Intermediate spanning tree

03/14/10 • -1 min

We cover intermediate spanning tree concepts. The importance of specifying your root bridge and backup root bridge with spanning-tree priority. Using portfast to allow host ports to start forwarding without waiting for 30 seconds. Using bpduguard to disable portfast-enabled ports where someone erroneously plugs in a switch. Using errdisable timeout to automatically reenable those ports after 15 minutes. Using rootguard to prevent improper switches from becoming your spanning-tree root. The dangers of using bpdufilter to ignore and delete BPDUs. How uplinkfast can enable access switches to failover between uplinks without delay. And using backbone fast to improve responsiveness to indirect link failures by eliminating the need for the 20 second maxage timeout.

Previous Episode

VLANs and spanning tree

VLANs are a feature of ethernet switches which makes them act like multiple "virtual switches". Each VLAN is a separate broadcast domain and could be configured with a separate subnet. That way could could have separate subnets for separate purposes (IT, accounting, network management) on one physical switch. This saves money and cabling while decreasing complexity.

Spanning tree is a protocol which allows you to build redundant loops out of ethernet switches without suffering a bandwidth outage due to looping ethernet frames. Spanning tree blocks ports in your switch mesh to change a topology of loops into a non-looping tree. Then if you suffer a link outage, spanning tree will reconverge in a new fully operational tree. This reconvergence make take significant time (30-50 seconds) with the old spanning tree protocol.

More modern improvements to the spanning tree protocol, including RSTP (rapid spanning tree) and MST (multiple spanning tree) will be covered in a later episode.

Next Episode

802.1q and ISL trunks

Switches can have multiple vlans. When we connect switches together we use 802.1q trunks (or older ISL trunks) to run multiple vlans over one physical link. With either trunking protocol, a tag is added to the ethernet frame with the vlan information. ISL is an older Cisco-proprietary trunking protocol. Newer switches do not even support ISL. Newer switches use the 802.1q vendor-indepentend trunking protocol. Cisco switches also speak the dynamic trunk protocol (DTP) to dynamically negotiate whether to enable a trunk.