Enterasys Networks X-PeditionTM manual ARP & Secondary IP, Icmp & Secondary IP

General IP Features

An XSR interface can support one primary IP address and multiple secondary IP addresses. Including all XSR interfaces, the total of supported secondary IP addresses allowed depends on the amount of the installed memory, although at present ten secondary IP addresses are supported despite the memory size. All system interfaces share the pool of secondary addresses. For example, if FastEthernet 1 uses eight secondary addresses, FastEthernet 2 is allowed no more than two secondary addresses.

Secondary IP is subject to the following rules:

•Primary and secondary IP addresses on the same interface are not allowed to exist in the same subnet, nor allowed to exist in the same subnets already occupied by other interfaces.

•Packets generated by the XSR, except the route update packet, are always sourced by the IP address of the outgoing interface which is in the same subnet as the IP address of the next-hop the packet should be forwarded to.

•All routers on the same segment should share the primary network number or some protocols, such as OSPF, may not work properly.

•If any router on a network segment uses a secondary address, all other devices on the same segment must also use a secondary address from the same network or subnet. Inconsistent use of secondary addresses on a network segment can quickly cause routing loops.

•Specify the primary IP address before any secondary IP addresses on the same interface. Conversely, before deleting a primary address, all secondary IP addresses should be removed.

•You can configure OSPF, RIP or static routes on each primary and secondary IP address.

•A secondary IP address is configured using the ip address secondary command.

ARP & Secondary IP

For each IP address configured on the interface, including primary and secondary IP addresses, the corresponding static ARP entry should be maintained in the static ARP table. Primary and secondary IP addresses on the same interface share the same MAC address of the interface.

When an ARP request is received, the destination IP address in the ARP packet will be checked against the primary IP and all secondary IP addresses. If found, an ARP reply will be sent back with the MAC address of the interface. When sending an ARP request, the source IP address used in the ARP packet should be on the same subnet as the destination IP.

ICMP & Secondary IP

When ICMP Echo packets are received by the XSR, the destination IP address is checked against all configured IP addresses including primary and secondary addresses. Any ICMP Echo packet addressed to the subnet broadcast addresses will be dropped without returning a response.

ICMP Echo Replies are generated by swapping the destination and source IP addresses in the received ICMP Echo packets.

By default, ICMP Echo packets generated by the XSR’s ping command will be sourced by the IP address of the outgoing interface which is in the same subnet as the IP address of the next-hop the ICMP packet should be forwarded to.

When ICMP Mask request packets are received, the destination IP address will be matched against the entire subnet network associated with the primary and secondary IP addresses. The matched IP address will then be used as the source IP address of the reply packet.

5-8 Configuring IP