AS_Path Manipulation

AS_path information is manipulated to affect interdomain routing behavior. Because BGP prefers a shorter path over a longer one, system operators are tempted to change the path information by including dummy AS path numbers that would increase the path length and influence the traffic trajectory one way or the other. Cisco's implementation enables a user to insert AS numbers at the beginning of an AS_path to make the path length longer. The following example shows how this feature can be used.

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Troubleshooting:  
Example: Ch. 10, pp. 332-333. AS_Path Manipulation

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In figure 5-17, AS50 is connected to two providers, AS200 and AS100. AS100 is directly connected to the NAP, whereas AS200 has to go through an extra hop via AS300 to reach the NAP. Figure 5-17 shows instances of prefix 192.213.1.0/24 as it traverses the ASs in its way to the NAP. When the 192.213.1.0/24 prefix reaches the NAP via AS300, it would have an AS_path of 300 200 50. If the same prefix reaches the NAP via AS100, it would have an AS_path of 100 50, which is shorter. ASs upstream from the NAP would prefer the shorter AS_path length and would direct their traffic toward AS100 at all times for destination 192.213.1.0/24.

Figure 5-17  Routing environment before prepending dummy AS.

AS50 is not too happy about this behavior because it prefers the traffic to come via its higher bandwidth T3 link to AS200. AS50 will manipulate the AS_path information by inserting dummy AS numbers when sending routing updates to AS100. One common practice is for AS50 to repeat its AS number as many times as necessary to tip the balance and make the path via AS200 become shorter.

In figure 5-18, AS50 will insert two AS numbers 50 50 at the beginning of the AS_path of prefix 192.213.1.0/24. When the prefix 192.213.1.0/24 reaches the NAP via AS100, it would have the AS_path 100 50 50 50, which is longer than the AS_path 300 200 50 via AS300. ASs upstream of the NAP would prefer the shortest path and would direct the traffic toward AS300 for destination 192.213.1.0/24.

Figure 5-18  Routing environment after prepending dummy AS.

The bogus number should always be a duplicate of the AS announcing the route or the neighbor the route is learned from (in case an AS is increasing the path length for incoming updates). Adding any other number is misleading and could potentially lead to routing loops. Note the insertion point in figure 5-18.

The Local Preference Attribute

The local preference is a well-known discretionary attribute (type code 5). The local preference attribute is a degree of preference given to a route to compare it with other routes for the same destination. A higher local preference value is an indication that the route is more preferred. Local preference, as indicated by the name, is local to the autonomous system and gets exchanged between IBGP peers only and is not passed to EBGP peers.

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Troubleshooting:  
Example: Ch. 10, pp. 335-337. The Local Preference Attribute

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An AS connected via BGP to multiple other ASs will get routing updates about the same destinations from different ASs. Local preference is usually used to set the exit point of an AS to reach a certain destination. Because this attribute is communicated within all BGP routers inside the AS, all BGP routers will have a common view on how to exit the AS.

Consider the environment illustrated in figure 5-19. Suppose that company ANET has purchased Internet connections via two service providers XNET and YNET. ANET is connected to YNET via a primary T3 link and to XNET via a backup T1 link.

Figure 5-19  Local preference attribute example.

It is important for ANET to decide what path its outbound traffic is going to take. Of course ANET prefers to use the T3 link via YNET in normal operation because it is a high-speed link.

This is where local preference comes into play: the LA router will give the routes coming from YNET a local preference of 300. The SJ router will give the routes coming from XNET a lower value, say 200. Because both the LA and SJ routers are exchanging routing updates via IBGP, they both agree that the exit point of the AS is going to be via YNET because of the higher local preference. In figure 5-19, ANET learns route 128.213.0.0/16 via XNET and YNET. The SJ and LA routers will agree on using YNET as the exit point for destination 128.213.0.0/16 because of the higher local preference value of 300. The local preference manipulation discussed in this case affects the traffic going out of the AS and not traffic coming into the AS. Inbound traffic can still come via the T1 link.

Cisco's proprietary weight parameter is similar to the local preference in that it gives higher preference to the route that has a higher weight. The difference is that the weight parameter is local to the router and does not get exchanged between routers. The weight parameter influences routes coming from different providers to the same router (one router with multiple connections to two or more providers). The weight parameter has a higher precedence than any other attribute; it is used as a main (proprietary) switch to determine route preference.