IPv6 MTU size

[Eric Guo]

Hi All,

I setup IPv6 lab test for MTU size change.  I can see the physical interface and IPv6 interface MTU size are under 9178, but when I did PING test for peering IPv6 adddress, I still could ping through with large packet size up to 18024. I run ASR9K OS version 3.9.2

is there something wrong here ? becuase I couldnt' understand IPv6 MTU command purpose.

thanks advance for everyone if you could share with your idea!

GigabitEthernet0/0/0/0 is Up, ipv6 protocol is Up, Vrfid is default (0x60000000)

  IPv6 is enabled, link-local address is fe80::da67:d9ff:fe7c:d5d0

  Global unicast address(es):

    2002::1, subnet is 2002::/64

  Joined group address(es): ff02::1:ff00:1 ff02::1:ff7c:d5d0 ff02::2

      ff02::1

  MTU is 9192 (7000 is available to IPv6)

ping ipv6 2002::2 size 10000

Sun Dec  8 16:39:04.326 UTC

Type escape sequence to abort.

Sending 5, 10000-byte ICMP Echos to 2002::2, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 12/13/15 ms

Eric

[Harold Ritter]

Hi Eric,

This is normal behavior since the ASK9k in this case is used as a source host and can then fragment the packets. The ASR9k will create mutiple frames with fragmentation header and send them across the link. Each fragment will be smaller than or equal to the ipv6 MTU.

Regards

[Eric Guo]

Many thanks Harold for your explanation!

if IPv6 packet is transmitting these 2 ASR9K router2, does the ASR9K router do same thing ? shown below

IPv6 packet IN --> IOS router A--Ipv6 enable ---ASR 9K router 1---Ipv6 enable router 2 ----Ipv6 enable ----IOS router B----> IPv6 packet OUT

I did PING test between  IOS router A and to ASR9K router 1, I can ping IPv6 packet size up to 18024 same result  as do ping test between ASR9K with ASR9K. so my question is the  IPv6  MTU command  under ASR router interface and IOS router interface   when I  can use for packet limit ?

thanks,

Eric

[Harold Ritter]

Hi Eric,

Again in this scenario, router A will fragment the packets depending on the outbound interface ipv6 mtu, so everything should work even with a packet size larger than the outbound interface ipv6 mtu.

The ipv6 mtu would imposed a limit for transit traffic. For instance if the egress interface ipv6 mtu is lower than the ingress interface and received packet is too big to be forwarded out the egress interface, the router would discard the packet and send an ICMPv6 Packet Too Big (PTB) message to the source.

Regards

[Eric Guo]

Great ! I am clear now.

thanks very much, Harold!