Help to understand L3 SW
Question:
I hope you can help me as usuall , i designed a layer 3 lab contains 4 L3 WS-C3560X-48T-S switches + Vlans , the issue is as the following : i can ping any pc from sw0 to sw1 but i can't which is weird ping any thing else like from sw0 to sw2 or sw3 , & i need to ask if we solved this issue then what is the purposes of the routing protocols if we can connect devices together via vlans ,
Thanks for help in advance
Answer:
If you connect everything in one big VLAN (Layer 3) you will end up with a very slow network as everything is broadcast. If a lot of broadcast is doen in the network all the PC will use CPU cycles to process the broadcast slowing them down. Ethernet is built this way.
That why you segment you network in mutiple segments (VLAN's). You need IP addressing to get out of your VLAN to communicate with other devices in the network (Other VLAN's) or outside the network.
I wrote up a sample picture. If you take a look. the router on the left would install a route to the 10.1.0.0/16 network and the 10.2.0.0/16 network.
However, it will have no knowledge of the 10.3.0.0/16 network. This is because it is not directly connected. There are two ways around this: setting static routes, or using dynamic routing protocols.
Static routes must be set manually by a network engineer. Imagine if there were hundreds of networks each router had to have a route. Also imagine if there were 20-30 routers that needed to have their routes manually learned. Manually setting routes would take way too long.
Dynamic Routing Protocols will advertise these routes for you and select the best path to each route when configured properly.
You said you can ping pcs from SW0 and SW1 that means the link is good there. This means the problem lines between SW1 and SW2. What do you need in order to make communication happen? (not in order)
- Required VLAN must exist in SW1 and SW2 VLAN database
- Ports between SW1 and SW2 need to be in Trunking mode and the required VLAN must be allowed across the trunk.
- To facilitate inter VLAN communication at least one switch needs to have an SVI configured with a L3 address hosts can use as a default gateway.
Run through those steps on every switch and you should discover the problem. Remember the "show interface trunk" command can be your best friend when troubleshooting inter VLAN routing.
I will also make a special note about your topology. Currently, you have SVIs on all of the L3 switches and they all are configured with the same IP Address for each SVI. (Example: SVI for VLAN 1 on every switch has the IP address 10.1.1.1. This is incorrect and really bad for Routing as packets can be confused. Do not duplicate IP addresses on links.
To answer your question about Routing Protocols. Routing Protocols help a router build the routing table. On its own, a router (and L3 switch) will only install routes in the routing table that are directly connected to it. It would have no knowledge of how to reach other networks. Routing protocols advertise routes the local rouer knows to neighboring routers so they can add them to their own routing tables.