CCNP Study Notes – Switch Port Configuration

By | March 4, 2012


Ethernet (10Mbps) – IEEE 802.3, 10mbps bandwidth between users, shared medium, both a collison domain and and a broadcast domain, CSMA/CD = half-duplex operation, more devices sharing the medium = more collisions = poor performance.  Ethernet switching allows full-duplex operation due to the fact that the switch dynamically allocates a dedicated 10Mbps to each port – this means that becasue collisions are not likely, devices can operate in full-duplex mode, which increases performance further as throughput is 10Mbps in both directions giving 20Mbps total throughput.  Cabling used is UTP 10Base-T, which is restricted to 100 metres end-to-end distance.

Fast Ethernet (100Mbps) –  IEEE.3u.  CSMS/CD and upper layer protocol operations are the same as with Ethernet.  Cabling can be UTP or Fibre:

100Base-TX – EIA/TIA Cat 5 UTP       – 2 pairs – 100m

100Base-T2 – EIA/TIA Cat 3,4,5 UTP – 2 pairs – 100m

100Base-T4 – EIA/TIA Cat 3,4,5 UTP – 4 pairs – 100m

100Base-FX – MM Fibre 62.5/125      – 1 pair –   400m half duplex  / 2000m full duplex

– SM Fibre                       – 1 pair –   10KM

Full-Duplex Fast Ethernet – 100Mbps in each direction giving 200Mbps total throughput, if devices at both ends of the link support it.  Fast Ethernet is also backwards compatible to 10Mbps, so if both ends of a link are set to aut-negotiate and one end is only cabpable of 10Mbps, then this will become then negotiated speed.  For auto-negiation to work, both ends of a link must be configured for it.  If duplex negotiation fails then a switchport always falls back to it’s default setting og half-duplex.  FEC (Fast Etherchannel) is the bundling together of up to 8 full-duplex fast ethernet links, giving 400Mbps – 1600Mbps duplex bandwidth.

Gigabit Ethernet – 1000Mbps, same 802.3 Ethernet frame format as 10 & 100, Physical layer has been modified to increase data speeds – the result is IEEE 802.3z.  Gigabit Ethernet port duplex is always set to full on Cisco switches, so dupplex auto-neg is not possible.  GEC (Gigabit EtherChannel) allows aggregation of 2 to 8 connectionsto give up to 16Gbps throughput.

10-Gigabit Ethernet – 10,000Mbps, IEEE 802.3ae, most 10Gbps cables are fibre, with the exception of Copper type CX4 with Infiniband connectors (up to 15 metres)


Switchport Error Conditions

By default Catalyst switches detects error conditions for every possible cause.  If an error is detected the port is put into state errdisable and is disabled.  To fine tune this behaviour and ensure that only certain causes can trigger a port being disabled use the following command from Global Config mode:

Switch(config)# [no] errdisable detect cause [all | cause-name]

To re-enable a port, issue a shut/no-shut command.  You can specifiy that ports will be automatically re-enabled (default after 300 Secs), with the following command:

Switch(config)#errdisbale recovery cause [all | cause-name]

Set the auto recovery timer:

Switch(config)# errdisable recovery interval seconds


Troubleshooting Port Connectivity

S4#sh interfaces e0/1

Ethernet0/1 is up, line protocol is up (connected)

Hardware is AmdP2, address is aabb.cc00.0410 (bia aabb.cc00.0410)

MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec,

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


1st up = Physical/Data Link Layer, if down then the link is disconnected for cann to be detected.

2nd up = Layer 2


Switch# show interface status (shows states for all switchports)

Switch# show interface status err-disabled (shows all switchports in the err-disabled state and the cause)

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