The Truth About GRE Tunnel Overhead
Question
Which two statements are true regarding GRE tunnel overhead? (Choose two.)
Answers
Explanations
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A. B. C. D.CD
The two statements that are true regarding GRE tunnel overhead are:
B. Packets are not dropped because the router clears the DF bit for GRE traffic by default. D. You can change the tunnel MTU to avoid packet fragmentation or drops.
Explanation:
GRE (Generic Routing Encapsulation) is a tunneling protocol that encapsulates one protocol over another protocol to transport packets over an IP network. The protocol being encapsulated is called the payload, and the protocol used for encapsulation is called the encapsulating protocol.
When packets are encapsulated with GRE, additional overhead is added to the original packet. This overhead includes a GRE header with a new IP header, which adds an additional 24 bytes to the original packet.
Regarding the MTU (Maximum Transmission Unit) of the GRE tunnel, there are several factors to consider:
If the host sends a packet larger than the MTU, that packet must be dropped (Option A is false). This is because the router can't fragment the GRE packet, as the fragmentation would happen inside the tunnel and not at the network layer.
By default, the router clears the DF (Don't Fragment) bit for GRE traffic, which means that packets larger than the MTU are fragmented before they are encapsulated (Option B is true). This fragmentation is done at the network layer, and the resulting fragments are then encapsulated into separate GRE packets.
However, if the DF bit is set in the original IP packet, the router won't fragment the packet, and it will be dropped if it's larger than the MTU (Option C is false).
To avoid packet fragmentation or drops, you can change the tunnel MTU to a larger value (Option D is true). This will allow larger packets to be encapsulated without being fragmented or dropped. However, changing the MTU can cause other issues, such as increasing the amount of overhead and reducing the available bandwidth.
In summary, GRE tunnel overhead includes the addition of a GRE header and an IP header, which adds an additional 24 bytes to the original packet. To avoid fragmentation or drops, the tunnel MTU can be increased. The DF bit is cleared by default for GRE traffic, which allows for fragmentation if necessary.
