CO5023 LAN Redundancy

Issues with Redundancy Broadcast storms and MAC instability In a network with redundant links it is possible for switching loops to occur. This can result in: Exponential growth in broadcast frames, brings network down very fast. Duplicate records for unicast addresses in MAC table

Spanning tree protocol This can be fixed by defining a ‘tree’ of active links which spans the network. Links which are not in the tree are effectively on standby and can be introduced into the network is another link fails. Spanning Tree Protocol is responsible for managing the status of switch ports on the network. By stopping some ports from forwarding, only one path is available between devices at any given time.

Spanning Tree Port Roles The spanning tree algorithm defines a ‘root’ switch (aka root bridge) and a path from all other switches to it. The root switch is elected based on the lowest bridge ID among all switches in the LAN. Root bridge elections can be manipulated by changing the priority value at the switches. All other switches then have a root port, which is the port giving the shortest cost path to the root switch, all other ports are either designated ports or alternate ports. Designated ports may still forward traffic Alternate ports may not An inactive link usually only has an alternate port at one end (quicker to bring it up when needed)

Bridge Protocol Data Units (BPDUs) The election of the root bridge and the path costs are updated by the propagation of BPDUs. A typical BPDU frame is given opposite. When a switch receives a BPDU, it checks to see if the root ID in the BPDU is less than its current root ID If it is, it changes the root ID, root port (if applicable) and changes the path cost to that indicated by the BPDU plus the cost for the interface where the BPDU was received. The BPDU is then forwarded to neighbouring switches, with the new path cost value. Otherwise, the BPDU is discarded.

Types of Spanning Tree Protocol STP: The original. Only one spanning tree is computed regardless of the number of VLANs PVST+: Per VLAN spanning tree. Provides one spanning tree instance per VLAN. Also supports handy technologies such as PortFast* RSTP: A newer version of STP that converges on a solution more quickly Rapid PVST+ a per VLAN spanning tree protocol which uses the RSTP algorithm. Portfast etc included Multiple Spanning Tree Protocol (MSTP) – maps multiple VLANs into the same spanning tree instance. Each instance applies to a set of VLANs and not just an individual one. *Portfast is a technology which automatically disables STP activity on access ports. All access ports are forwarding ports and stay that way.

PVST+ PVST+ creates one instance of STP per VLAN. This is rather useful: In a network with a single spanning tree for all VLANs, some links would just never be used. By having different trees for each VLAN, all links (and therefore more capacity) can be used with some links active on one VLAN but not another. The VLAN ID (aka extended system ID) is recorded in the bridge ID field of the BPDU

Rapid PVST+ The main advantage of Rapid PVST+ is that it runs the rapid spanning tree protocols for each VLAN This means that ports can transition into the forwarding state without waiting for a timer Results in much faster network convergence BPDU is the same apart from the version field (2 instead of 0)

Configuration PVST+ commands (global config mode): spanning-tree mode pvst Set primary and secondary root bridges: spanning-tree vlan vlan-id root primary spanning-tree vlan vlan-id root secondary Enable PortFast: spanning-tree portfast default Verification: show spanning-tree vlan vlan-id Rapid PVST+ commands (global config mode): spanning-tree mode rapid-pvst (the only difference) Set primary and secondary root bridges: spanning-tree vlan vlan-id root primary spanning-tree vlan vlan-id root secondary Enable PortFast: spanning-tree portfast default Verification: show spanning-tree vlan vlan-id

First Hop Redundancy Protocols An end device can have more than one device acting as default gateway. A virtual router group can create automated backup for a default gateway by responding to ARP requests with a virtual MAC address. Then when frames are sent to the virtual MAC, it can be processed by the currently active router within the group. Several protocols can be used for this, including HSRP, VRRPv2 and GLBP