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Configuration Guide - IP Service

CloudEngine 8800, 7800, 6800, and 5800 V200R003C00

This document describes the configurations of IP Service, including IP address, ARP, DHCP, DNS, IP performance optimization, IPv6, DHCPv6, and IPv6 DNS.
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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
Distributed VXLAN Gateways Functioning as DHCP Relay Agents

Distributed VXLAN Gateways Functioning as DHCP Relay Agents

After DHCP relay is enabled on a VXLAN gateway, terminal tenants can dynamically obtain IP addresses using DHCP. This section describes the forwarding principles of distributed VXLAN gateways that function as DHCP relay agents in intra-VPN (the DHCP client and server reside on the same VPN) and inter-VPN (the DHCP client and server reside on different VPNs) scenarios.

DHCP Client and Server on the Same VPN

On the network shown in Figure 3-9:
  • Distributed Layer 3 VXLAN gateways are deployed on leaf1, leaf2, and leaf3, and they are connected over a Layer 3 VXLAN tunnel.
  • VM1 and VM3 belong to the subnet with VNI 10. VM2 and VM4 belong to the subnet with VNI 20.
  • The DHCP server connected to leaf3 belongs to the same VPN but different subnet as VM1, VM2, VM3, and VM4.
After DHCP relay is enabled on leaf1 and leaf2, VM1, VM2, VM3, and VM4 can all function as DHCP clients to dynamically request for IP addresses from the DHCP server.
Figure 3-9 Support for DHCP Relay on VXLAN Gateways in Intra-VPN Scenarios

DHCP relay is enabled on the VBDIF interfaces of leaf nodes to which DHCP clients are connected. In distributed VXLAN scenarios, the gateway address is the same for all the users in a subnet. On the network shown in Figure 3-9, the gateway address for all the users in the subnet with VNI 10 where leaf1 and leaf2 reside is 10.1.1.1.

DHCP relay is required if clients go online and broadcast renewal requests are sent. The following uses client access on VM1 as an example to describe the forwarding process of DHCP relay-enabled gateways. Generally, VM1 sends an address request to the DHCP server through leaf1. The DHCP server sends a response packet to leaf1. There is a possibility that the response packet is sent to leaf2. In this case, VM1 can still obtain an IP address. The detailed forwarding procedures are as follows:
  1. VM1 broadcasts a Discover packet to locate a DHCP server.
  2. Upon receipt of the broadcast Discover packet, leaf1 converts it to a unicast Discover packet. In the meantime, leaf1 uses the VBDIF address of the corresponding gateway as the GiAddr address and encapsulates the configured Option 82 suboption 9 which carries the IP address that uniquely identifies the gateway location into the Discover packet. Leaf1 encapsulates a VXLAN header into the unicast Discover packet and forwards it over a VXLAN tunnel.
  3. Leaf3 decapsulates the received packet and forwards the packet to the DHCP server based on the destination IP address.
  4. Upon receipt of the unicast Discover packet, the DHCP server assigns an IP address from the local address pool. The DHCP server replies with a unicast Offer packet in which the destination IP address is the GiAddr address of the unicast Discover packet.
  5. Upon receipt of the Offer packet, leaf3 searches the routing table based on the destination IP address (VBDIF interface address of the VXLAN Layer 3 gateway) and finds two available routes to the destination IP address. Leaf3 encapsulates the Offer packet based on the next hop of the selected route and forwards the packet.
    • If the route to leaf1 is selected, leaf3 directly forwards the Offer packet to leaf1.
    • If the route to leaf2 is selected, leaf2 decapsulates the Offer packet upon receipt of the packet, encapsulates the packet into a VXLAN packet based on the IP address carried in the Option 82 suboption 9, and then forwards the packet to leaf1.
  6. Upon receipt of the Offer packet, leaf1 decapsulates the packet and then forwards it to VM1.
DHCP relay is not required if clients go offline or unicast renewal requests are sent. The following uses unicast client renewal on VM1 as an example to describe the packet forwarding process:
  • Before the lease of the assigned IP address expires, VM1 unicasts a Request packet for lease renewal in which the source IP address is the DHCP user's IP address and the destination IP address is the DHCP server's IP address.
  • Upon receipt of the Request packet, leaf1 searches the routing table and forwards the Request packet to the DHCP server over a Layer 3 VXLAN tunnel.
  • The DHCP server replies with a unicast ACK packet destined for the DHCP user's IP address. Therefore, the DHCP server also searches the routing table and forwards the ACK packet to the DHCP client over a Layer 3 VXLAN tunnel.

DHCP Client and Server on Different VPNs

Figure 3-10 shows an inter-VPN scenario where VM1, VM2, VM3, and VM4 are in VPN 10, while the DHCP server is in VPN 20.

Leaf3 cannot identify the destination VPN of a DHCP response packet because VPN information is not carried in the packet. Therefore, network segments in VPNs must not overlap. For example, if another network VPN 30 exists, the VBDIF interface address of the DHCP relay-enabled VXLAN Layer 3 gateway in VPN 30 cannot overlap with that (10.1.1.1) of VPN 10.

To allow overlapping between network segments in different VPNs, insert suboption 151 and suboption 5 of Option 82. Table 3-6 describes the key fields in a DHCP relay packet.
Table 3-6 Description of the key fields in a DHCP relay packet

Field

Description

Processing of the DHCP Relay Agent

Processing of the DHCP Server

giaddr

This field indicates an IP address that uniquely identifies a DHCP relay agent and that is reachable to the DHCP server.

NOTE:
The GiAddr field originally indicates a DHCP client's gateway address which is used as a reference for the DHCP server to assign IP addresses. However, the DHCP server may also use the GiAddr address as the destination IP address of a response packet. Therefore, this field is changed to indicate an IP address that uniquely identifies a DHCP relay agent and that is reachable to the DHCP server. As an alternative, the Option 82 suboption 5 is used to indicate a DHCP client's gateway address.

The DHCP relay agent inserts the VBDIF interface address of leaf1's Layer 3 VXLAN gateway into this field.

The DHCP server inserts the gateway address of a DHCP client as the destination IP address in a DHCP response packet.

Suboption 5 of Option 82

This field indicates the gateway address of a DHCP client.

The DHCP relay agent inserts the VBDIF interface address of the Layer 3 VXLAN gateway into suboption 5.

During address assignment, the DHCP server searches for desired address pools based on suboption 5 and suboption 151. After address assignment succeeds, the DHCP server inserts suboption 5 and suboption 151 into a response packet.

Suboption 151 of Option 82

This field indicates the VPN information of a DHCP client.

The DHCP relay agent inserts the client's VPN instance into this field.

Suboption 11 of Option 82

This field indicates the IP address of the interface on which DHCP relay is enabled.

The DHCP relay agent inserts the VBDIF interface address of the Layer 3 VXLAN gateway into suboption 11. The DHCP server encapsulates the content of suboption 11 into Option 54 in a response packet.
Figure 3-10 Support for DHCP Relay on VXLAN Gateways in Inter-VPN Scenarios

The forwarding procedures in inter-VPN scenarios are as follows:

  1. VM1 broadcasts a Discover packet to locate a DHCP server.
  2. Upon receipt of the broadcast Discover packet, leaf1 converts it to a unicast Discover packet. In the meantime, leaf1 uses the VBDIF interface address of the corresponding gateway as the GiAddr address and encapsulates the configured Option 82 suboption 9 which carries the IP address that uniquely identifies the gateway location into the Discover packet. Leaf1 encapsulates a VXLAN header in the unicast Discover packet and forwards it over a VXLAN tunnel.
    NOTE:

    To allow overlapping between network segments in different VPNs, insert suboption 151 and suboption 5 of Option 82 into a Discover packet.

  3. Leaf3 decapsulates the received packet and forwards the packet to the DHCP server based on the destination IP address.
  4. Upon receipt of the unicast Discover packet, the DHCP server assigns an IP address from the local address pool. The DHCP server replies with a unicast Offer packet in which the destination IP address is the GiAddr address of the unicast Discover packet.
  5. Upon receipt of the Offer packet, leaf3 searches the routing table based on the destination IP address (VBDIF interface address of the Layer 3 VXLAN gateway) and finds two available routes to the destination IP address. Leaf3 encapsulates the Offer packet based on the next hop of the selected route and forwards the packet.
    • If the route to leaf1 is selected, leaf3 directly forwards the Offer packet to leaf1.
    • If the route to leaf2 is selected, leaf2 decapsulates the Offer packet upon receipt of the packet, encapsulates the packet into a VXLAN packet based on the IP address carried in the Option 82 suboption 9, and then forwards the packet to leaf1.
  6. Upon receipt of the Offer packet, leaf1 decapsulates the packet and then forwards it to VM1.
Figure 3-11 Lease Renewal Process in Inter-VPN Scenarios

In inter-VPN scenarios, due to the lack of VPN information, leaf1 cannot find the next hop IP address based on the destination IP address (DHCP server's IP address) of renewal packets. Therefore, a DHCP relay agent is required.

Lease renewal procedures in inter-VPN scenarios are as follows:

  1. VM1 broadcasts a Discover packet to locate a DHCP server.
  2. Upon receipt of the broadcast Discover packet, leaf1 converts it to a unicast Discover packet. In the meantime, leaf1 uses the VBDIF address of the corresponding gateway as the GiAddr address and encapsulates the configured IP addresses carried in the suboption 9 and suboption 11 of Option 82 into the Discover packet. Leaf1 encapsulates a VXLAN header in the unicast Discover packet and forwards it over a VXLAN tunnel.
    NOTE:

    The Option 82 suboption 11 must be inserted into a Discover packet in inter-VPN scenarios. Otherwise, lease renewal of DHCP clients will be affected.

  3. Leaf3 decapsulates the received packet and forwards the packet to the DHCP server based on the destination IP address.
  4. Upon receipt of the unicast Discover packet, the DHCP server assigns an IP address from the local address pool. The DHCP server inserts the content of the Option 82 suboption 11 into the Option 54 field in an Offer packet. Upon receipt of the Offer packet, the DHCP client uses the IP address of the DHCP relay-enabled interface as the destination IP address of a lease renewal packet. The destination IP address of the packet is the GiAddr address of the request packet.
  5. Upon receipt of the Offer packet, leaf3 searches the routing table based on the destination IP address (VBDIF interface address of the Layer 3 VXLAN gateway) and finds two available routes to the destination IP address. Leaf3 encapsulates the Offer packet based on the next hop of the selected route and forwards the packet.
    • If the route to leaf1 is selected, leaf3 directly forwards the Offer packet to leaf1.
    • If the route to leaf2 is selected, leaf2 decapsulates the Offer packet upon receipt of the packet, encapsulates the packet into a VXLAN packet based on the IP address carried in Option 82 suboption 9, and then forwards the packet to leaf1.
  6. Upon receipt of the Offer packet, leaf1 decapsulates the packet and then forwards it to VM1.
  7. After receiving the Offer packet, VM1 uses the IP address carried in the Option 54 field as the destination IP address in subsequent lease renewal packets. The subsequent lease renewal packets will be sent to the DHCP relay-enabled gateway for inter-VPN forwarding.
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Updated: 2019-05-08

Document ID: EDOC1100004354

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