User`s guide

XSR User’s Guide 71
Chapter 5 General IP Features
Configuring IP
Secondary IP can be used when there are insufficient host addresses on a
particular network segment. Configuring several subnets on the router interface
which connects the network segment allows you to combine these logical subnets
into one physical segment and make more host addresses available.
Interface & Secondary IP
The XSR supports seconday IP on Ethernet networks only. All other ports,
including loopback interfaces, support one IP address per interface only.
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.
Table 7
Installed Memory
Total Secondary IP
addresses Supported
16 MBytes 10
32 MBytes 10
64 MBytes 10
128 MBytes 10