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Why is the MTU size different between these two outputs?

Hello.

Below output is from a 2901 router DMVPN tunnel interface.

QUESTIONS:

Why is the MTU size different between these outputs? 

What exactly means "Tunnel transport MTU 1472 bytes"?

Why is the second bold data so specifically "MTU 17912?"

Thank you!

---

(obfuscated)
2901#sh ip int tu33
Tunnel33 is up, line protocol is up
Internet address is 192.168.0.1/24
Broadcast address is 255.255.255.255
Address determined by non-volatile memory
MTU is 1400 bytes
Helper address is not set
!! (output omitted) !!
2901#
2901#
2901#sh int tu33
Tunnel33 is up, line protocol is up
Hardware is Tunnel
Internet address is 192.168.0.1/24
MTU 17912 bytes, BW 20000 Kbit/sec, DLY 50000 usec,

!! (output omitted) !!

Tunnel transport MTU 1472 bytes

2 Accepted Solutions

Accepted Solutions

there are two 
MTU and IP MTU
MTU is L2 total frame size 
IP MTU is L3 total packet size 

show interface <<- show mtu l2 status 
show ip interface <<- show ip mtu l3 status  

View solution in original post

Joseph W. Doherty
Hall of Fame
Hall of Fame

The 17k tunnel MTU, which appears to vary slightly across different Cisco platforms, is the MTU supported by the tunnel interface.  As the tunnel interface is a logical interface, Cisco chose a value, I believe, to support largest possible physical interface MTU (i.e. the 17k 16 Mbps token-ring [?]).

I believe the transport MTU is effectively the MTU being provided to the tunnel traffic.  I believe this is the tunnel's path MTU less encapsulation overhead.  (Note you can run PMTUD for tunnels.)

View solution in original post

7 Replies 7

there are two 
MTU and IP MTU
MTU is L2 total frame size 
IP MTU is L3 total packet size 

show interface <<- show mtu l2 status 
show ip interface <<- show ip mtu l3 status  

"IP MTU is L3 total packet size"

Just a comment to clarify - IP MTU, for the IP protocol, "pretends" the interface is that MTU.  IP packet size (max 64 KB) is NOT limited by L2 MTU although an IP source will almost always "honor" L2 MTU so to avoid the need to fragment a packet larger than the L2 MTU.

The classic example of how these two might be use:

interface e0
!inside
MTU 1500

interface t0
!gre
IP MTU 1476 !or MTU 1500 - the IP variant would only be limited to IP traffic
tunnel source e1

interface e1
!outside
MTU 1500

Traffic using the tunnel, if larger that 1476, will need to be fragmented being sent out the physical 1500 MTU.

If IP packet has DF set, and if larger than 1476, it will be dropped and IGMP (too large) message sent back to source IP in packet.

Given:

2901#sh ip int tu20
MTU is 1400 bytes

--

QUESTION:

If "MTU is L2 total frame size", then why is it ever suggested to reduce this from 1500?

Thank you.

Since this is output from show ip interface the MTU is the layer 3 view of the frame size. It might be suggested to reduce this below 1500 because of the bytes added to the frame for tunneling. Also might be suggested to reduce this because some link between source and destination might have a smaller max frame size.

HTH

Rick

"If "MTU is L2 total frame size", then why is it ever suggested to reduce this from 1500?"

Normally, you would never reduce a "real" MTU.

However, (again) on something like tunnel interfaces, the MTU is not really physical, but logical.  So, if we reduce it there, we can allow for tunnel encapsulation overhead.  This way, packets being sent across the physical interface can avoid being fragmented.

Joseph W. Doherty
Hall of Fame
Hall of Fame

The 17k tunnel MTU, which appears to vary slightly across different Cisco platforms, is the MTU supported by the tunnel interface.  As the tunnel interface is a logical interface, Cisco chose a value, I believe, to support largest possible physical interface MTU (i.e. the 17k 16 Mbps token-ring [?]).

I believe the transport MTU is effectively the MTU being provided to the tunnel traffic.  I believe this is the tunnel's path MTU less encapsulation overhead.  (Note you can run PMTUD for tunnels.)

Two helpful responses. I find the L2/L3 observation especially convincing.

HTH

Rick
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