BCMSN - New stuff on Spanning-Tree [CCNP]

& some earlier revision from CCNA...

For more info on STP and L2/L3 switching in general, refer to chapter 3 of the CCNP BCMSN Official Exam Certification Guide. A lot of it covers on how frames travel through a switched network, and the CAM table (content addressable memory better known as the MAC address-table), along with the TCAM table (ternary content addressable memory; which is used for ACL storage within a table format). Overall, a good read.

STP Costs [CCNA Rev.]

Important ones are in blue italics...

4 Mbps - 250
10 Mbps - 100
16 Mbps - 62
45 Mbps - 39
100 Mbps - 19
155 Mbps - 14
622 Mbps - 6
1 Gbps - 4
10 Gbps - 2

The cost can be adjusted manually using the following command:

(a) Change STP cost for ALL VLANs's (STP)

S1#conf t
S1(config)#int fa0/1
S1(config-if)#spanning-tree cost [1-200000000]

(b) Change STP cost per-vlan (PVSTP)

S1#conf t
S1(config)#int fa0/1
S1(config-if)#spanning-tree vlan [id] cost [1-200000000]

More on Spanning-Tree...

BPDU's - BPDU's are transmitted every 2 seconds to the multicast address 01-80-c2-00-00-00 in the following two types:

(1) Topology Change Notification (TCN) BPDU - When a change takes place in the network topology.
(2) Configuration BPDU - Used for STP calculations. The root bridge originates these while the non-root bridges forward them.

STP Timers: Description - Following is a short description of various STP timers :

(1) Hello - Hello timer originates 'Configuration BPDU's'. Default hello timer = 2 seconds.
(2) Forward Delay - Forward Delay timer sets the amount of time an interface spends in the STP listening and learning stage. Default timer = 15 seconds.
(3) Max-Age - MaxAge Timer defines how long a bridge or switch should wait after the last received hello message before believing that the network topology has changed and it can no longer hear the hello messages sent by the root bridge or switch. Default Timer = 20 seconds.
(Basically, Max-Age is the aging time of superior switch's BPDU's.)

STP Timers: Commands - Following are the commands used to set the timers:

(a) Hello:
S1#conf t
S1(config)#spanning-tree vlan [id] hello-time [seconds]

(b) Forward Delay
S1#conf t
S1(config)#spanning-tree vlan [id] forward-time [seconds]

(c) Max-Age
S1#conf t
S1(config)#spanning-tree vlan [id] max-age [seconds]

STP: Further spanning-tree concepts...

Portfast - Portfast is a tool that essentially turns off spanning-tree on your switches, so the port will become active as soon as you plug in but that will cause loops in the network. Basically, Portfast allows a switch to go directly from blocking to forwarding mode. It is suitable for ports where 1 host is connected.

(a) Enable Portfast on the port where 1 host is connected:

S1#config t
S1(config)#int [interface-id that is connected to 1 host]
S1(config-if)#spanning-tree portfast

(b) Enable Portfast on ALL Ports:
S1#config t
S1(config)#spanning-tree portfast default

I find (a) to be most commonly used. This is where you enable portfast only on the ports that are connected to a 'HOST'. (Remember when it comes to RSTP, you need to enable portfast for it to work in your network.)

uplinkfast - uplinkfast reduces STP 'convergence' time from 50 seconds (15 secs for listening + 15 secs for learning + 20 secs for max-age = 50 secs of delay-time by default) to around 1-3 seconds on trunk links between switches. It is enabled globally and for ALL VLAN's. Should be done between access and distribution layer switches rather than distro and core layers.

(a) Enable uplinkfast - Priority becomes 49152, Cost is updated by +3000
S1#conf t
S1(config)#spanning-tree uplinkfast

Increases priority to 49152 so if this cmd is executed on the root switch, make sure to increase the priority on the other secondary switches from 32768 default-value to > 49152.

backbonefast - backbonefast should be enabled on ALL switches in the network. This is so all switches can respond to RLQ's a.k.a. root link queries.

(a) Enable backbonefast on ALL switches in the LAN - Cuts 'convergence time' to 30 seconds (by eliminating max-age-time)
S1#conf t
S1(config)#spanning-tree backbonefast

BPDU Guard - BPDU Guard actually shuts down a port in an 'err-disabled state' if a bpdu comes in on that particular port/s.

(a) Enable BPDU Guard on a single port:

S1#conf t
S1(config)#int [interface-id]
S1(config-if)#spanning-tree bpduguard [enable/disable]

(b) Enable BPDU Guard on ALL ports running portfast:
S1#conf t
S1(config)#spanning-tree portfast bpduguard default

BPDU Filter - BPDU Filtering stops the port from going into an err-disabled state when it receives a bpdu.
If enabled globally: a port running portfast will STOP running portfast if a BPDU is received.
If enabled at the interface level: the bpdu's will simply be ignored and dropped.

(a) Enable BPDU Filter on a single port running portfast:

S1#conf t
S1(config)#int [interface-id]
S1(config-if)#spanning-tree bpdufilter [enable/disable]

(b) Enable BPDU Filter on ALL ports running portfast:

S1#conf t
S1(config)#spanning-tree portfast bpdufilter default

This is not to be confused with BPDU Guard as it's exactly the opposite of it. BPDU Filter actually stops running Portfast and thus should never be enabled in an RSTP network.

Also note, for BPDU Guard and BPDU Filter, you have to type in'spanning-tree portfast' followed by 'bpduguard' or 'bpdufilter' to enable either two in global-config mode (for ALL ports running portfast). Note especially the keyword 'portfast' after 'spanning-tree'.

Root Guard - Root Guard ignores superior bpdu's by going into root-inconsistent state (so I think basically, the Max-Age timer of 20 secs is ignored).

(a) Enable Root-Guard to ignore superior BPDU's on an interface:
S1#conf t
S1(config)#int [int-id]
S1(config-if)#spanning-tree guard root

S1#show spanning-tree inconsistentports (to show ports blocking superior bpdu's)

STP: Show Commands...

S1#show spanning-tree

S1#show spanning-tree summary totals (pretty good to see what's running/enabled)
S1#show spanning-tree interface [num] detail
S1#show spanning-tree inconsistentports (to show ports blocking superior bpdu's)
S1#show interface [interface-id] switchport

Those are some of the main concepts with regards to spanning-tree in the BCMSN curriculum.