BROCADE IP PRIMER

49F Link 50FPower11F 2F 3F 4F32Console5476981110131215141716191821202322252427262928313033323534373639384140434245444746 48Console49F Link 50F1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47Power1F 2F 3F 4F2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48BI4XGBI4XGBI24CBI24CBI24C16T X RX T X RX T X RX T X RXT X RX T X RX T X RX T X RX12BigIron RX-8NETWORKS7 131819242530313637424348AC OK DC OK ALMEJECT SYS AC OK DC OK ALMEJECT SYS AC OK DC OK ALMEJECT SYS AC OK DC OK ALMBI24CBI24CBI24CBI24CBI24CBI4XGBI4XGT X RX T X RX T X RX T X RXT X RX T X RX T X RX T X RXEJECT SYS49F Link 50FPower11F 2F 3F 4F32Console5476981110131215141716191821202322252427262928313033323534373639384140434245444746 48User Datagram Protocol (UDP)What about the Network Layer? What will it use for an address? For a sourceaddress, it will use, well, nothing: 0.0.0.0. For a destination address, it usesthe universal broadcast address: 255.255.255.255. This is a broadcastaddress no matter what subnet you may find yourself in.And what about the Data Link Layer? The frame will use the client's MACaddress (because that's burned into the NIC's ROM, remember?) as a sourceMAC address, and it will use the broadcast MAC address, FF:FF:FF:FF:FF:FF, asthe destination MAC address.The reply is sent on destination UDP port 68 (well-known port for DHCP reply).The DHCP server uses its own IP address and MAC for the source addresses.On the Network Layer, it uses 255.255.255.255 as the destination address,but it uses the proper destination MAC address in the frame. It learned theMAC address from the DHCP request. The reply contains all of the informationrequested, which usually includes at least an IP address, subnet mask, anddefault gateway.SourceUDP 670.0.0.012:34:56:78:9A“I need an IP address”DestinationUDP 67255.255.255.255FF:FF:FF:FF:FF:FFMAC+ 12:34:56:78:9ADestinationUDP 68255.255.255.25512:34:56:78:9A“Here’s an IP address”SourceUDP 6710.0.0.1032:10:FE:DC:BA:9832:10:FE:DC:BA:9810.0.0.10But notice, for the client to use DHCP, it must send a broadcast. This meansthat a DHCP server must have an address in the broadcast domain. What ifyou have several broadcast domains. Does this mean you have to have a DHCPserver in each domain?Well, you can do that, but most engineers find that using a DHCP helperaddress is the better way to go. This acts as kind of a DHCP proxy. The helperaddress is usually configured on the broadcast domain's gateway. Essentially,you're telling the router that it should listen for DHCP broadcasts. If it sees one,it should encapsulate it and send it to the DHCP helper address (wherever thatmay be), which is the address of the remote DHCP server. The DHCP serversends an encapsulated DHCP reply to the router, and the router sends thereply to the client. The client will not know that its broadcast ever left thebroadcast domain.SourceUDP 670.0.0.012:34:56:78:9A“I need an IP address” “Hey, this guy needs an IP address”DestinationUDP 67255.255.255.255FF:FF:FF:FF:FF:FFSourceUDP 67192.168.100.1DestinationUDP 6710.0.0.10MAC+ 12:34:56:78:9A192.168.100.1 10.0.0.1“Here, have 192.168.100.5”“Okay, give him 192.168.100.5”DestinationUDP 68255.255.255.25512:34:56:78:9ASourceUDP 67192.168.100.1DestinationUDP 67192.168.100.1SourceUDP 6710.0.0.1032:10:FE:DC:BA:9810.0.0.10Brocade IP Primer 61