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Book Contents Book ContentsIP Addressing Configuration Guide, Cisco IOS XE 17.x
The Asymmetric Lease for DHCPv6 Relay Prefix Delegation feature is used to manage or change the lease renewal by the relay agent.
Asymmetric lease is also referred as short lease which is shorter than the actual lease that is granted by the server. You can configure the short lease on a relay agent which overrides the actual lease. The short lease provides an option to force a lease renewal for clients before the original lease expires. It detects the lease expiry early and helps to keep the clients status live.
The Cisco DHCPv6 prefix delegation client receives the IPv6 prefix from the DHCPv6 server. The prefix delegation client uses the allotted prefix to assign IPv6 addresses to LAN side hosts. The prefix delegation client, relay agent, and server retain the allotted prefix even when the connnection is down in the following scenarios:
In both scenarios, the prefix is retained until the valid lifetime timer is expired. The short lease enables the client to reclaim the unused address prefixes earlier. Also, it enables the client to detect the failure of the relay or server earlier and allows prefix delegation client to reinitiate the DHCP6 prefix delegation prefix assignment.
In DHCPv6 Prefix Delegation with Asymmetric Lease, the client initiates the prefix assignment by sending the multicast desired packet to the server. The Relay intercepts and encapsulates the desired request message in Relay-Forward and forwards it to the server. The server responds with the advertise or reply message encapsulated in relay-reply to a Relay agent. When the Relay agent receives the relay-reply message, it performs the following:
The following sequence diagram describes the prefix delegation flow with short lease.
The IA-PD option contatins the T1 and T2 values. The T1 represents the time at which prefix delegation client renews the IA_PD prefix and lifetime values. It sends the renewal message to the server which is provided with the client's addresses and configuration parameters to extend the lifetimes on the IA_PD prefix assigned to the client.
T2 represents the time at which the prefix delegation client tries to rebind. For example, contact any available DHCPv6 server to extend the lifetime of IA_PD prefix. The rebind message is sent after a client receives no response to a renewal message. Both T1 and T2 are time duration relative to the current time expressed in units of seconds.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | OPTION_IA_PD | option-length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IAID (4 octets) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | T1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | T2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . IA_PD-options . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
On the DHCPv6 relay agent, the minimum allowed short lease value is 60 seconds. The T1 can be assigned with the configured short lease value. T2 is derived from T1 as shown below.
T2 = minimum (2 * T1 * 0.8, DHCPv6 Server assigned T2 value) |
If the DHCPv6 server sets T1 and T2 to the value 0, the T1 and T2 calculation is determined by the client. In this case, T1 can be assigned with the configured short lease.
T2 shall be calculated as shown below.
T2 = 2 * T1 * 0.8 |
DHCPv6 relay agent uses timer wheel for handling the server assigned T1 and T2 values.
The relay agent stores the actual T1 and T2 values received from DHCPv6 Server. For every reply with IA options received from the server, relay will update the T1 and T2 values which will also take care of any changes in the server side configurations. In this sequence diagrams, CPE is the DHCPv6 Prefix Delegation client and Cisco ASR 1000 Series is the DHCPv6 relay agent.
The following sequence diagram depicts the short lease renewal before the expiry of the server assigned T1 value.
The following message sequence diagram depicts the short lease renewal after the expiry of the server assigned T1 value.
The following message sequence diagram depicts the short lease rebind scenario.
The following message sequence diagram depicts how to handle the short lease renewal and rebind when the server is down.
The DHCPv6 server sends a reconfigure message to the client to inform that the server has a new or updated configuration parameters. With this information, the client initiates a renewal or reply or information-request and reply transaction with the server to receive the updated information.
The server tries to send the reconfigure message to an IPv6 unicast address of the DHCP client. If the server does not have an address to which it can send the reconfigure message directly to the client, the server uses Relay-reply option to send the message to a relay agent that will relay the message to the client.
When responding to a reconfigure message, the client creates and sends the information-request message with the exception that the client includes a server identifier option with the identifier from the reconfigure message to which the client is responding.
When responding to a reconfigure, the client creates and sends the renew message with the exception that the client copies the Option Request option and any IA options from the reconfigure message into the renew message.
You can apply the Asymmetric lease configuration on per interface or globally for all interfaces.
To configure the asymmetric lease on an interface, perform these steps:
Device> enable
Enables privileged EXEC mode.
Device# configure terminal
Enters global configuration mode.
interface type number
Device(config)# interface Ethernet 0/0
Specifies an interface type and number, and enters interface configuration mode.
ip dhcp relay destination option short-lease source-information time in seconds
Router(config-if)# ip dhcp relay short-lease 500
Sets and enables the short lease for the client on the interface. You can set the lease time in seconds. The range is from 60 to 3600 seconds.
Device(config-if)# end
Returns to privileged EXEC mode.
To configure the asymmetric lease globally:
Device> enable
Enables privileged EXEC mode.
Device# configure terminal
Enters global configuration mode.
ip dhcp relay destination option short-lease source-information time in seconds
Router(config)# ip dhcp relay short-lease 500
Sets and enables the short lease for the client globally. You can set the lease time in seconds. The range is from 60 to 3600 seconds.
Device(config-if)# end
Returns to privileged EXEC mode.
The show running-config interface Ethernet command displays the interface where short lease is configured:
Router# show running-config interface Ethernet0/0 Building configuration. Current configuration : 215 bytes ! interface Ethernet0/0 no ip address ipv6 address 2001:DB8:10::1/64 ipv6 enable ipv6 dhcp relay destination 2001:DB8:10::1 ipv6 dhcp relay short-lease 500 end
To verify the configuration on the interface, use the show ipv6 dhcp interface GigabitEthernet 2 and show ipv6 dhcp relay binding commands.
Router# show ipv6 dhcp interface GigabitEthernet 2 GigabitEthernet2 is in client mode Prefix State is OPEN Renew will be sent in 11:38:28 Address State is IDLE List of known servers: Reachable via address: FE80::250:56FF:FEAE:17A8 DUID: 00030001001EE6383500 Preference: 0 Configuration parameters: IA PD: IA ID 0x00080001, T1 120, T2 240 Prefix: 8001:DB8::/48 preferred lifetime 120, valid lifetime 240 expires at Mar 31 2020 09:24 PM (51 seconds) DNS server: 2000:3000:4000::1 Information refresh time: 0 Prefix name: pd-client Prefix Rapid-Commit: disabled Address Rapid-Commit: disabled end
Device# show ipv6 dhcp relay binding Relay Bindings associated with default vrf: Prefix: 8001:DB8::/48 (Ethernet0/0) DUID: 00030001AABBCC000A00 IAID: 131073 Short Lease T1: 60 (Expired) Short Lease T2: 96 (Active) T1: 3600 expiration: 19:47:00 IST Jul 15 2020 T2: 7200 expiration: 20:47:00 IST Jul 15 2020 lifetime: 150 expiration: 21:17:00 IST Jul 15 2020 Summary: Total number of Relay bindings = 1 Total number of IAPD bindings = 1 Total number of IANA bindings = 0 Total number of Relay bindings added by Bulk lease = 0 RELAY#
The following table provides the performance scaling information for the DHCPv6 short lease.