Using Default Gateways

When a default gateway is configured on most devices, there is no means by which to configure a secondary gateway, even if a second route exists to carry packets off the local segment.

For example, primary and secondary paths between the Building Access submodule and the Building Distribution submodule provide continuous access in the event of a link failure at the Building Access layer. Primary and secondary paths between the Building Distribution layer and the Building Core layer provide continuous operations should a link fail at the Building Distribution layer.

In this example, router A is responsible for routing packets for subnet A, and router B is responsible for handling packets for subnet B. If router A becomes unavailable, routing protocols can quickly and dynamically converge and determine that router B will now transfer packets that would otherwise have gone through router A. Most workstations, servers, and printers, however, do not receive this dynamic routing information.

End devices are typically configured with a single default gateway IP address that does not change when network topology changes occur. If the router whose IP address is configured as the default gateway fails, the local device will be unable to send packets off the local network segment, effectively disconnecting it from the rest of the network. Even if a redundant router exists that could serve as a default gateway for that segment, there is no dynamic method by which these devices can determine the address of a new default gateway.

5-4 Building Cisco Multilayer Switched Networks (BCMSN) v3.0 © 2006 Cisco Systems, Inc.

Using Proxy ARP

This subtopic describes proxy ARP.

Routing Issues: Using Proxy ARP

Systems, Inc. All rights reserved.

Systems, Inc. All rights reserved.

Cisco IOS software runs proxy ARP to enable hosts that have no knowledge of routing options to obtain the MAC address of a gateway that is able to forward packets off the local subnet.

For example, if the proxy ARP router receives an ARP request for an IP address that it knows is not on the same interface as the request sender, it will generate an ARP reply packet giving its own local MAC address as the destination MAC address of the IP address that is being resolved. The host that sent the ARP request sends all packets that are destined for the resolved IP address to the MAC address of the router. The router then forwards the packets toward the intended host, perhaps repeating this process along the way. Proxy ARP is enabled by default.

With proxy ARP, the end-user station behaves as if the destination device were connected to its own network segment. If the responsible router fails, the source end station continues to send packets for that IP destination to the MAC address of the failed router, and the packets are therefore discarded.

Eventually, the proxy ARP MAC address will age out of the workstation's ARP cache. The workstation may eventually acquire the address of another proxy ARP failover router, but the workstation cannot send packets off the local segment during this failover time.

For further information on proxy ARP, refer to RFC 1027, Using ARP to Implement Transparent Subnet Gateways.

© 2006 Cisco Systems, Inc. Implementing High Availability in a Campus Environment 5-5

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