Network Technology Basics · Topic 2 of 2
Switching and Routing Basics
Learn MAC learning and flooding, VLANs and 802.1Q tags, access, trunk and hybrid ports, routing tables, route preference, static routes and VLANIF.
- 6 min
- Switches learn source MAC addresses, forward known unicast and flood broadcast and unknown unicast.
- 802.1Q tag: 4 bytes, 12-bit VLAN ID, usable range 1 to 4094; VLAN 1 is the default.
- Huawei port types: access (one untagged VLAN), trunk (many tagged VLANs), hybrid (per VLAN choice).
What it is
Switching moves Ethernet frames between hosts in the same Layer 2 network, using MAC addresses. VLANs split one physical switch network into several separate broadcast domains.
Routing moves IP packets between different networks. A router looks up the destination address in its routing table and sends the packet to the next hop.
How it works
How a switch forwards frames
A switch keeps a MAC address table that maps MAC addresses to ports. It handles every frame in three steps.
| Step | What the switch does |
|---|---|
| Learn | Records the frame's source MAC address and the port it arrived on |
| Forward | Sends a frame for a known unicast MAC out of that one port |
| Flood | Sends broadcast, unknown unicast and (by default) multicast frames out of all other ports in the VLAN |
Entries that are not refreshed age out, after 300 seconds by default on Huawei switches. A switch port is its own collision domain, but all ports in one VLAN share one broadcast domain.
VLANs
A VLAN is a logical Layer 2 network. Hosts in different VLANs cannot talk directly, even on the same switch; their traffic must be routed. VLANs limit broadcasts and separate traffic.
IEEE 802.1Q marks frames with a 4-byte VLAN tag inserted after the source MAC address. The tag holds a 12-bit VLAN ID, so IDs run from 0 to 4095, and 1 to 4094 can be used. VLAN 1 is the default VLAN on Huawei switches.
| Port type (Huawei) | Frames it sends | Typical connection |
|---|---|---|
| Access | Untagged, one VLAN only | PCs and servers that do not tag |
| Trunk | Tagged for many VLANs; only the PVID's VLAN is sent untagged | Switch to switch, switch to virtualization host |
| Hybrid | Tagged or untagged per VLAN, as configured | Mixed cases; Huawei specific |
The PVID (port default VLAN ID) is the VLAN an untagged incoming frame is placed in.
Example on a Huawei switch (VRP):
[Huawei] vlan batch 10 20
[Huawei] interface GigabitEthernet0/0/1
[Huawei-GigabitEthernet0/0/1] port link-type access
[Huawei-GigabitEthernet0/0/1] port default vlan 10
[Huawei-GigabitEthernet0/0/1] quit
[Huawei] interface GigabitEthernet0/0/24
[Huawei-GigabitEthernet0/0/24] port link-type trunk
[Huawei-GigabitEthernet0/0/24] port trunk allow-pass vlan 10 20In FusionCompute, each port group on a distributed virtual switch can carry a VLAN ID. The host uplinks then connect to physical trunk ports that allow those VLANs.
The routing table
A router builds its routing table from three sources.
| Source | How the route appears |
|---|---|
| Direct routes | Automatically, for the subnets of the router's own up interfaces |
| Static routes | Configured by hand by the administrator |
| Dynamic routes | Learned from routing protocols such as RIP, OSPF, IS-IS and BGP |
Each entry holds the destination and mask, the protocol, the preference, the cost, the next hop and the outbound interface. On Huawei devices display ip routing-table shows it.
Choosing a route
The router picks a route in a fixed order.
| Route source | Default preference on Huawei |
|---|---|
| Direct | 0 |
| OSPF | 10 |
| IS-IS | 15 |
| Static | 60 |
| RIP | 100 |
| OSPF ASE (external) | 150 |
| BGP | 255 |
Lower preference is more trusted. Cost (metric) compares routes from the same protocol only.
Static and default routes
A static route names the destination, its mask and the next hop. A default route, 0.0.0.0/0, matches every destination and is used only when no more specific route exists.
[Huawei] ip route-static 10.2.2.0 24 192.168.1.2
[Huawei] ip route-static 0.0.0.0 0 203.0.113.1Routing between VLANs
Each VLAN is its own subnet, so traffic between VLANs needs a Layer 3 hop. On a Huawei Layer 3 switch, a VLANIF interface gives a VLAN an IP address, which hosts use as their gateway.
[Huawei] interface Vlanif 10
[Huawei-Vlanif10] ip address 192.168.10.1 24A worked example
PC A (192.168.10.5, VLAN 10) sends to PC B (192.168.20.5, VLAN 20). PC A sees a different subnet and sends the frame to its gateway, VLANIF 10 at 192.168.10.1.
The Layer 3 switch looks up 192.168.20.0/24, finds the direct route through VLANIF 20, and forwards the packet into VLAN 20. A static route to 192.168.0.0/16 or a default route would lose to this /24, the longest match.
How the exam asks about this
- "What does a switch do with a frame whose destination MAC address is not in its MAC table?" → It floods the frame out of all other ports in the same VLAN.
- "How many bits is the VLAN ID in an 802.1Q tag?" → 12 bits, giving usable VLANs 1 to 4094.
- "A router has routes to 10.0.0.0/8 and 10.1.1.0/24. Which does it use for 10.1.1.9?" → 10.1.1.0/24, the longest prefix match.
What to remember for the exam
- Switches learn source MAC addresses, forward known unicast and flood broadcast and unknown unicast.
- 802.1Q tag: 4 bytes, 12-bit VLAN ID, usable range 1 to 4094; VLAN 1 is the default.
- Huawei port types: access (one untagged VLAN), trunk (many tagged VLANs), hybrid (per VLAN choice).
- Route selection: longest prefix first, then lowest preference, then lowest cost.
- Huawei preferences: direct 0, OSPF 10, static 60, RIP 100; the default route is 0.0.0.0/0.
Preference and cost are different. Preference compares routes from different protocols; cost compares routes within one protocol. Neither overrides the longest prefix match.
Common mistakes
Thinking a switch learns destination addresses. A switch learns only source MAC addresses. Destinations are looked up, never learned.
Thinking VLANs on one switch can talk directly. Different VLANs are different broadcast domains and subnets. Their traffic must pass a router or a VLANIF interface.
Choosing the route with the best preference first. The router first takes the most specific prefix. Preference only breaks ties between routes to the same prefix.
Check yourself
What is a PVID used for on a Huawei switch port?
It is the default VLAN of the port. Untagged frames that arrive on the port are placed in that VLAN.
Which route does a Huawei router prefer for the same prefix: a static route or an OSPF route?
The OSPF route, because its default preference of 10 is lower than the static route's 60.
What gives a VLAN an IP gateway on a Huawei Layer 3 switch?
A VLANIF interface configured with an IP address in that VLAN's subnet.
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