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Understanding the network.pdf - Back to Home

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externally destined traffic arrives, it will decide which ISP gateway <strong>to</strong> forward <strong>the</strong><br />

traffic <strong>to</strong> based on its local routing table. Alternatively, we can redistribute <strong>the</strong> BGP<br />

derived routing information in<strong>to</strong> <strong>the</strong> local IGP process.<br />

When using BGP <strong>to</strong> provide reachability information about <strong>network</strong>s within <strong>the</strong> AS<br />

(in perhaps a large global private inter<strong>network</strong>), two approaches can be taken:<br />

• The first is <strong>to</strong> create BGP peer groups, which will exchange internal BGP<br />

routing information and provide gateway services for <strong>the</strong> local<br />

inter<strong>network</strong>s.<br />

• The second is <strong>to</strong> redistribute <strong>the</strong> BGP process in<strong>to</strong> <strong>the</strong> IGP process and have<br />

<strong>the</strong> IGP routers determine <strong>the</strong> destination path.<br />

The IBGP approach will be discussed in <strong>the</strong> next section. BGP redistribution will be<br />

covered in <strong>the</strong> section "Route Control and Redistribution." In most cases, you want<br />

<strong>to</strong> avoid BGP-<strong>to</strong>-IGP and IGP-<strong>to</strong>-BGP redistribution.<br />

Intra-Au<strong>to</strong>nomous BGP Configuration<br />

In <strong>the</strong> inter-au<strong>to</strong>nomous BGP example, we used a single "distribution" router <strong>to</strong> act<br />

as a gateway between <strong>the</strong> BGP and IGP processes. In large private enterprise<br />

<strong>network</strong> environments, it is not uncommon for each location <strong>to</strong> have an Internet<br />

gateway and one or more internal access gateways <strong>to</strong> some or all of <strong>the</strong> o<strong>the</strong>r<br />

internal <strong>network</strong> sites. Our testnet example uses just such a <strong>network</strong> model. When<br />

configuring Internal BGP (IBGP), all <strong>the</strong> IBGP routers need <strong>to</strong> have peering sessions<br />

with all <strong>the</strong> o<strong>the</strong>r IBGP routers.<br />

Let's examine <strong>the</strong> IBGP/EBGP configurations on asbr-a1 and asbr-a2. For this<br />

example, <strong>the</strong> test <strong>network</strong> uses AS number 30001. Both Albany and Bos<strong>to</strong>n use an<br />

IGP pro<strong>to</strong>col <strong>to</strong> manage local route announcement. BGP is used <strong>to</strong> advertise routes<br />

over <strong>the</strong> private WAN links and with <strong>the</strong> ISPs. asbr-a1/a2 and asbr-b1/b2 use<br />

identical BGP configurations. Here are <strong>the</strong> BGP configurations for asbr-a1 and<br />

asbr-b1:<br />

hostname asbr-a1<br />

!<br />

au<strong>to</strong>nomous-system 30001<br />

!<br />

router bgp 30001<br />

<strong>network</strong> 192.168.0.0 mask 255.255.255.252<br />

<strong>network</strong> 192.168.0.4 mask 255.255.255.252<br />

<strong>network</strong> 192.168.0.128 mask 255.255.255.128<br />

<strong>network</strong> 192.168.1.0 mask 255.255.255.0

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