Compatible Systems 5.4 manual Update Method

Chapter 2 - IP Routing & Bridging 35

default router see the discussion of the IP Static Route Dialog Box later in this chapter.

ϖ Note: Some routers, in particular those designed to create very large corporate backbones, may use other routing protocols such as OSPF (Open Shortest Path First). These routers can simultaneously use RIP 1 (and in some cases RIP 2) to communicate with smaller routers, or each of the smaller routers can be set to use one of these backbone routers as their default router.

>Update Method

VPN links which are configured to provide “dial-on-demand” service will bring a connection up (i.e. dial the other end) when there are network packets which must be transferred over the link. Once a dial-on-demand connection is up, network traffic passing across the link causes the inactivity timer for the link to be reset, keeping the connection up.

The RIP protocol periodically sends out update information across a link. These periodic update packets will cause a VPN link set for dial-on-demand operation to stay up indefinitely.

• If Triggered is selected with this pull-down menu, the router will modify the standard RIP behavior for this link to send RIP packets only when there has been an update to its routing table information, or when it has detected a change in the accessibility of the next hop router.

• If Periodic is selected with this pull-down menu, the router will use the standard RIP protocol, which sends RIP packets over the link every 30 seconds.

RIP Split Horizon

Normally, RIP uses a technique called split horizon to avoid routing loops and allow smaller update packets. This technique specifies that when the device sends a RIP update out a particular network interface, it should never include routing information acquired over that same interface.

There is a variation of the split horizon technique called “poison reverse” which specifies that all routes should be included in an update out a particular interface, but that the metric should be set to infinity for those routes acquired over that interface. One drawback is that routing update packet sizes will be increased when using poison reverse.

•If Split Horizon is selected with this pull-down menu, the device will apply the split horizon technique to routes being output over this inter- face.