【答题赢鸿鹄币】 GRE features

小乔

QUESTION 86
Which two GRE features are configured to prevent fragmentation? (Choose two.)
A. TCP window size
B. IP MTU
C. TCP MSS
D. DF bit clear
E. MTU ignore


选详细解释给予20鸿鹄币奖励

itwong

答案是B跟C，MTU值是关键，一般MTU为1500, 但在使用GRE时候，需要考虑overhead，例如overhead为50字节，那就需要ip mtu 改为1450或以下。 TCP mss可以让router自己选择合适的MTU来避免TCP framentation.  Cisco提供IPSEC overhead calculator -> https://cway.cisco.com/ipsec-overhead-calculator