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Chapter 13 Configuring Optional Spanning-Tree Features
Unders tanding Optional Spanning -Tree Features

Understanding UplinkFast

Switches i n hie rarchi cal ne tworks can be g rou ped int o b ackb one sw itch es, d istr ibution swi tc hes, an d
acces s swit ches. Figure 13-2 sho ws a complex network where distribution switches and access switches
each have at least one redundant link that spanning tree blocks to prevent loops.
Figure 13-2 Switches in a Hierarchical Network
If a switch looses connectivity, it begins using the alternate paths as soon as the spanning tree selects a
new root port . By e nabl ing U plinkFas t w ith t he s panning-tree uplinkfast global configura tion
command, you can accelerate the choice of a new root port when a link or switch fails or when the
spanning tree reconfigures itself. The root port transitions to the forwarding state immediately without
going t hro ugh th e lis tenin g a nd le arni ng stat es, a s it wou ld wi th th e no rma l sp an ni ng-t ree pr ocedu re s.
The Upl inkFast feature is supporte d only when th e switch is runni ng PVST.
When the span ning tree re config ures the ne w root po rt, other inter faces fl ood the netw ork with multica st
packets, one for each address that was learned on the interface. You can limit these bursts of multicast
traffic by reducing t he m ax- up date -rat e p ar amet er (t he d efaul t f or thi s p ar amete r is 15 0 p ac kets pe r
second). However, if you enter zero, station-learning frames are not generated, so the spanning-tree
topology converges more slowly after a loss of co nnectivity.
Note U plinkFa st i s m ost u sef ul i n wi ring -clo set sw itche s a t t h e ac cess or edge o f the net work. It is not
appro priate fo r backb one devices. Th is feature might not be useful for ot her types of app lica tions.
UplinkFast provides fast convergence after a direct link failure and achieves load balancing between
redundant Layer 2 links using uplink groups. An uplink group is a set of Layer 2 interfaces (per VLAN),
only one of whic h is for war ding at any gi v en ti me. Spec if ically, an up link gro up consists o f the r oot por t
(which is forwarding) and a set of blocked ports, except for self-looping ports. The uplink group provides
an alternate path in case the currently forwarding link fails.
Figure 13-3 shows an example topology with no link failures. Switch A, the root switch, is conn ected
directly to Switch B over link L1 and to Switch C over link L2. The Layer 2 interface on Switch C that
is connected directly to Switch B is in a blocking state.
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Active link
Blocked link
Root bridge
Backbone switches
Distribution switches
Access switches
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