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JUNOS Cookbook by Aviva Garrett

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13.2. Configuring BGP on Routers Within an AS

Problem

You want to propagate the routes learned by your EBGP peering sessions to your IBGP routers.

Solution

Configure IBGP on the border router and on all the routers within your AS. On each router, configure an IBGP group:

	[edit protocols bgp]
	aviva@RouterF# set group internal-within-AS65500 type internal
	aviva@RouterF# set group internal-within-AS65500 local-address 192.168.16.1
	aviva@RouterF# set group internal-within-AS65500 neighbor 192.168.15.1
	aviva@RouterF# set group internal-within-AS65500 neighbor 192.168.17.1

Discussion

Once you have set up an external BGP connection that runs between two different ASs, the two border routers are able to exchange routing information, but you still need a way to propagate these routes within your AS. One way to do this is to inject all the external routes into your IGP. Generally, you do not want to use this approach, especially if you are an ISP and are carrying the full Internet routing table; there are too many routes (on the order of 170,000 and climbing), and they change too often, so you would end up overwhelming the IGP. The standard way to inject external routes into your AS is to set up IBGP sessions on all routers in your AS. The connections between all IBGP routers must be fully meshed to prevent routing loops within the AS. This full mesh is a virtual mesh, completely independent of any actual physical connections. To do this, you create an IBGP group in which you list as neighbors ...

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