Providing Connectivity Using ATM Routed Bridge Encapsulation over PVCs

The Providing Connectivity Using ATM Routed Bridge Encapsulation over PVCs feature provides the functionality of bridged ATM interface support to ATM switched virtual circuits (SVCs). Unlike permanent virtual circuits (PVCs), SVCs must be triggered by ongoing traffic and can be brought down when idle for some time. The SVCs are triggered, if down, and the traffic is passed on to the SVCs belonging to bridged ATM interface.

ATM routed bridge encapsulation (RBE) is used to route IP over bridged RFC 1483 Ethernet traffic from a stub-bridged LAN.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and 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.

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.

Prerequisites for Providing Connectivity Using ATM Routed Bridge Encapsulation over PVCs

  • When ATM SVCs are used, support for a form of bridging, such as integrated routing and bridging, is required.

  • Before configuring connectivity from a remote bridged Ethernet network to a routed network using ATM routed bridge encapsulation, you must understand the concepts in the Understanding Broadband Access Aggregation module.

Restrictions for Providing Connectivity Using ATM Routed Bridge Encapsulation over PVCs

  • Unlike PVCs, SVCs must be triggered by ongoing traffic and might be brought down after they have been idle for some time. The Bridged 1483 Encapsulated Traffic over ATM SVCs feature allows for the SVC to be triggered if down, and to pass the traffic on to the SVCs belonging to the bridged ATM interface.

  • ATM RBE does not support MAC-layer access lists; only IP access lists are supported.

Information About Providing Connectivity Using ATM Routed Bridge Encapsulation over PVCs

Overview on Bridged 1483 Encapsulated Traffic over ATM SVCs

ATM RBE is used to route IP over bridged RFC 1483 Ethernet traffic from a stub-bridged LAN.

The figure below shows an ATM subinterface on a headend router that is configured to function in ATM routed-bridge encapsulation mode. This configuration is useful when a remote bridged Ethernet network device needs connectivity to a routed network via a device bridging from an Ethernet LAN to an ATM RFC 1483 bridged encapsulation.

Figure 1. ATM Routed Bridge Encapsulation

Because PVCs are statically configured along the entire path between the end systems, it would not be suitable to route bridged encapsulated traffic over them when the user wants to configure the virtual circuits (VCs) dynamically and tear down the VCs when there is no traffic.

ATM RBE Subinterface Grouping by PVC Range

You can configure ATM routed bridge encapsulation using an ATM PVC range rather than individual PVCs. When you configure a PVC range for routed bridge encapsulation, a point-to-point subinterface is created for each PVC in the range. The number of PVCs in a range can be calculated using the following formula:

number of PVCs = (end-vpi - start-vpi + 1) x (end-vci - start-vci +1)

Subinterface numbering begins with the subinterface on which the PVC range is configured and increases sequentially through the range.


Note

You cannot explicitly configure the individual point-to-point subinterfaces created by the PVC range on a point-to-point subinterface. All the point-to-point subinterfaces in the range share the same configuration as the subinterface on which the PVC range is configured.

DHCP Option 82 Support for RBE

The DHCP relay agent information option (option 82) enables a Dynamic Host Configuration Protocol (DHCP) relay agent to include information about itself when forwarding client-originated DHCP packets to a DHCP server. The DHCP server can use this information to implement IP address or other parameter-assignment policies.

The DHCP Option 82 Support for RBE feature provides support for the DHCP relay agent information option when ATM RBE is used. The figure below shows a typical network topology in which ATM RBE and DHCP are used. The aggregation router that is using ATM RBE is also serving as the DHCP relay agent.

Figure 2. Network Topology Using ATM RBE and DHCP

This feature communicates information to the DHCP server using a suboption of the DHCP relay agent information option called agent remote ID . The information sent in the agent remote ID includes an IP address identifying the relay agent and information about the ATM interface and the PVC over which the DHCP request came in. The DHCP server can use this information to make IP address assignments and security policy decisions.

The figure below shows the format of the agent remote ID suboption.

Figure 3. Format of the Agent Remote ID Suboption

The table below describes the agent remote ID suboption fields displayed in the figure above.

Table 1 Agent Remote ID Suboption Field Descriptions

Field

Description

Port Type

Port type. The value 0x01 indicates RBE. (1 byte)

Version

Option 82 version. The value 0x01 specifies the RBE version of Option 82 (1 byte).

Reserved

RBE reserved (2 bytes).

NAS IP Address

One of the interfaces on the DHCP relay agent. The rbe nasip command can be used to specify which IP address will be used. (4 bytes)

NAS Port

RBE-enabled virtual circuit on which the DHCP request has come in. See the figure below for the format of this field. (4 bytes)

The figure below shows the format of the network access server (NAS) port field in the agent remote ID suboption.

Figure 4. Format of the NAS Port Field

The figure below shows the format of the interface field. If there is no module, the value of the module bit is 0.

Figure 5. Format of the Interface Field

DHCP Lease Limit per ATM RBE Unnumbered Interface

The DHCP lease limit per ATM RBE Unnumbered Interface feature is enabled on a Cisco IOS DHCP relay agent connected to clients through unnumbered interfaces. The relay agent keeps information about the DHCP leases offered to the clients per subinterface. When a DHCPACK message is forwarded to the client, the relay agent increments the number of leases offered to clients on that subinterface. If a new DHCP client tries to obtain an IP address and the number of leases has already reached the configured lease limit, DHCP messages from the client will be dropped and will not be forwarded to the DHCP server.

If this feature is enabled on the Cisco IOS DHCP server directly connected to clients through unnumbered interfaces, the server allocates addresses and increments the number of leases per subinterface. If a new client tries to obtain an IP address, the server will not offer an IP address if the number of leases on the subinterface has already reached the configured lease limit.

Benefits of Providing Connectivity Using ATM Routed Bridge Encapsulation

Bridged IP packets received on an ATM interface configured in routed-bridge mode are routed via the IP header. Such interfaces take advantage of the characteristics of a stub LAN topology commonly used for digital subscriber line (DSL) access and offer increased performance and flexibility over integrated routing and bridging (IRB).

Another benefit of ATM RBE is that it reduces the security risk associated with normal bridging or IRB by reducing the size of the nonsecured network. By using a single VC allocated to a subnet (which could be as small as a single IP address), ATM RBE uses an IP address in the subnet to limit the "trust environment" to the premises of a single customer.

ATM RBE supports Cisco Express Forwarding (CEF), fast switching, and process switching.

The DHCP Option 82 Support for RBE feature enables those service providers to use DHCP to assign IP addresses and DHCP option 82 to implement security and IP address assignment policies.

The DHCP Lease Limit per ATM RBE Unnumbered Interface feature allows an Internet service provider (ISP) to globally limit the number of leases available to clients per household or connection.

How to Configure ATM Routed Bridge Encapsulation over PVCs

Configuring ATM Routed Bridge Encapsulation Using PVCs

Perform the following task to configure ATM RBE using PVCs. Only the specified network layer (IP) is routed. Any remaining protocols can be passed on to bridging or other protocols. In this manner, ATM RBE can be used to route IP, while other protocols (such as IPX) are bridged normally.

or

show ip cache verbose

SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    interface atm slot / 0 . subinterface-number point-to-point

    4.    Do one of the following:

    • pvc vpi /vci
    • range [range-name] pvc start-vpi / start-vci end-vpi / end-vci

    5.    exit

    6.    ip address ip-address mask [secondary]

    7.    end

    8.    Do one of the following:

    • show arp
    • or
    • show ip cache verbose


DETAILED STEPS
     Command or ActionPurpose
    Step 1 enable


    Example:
    Router> enable
     

    Enables privileged EXEC mode.

    • Enter your password if prompted.

     
    Step 2 configure terminal


    Example:
    Router# configure terminal
     

    Enters global configuration mode.

     
    Step 3 interface atm slot / 0 . subinterface-number point-to-point


    Example:
    Router(config)# interface atm 5/0.5 point-to-point
     

    Specifies an ATM point-to-point subinterface and enters subinterface mode.

     
    Step 4Do one of the following:
    • pvc vpi /vci
    • range [range-name] pvc start-vpi / start-vci end-vpi / end-vci


    Example:
    Router(config-subif)# pvc 0/32


    Example:
    Router(config-subif)# range range1 pvc 1/200 1/299
     

    Configures a PVC to carry the routed bridge traffic and enters ATM VC class configuration mode.

    Configures a range of PVCs to carry the routed bridge traffic and enters ATM PVC range configuration mode.

     
    Step 5 exit


    Example:
    Router(config-if-atm-vc)# exit
     

    Exits to subinterface configuration mode.

     
    Step 6 ip address ip-address mask [secondary]


    Example:
    Router(config-subif)# ip address 209.165.200.224 255.255.255.0
     

    Provides an IP address on the same subnetwork as the remote network.

     
    Step 7 end


    Example:
    Router(config-subif)# end
     

    Exits to privileged EXEC mode.

     
    Step 8Do one of the following:
    • show arp
    • or
    • show ip cache verbose


    Example:
    Router# show arp


    Example:
    Router# show ip cache verbose
     

    (Optional) Displays ATM RBE configuration information.

     

    Examples

    To confirm that ATM RBE is enabled, use the show arp command and the show ip cache verbose command in privileged EXEC mode: 

    Router# show arp
 
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet 	209.165.201.51          6   0001.c9f2.a81d  ARPA   Ethernet3/1
Internet 	209.165.201.49          -   0060.0939.bb55  ARPA   Ethernet3/1
Internet 209.165.202.128         30   0010.0ba6.2020  ARPA   Ethernet3/0
Internet 209.165.201.52           6   00e0.1e8d.3f90  ARPA   ATM1/0.4
Internet 209.165.201.53           5   0007.144f.5d20  ARPA   ATM1/0.2
Internet 209.165.202.129          -   0060.0939.bb54  ARPA   Ethernet3/0
Internet 209.165.201.125         30   00b0.c2e9.bc55  ARPA   Ethernet3/1#
Router# show ip cache verbose
IP routing cache 3 entries, 572 bytes
   9 adds, 6 invalidates, 0 refcounts
Minimum invalidation interval 2 seconds, maximum interval 5 seconds,
   quiet interval 3 seconds, threshold 0 requests
Invalidation rate 0 in last second, 0 in last 3 seconds
Last full cache invalidation occurred 00:30:34 ago
Prefix/Length       Age       Interface 	Next Hop
209.165.201.51/32-24 00:30:10  Ethernet3/1 10.1.0.51 14  0001C9F2A81D00600939 BB550800
209.165.202.129/32-24 00:00:04 ATM1/0.2 	10.8.100.50 28  00010000AAAA030080C2000700000007144F5D2000600939 BB1C0800
209.165.201.125/32-24 00:06:09 ATM1/0.4 	10.8.101.35 28  00020000AAAA030080C20007000000E01E8D3F9000600939 BB1C0800

    Configuring DHCP Option 82 for RBE

    Perform this task to configure the DHCP Option 82 Support for RBE feature.

    Before You Begin

    DHCP option 82 support must be configured on the DHCP relay agent using the ip dhcp relay information option command before you can use the DHCP Option 82 Support for RBE feature.

    SUMMARY STEPS

      1.    enable

      2.    configure terminal

      3.    ip dhcp relay information option

      4.    rbe nasip source-interface

      5.    end


    DETAILED STEPS
       Command or ActionPurpose
      Step 1 enable


      Example:
      Router> enable
       

      Enables privileged EXEC mode.

      • Enter your password if prompted.

       
      Step 2 configure terminal


      Example:
      Router# configure terminal
       

      Enters global configuration mode.

       
      Step 3 ip dhcp relay information option


      Example:
      Router(config)# ip dhcp relay information option
       

      Enables the DHCP option 82 support on relay agent.

      • Enabling the DHCP option 82 support allows the system to insert the DHCP relay agent information option in forwarded BOOT REQUEST messages to a Cisco IOS DHCP server.

       
      Step 4 rbe nasip source-interface


      Example:
      Router(config)# rbe nasip loopback0
       

      Specifies the IP address of an interface on the DHCP relay agent that will be sent to the DHCP server via the Agent Remote ID suboption.

       
      Step 5 end


      Example:
      Router(config)# end
       

      Exits global configuration mode and enters privileged configuration mode.

       

      Configuring the DHCP Lease Limit

      Perform this task to limit the number of DHCP leases allowed on ATM RBE unnumbered or serial unnumbered interfaces.

      SUMMARY STEPS

        1.    enable

        2.    configure terminal

        3.    ip dhcp limit lease per interface lease-limit

        4.    end


      DETAILED STEPS
         Command or ActionPurpose
        Step 1 enable


        Example:
        Router> enable
         

        Enables privileged EXEC mode.

        • Enter your password if prompted.

         
        Step 2 configure terminal


        Example:
        Router# configure terminal
         

        Enters global configuration mode.

         
        Step 3 ip dhcp limit lease per interface lease-limit


        Example:
        Router(config)# ip dhcp limit lease per interface 2
         

        Limits the number of leases offered to DHCP clients behind an ATM RBE unnumbered or serial unnumbered interface.

         
        Step 4 end


        Example:
        Router(config)# end
         

        Exits global configuration mode and returns to privileged EXEC mode.

         

        Troubleshooting the DHCP Lease Limit

        Perform this task to troubleshoot the DHCP lease limit.

        SUMMARY STEPS

          1.    enable

          2.    debug ip dhcp server packet

          3.    debug ip dhcp server events


        DETAILED STEPS
           Command or ActionPurpose
          Step 1 enable


          Example:
          Router> enable
           

          Enables privileged EXEC mode.

          • Enter your password if prompted.

           
          Step 2 debug ip dhcp server packet


          Example:
          Router# debug ip dhcp server packet
           

          (Optional) Decodes DHCP receptions and transmissions.

           
          Step 3 debug ip dhcp server events


          Example:
          Router(config)# debug ip dhcp server events
           

          (Optional) Displays server events.

           

          Configuration Examples for Providing Connectivity Using ATM Routed Bridge Encapsulation

          The following examples show various ways to provide connectivity from a remote bridged network to a routed network using ATM RBE.

          Example Configuring ATM RBE on PVCs

          The following example shows a typical ATM routed bridge encapsulation configuration:

          enable
 configure terminal
 interface atm 4/0.100 point-to-point
  ip address 209.165.200.225 255.255.255.224
   pvc 0/32
    end

          Example Configuring ATM RBE on an Unnumbered Interface

          The following example uses a static route to point to an unnumbered interface:

           enable
  configure terminal
  interface loopback 0
  ip address 209.165.200.226 255.255.255.224
  interface atm 4/0.100 point-to-point
   ip unnumbered loopback 0
   pvc 0/32
    atm route-bridge ip
    exit
  ip route 209.165.200.228 255.255.255.224 atm 4/0.100
  end

          Example Concurrent Bridging and ATM RBE

          The following example shows concurrent use of ATM RBE with normal bridging. IP datagrams are route-bridged, and other protocols (such as IPX or AppleTalk) are bridged.

          bridge 1 protocol ieee
interface atm 4/0.100 point-to-point
 ip address 209.165.200.225 255.255.255.224
 pvc 0/32
 bridge-group 1
 atm route-bridge ip

          Example DHCP Option 82 for RBE Configuration

          In the following example, DHCP option 82 support is enabled on the DHCP relay agent using the ip dhcp relay information option command. The rbe nasip command configures the router to forward the IP address for Loopback0 to the DHCP server. 

          ip dhcp-server 209.165.200.225
!
ip dhcp relay information option
!
interface Loopback0
 ip address 209.165.201.0 255.255.255.248
!
interface atm 4/0
 no ip address
!
interface atm 4/0.1 point-to-point
 ip unnumbered Loopback0
 ip helper-address 209.165.201.3
 atm route-bridged ip
 pvc 88/800
  encapsulation aal5snap
!
!
interface Ethernet5/1
 ip address 209.165.201.4 255.255.255.248
!
router eigrp 100
 network 209.165.201.0
 network 209.165.200.0
!
rbe nasip Loopback0

          For the configuration example, the value (in hexadecimal) of the agent remote ID suboption would be 010100000B01018140580320. The table below shows the value of each field within the agent remote ID suboption.

          Table 2 Agent Remote ID Suboption Field Values

          Agent Remote ID Suboption Field

          Value

          Port Type

          0x01

          Version

          0x01

          Reserved

          undefined

          NAS IP Address

          0x0B010181 (hexadecimal value of 11.1.1.129)

          NAS Port

          • Interface (slot/module/port)

          • VPI

          • VCI

          • 0x40 (The slot/module/port values are 01 00/0/000.)

          • 0x58 (hexadecimal value of 88)

          • 0x320 (hexadecimal value of 800)

          Example DHCP Lease Limit

          In the following example, if more than three clients try to obtain an IP address from interface ATM4/0.1, the DHCPDISCOVER packets will not be forwarded to the DHCP server. If the DHCP server resides on the same router, DHCP will not reply to more than three clients.

          ip dhcp limit lease per interface 3
!
interface loopback0
 ip address 209.165.201.3 255.255.255.248
!
interface atm 4/0.1
 no ip address
!
interface atm 4/0.1 point-to-point
 ip helper-address 172.16.1.2
 ip unnumbered loopback0
 atm route-bridged ip
  pvc 88/800
  encapsulation aal5snap

          Additional References

          Related Documents

          Related Topic

          Document Title

          Cisco IOS commands

          Cisco IOS Master Commands List, All Releases

          Broadband Access Aggregation and DSL commands

          Cisco IOS Broadband Access Aggregation and DSL Command Reference

          Broadband access aggregation concepts

          Understanding Broadband Access Aggregation

          Preparing for broadband access aggregation task

          Preparing for Broadband Access Aggregation

          DHCP commands

          Cisco IOS IP Addressing Services Command Reference

          DHCP configuration tasks

          "Configuring the Cisco IOS DHCP Server" module in the Cisco IOS IP Addressing Services Configuration Guide

          Standards

          Standards

          Title

          None

          --

          MIBs

          MIBs

          MIBs Link

          None

          To locate and download MIBs for selected platforms, Cisco software releases, and feature sets, use Cisco MIB Locator found at the following URL:

          http:/​/​www.cisco.com/​go/​mibs

          RFCs

          RFCs

          Title

          None

          --

          Technical Assistance

          Description

          Link

          The Cisco Support website provides extensive online resources, including documentation and tools for troubleshooting and resolving technical issues with Cisco products and technologies.

          To receive security and technical information about your products, you can subscribe to various services, such as the Product Alert Tool (accessed from Field Notices), the Cisco Technical Services Newsletter, and Really Simple Syndication (RSS) Feeds.

          Access to most tools on the Cisco Support website requires a Cisco.com user ID and password.

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

          Feature Information for Providing Connectivity Using ATM Routed Bridge Encapsulation

          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 3 Feature Information for Providing Connectivity Using ATM Routed Bridge Encapsulation

          Feature Name

          Releases

          Feature Information

          Bridged 1483 Encapsulated Traffic over ATM SVCs

          12.4(15)T 12.2(33)SRE

          The Bridged 1483 Encapsulated Traffic over ATM SVCs feature provides support for bridged 1483 encapsulated packets to trigger ATM SVC and also support for sending this traffic on triggered ATM SVCs.

          DHCP Option 82 Support for Routed Bridge Encapsulation

          15.1(1)S 12.2(2)T

          This feature provides support for the DHCP relay agent information option when ATM RBE is used.

          The following command was introduced:rbe nasip

          DHCP Lease Limit per ATM RBE Unnumbered Interface

          12.3(2)T

          This feature limits the number of DHCP leases per subinterface offered to DHCP clients connected from an ATM RBE unnumbered interface or serial unnumbered interface of the DHCP server or DHCP relay agent.

          The following command was introduced:ip dhcp limit lease per interface