Routing Protocols and Switching Technologies
This domain tests how switches build forwarding tables and how routers choose paths among connected, static, and dynamic routes. Expect VLAN trunks, STP, EtherChannel, OSPF, EIGRP, BGP, first-hop redundancy, and administrative distance.
Core rules
- A switch learns source MAC addresses, floods unknown unicast and broadcasts, and filters known destinations by MAC table entry.
- STP elects one root bridge by lowest priority, then lowest bridge ID. RSTP converges faster with alternate and backup ports.
- 802.1Q tags trunk frames; native VLAN mismatch causes VLAN leakage and STP instability. EtherChannel requires matching speed, duplex, and VLAN configuration.
- Route selection order: longest prefix match first, then lowest administrative distance for the same prefix, then lowest metric.
- Common AD values: connected 0, static 1, eBGP 20, EIGRP 90, OSPF 110, RIP 120, iBGP 200. OSPF cost is reference bandwidth divided by interface bandwidth.
- HSRP and VRRP provide a virtual default gateway; the active or master router answers ARP for the virtual IP.
Worked example
A packet to 10.1.5.9 matches 10.1.0.0/16 via R1 with metric 10, 10.1.5.0/24 via R2 with metric 100, and 0.0.0.0/0 via R3. The router chooses R2 because /24 is the longest prefix. Metric and AD are not compared across different prefix lengths. If two OSPF routes to 10.1.5.0/24 existed, AD would matter only if the protocols differed; same-protocol routes compare metric.
Traps
Longest prefix wins before AD or metric. STP root selection is not based on port speed. A trunk carrying an allowed VLAN list can still fail if the native VLAN mismatches. BGP does not select by lowest metric by default; it evaluates path attributes such as weight, local preference, and AS path length.