Question about RSTP & SVIs

Question:

This is something I'm looking WS-C3750V2-48PS-S Price to do on a weekend but am curious about its operation.  I'm going to be created new VLANs and will need them to be routable.  Therefore, I'm going to be creating VLAN interfaces for these VLANs.  Our current VLAN interfaces support Multicast with the ip pim dense-mode command.  They're also configured with HSRP.  My theory in doing this is to configure the active HSRP host first then the others so that they don't have to exchange active/standby messages when the active host comes up.  My question is how will adding new SVIs to my core switches affect RSTP?  I'm assuming since SVIs support both L2/L3 protocols, RSTP would have to calculate for the new VLANs and properly reconverge.  Is this accurate?

Answer:

Adding new SVI's wont effect anything but your layer 3 - remember these are only interfaces, not vlans.

You need your vlans before your interfaces.

When you create vlans on the L2/L3 switches then we might need to take a look at spanning-tree to avoid loops (although stp should work itself out) - but safe to check and be sure. Checking where the root bridge is and which ports are root ports, forwarding ports and blocking... will be beneficial.

When the switches first come up, they start the root switch selection process. Each switch transmits a BPDU to the directly connected switch on a per-VLAN basis.

As the BPDU goes out through the network, each switch compares the BPDU that the switch sends to the BPDU that the switch receives from the neighbors. The switches then agree on which switch is the root switch. The switch with the lowest bridge ID in the network wins this election process. This can be influenced by changing the priority (lower one wins)

You havent stated whether your switches are using VTP in server/client configuration, or if they're in transparent mode.

I recommend having a really good understanding of your Layer 2 topology because thats probably more important than layer 3 (since layer 3 relies on layer 2 to operate).

Generally, adding a new vlan on switch is fine since STP will calculate for the vlan - RSTP has much improved convergance times than STP (seconds if not - miliseconds) instead of the normal  STP around 30sec, but you need to be sure about adding the vlan to the relevant trunks. If you have manually configured which vlans are allowed on trunks you must specify the switchport mode trunk allowed vlan add command to avoid an outage for the other vlans.

Once you have your layer 2 in place with your vlan created and allowed on the relevant trunks, you can then create your SVI's and implement your HSRP configuration and so on...

White paper on Rapid Spanning-Tree:

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_configuration_example09186a008009467c.shtml

Also, with regards to dense-mode for multicast there can be implications on network performance/traffic - I'd consider sparse-mode or at least sparse-dense mode. But if this dense-mode is required because multicast is used absolutely everywhere in your network then there isn't a problem with this.

Hope this helps

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1841 Router Question

Question:

Does anyone know if the Cisco 1841 router has a web management interface? I have searched and searched for a way to turn it on through TelNet, but I can't find anything about it, which got me wondering if it even has an interface built in.

I have two of these I need to manage, and am basically an illiterate newby when it comes to telnet or CLI.

Answer:

Do you mean Cisco Security Device Manager ("Web-based device management tool embedded within the Cisco IOS Software access routers can be accessed remotely for faster and easier deployment of Cisco routers for both WAN access and security features")? If yes, then 1841 does have it.

See this guide for more WS-C3750V2-24PS-S details:

http://www.cisco.com/en/US/docs/routers/access/1800/1841/software/configuration/guide/1800over.html#wp44489