Overview

NOTE:

GRE is not supported on Dell N22xx series switches, N3208PX-ON switch and FS S5860 series switches.
PicOS GRE supports tunneling of IPv4 over IPv4 and IPv6 over IPv4.
One GRE tunnel source address and destination address form an address pair. Different GRE tunnels cannot be configured with the same address pair.
In the same VRF, it is not allowed to configure different GRE tunnel interface addresses with the same network segment IP address.
If a large number of static routes for the GRE tunnel are configured and committed at one time, the background process will take several minutes to successfully complete all these configurations, although the print may immediately show that the commit is OK.
On AS4610 serial switches, the number of static routes for the GRE tunnel should be not more than 20.
By default, the MTU value of an L3 interface is 1500. If you want to change the MTU value of the GRE tunnel source interface, its value should be less than 1304.

Generic Routing Encapsulation (GRE) is a tunneling protocol that can encapsulate almost any network layer protocol and delivers it over an IP network. GRE is defined by two tunnel endpoints, the GRE source and destination endpoint. From the common user perspective the two endpoints appear as if they are directly connected in a point-to-point configuration but in reality the packet may traverse a number of hops before it reaches the destination endpoint. The generic packet format of a GRE encapsulated packet is show in Figure 1.

Figure 1. GRE Packet Format

The original payload packet is first encapsulated with the GRE header and then encapsulated again with the delivery protocol header to transport it over the network. For example, if the network doesn't support routing IPv6 packets, the packet can be encapsulated with GRE header and then use IPv4 as the delivery protocol to transport the packet. The packet is decapsulated at the destination endpoint and the actual IPv6 packet is forwarded to the destination. One of the most common issue in enterprise networks is that on local subnets private IP addresses are used but routing packets between different subnets via private IP headers is not supported over public networks such as the internet. This issue is solved by using GRE which can encapsulate packets into a delivery protocol header that can be transported over the public network.

Application Scenario Limitation

In L2 GRE or VXLAN networks, only one next hop is allowed for the same egress interface. In the following figure, the same egress interface on Switch1 has two tunnels, that is, two next hops, which is not allowed.

However, multiple L2 GRE or VXLAN tunnels can exist from the same egress port on Switch1 if connected via the IP router, ensuring that one egress interface has only one next hop, as shown in the figure below.