Bandwidth of an interface

[gowdakssujan]

I have confusion in finding the bandwidth for an interface, between the bolded values below...

BW 1000000 Kbit or 100Mb/s

Also, what is the difference between them?

Router#sh int g0/1

GigabitEthernet0/1 is up, line protocol is up (connected)

Hardware is CN Gigabit Ethernet, address is 0040.0b1e.c202 (bia 0040.0b1e.c202)

Internet address is 11.0.0.1/8

MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,

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

Encapsulation ARPA, loopback not set

Keepalive set (10 sec)

Full-duplex, 100Mb/s, media type is RJ45

output flow-control is unsupported, input flow-control is unsupported

ARP type: ARPA, ARP Timeout 04:00:00,

Last input 00:00:08, output 00:00:05, output hang never

Last clearing of "show interface" counters never

Input queue: 0/75/0 (size/max/drops); Total output drops: 0

Queueing strategy: fifo

Output queue :0/40 (size/max)

5 minute input rate 42 bits/sec, 0 packets/sec

5 minute output rate 45 bits/sec, 0 packets/sec

29 packets input, 2036 bytes, 0 no buffer

Received 0 broadcasts, 0 runts, 0 giants, 0 throttles

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort

0 watchdog, 1017 multicast, 0 pause input

0 input packets with dribble condition detected

27 packets output, 1888 bytes, 0 underruns

0 output errors, 0 collisions, 1 interface resets

0 unknown protocol drops

0 babbles, 0 late collision, 0 deferred

0 lost carrier, 0 no carrier

0 output buffer failures, 0 output buffers swapped out

[Giuseppe Larosa]

Hello gowdakssujan,

the bandwidth of an interface and the actual speed of an interface are not the same concept, even if in most cases they have the same value.

The actual speed of the physical interface can be the result of a negotiation with the connected device or it can be manually configured (not recommended anymore unless the connecting device does not support autonegotiation).

>> Full-duplex, 100Mb/s, media type is RJ45

This is the actual speed of the interface that is operating like a FastEthernet 100 Mbps Full duplex and using media type RJ45, this means a command like media-type rj45 in interface configuration. This is likely a router interface that can use an RJ45 or an SFP optics.

The bandwidth parameter expressed in kbps is an administrative value:

>> MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,

The bandwidth value follows the actual speed, but it can be set manually using

bandwidth <value in Kbps> in interface configuration mode.

This administrative parameter is used by:

- EIGRP as the bandwdith metric component of its composite metric

- Modular QoS: it uses the bandwidth value as a reference for all commands that instead of specifying a value in kbps use something like bandwidth percent or bandwidth remaining percent in defining a policy-map that will use CBWFQ or LLQ queue schedulers. In this context the 100% of bandwidth refers to the bandwidth parameter on the interface not the actual speed.

Most of people when refer to the bandwidth of an interface are actually meaning the actual speed of the interface.

In Cisco IOS the bandwidth parameter is an administrative value that has no direct effect on the amount of traffic that can go out of the interface, but it is used by some features like EIGRP or modular QoS as explained above.

Hope to help

Giuseppe

[gowdakssujan]

Metric =>OSPF cost = 10^8/BW
=10^8/(1000000k) = 1/10 = 0.1
Is the cost calculation right ???

[Giuseppe Larosa]

Hello gowdakssujan,

OSPF metric is an integer between 1 and 65535 in single area.

in the formula you divide 10^8 by 100000 K = 10^8 / 10^5 * 10^3 = 10^8 /10^ 8 = 1.

In OSPF with default reference bandwidth a 100 Mbps link like a FE has cost 1. But also all links faster then 100 Mbps have cost 1.

In order to make OSPF work correctly in modern networks we need to change the reference bandwidth for automatic cost calculation using:

router ospf 1

auto-cost reference-bandwith 100000   ! note this value is expressed in Mbps so 100 is the default setting

This has to be done on all routers in the OSPF domain to have consistent cost calculations

see the following thread

https://community.cisco.com/t5/switching/ospf-auto-cost-reference-bandwidth/td-p/1220819

Hope to help

Giuseppe