Extreme Networks ExtremeWare XOS 10.1, Manual

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178 ExtremeWare XOS 10.1 Concepts Guide
IP Unicast Routing
Multiple Routes
When there are multiple, conflicting choices of a route to a particular destination, the router picks the
route with the longest matching network mask. If these are still equal, the router picks the route using
the following criteria (in the order specified):
• Directly attached network interfaces
• ICMP redirects
• Static routes
•
Directly attached network interfaces that are not active.
NOTE
If you define multiple default routes, the route that has the lowest metric is used. If multiple default
routes have the same lowest metric, the system picks one of the routes.
You can also configure blackhole routes—traffic to these destinations is silently dropped.
Proxy ARP
Proxy Address Resolution Protocol (ARP) was first invented so that ARP-capable devices could respond
to ARP Request packets on behalf of ARP-incapable devices. Proxy ARP can also be used to achieve
router redundancy and simplify IP client configuration. The switch supports proxy ARP for this type of
network configuration. The section describes some example of how to use proxy ARP with the switch.
ARP-Incapable Devices
To configure the switch to respond to ARP Requests on behalf of devices that are incapable of doing so,
you must configure the IP address and MAC address of the ARP-incapable device using the use the
following command:
configure iparp add proxy <ip_addr> {vr <vr_name>} {<mask>} {<mac>} {always}
After it is configured, the system responds to ARP Requests on behalf of the device as long as the
following conditions are satisfied:
• The valid IP ARP Request is received on a router interface.
• The target IP address matches the IP address configured in the proxy ARP table.
• The proxy ARP table entry indicates that the system should always answer this ARP Request,
regardless of the ingress VLAN (the
always
parameter must be applied).
When all the proxy ARP conditions are met, the switch formulates an ARP Response using the
configured MAC address in the packet.
Proxy ARP Between Subnets
In some networks, it is desirable to configure the IP host with a wider subnet than the actual subnet
mask of the segment. Proxy ARP can be used so that the router answers ARP Requests for devices
outside of the subnet. As a result, the host communicates as if all devices are local. In reality,
communication with devices outside of the subnet are proxied by the router.