Layer 2 technologies – VTP

Lessons learned today:

Once again - as I just learned in my lab - Always make sure the basic layer 1 and layer 2 status is working. 

VTP
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Three Modes: Server / Client / Transparent

SERVER Mode:

All Switches by default are set in Server mode. This means they can make changes to the Vlan Database. The Version / Config revision tracks Vlan / database changes.

VTP – does authentication by default. Basically using a “null” authentication ( Like OSPF). The password by default is “null” à The MD5 checksum is a combination of the “null” password and the revision number. The revision number is basically a “seed” for the MD5 output.

Note: To force an update between switches, you can simple add and or remove a Vlan. Also a reboot of the switched will force a reload and bring up VTP in the correct order of operation. This update should reflect in the revision number.

CLIENT Mode:

Can receive updates from Servers but cannot make any changes. It can also not create or delete a learned Vlan from the server.

TRANSPARENT Mode:

Does not participate in VTP with other device. A received update – will not update the local Vlan database. It can however send or forward to other switches. Transparent is essentially VTP off!

VTP – Applies to switches not in transparent mode.

Note: A goof verification for layer 2 forwarding issues:

# Sh spanning-tree dynamic vlan (vlan #)

          This command will show if Vlans coming across a trunk link are dropped or not. The final say for forwarding lies in the MAC / CAM table. If a MAC address is not in the table – traffic will not be forwarded. Spanning-tree will prevent loops – it does this by disabling Mac address learning on interfaces that not in the forwarding state.

VTP Authentication:

Note: Authentication is on by default.

# Sh vtp password – shows configured vtp password for the domain.

# Sh vtp status

# Sh vtp statu | inc MD5 – as long as the configuration revision on each switch is the same, even with a “null” password. The MD5 hash should match on all switches.

Note: If there is a password mismatch , the parser should respond with an error: “MD5 digest checksum mismatch on receipt of equal revision summary on trunk Fa x/x”

VTP Pruning:

Used to reduce unnecessary traffic that’s forwarded through thr layer 2 domain.

Reduces unnecessary replication of:   

Broadcasts

Unknown Unicasts

Unknown Multicasts

Pruning is only supported in Server/ Client mode.

To configure pruning – from global mode - # vtp pruning.

Verification:

#sh int trunk

#sh int pruning

Note:

Pruning assumes you’re running vtp, it also assumes you’re running in client server mode. If there’s a transparent switch in the network / transparent path you could run into an issue.

Always verify what links are forwarding and what Vlans are forwarding across those links. 

To verify what’s forwarding on what port.

#sh int trunk –

shows if trunking is working,  “status”

shows what Vlans are allowed to forward, “vlans allowed on trunk”

shows – out of all possible Vlans 1-4096, which ones are active and which ones are created.

Shows which Vlans are forwarding and not pruned by VTP and which Vlans that spanning-tree is running on.

# sh spanning-tree int FA x/x

 Will also show what Vlans are forwarding out what interfaces. .

# sh mac-address-table interface FA x/x

Mac addressee can only exist in the forwarding Vlans. With pruning enabled – all switches should only forward Vlans that have local and or are in the transit pat for.