Reference Guide

Table 12: Default Ports - UDP Port Numbers Implied by Wildcard
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 (TFTP) 69
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, 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.
clear ip helper statistics
Use this command to reset to zero the statistics displayed in the show ip helper statistics
command.
Routing Commands
ExtremeSwitching 200 Series: Command Reference Guide for version 01 .02.04.0007 558