04-20-2008 06:40 PM - edited 03-05-2019 10:29 PM
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
Solved! Go to Solution.
04-22-2008 12:17 AM
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.
04-20-2008 09:09 PM
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
04-21-2008 07:50 PM
Hi thanks for reply
so right now MTU size being used is 1476?
please explain
04-21-2008 12:35 AM
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.
04-21-2008 07:32 PM
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?
04-22-2008 12:17 AM
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.
04-22-2008 04:54 AM
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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