automatically assumed.
Content 6.4 Advanced BGP Configuration and Verification 6.4.12 The debug ip bgp Command Figure shows partial output from the debug ip bgp updates command on router A after the clear ip bgp command was issued to clear BGP sessions with its IBGP neighbor 10.1.0.2. After the neighbor adjacency is reestablished, router A creates and sends updates to 10.1.0.2. The first update highlighted in the figure, “10.1.1.0/24, next 10.1.0.1,” is an update about network 10.1.1.0/24, with a next hop of 10.1.0.1, which is the address of router A. The second update highlighted in the figure, “10.97.97.0/24, next 172.31.11.4,” is an update about network 10.97.97.0/24, with a next hop of 172.31.11.4, which is the address of one of router A’s EBGP neighbors. The EBGP next-hop address is being carried into IBGP. Router A later receives updates from 10.1.0.2. The update highlighted in the figure contains a path to two networks: 10.1.2.0/24 and 10.1.0.0/24. The attributes shown in this update are described in the next lesson. Note
Debugging uses up router resources and should be turned on only when necessary.
Content 6.5 Selecting a BGP Path 6.5.1 Characteristics of BGP Attributes BGP routers send BGP update messages about destination networks to other BGP routers. The update messages contain one or more routes and a set of BGP metrics, which are called path attributes, attached to the routes. An attribute is either well-known or optional, mandatory or discretionary, and transitive or nontransitive. An attribute may also be partial. Not all combinations of these characteristics are valid. Path attributes fall into the following four categories: Only optional transitive attributes can be marked as partial. All BGP routers must recognize a well-known attribute and propagate it to the other BGP neighbors. Well-known attributes are either mandatory or discretionary. A well-known mandatory attribute must be present in all BGP updates. A well-known discretionary attribute does not have to be present in all BGP updates. Attributes that are not well-known are called optional. BGP routers do not have to support an optional attribute. Optional attributes are either transitive or nontransitive. The following statements apply to optional attributes:
Content 6.5 Selecting a BGP Path 6.5.2 BGP Attributes The following is a list of the common BGP attributes according to categories that they belong to: Note
In addition, Cisco defines a weight attribute for BGP. The weight is configured locally on a router and is not propagated to any other BGP routers.
Note
The attributes in this list are detailed in the following topics, except for the atomic aggregate and aggregator attributes. These two attributes relate to BGP summarization (or aggregation), and you can find more information on them on Cisco.com.
Interactive Media Activity Point and Click: Classify the Attributes Upon completion of this activity, the student will be able to identify the different BGP attribute types.
Content 6.5 Selecting a BGP Path 6.5.3 AS Path Attribute The AS path is a well-known mandatory attribute. Whenever a route update passes through an autonomous system, the autonomous system number is prepended (added) to that update when it is advertised to the next EBGP neighbor. The AS path attribute is actually the list of autonomous system numbers that a route has traversed to reach a destination, with the number of the autonomous system that originated the route at the end of the list. In Figure , router A in AS 64520 advertises network 192.168.1.0. When that route traverses AS 65500, router C prepends its own autonomous system number to it. When 192.168.1.0 reaches router B, it has two autonomous system numbers attached to it. From router B’s perspective, the path to reach 192.168.1.0 is (65500, 64520). A similar process applies for the paths to networks 192.168.2.0 and 192.168.3.0. The path from router A to 192.168.2.0 is (65500, 65000), which means traverse AS 65500 and then AS 65000. Router C must traverse path (65000) to reach 192.168.2.0, and path (64520) to reach 192.168.1.0.
Content 6.5 Selecting a BGP Path 6.5.4 Next-Hop Attribute The BGP next-hop attribute is a well-known mandatory attribute that indicates the next-hop IP address that is to be used to reach a destination. BGP routes autonomous system by autonomous system, not router by router. The next-hop attribute defines the IP address of the border router that should be used as the next hop to the destination. For EBGP, the next hop is the IP address of the neighbor that sent the update. In Figure , router A advertises 172.16.0.0 to router B, with a next hop of 10.10.10.3, and router B advertises 172.20.0.0 to router A, with a next hop of 10.10.10.1. For IBGP, the protocol states that the next hop that is advertised by EBGP should be carried into IBGP. Because of that rule, router B advertises 172.16.0.0 to its IBGP peer router C with a next hop of 10.10.10.3 (router A’s address). Therefore, router C knows that the next hop to reach 172.16.0.0 is 10.10.10.3, not 172.20.10.1, as you might expect. It is very important for router C to know how to reach the 10.10.10.0 subnet, either via an IGP or a static route. Otherwise, it will drop packets destined to 172.16.0.0, because it cannot get to the next-hop address for that network. Alternatively, router B can change the next-hop attribute to itself if you use the neighbor next-hop-self command.
Content 6.5 Selecting a BGP Path 6.5.5 Origin Attribute The origin attribute defines the origin of the path information. The origin attribute can be one of these three values: Figure shows example output of the show ip bgp command. The origin code, reflecting the origin attribute, is in the last column at the end of each line. In this example, all origin codes are “i,” indicating an origin attribute of IGP; the routes are interior to the originating autonomous system.
Content 6.5 Selecting a BGP Path 6.5.6 Local Preference Attribute Local preference is a