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IP 101: All About IP Addresses
By Chris Lewis Static Routing Usually an experienced network administrator will seek to minimize any manual configuration. In the case of Exterior routing, this may be different, as static routing offers a number of advantages when routing between Autonomous Systems. These advantages can be summarized as follows:
Static Routing Between Autonomous Systems To complete static routing for connecting these two autonomous systems together, use the following commands:
RouterA(config)#ip route 47.0.0.0 255.0.0.0 80.4.4.5
RouterB(config)#ip route 45.0.0.0 255.0.0.0 80.4.4.4
This tells each Autonomous System how to get to networks in the other Autonomous System.
Exterior Gateway Protocol
The routing information of EGP is similar to distance vector protocols, but it omits the metric for routes advertised. EGP was implemented like this because it was designed for the Internet, when it was assumed that there would be a core network, with separate routing domains connected to this core by one router. The major problem with using EGP in a more generalized network is that, since no use is made of metrics, if there is more than one path to a destination, packets can very easily get caught in routing loops.
EGP has been superseded by the Border Gateway Protocol, BGP.
Border Gateway Protocol - BGP
BGP works on the principle of enforcing policies. A policy is manually configured and allows a BGP enabled router to rank possible routes to other Autonomous Systems, selecting the best path.
Configuring BGP
Network Configuration for BGP Example
This is a basic configuration for BGP, there are however many many further configuration customizations that can be made. For a fuller discussion of BGP configuration, refer to "Multi-Homing Your Internet Service With BGP" .
The following commands are entered in to router R6.
Router6(config)#router bgp 3
The first line in this configuration defines BGP for Autonomous System 3 on router 6. The next two lines define the network numbers internal to AS 3 that will be advertised via BGP. The fourth line defines an internal neighbor, that is in the same AS. The BGP process on router 6 will now exchange information with a BGP process defined on R5. The fifth line defines the neighbor in a different AS that router 6 will exchange information with.
The effect of this configuration is that R6 will share information about network s 147.30.0.0 and 150.1.0.0 with the two specified routers via BGP updates.
The last thing left to do in our basic configuration of BGP, is to assign administrative weights to control the path selection process. In the following example, a weight of 40,000 is assigned to the path to router R4
Router6(config-router)#neighbor 160.4.5.5 40000
This administrative weight can vary between 0 and 65535, the default being 32768. The effect of increasing the weight to R4 is to make it less attractive when R6 is calculating which paths to use.
Redistributing Route Information Between Protocols
The answer is Redistribution. A router can be configured to run more than one routing protocol and Redistribute route information between the two protocols. The idea is that there will be multiple domains on the internetwork, each operating with a different routing protocol. At the border between these domains, one router has the responsibility of running both routing protocols and informing each domain about the other's networks in the appropriate routing protocol. This is illustrated below:
A Border Router Configured to Redistribute Between RIP and IGRP
In this example, router 1 has to run both RIP and IGRP, then inform Domain A about the networks in Domain B with RIP updates and inform Domain B about Domain A's networks using IGRP updates. The router in this figure will only be able to assign one metric to all the routes that it redistributes from one Domain to another. It cannot translate metrics between protocols. At first this may seem to be a drawback, that all networks are redistributed with the same metric value, no matter where they are located in the other Domain. In reality this is not a problem, since to get from Domain A to Domain B, all connections have to go through the same router, so the initial part of the journey is identical when sending packets between Domains.
In this example, a packet destined for Domain A, originating in Domain B reaches router 1. Router 1 then has a routing table filled with entries for the networks in Domain A that have been calculated using RIP updates. The packet will then follow the best path to it's destination network.
The following is an example of how the routing protocol processes could be configured on router 1, to redistribute routes between the RIP and IGRP Domains.
Given a basic configuration for both RIP and IGRP, the redistribute commands are in bold:
router igrp 12
router rip
This assumes that Domain A has network numbers 150.1.0.0 and 120.0.0.0 in it and Domain B has networks 164.8.0.0 and 193.1.1.0 in it.
The five values following the default-metric entry in the router IGRP section are the metrics that will be sent out in IGRP updates, for routes learned about via RIP. In the router RIP section, routes learned from IGRP updates will be advertised with a metric of 3. It must be noted that the numbers shown here are random. In most instances this does not matter, as all cross domain traffic has to go through router 1 as previously explained.
As you can see, once you start to configure subnets and use multiple network numbers, autonomous systems and redistribution, life can get tricky! The best advice we can leave you with is to be careful out there!
Answers to subnet questions
Subnet 164.2.34.32
Subnet 101.2.0.0
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