948 IP Helper/DHCP Relay Commands
Certain preexisting configurable DHCP relay options do not apply to relay of
other protocols. These options are unchanged. The user may optionally set a
DHCP maximum hop count or minimum wait time.
The relay agent relays DHCP packets in both directions. It relays broadcast
packets from the client to one or more DHCP servers, and relays packets to
the client that the DHCP server unicasts back to the relay agent. For other
protocols, the relay agent only relays broadcast packets from the client to the
server. Packets from the server back to the client are assumed to be unicast
directly to the client. Because there is no relay in the return direction for
protocols other than DHCP, the relay agent retains the source IP address from
the original client packet. The relay agent uses a local IP address as the source
IP address of relayed DHCP client packets.
When a switch receives a broadcast UDP packet on a routing interface, the
relay agent verifies that the interface is configured to relay to the destination
UDP port. If so, the relay agent unicasts the packet to the configured server IP
addresses. Otherwise, the relay agent verifies that there is a global
configuration for the destination UDP port. If so, the relay agent unicasts the
packet to the configured server IP addresses. Otherwise the packet is not
relayed.
The relay agent only relays packets that meet the following conditions:
• The destination MAC address must be the all-ones broadcast address
(FF:FF:FF:FF:FF:FF).
• The destination IP address must be the limited broadcast address
(255.255.255.255) or a directed broadcast address for the receive interface.
• The IP time-to-live (TTL) must be greater than 1.
DNS 53
NetBIOS Name Server 137
NetBIOS Datagram Server 138
TACACS Server 49
Time Service 37
DHCP 67
Trivial File Transfer Protocol 69
Protocol UDP Port Number
2CSPC4.XCT-SWUM2XX1.book Page 948 Monday, October 3, 2011 11:05 AM