Dell PowerConnect B-RX - GRE IP Tunnel

BigIron RX Series Configuration Guide 165
53-1002253-01
Configuring IP parameters
7
GRE IP tunnel
The BigIron RX allows the tunneling of packets of the following protocols over an IP network using
the Generic Router Encapsulation (GRE) mechanism as described in RFC 2784:
• OSPF
• BGP
• IS-IS point-to-point
Using this feature, packets of these protocols can be encapsulated inside a transport protocol
packet at a tunnel source and delivered to a tunnel destination where it is unpacked and made
available for delivery. Figure 6 describes the GRE header format.
FIGURE 6 GRE header format
Checksum – This field is assumed to be zero in this version. If set to 1 means that the Checksum
(optional) and Reserved (optional) fields are present and the Checksum (optional) field contains
valid information.
Reserved0 – Bits 6:0 of the field are reserved for future use and must be set to zero in transmitted
packets. If bits 11:7 of the field are non-zero, then a receiver must discard the packet unless RFC
1701 is implemented. This field is assumed to be zero in this version.
Ver – This field must be set to zero. This field is assumed to be zero in this version.
GRE MTU configuration considerations
The default value of IP GRE tunnel MTU is 1476 bytes. The MTU of the GRE tunnel is compared with
the outgoing packet before the encapsulation is done. After the encapsulation, the packet size
increases by 24 bytes. If a user wants to change the GRE tunnel MTU, the MTU should be at least
24 bytes less than the IP MTU of the outgoing interface. Otherwise, the size of the encapsulated
packet will exceed the IP MTU of the outgoing interface. In that case, the packet is dropped if the
DF (Do-Not-Fragment) bit is set in the original IP packet, otherwise, the packet is sent to CPU for
fragmentation.
NOTE
The encapsulated packets sent on a GRE tunnel have the DF bit set. Setting a GRE tunnel MTU to
be greater than 1476 will cause the encapsulated packet to be greater than 1500 bytes. This may
cause the transit routers to drop the encapsulated packet if that transit router's IP MTU is 1500
bytes (a typical default MTU value) since transit routers can not fragment a GRE packet.
Configuring a GRE IP tunnel
To configure a GRE IP Tunnel, the following parameters must be configured:
• Tunnel inter face
• Source Address for the Tunnel
• Destination address for the Tunnel
Checksum
Reserved0
Ver
Protocol Type
Checksum
(optional)
Reserved
(optional)
1 bit
12 bits
3 bits
16 bits
16 bits16 bits