Subinterfaces

Subinterfaces

Subinterfaces in Cisco Catalyst SD-WAN are logical interfaces that are configured on a physical interface to enable flexible network segmentation.

Interface speed

Interface speed is the rate at which data is transmitted over a physical or logical interface. When a Cisco IOS XE Catalyst SD-WAN device starts, the Cisco Catalyst SD-WAN software autodetects the SFPs present in the router and sets the interface speed accordingly. The software then negotiates the interface speed with the device at the remote end of the connection to establish the actual operating speed. For non-physical interfaces, such as those used for the system IP address and loopback interfaces, the interface speed defaults to 10 Mbps. In Cisco SD-WAN Controller and Cisco SD-WAN Manager systems, the initial interface speed is 1000 Mbps, with the operating speed negotiated with the remote device. The controller interface speed may vary depending on the virtualization platform, the NIC used, and the drivers present in the software

Interface MTU

MTU (Maximum Transmission Unit) is the largest size, in bytes, of a packet that can be sent on an interface without fragmentation. In Cisco Catalyst SD-WAN, by default, all interfaces have an MTU of 1500 bytes.

Here is the lookup table summarizing the MTU range for Cisco IOS XE Catalyst SD-WAN devices by release and interface type:

Release

Interface Type

MTU Range (bytes)

Cisco IOS XE Catalyst SD-WAN Release 17.4.1a and earlier

576 through 2000

Cisco IOS XE Catalyst SD-WAN Release 17.4.1a and later

1 GE interfaces

576 through 9216

Cisco IOS XE Catalyst SD-WAN Release 17.5.1a and later

10 GE and 100 GE interfaces

576 through 9216

For Cisco SD-WAN Validator, Cisco SD-WAN Manager, and Cisco SD-WAN Controllers, you can configure interfaces to use ICMP to perform path MTU (PMTU) discovery. When PMTU discovery is enabled, the device automatically negotiates the largest MTU size that the interface supports in an attempt to minimize or eliminate packet fragmentation.

On Cisco IOS XE Catalyst SD-WAN device, the Cisco Catalyst SD-WAN BFD software automatically performs PMTU discovery on each transport connection (that is, for each TLOC, or color). BFD PMTU discovery is enabled by default, and it is recommended that you use it and not disable it.


Note

BFD is a data plane protocol and so does not run on Cisco SD-WAN Validator, Cisco SD-WAN Manager, and Cisco SD-WAN Controllers.

Configuration examples for subinterfaces

This section provides configuration examples for subinterfaces.

When you create a subinterface that does not specify an IP MTU value, the subinterface inherits the IP MTU value from the parent interface. If you want the subinterface to have a different IP MTU value, use the ip mtu command in the subinterface configuration to set the IP MTU for the sub interface.

The following is a configuration example for subinterfaces:


interface GigabitEthernet0/0/0
 mtu 1504
 no ip address
!
interface GigabitEthernet0/0/0.9
 encapsulation dot1Q 9
 no shutdown
 ip address 192.168.9.32 255.255.255.0
!
interface Tunnel9
 no shutdown
 ip unnumbered GigabitEthernet0/0/0.9
 no ip redirects
 ipv6 unnumbered GigabitEthernet0/0/0.9
 no ipv6 redirects
 tunnel source GigabitEthernet0/0/0.9
 tunnel mode sdwan
!
sdwan
 interface GigabitEthernet0/0/0.9
  tunnel-interface
   encapsulation ipsec
   color private1
  !
 !

Verify configurations on subinterfaces

This section provides details on how to verify configurations on subinterfaces.

Configuration example to display information about interface speed and MTU

To display the actual speed of each interface, use the show interfaces command. The following example displays interface information on all interfaces.

Device# show interfaces
GigabitEthernet0/0/0 is up, line protocol is up
  Hardware is ISR4331-3x1GE, address is 084f.f99b.267c (bia 084f.f99b.267c)
  Description: INET
  Internet address is 10.3.6.2/24
  MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive not supported
  Full Duplex, 1000Mbps, link type is auto, media type is RJ45
  output flow-control is off, input flow-control is off
  ARP type: ARPA, ARP Timeout 00:20:00
  Last input 00:00:00, output 00:00:00, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/375/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 2000 bits/sec, 2 packets/sec
  5 minute output rate 1000 bits/sec, 1 packets/sec
     235182 packets input, 23708237 bytes, 0 no buffer
     Received 1 broadcasts (0 IP multicasts)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 170048 multicast, 0 pause input
     71585 packets output, 12131971 bytes, 0 underruns
     Output 6 broadcasts (0 IP multicasts)
     0 output errors, 0 collisions, 1 interface resets
     1 unknown protocol drops
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier, 0 pause output
     0 output buffer failures, 0 output buffers swapped out
GigabitEthernet0/0/1 is up, line protocol is up
  Hardware is ISR4331-3x1GE, address is 084f.f99b.267d (bia 084f.f99b.267d)
  Description: Service
  Internet address is 10.3.13.2/24
  MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive not supported
  Full Duplex, 1000Mbps, link type is auto, media type is RJ45
  output flow-control is off, input flow-control is off
  ARP type: ARPA, ARP Timeout 00:20:00
  Last input 00:00:00, output 00:00:14, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/375/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
     144332 packets input, 13390830 bytes, 0 no buffer
     Received 0 broadcasts (0 IP multicasts)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 144332 multicast, 0 pause input
     13613 packets output, 5135370 bytes, 0 underruns
     Output 1 broadcasts (0 IP multicasts)
     0 output errors, 0 collisions, 1 interface resets
     1 unknown protocol drops
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier, 0 pause output
     0 output buffer failures, 0 output buffers swapped out
<output truncated>