Network Ninja

This subject ties into the Campus Network Model and ECNM

Default Gateways

Typically when a default gateway is configured on network devices there is no means to configure a second default gateway (not always the case). On end-devices a single default gateway is typically configured (single IP Address) that does not change when the IP topology changes due to failure. If (or when) the router that is acting as default gateway fails the end device cannot send packets off the local network segment this would be the case even if there exists a back-up default gateway or network device (router or MLS) that could act as a secondary gateway.

Proxy Address Resolution Protocol (ARP)

The IOS software on Routers runs proxy ARP to provide end-devices with the routing option (MAC Address) of a gateway that is able to forward a packet off the local subnet.

With Proxy ARP, end-devices behave as if the destination device were connected to its own network segment.

If the default gateway router fails (responsible router), the source end-devices continue to send packets for the IP destination to the MAC Address of the failed router, and the packets are discarded.

Eventually, the Proxy ARP address will age out the end-devices ARP cache and the end-device may eventually acquire the address of another proxy ARP failover router (secondary gateway). During this time the end-device cannot send packets off the local segment. (RFC 1027)

Router Redundancy

With router device redundancy a set of routers (two or more) can present themselves as a single virtual router to end-devices on the LAN. By sharing an IP Address (layer-3) and a MAC Address (layer-2), two or more routers can act as a single “virtual” router.

The IP address of the virtual router will be configured as the default gateway for end-devices on a particular segment. When frames are to be sent from an end-device to the default gateway, the end-device will use ARP to resolve the MAC Address of the default gateway (which will return the virtual router MAC Address).

Frames in turn sent to the virtual router can be physically processed by any active or standby router that is part of that virtual router group. The physical router that receives the forwarded traffic is transparent to the end-device as the virtual router IP and MAC Address is presented to end-devices.

A protocol is used to identify two or more routers as the devices responsible for processing frames for the virtual router group. This redundancy protocol provides the mechanism by which the router determines what router will forward traffic and determines when that role must be taken by another router. This transition as with the processing is transparent to the end-user.

The next post will detail Hot Standby Router Protocol (HSRP) that will elaborate on this topic

Notes and Notices:

This is a part of my personal BCMSN notes and research to assist myself in learning and understanding the concepts and theory for the BCMSN exam. I learn by making notes reading and writing things down and wish to file them where I can’t lose them. These notes are not to be seen, judged or mistaken for replacements to Cisco recognized and authorized training which I personally support and attend and suggest you undertake if you are going for the BCMSN Certification.