{"id":368,"date":"2020-10-02T21:32:19","date_gmt":"2020-10-02T21:32:19","guid":{"rendered":"https:\/\/dft.wiki\/?p=368"},"modified":"2026-06-09T14:37:15","modified_gmt":"2026-06-09T18:37:15","slug":"routing-between-vlans-with-sub-interfaces-and-dhcp","status":"publish","type":"post","link":"https:\/\/dft.wiki\/?p=368","title":{"rendered":"Routing between VLANs with Sub-Interfaces and DHCP"},"content":{"rendered":"

\"\"<\/p>\n

At first glance at the diagram above, it may seem like Router R1 is not needed for the computers to communicate. However, the network is segmented into VLANs.<\/p>\n

You can learn what a VLAN is and how it works in this other post [Read It<\/a>].<\/p>\n

\"\"<\/p>\n

Switch configuration:<\/p>\n

interface GigabitEthernet0\/1\r\nswitchport mode trunk\r\ninterface FastEthernet0\/1\r\nswitchport access vlan 10\r\nswitchport mode access\r\ninterface FastEthernet0\/7\r\nswitchport access vlan 20\r\nswitchport mode access\r\ninterface FastEthernet0\/13\r\nswitchport access vlan 30\r\nswitchport mode access\r\ninterface FastEthernet0\/19\r\nswitchport access vlan 40\r\nswitchport mode access<\/pre>\n
Router configuration:<\/p>\n
int g0\/0\r\nno shut\r\nint g0\/0.10\r\nencapsulation dot1q 10\r\nip add 172.17.7.1 255.255.255.192\r\nint g0\/0.20\r\nencapsulation dot1q 20\r\nip add 172.17.7.65 255.255.255.192\r\nint g0\/0.30\r\nencapsulation dot1q 30\r\nip add 172.17.7.129 255.255.255.192\r\nint g0\/0.40\r\nencapsulation dot1q 40\r\nip add 172.17.7.193 255.255.255.192\r\nservice dhcp\r\nip dhcp pool POOL10\r\nnetwork 172.17.7.0 255.255.255.192\r\ndefault-router 172.17.7.1\r\ndns-server 172.17.7.1\r\nip dhcp excluded-address 172.17.7.1\r\nip dhcp pool POOL20\r\nnetwork 172.17.7.64 255.255.255.192\r\ndefault-router 172.17.7.65\r\ndns-server 172.17.7.65\r\nip dhcp excluded-address 172.17.7.65\r\nip dhcp pool POOL30\r\nnetwork 172.17.7.128 255.255.255.192\r\ndefault-router 172.17.7.129\r\ndns-server 172.17.7.129\r\nip dhcp excluded-address 172.17.7.129\r\nip dhcp pool POOL40\r\nnetwork 172.17.7.192 255.255.255.192\r\ndefault-router 172.17.7.193\r\ndns-server 172.17.7.193\r\nip dhcp excluded-address 172.17.7.193<\/pre>\n
Useful commands for this exercise:<\/p>\n
show ip dhcp binding\r\nshow arp\r\ninterface range f0\/1-24\r\nencapsulation dot1Q 1<\/strong> native<\/pre>\n
Note: the last command tells the router which VLAN a frame belongs to when it arrives without dot1Q encapsulation. Replace 1<\/strong> with the desired Native (default) VLAN.<\/p>\n
Bonus: if a router sits between the client and the DHCP server, it needs to be configured as a DHCP Relay:<\/p>\n
interface g0\/0\r\nip helper-address 10.0.0.1<\/strong><\/pre>\n
Replace 10.0.0.1<\/strong> with the IP address of the DHCP server.<\/p>\n
The ‘ip helper-address’ command does not only relay DHCP, but also other services:<\/p>\n
    \n
  • Time (port 37)<\/li>\n
  • TACACS (port 49)<\/li>\n
  • DNS (port 53)<\/li>\n
  • BOOTP\/DHCP Server (port 67)<\/li>\n
  • BOOTP\/DHCP Client (port 68)<\/li>\n
  • TFTP (port 69)<\/li>\n
  • NetBIOS name service (port 137)<\/li>\n
  • NetBIOS datagram service (port 138)<\/li>\n<\/ul>\n
    You can control what gets forwarded (relayed) by using:<\/p>\n
    ip forward-protocol udp 37\r\nno ip forward-protocol udp 53\r\nip directed-broadcast<\/pre>\n","protected":false},"excerpt":{"rendered":"
    At first glance at the diagram above, it may seem like Router R1 is not […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-368","post","type-post","status-publish","format-standard","hentry","category-ccna"],"_links":{"self":[{"href":"https:\/\/dft.wiki\/index.php?rest_route=\/wp\/v2\/posts\/368","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/dft.wiki\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/dft.wiki\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/dft.wiki\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/dft.wiki\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=368"}],"version-history":[{"count":13,"href":"https:\/\/dft.wiki\/index.php?rest_route=\/wp\/v2\/posts\/368\/revisions"}],"predecessor-version":[{"id":5830,"href":"https:\/\/dft.wiki\/index.php?rest_route=\/wp\/v2\/posts\/368\/revisions\/5830"}],"wp:attachment":[{"href":"https:\/\/dft.wiki\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=368"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/dft.wiki\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=368"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/dft.wiki\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=368"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}