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ping only success with 1 byte lesser then the mtu i set on ppp multilink interface

Hi All,

  i have a stituation which i needed help in explain on this situation.i have 2 routers and i have configure 2 serial links on each router into  a ppp multilink 1, when i ping both ways from both router i need to ping with 1 byte lesser then the mtu size i set. As show below

routerA#ping 202.122.151.129 df si 1604

Type escape sequence to abort.

Sending 5, 1604-byte ICMP Echos to 202.122.151.129, timeout is 2 seconds:

Packet sent with the DF bit set

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 12/12/12 ms

routerA#ping 202.122.151.129 df si 1605

Type escape sequence to abort.

Sending 5, 1605-byte ICMP Echos to 202.122.151.129, timeout is 2 seconds:

Packet sent with the DF bit set

.....

Success rate is 0 percent (0/5)

routerA#sh run int multil 1

Building configuration...

Current configuration : 384 bytes

!

interface Multilink1

mtu 1605

ip address 202.122.151.130 255.255.255.252

load-interval 30

ppp multilink

ppp multilink group 1

end

routerA#

============================================

routerB#ping 202.122.151.130 df si 1604

Type escape sequence to abort.

Sending 5, 1604-byte ICMP Echos to 202.122.151.130, timeout is 2 seconds:

Packet sent with the DF bit set

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 8/10/12 ms

routerB#ping 202.122.151.130 df si 1605

Type escape sequence to abort.

Sending 5, 1605-byte ICMP Echos to 202.122.151.130, timeout is 2 seconds:

Packet sent with the DF bit set

.....

Success rate is 0 percent (0/5)

routerB#

routerB#

routerB#sh run int multil 1

Building configuration...

Current configuration : 380 bytes

!

interface Multilink1

mtu 1605

ip address 202.122.151.129 255.255.255.252

load-interval 30

ppp multilink

multilink-group 1

end

Can some one explain to me why i have to ping with 1 byte lesser then mtu i set on?

Thanks

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