Mobile IP - Generic Routing Encapsulation for Cisco Mobile Networks

Generic routing encapsulation (GRE) is a tunneling protocol used by Mobile IP. The GRE tunnel interface creates a virtual point-to-point link between two routers at remote points over an IP internetwork. GRE tunnels can transport a passenger protocol or encapsulated protocol.

Unlike IP-in-IP encapsulation, GRE provides the following:

• Explicit protection against recursive encapsulation, a condition in which tunneled packets reenter the same tunnel before exiting.
• Configurable keepalive messages to monitor the end-to-end status of the tunnel.

GRE is beneficial for certain applications because of its support for multiprotocol tunneling and explicit prevention of recursive encapsulation.

To understand the components of the Cisco Mobile Networks solution, refer to the http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122newft/122t/122t4/ftmbrout.htm Cisco Mobile Networks feature documentation.

During agent discovery, HAs and FAs advertise their presence on their attached links by periodically multicasting or broadcasting messages called agent advertisements. The agent advertisements are ICMP Router Discovery Protocol (IRDP) messages with one or more extensions specific to Mobile IP. The agent advertisement extension consists of several fields including the following field that is relevant to this feature:

• G: This agent can receive tunneled IP datagrams that use GRE (referred to as the G bit)

If the GRE for Cisco Mobile Networks feature is enabled, the mobile router will request GRE encapsulation in the registration request only if the FA advertises that it is capable of GRE encapsulation (the G bit is set in the advertisement). If the registration request is successful, packets will be tunneled using GRE encapsulation.

If the GRE for Cisco Mobile Networks feature is enabled and the mobile router is using collocated care-of address (CCoA), the mobile router will attempt to register with the HA using GRE encapsulation. If the registration request is successful, packets will be tunneled using GRE encapsulation.

If the mobile router receives a denied registration reply with error code 72 (foreign agent required encapsulation unavailable) or error code 139 (home agent unsupported encapsulation), the mobile router will send another registration request with the G bit unset and the default IP-in-IP encapsulation will be used.

GRE tunnels support keepalive messages, which are messages sent periodically to the HA that allow the detection of an interruption in the end-to-end tunnel. Tunnels that use IP-in-IP encapsulation do not use keepalive messages. If a tunnel that is using IP-in-IP encapsulation loses its connection to the HA, the mobile router will not be aware of the disruption until it tries to register with the HA again. This can take up to one half of the mobile router's registration lifetime. GRE keepalive messages allow the status of the end-to-end tunnel to be checked at a configurable interval. If the mobile router detects an interruption in the connection to the HA, it will tear down the existing tunnel and attempt to reregister using the best interface. Typically this is the same interface on which the connection was previously established. If the registration attempt is unsuccessful, the mobile router will then try to register on the next best interface if one exists.

The GRE for Cisco Mobile Networks feature introduces the ability for a mobile router to use GRE tunneling in addition to the default encapsulation method of IP-in-IP. GRE is a widely supported tunneling protocol, and some platforms support GRE tunnels in hardware. Hardware support of GRE tunneling offloads software operations, such as Cisco Express Forwarding (CEF) switching, from the CPU and increases the performance of the router. In addition, GRE supports multiprotocol tunneling and provides explicit protection against recursive encapsulation. Finally, the ability to configure keepalive messages with GRE allows the status of the end-to-end tunnel to be checked at a configurable interval, and reregistration can be attempted as soon as an interruption is detected.

GRE encapsulation can be configured per interface or globally. Configuring GRE encapsulation on an interface allows only that interface to attempt to register with GRE encapsulation enabled. Configuring GRE encapsulation globally allows all roaming interfaces to attempt to register with GRE encapsulation enabled, unless the interface is configured for IP-in-IP encapsulation. The interface-level configuration overrides the global configuration.

Perform one of the following tasks to configure GRE on the mobile router:

• Configuring GRE Globally on the Mobile Router
• Configuring GRE per Interface on the Mobile Router

1.    enable

2.    configure terminal

3.    interface tunnel interface-number

4.    keepalive [period [retries]]

5.    exit

6.    ip mobile router

7.    template tunnel interface-number

The following example globally configures GRE encapsulation on a mobile router and enables GRE keepalive messages:

router mobile
!
ip mobile secure home-agent 10.40.40.1 spi 101 key hex 12345678123456781234567812345678      algorithm md5 mode prefix-suffix
ip mobile router
 address 10.80.80.1 255.255.255.0
 home-agent 10.40.40.1
 mobile-network Ethernet1/3
 mobile-network FastEthernet0/0
 template Tunnel 121
 tunnel mode gre
!
interface tunnel 121
 keepalive 5 3

The following example configures GRE encapsulation on an interface of a mobile router and enables GRE keepalive messages:

interface FastEthernet0/0
 ip address 10.52.52.2 255.255.255.0
 ip mobile router-service roam
 ip mobile router-service tunnel mode gre
!
interface tunnel 121
 keepalive 5 3
!
ip mobile router
 template tunnel 121

The following example shows display output from the show ip mobile router registration command when GRE encapsulation is configured on the mobile router. The Flags field shows that GRE encapsulation is enabled by displaying a capital "G." If GRE encapsulation were not enabled, a lowercase "g" would be displayed.

Router# show ip mobile router registration
Mobile Router Registrations:
Foreign agent 10.52.52.1:
    Registration accepted 01/11/00 07:01:24, On FastEthernet0/0
    Care-of addr 10.52.52.1, HA addr 10.40.40.1, Home addr 10.80.80.1 
    Lifetime requested 10:00:00 (36000), Granted 01:00:00 (3600) 
    Remaining 00:59:47 
    Flags sbdmG-t-
, Identification B68B7673.81565B8
    Register next time 00:59:27 
    Extensions:
        Mobile Network 172.16.153.0/24 
        Mobile Network 172.16.143.0/24 
        MN-HA Authentication SPI 101 

The following example shows display output from the show ip mobile routercommand when GRE encapsulation is globally configured on the mobile router. When GRE encapsulation is enabled, the line "Request GRE tunnel" is displayed in the output and the tunnel mode is shown as "GRE/IP".

Router# show ip mobile router
Mobile Router 
    Enabled 01/11/00 06:59:19 
    Last redundancy state transition NEVER 
Configuration:
    Home Address 10.80.80.1 Mask 255.255.255.0 
    Home Agent 10.40.40.1 Priority 100 (best) (current) 
    Registration lifetime 65534 sec 
    Retransmit Init 1000, Max 5000 msec, Limit 3 
    Extend Expire 20, Retry 10, Interval 1 
    Request GRE tunnel
    Mobile Networks:Ethernet1/3 (172.16.143.0/255.255.255.0) 
                     FastEthernet0/0 (172.16.153.0/255.255.255.0) 
Monitor:
    Status -Registered- 
    Active foreign agent 10.52.52.1, Care-of 10.52.52.1 
    On interface FastEthernet0/0
    Tunnel0 mode GRE/IP

The following example shows display output from the show ip mobile router interface command when GRE encapsulation is configured on an interface of the mobile router. When GRE encapsulation is enabled on the interface, the line "Request GRE tunnel" is displayed in the output.

Router# show ip mobile router interface
FastEthernet0/0:
    Priority 110, Bandwidth 100000, Address 10.52.52.2 
    Periodic solicitation disabled, Interval 600 sec 
    Retransmit Init 1000, Max 5000 msec, Limit 3 
    Current 2000, Remaining 0 msec, Count 2 
    Hold down 0 sec 
    Routing disallowed 
    Collocated CoA disabled 
    Request GRE tunnel