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Routing Configuration 101
Table 4-1. Default Ports - UDP Port Numbers Implied By Wildcard
The switch limits the number of relay entries to four times the maximum number of VLAN routing
interfaces (512 relay entries). 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, UDP port} pair.
NOTE: DHCP relay cannot be enabled and disabled globally. IP helper can be enabled or disabled globally.
Enabling IP helper enables DHCP relay.
Certain pre-existing configurable DHCP relay options do not apply to relay of other protocols. These
options are unchanged. You can optionally set a maximum hop count or minimum wait time using the
bootpdhcprelay maxhopcount and bootpdhcprelay minwaittime commands.
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.
NOTE: If the packet matches a discard relay entry on the ingress interface, the packet is not forwarded,
regardless of the global configuration.
Protocol UDP Port Number
IEN-116 Name Service 42
DNS 53
NetBIOS Name Server 137
NetBIOS Datagram Server 138
TACACS Server 49
Time Service 37
DHCP 67
Trivial File Transfer Protocol 69