface configuration mode and enters global configuration mode.

           
          Step 7 redundancy


          Example:
          Device(config)# redundancy
           

          Configures redundancy and enters redundancy configuration mode.

           
          Step 8 application redundancy


          Example:
          Device(config-red)# application redundancy
           

          Configures application redundancy and enters redundancy application configuration mode.

           
          Step 9 group id


          Example:
          Device(config-red-app)# group 1
           

          Configures a redundancy group and enters redundancy application group configuration mode.

           
          Step 10 asymmetric-routing always-divert enable


          Example:
          Device(config-red-app-grp)# asymmetric-routing always-divert enable
           

          Diverts the traffic to the active device.

           
          Step 11 end


          Example:
          Device(config-red-app-grp)# end
           

          Exits redundancy application group configuration mode and enters privileged EXEC mode.

           
          Step 12 configure terminal


          Example:
          Device# configure terminal
           

          Enters global configuration mode.

           
          Step 13 ip nat pool name start-ip end-ip {mask | prefix-length prefix-length}


          Example:
          Device(config)# ip nat pool pool1 prefix-length 24
           
          Defines a pool of global addresses.
          • Enters IP NAT pool configuration mode.
           
          Step 14 exit


          Example:
          Device(config-ipnat-pool)# exit
           

          Exits IP NAT pool configuration mode and enters global configuration mode.

           
          Step 15 ip nat inside source list acl-number pool name redundancy redundancy-id mapping-id map-id


          Example:
          Device(config)# ip nat inside source list pool pool1 redundancy 1 mapping-id 100
           

          Enables NAT of the inside source address and associates NAT with a redundancy group by using the mapping ID.

           
          Step 16 access-list standard-acl-number permit source-address wildcard-bits


          Example:
          Device(config)# access-list 10 permit 10.1.1.1 255.255.255.0
           

          Defines a standard access list for the inside addresses that are to be translated.

           
          Step 17 end


          Example:
          Device(config)# end
           

          Exits global configuration mode and enters privileged EXEC mode.

           

          Configuration Examples for Interchassis Asymmetric Routing Support for Zone-Based Firewall and NAT

          Example: Configuring a Redundancy Application Group and a Redundancy Group Protocol

          Device# configure terminal	
Device(config)# redundancy
Device(config-red)# application redundancy
Device(config-red-app)# group 1
Device(config-red-app-grp)# name group1
Device(config-red-app-grp)# priority 100 failover threshold 50
Device(config-red-app-grp)# preempt
Device(config-red-app-grp)# track 50 decrement 50
Device(config-red-app-grp)# exit
Device(config-red-app)# protocol 1
Device(config-red-app-prtcl)# timers hellotime 3 holdtime 10
Device(config-red-app-prtcl)# authentication md5 key-string 0 n1 timeout 100
Device(config-red-app-prtcl)# bfd
Device(config-red-app-prtcl)# end

          Example: Configuring Data, Control, and Asymmetric Routing Interfaces

          Device# configure terminal
Device(config)# redundancy 
Device(config-red)# application redundancy
Device(config-red-app)# group 1
Device(config-red-app-grp)# data GigabitEthernet 0/0/1
Device(config-red-app-grp)# control GigabitEthernet 1/0/0 protocol 1
Device(config-red-app-grp)# timers delay 100 reload 400 
Device(config-red-app-grp)# asymmetric-routing interface GigabitEthernet 0/1/1 
Device(config-red-app-grp)# asymmetric-routing always-divert enable
Device(config-red-app-grp)# end

          Example: Configuring a Redundant Interface Identifier and Asymmetric Routing on an Interface

          Device# configure terminal
Device(config)# interface GigabitEthernet 0/1/3
Device(config-if)# redundancy rii 600
Device(config-if)# redundancy group 1 decrement 20
Device(config-if)# redundancy asymmetric-routing enable 
Device(config-if)# end

          Example: Configuring Dynamic Inside Source Translation with Asymmetric Routing

          Device(config)# interface gigabitethernet 0/1/3
Device(config-if)# ip address 10.1.1.1 255.255.255.0
Device(config-if)# ip nat outside
Device(config-if)# exit
Device(config)# redundancy
Device(config-red)# application redundancy
Device(config-red-app)# group 1
Device(config-red-app-grp)# asymmetric-routing always-divert enable
Device(config-red-app-grp)# end
Device# configure terminal
Device(config)# ip nat pool pool1 prefix-length 24
Device(config-ipnat-pool)# exit
Device(config)# ip nat inside source list pool pool1 redundancy 1 mapping-id 100
Device(config)# access-list 10 permit 10.1.1.1 255.255.255.0

          Additional References for Interchassis Asymmetric Routing Support for Zone-Based Firewall and NAT

          Related Documents

          Related Topic

          Document Title

          Cisco IOS commands

          Cisco IOS Master Command List, All Releases

          Security commands

          Firewall inter-chassis redundancy

          “Configuring Firewall Stateful Inter-Chassis Redundancy” module

          NAT inter-chassis redundancy

          “Configuring Stateful Inter-Chassis Redundancy” module

          Technical Assistance

          Description

          Link

          The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

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

          Feature Information for Interchassis Asymmetric Routing Support for Zone-Based Firewall and NAT

          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 Interchassis Asymmetric Routing Support for Zone-Based Firewall and NAT

          Feature Name

          Releases

          Feature Information

          Interchassis Asymmetric Routing Support for Zone-Based Firewall and NAT

          Cisco IOS XE Release 3.5S

          The Interchassis Asymmetric Routing Support for Zone-Based Firewall and NAT feature supports the forwarding of packets from a standby redundancy group to the active redundancy group for packet handling.

          The following commands were introduced or modified: asymmetric-routing, redundancy asymmetric-routing enable.