{"id":1053,"date":"2008-11-15T16:05:37","date_gmt":"2008-11-15T15:05:37","guid":{"rendered":"http:\/\/www.corelan.be:8800\/index.php\/2008\/11\/15\/juniper-screenos-bgp-basics-a-simple-ibgp-test-case\/"},"modified":"2008-11-15T16:05:37","modified_gmt":"2008-11-15T15:05:37","slug":"juniper-screenos-bgp-basics-a-simple-ibgp-test-case","status":"publish","type":"post","link":"https:\/\/www.corelan.be\/index.php\/2008\/11\/15\/juniper-screenos-bgp-basics-a-simple-ibgp-test-case\/","title":{"rendered":"Juniper ScreenOS BGP Basics : a simple iBGP test case"},"content":{"rendered":"

As explained in one of my earlier posts<\/a>, ScreenOS supports a couple of ways to provide for dynamic routing. Today, I\u2019m going to explain some basic implementations of BGP<\/a> on ScreenOS. Unlike my post on OSPF, I won\u2019t be going too much into all the details of what BGP is and how it works, but we need to understand some important basic facts about BGP before looking at the configuration.<\/p>\n

Introduction<\/h3>\n

First of all, while OSPF is a broadcast-based mechanism, BGP is connection-oriented uses TCP sessions between BGP peers in order to exchange information. This means that BGP peers don\u2019t need to be in the same broadcast domain in order to communicate, form neighbors and exchange information.  This means that you can have BGP peers that are connected over one or more routers, even non-BGP enabled routers\/hosts, and that you can easily run BGP through firewalls as well.  BGP uses tcp 179 to communicate.<\/p>\n

It is important to know that BGP does not really have reliability mechanisms (OSPF has LSAck messages), so the TCP sessions between the peers must be up at all time, even if the peers are not exchange information.<\/p>\n

Next, BGP is a path-vector routing protocol.  Unlike interior routing protocols<\/a> (such as OSPF), BGP (which is an exterior routing protocol<\/a>) does not contain metric information, but merely consists of the AS path that can be used to reach a particular network.   By default, the best path is the one with the fewest number of AS  in the path between source and destination.<\/p>\n

BGP does not deal with hops or links, but with AS (Autonomous Systems).  BGP refers to networks as prefixes<\/em>.  When a prefix is advertised from an AS, BGP will send out the information about the prefix to the connected AS, until all AS are aware of the prefix.   BGP adds the AS number to the AS path (which is the equivalent of a route in a IGP such as OSPF). So when an AS learns about a new prefix (network), it really learns about the path associated with this network.  This is what happens on the internet, and within other EBGP networks.<\/p>\n

IBGP is somewhat easier to understand, because there is only one AS \u2013 the current AS. That\u2019s the main difference between IBGP and EBGP.  <\/p>\n

Since there is only one AS within IBGP, why would we choose BGP over OSPF ?   2 easy reasons :<\/p>\n