Path cost to root bridge - chnh l cost tnh t mt port switch khc root switch n root switch.

Gii thut chng loop Spanning tree ( STP)

trnh xy ra hin tng loop, cc SW s tin hnh bu chn ra mt SW c nhim v qun l, c ch ny gi l STP.

Bu chn Root Bridge

Vic bu chn Root Bridge da vo hai yu t theo th t sau:

Bridge Priority : nu Bridge no c priority thp nht s c chn lm root. Nu cc Bridge c priority bng nhau, th tin hnh chn la da vo a ch MAC

a ch MAC: Bridge no c MAC thp hn th s c chn lm root. y l trng hp cui cng trong vic chn root v khng c trng hp cc Bridge c a ch MAC ging nhau. Thng s priority c th c thay i:Switch(config)#spanning-tree vlan vlan-id priority bridge-priorityThng s priority nm trong khong t 0 65535, v gi tr mc nh l 32768.

Ta cng c th la chn root trc tip bng lnh:Switch(config)#spanning-tree vlan vlan-id root {primary | secondary}[diameter diameter]+ primary : s chn bridge lm root, khi chn thng s ny nu priority ca Root Bridge nh hn 24567 th Bridge ny s thit lp priority ca n nh hn so vi priority ca root hin ti mt lng l 4096, v d nhin bridge ny s l root. Cn nu priority ca Root Bridge ln hn 24567, bridge s thit lp priority ca n l 24567.+ secondary : la chn bridge lm root th hai.+ diameter: s lng bridge ni tng, ti a l 7.

Bu chn Root Port

Root Port : Port thuc Nonroot Bridge ni v Root Bridge sao cho chi ph ni v l thp nht. Root Port c bu chn da vo thng s Root Path Cost. Path cost c xem nh l chi ph ca lin kt, t l vi bng thng ca lin kt. Thng s ny c th c cu hnh bng dng lnh.Switch(config-if)#spanning-tree vlan vlan-id cost costPort no c tng s path cost v root l thp nht s c la chn lm root port.

Bu chn Designated Port (DP):Trong trng hp c hai hay nhiu bridge ni vo chung mt mng (xem hnh phn bridge loop) s xy ra trng hp loop. khc phc th cc bridge c port ni vo mng s tin hnh la chn designated port. Port ny c vai tr quan trng v n c nhim v chuyn cc gi tin i trong mng. Vic chn la da vo cc thng s theo trnh t:

Da vo Root Path Cost : port no c Root Path Cost thp nht s c chn lm DP. Trong trng hp Root Path Cost bng nhau, th chn da vo Bridge ID

Bridge ID : port thuc bridge no c Bridge ID thp hn s c chn lm DP. Nu thng s ny cng ging no th chn da vo a ch MACa ch MAC : port thuc bridge no c a ch MAC thp hn s c chn lm DP

Hi vng l bn bit cch tnh root-path-cost , n kh n gin : chnh l ng i c cost thp nht t 1 port trn switch khc n root-switch v y l bng i chiu : bn c cn c vo c th tnh root-path-cost