NETGEAR M6100 Series Reference Manual page 695

Table 10. Default ports—UDP port numbers implied by wildcard
Protocol
IEN-116 Name Service
DNS
NetBIOS Name Server
NetBIOS Datagram Server
TACACS Server
Time Service
DHCP
Trivial File Transfer Protocol (TFTP)
The system limits the number of relay entries to four times the maximum number of routing
interfaces. The network administrator can allocate the relay entries as he likes. There is no
limit to the number of relay entries on an individual interface, and no limit to the number of
servers for a given interface and UDP port pair.
The relay agent relays DHCP packets in both directions. It relays broadcast packets from the
client to one or more DHCP servers, and relays to the client packets 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
checks if the interface is configured to relay the destination UDP port. If so, the relay agent
unicasts the packet to the configured server IP addresses. Otherwise, the relay agent checks
if 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. Note
that if the packet matches a discard relay entry on the ingress interface, then the packet is
not forwarded, regardless of the global configuration.
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.
• The protocol field in the IP header must be UDP (17).
• The destination UDP port must match a configured relay entry.
M6100 Series Switches
UDP Port Number
42
53
137
138
49
37
67
69
Routing Commands
695