IP Routing Advanced Info

Howto by jmke @ 2003-01-23

This guide will be updated with more info now & then! Want to know what makes the internet work? Here you can find some info at least :)
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RIP Policy/IP Multi-Path&Net

RIP Accept Policies

- Listen to RIP updates from certain routers
- Listen only for specific networks
- assign specific mask to be included in the routing table
- change perferences of a RIP route

Updates -> RIP -> RIP accept policy -> RTM -> IP Routing Table

RIP Announce Policies

- Announce only certain routes out specific interfaces
- Conditionally generate a default route
- Announce only routes learned from specific gateways
- Announce only routes learned from specific interfaces
- Aggregate a range of networks into a single advertised route

IP Routing Table -> RTM -> RIP announce policy -> RIP -> Updates

IP MultiPath

IP Equal Cost Multipath (ECMP) allows the router to maintain up to five equal cost paths to the same destination in the routing table. Each path maintained has a unique next hop.

Equal cost can be maintained for static, RIP and/or OSPF routes.

- greater use of bandwidth, ability to use multiple routes for a single data stream. (increase overall throughput to the end user)
- Network resilience: a single point of failure is reduced

- Out-of-order delivery: whenever you send a single datastream over multiple paths out of order delivery is possible
- Performance degradation: increased packet handling by the router can cause this
- Greater memory use: routing tables become larger due to the increased number of routes to the same destination

IP MultiNetting

IP Multinetting is when you assign multiple IP addresses to the same physical network, you can do this under the following conditions:
- A LAN has more hosts then the host address space allows
- the network contains 2 departments and you want to maintain things as they are
- you are moving from a bridged network to a routed network and you do not want to re-address all hosts

Disadvantage of multinetting is that all traffic between 2 hosts may go through the router even though the hosts are on the same physical network. You can overcome this limitation by adding a static route on the workstations (using "route add" command) that points to the same interface for access to more than one network.
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