Category:Cisco Systems -> Routing and Switching
There are 3 types of MTU that can be configured on a switch:
- Layer 2 MTU that affects 10 and 100Mbit/s interfaces of a switch. Configured by system MTU {bytes} command in global config mode.
- Layer 2 MTU that affects affects 1000Mbit/s and higher speed interfaces of a switch. Configured by system MTU jumbo {bytes} command in global config mode.
- Layer 3 MTU that affects SVIs and routed interfaces of a switch with IP addresses on them and originating or transit IP traffic that uses these interfaces as GW for routing between networks. Configured by system mtu routing {bytes} command in global config mode.
The following MTU types can be configured on a Layer 3 switch 3750:
- System MTU
- System MTU jumbo
- System MTU routing
On 3550:
- System MTU
Let’s raise L2 MTU value to 1504 to accommodate Q in Q
3750:
Switch#sh system mtu
System MTU size is 1500 bytes
System Jumbo MTU size is 1500 bytes
Routing MTU size is 1500 bytes
Switch(config)#system mtu jumbo 1504
Changes to the system jumbo MTU will not take effect until the next reload is done
Switch#wr
Switch#reload
Proceed with reload? [confirm]
Switch#sh system mtu
System MTU size is 1500 bytes
System Jumbo MTU size is 1504 bytes
Routing MTU size is 1500 bytes
If instead of jumbo mtu 1504 we used system mtu 1504 first and reloaded, jumbo mtu would automatically be raised to 1504 even if we didn’t touch its value before:
Switch(config)#system mtu 1500
Changes to the system jumbo MTU will not take effect until the next reload is done
Switch#wr
Switch#reload
Proceed with reload? [confirm]
Switch#sh system mtu
System MTU size is 1504 bytes
System Jumbo MTU size is 1504 bytes
Routing MTU size is 1500 bytes
Following is the visual diagram of different possibilities that were tested by me:
Although I didn’t bother to copy all my logs from troubleshooting and they were lost, but here’s a recovered fragment regarding system MTU investigation on switches:
3550#sh system mtu
System MTU size is 1504 bytes
2960#ping 10.0.0.2 size 1504 df-bit
Type escape sequence to abort.
Sending 5, 1504-byte ICMP Echos to 10.0.0.2, timeout is 2 seconds:
Packet sent with the DF bit set
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/5/9 ms
2960#ping 10.0.0.2 size 1500 df-bit
Type escape sequence to abort.
Sending 5, 1500-byte ICMP Echos to 10.0.0.2, timeout is 2 seconds:
Packet sent with the DF bit set
!!!!!
Success rate is 60 percent (3/5), round-trip min/avg/max = 1/3/8 ms
Let’s see what happens if I changed system MTU of 3550:
3550#sh system mtu
System MTU size is 1500 bytes
2960#ping 10.0.0.2 size 1504 df-bit
Type escape sequence to abort.
Sending 5, 1504-byte ICMP Echos to 10.0.0.2, timeout is 2 seconds:
Packet sent with the DF bit set
.....
Success rate is 0 percent (0/5)
2960#ping 10.0.0.2 size 1500 df-bit
Type escape sequence to abort.
Sending 5, 1500-byte ICMP Echos to 10.0.0.2, timeout is 2 seconds:
Packet sent with the DF bit set
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/5/9 ms
If you turned on debug ip icmp you would have noticed that packets greater than 1500 bytes would not be allowed through without fragmentation.
By privilege15