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Community Member

MTU settings on GRE tunnel

sh run int tu162

Building configuration...

Current configuration : 251 bytes

!

interface Tunnel162

description GRE over MPLS to Dow

ip address 10.54.63.50 255.255.255.252

ip mtu 1476**********************

ip policy route-map clear-df-bit

load-interval 30

tunnel source Vlan60

tunnel destination 192.168.163.222

tunnel path-mtu-discovery

end

sh int tu162

Tunnel162 is up, line protocol is up

Hardware is Tunnel

Description: GRE over MPLS to Dow

Internet address is 10.54.63.50/30

MTU 1514 bytes, BW 9 Kbit, DLY 500000 usec,******************

reliability 255/255, txload 174/255, rxload 28/255

Encapsulation TUNNEL, loopback not set

Keepalive not set

Can someone explain me what is differnce

between two MTU config shows 1476 and sh int tu162 shows 1514????

Thanks

1 ACCEPTED SOLUTION

Accepted Solutions

Re: MTU settings on GRE tunnel

Hi,

As i've stated earlier, IP MTU affects only the IP traffic, accordingly there is also MPLS MTU, CLNS MTU and so on, while MTU affects all kind of traffic, thus if you change the IP MTU you'll affect only the IP traffic, while if you change the MTU you'll affect all the types of traffic, all the MTU values match the interface MTU unless explicitly changed, and if a specific MTU is changed it will override the interface MTU for this specific traffic type, accordingly in your example all the IP traffic will use the IP MTU 1476.

To prevent potential problems (as per Cisco precautions; data corruption, dropped packets, and high CPU conditions can occur), recent IOSs prevents the IP MTU (or any MTU) to exceed the interface MTU.

BR,

Mohammed Mahmoud.

6 REPLIES

Re: MTU settings on GRE tunnel

hi

The normal behaviour of mtu command under the interface overrides the value keyed in with ip mtu command.

If you want to keep the mtu value as 1476 try to use mtu 1476 under the interface tunnel config and check.

regds

Community Member

Re: MTU settings on GRE tunnel

Hi thanks for reply

so right now MTU size being used is 1476?

please explain

Re: MTU settings on GRE tunnel

Hi,

You have changed the IP MTU not the MTU, IP MTU affects only the IP traffic, accordingly there is also MPLS MTU, CLNS MTU and so on, while MTU affects all kind of traffic, you can see your configuration reflected with the "show ip int tun" not "show int tun":

R6(config-if)#do sh run int tun 13

Building configuration...

Current configuration : 79 bytes

!

interface Tunnel13

ip address 13.13.13.13 255.255.255.252

ip mtu 1524

end

R6(config-if)#do sh ip int tun 13

Tunnel13 is up, line protocol is down

Internet address is 13.13.13.13/30

Broadcast address is 255.255.255.255

Address determined by setup command

MTU is 1524 bytes

... output trimmed

R6(config-if)#do sh int tun 13

Tunnel13 is up, line protocol is down

Hardware is Tunnel

Internet address is 13.13.13.13/30

MTU 1514 bytes, BW 9 Kbit, DLY 500000 usec,

reliability 255/255, txload 1/255, rxload 1/255

... output trimmed

BR,

Mohammed Mahmoud.

Community Member

Re: MTU settings on GRE tunnel

Hi thanks for reply

i did

DJ01-rt1#sh ip int tun162

Tunnel162 is up, line protocol is up

Internet address is 10.54.63.49/30

Broadcast address is 255.255.255.255

Address determined by setup command

MTU is 1476 bytes

Helper address is not set

Directed broadcast forwarding is disabled

Multicast reserved groups joined: 224.0.0.10

Outgoing access list is not set

Inbound access list is not set

Proxy ARP is enabled

Local Proxy ARP is disabled

Security level is default

Split horizon is enabled

ICMP redirects are always sent

ICMP unreachables are always sent

ICMP mask replies are never sent

IP fast switching is enabled

IP fast switching on the same interface is disabled

IP Flow switching is enabled

IP CEF switching is enabled

IP CEF Flow Fast switching turbo vector

IP multicast fast switching is enabled

IP multicast distributed fast switching is disabled

IP route-cache flags are Fast, Flow cache, CEF, Full Flow

Router Discovery is disabled

IP output packet accounting is disabled

IP access violation accounting is disabled

TCP/IP header compression is disabled

RTP/IP header compression is disabled

Policy routing is enabled, using route map clear-df-bit

Network address translation is disabled

BGP Policy Mapping is disabled

so right now which MTU is used by tunnel 1524

or 1476?

Re: MTU settings on GRE tunnel

Hi,

As i've stated earlier, IP MTU affects only the IP traffic, accordingly there is also MPLS MTU, CLNS MTU and so on, while MTU affects all kind of traffic, thus if you change the IP MTU you'll affect only the IP traffic, while if you change the MTU you'll affect all the types of traffic, all the MTU values match the interface MTU unless explicitly changed, and if a specific MTU is changed it will override the interface MTU for this specific traffic type, accordingly in your example all the IP traffic will use the IP MTU 1476.

To prevent potential problems (as per Cisco precautions; data corruption, dropped packets, and high CPU conditions can occur), recent IOSs prevents the IP MTU (or any MTU) to exceed the interface MTU.

BR,

Mohammed Mahmoud.

Community Member

Re: MTU settings on GRE tunnel

Agree. The other thing is that in Cisco IOS, the tunnel interface default MTU value (1514

bytes) cannot be changed, so your only option is IP MTU, which you have already done.

So as Mohammed said all your IP traffic will be sent out with the MTU you have set manually.

Regards,

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