Network Access Fix · Medium ~12 min · 3 devices · 4 tasks · CCNA v1.1 and v2.0

Inter-VLAN routing broken on a Layer 3 switch

MLS1 is the core Layer 3 switch and the default gateway for VLAN 10 (192.168.10.1) and VLAN 20 (192.168.20.1). SW1 serves VLAN 10 users and SW2 serves VLAN 20 users, each over an 802.1Q trunk to MLS1. Nobody can reach the other VLAN, and VLAN 20 users cannot even reach their gateway. The access switches are fine: all three faults are on MLS1.

MLS1 SVI Vlan10 192.168.10.1/24 SVI Vlan20 192.168.20.1/24
MLS1 Gi0/1 trunk to SW1 Gi0/1 (VLAN 10 users on SW1 Fa0/1)
MLS1 Gi0/2 trunk to SW2 Gi0/1 (VLAN 20 users on SW2 Fa0/1)

Topology

Tasks

○MLS1 routes between VLANs (ip routing)
○SVI Vlan20 is enabled with 192.168.20.1
○The MLS1 trunk to SW2 carries VLAN 20
○Verified with show interfaces trunk

Hints (open one at a time)

Hint 1: can it route at all?

A Layer 3 switch only routes between SVIs when IP routing is on. Look for no ip routing in show running-config on MLS1 (press Space at --More-- to see the next page).

Hint 2: the gateway

Run show ip interface brief on MLS1. An SVI that shows administratively down was shut down on purpose.

Hint 3: the trunk

Run show interfaces trunk on MLS1 and read the Vlans allowed on trunk section. VLAN 20 has to be allowed on the link toward SW2. Use add so you do not remove VLAN 10.

Full solution

MLS1# configure terminal
MLS1(config)# ip routing
MLS1(config)# interface vlan 20
MLS1(config-if)# no shutdown
MLS1(config-if)# exit
MLS1(config)# interface gigabitethernet0/2
MLS1(config-if)# switchport trunk allowed vlan add 20
MLS1(config-if)# end
MLS1# show interfaces trunk

switchport trunk allowed vlan 20 without add would replace the list and drop VLAN 10.

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Inter-VLAN routing broken on a Layer 3 switch

Stuck on a lab, or not sure what to practice next?