BGP associates several different basic attributes with each route prefix. These attributes include useful pieces of information about the route, where it came from, and how to reach it. Well known attributes must be supported by every BGP implementation. Some well known attributes are mandatory. All of the mandatory attributes must be included with every route entry. A BGP router will generate an error message if it receives a route that is missing one or more well known mandatory attributes.
There are also well known discretionary attributes, which every BGP router must recognize and support, but that don't have to be present with every route entry. Whenever a router passes along a route that it has learned via BGP to another BGP peer, it must include all of the well known attributes that came with this route, including any discretionary attributes. Of course, the router may need to update some of these attributes before passing them along, to include itself in the path, for example.
BGP routes can also include one or more optional attributes. These are not necessarily supported by all BGP implementations. Optional attributes can be either transitive or nontransitive, which is specified by a special flag in the attribute type field. If a router receives a route with a transitive optional attribute, it will pass this information along intact to other BGP routers, even if it doesn't understand the option. The router will mark the Partial bit in the attribute flags to indicate that it was unable to handle this attribute, however.
The router will quietly drop any unrecognized nontransitive optional attributes from the route information without taking any action.
We will now describe several of the most common BGP attributes.
ORIGIN Well known, Mandatory
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This attribute can have one of three different values, reflecting how the BGP router that was responsible for originating the route first learned it. The possible values are:
IGP: the route came from an IGP interior to the originating AS.
EGP: the route came from an EGP other than BGP.
Incomplete: any other method.
AS_PATH Well known, Mandatory
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The AS_PATH is a list of ASNs, which show the path taken to reach the destination network. There are actually two types of AS_PATHs. An AS_SEQUENCE describes the literal path taken to reach the destination, while an AS_SET is an unordered list of ASNs along the path. Each time a BGP router passes a route update to an eBGP peer, it updates the AS_PATH variable to include its own ASN.
NEXT_HOP Well known, Mandatory
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This attribute carries the IP address of the first BGP router along the path to the destination network. When the router installs the route for the associated prefix in its routing table, it will use this attribute for the next hop router. This is where the router will forward its packets for this destination network.
By default, the NEXT_HOP router will be the router that announced this route to the AS. For routes learned from an external AS via eBGP, the NEXT_HOP router will be the first router in the neighboring AS. This information is passed intact throughout the AS by using iBGP, so all routers in the AS see the same NEXT_HOP router.
MULTI_EXIT_DISC Optional, Nontransitive
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The Multiple Exit Discriminatory (MED) option is also often called the BGP Metric. Because this 32-bit value is non-transitive, it is only propagated to adjacent ASs. Routers can use the MED to help differentiate between two or more equivalent paths between these ASs.
LOCAL_PREF Well known, Discretionary
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BGP only distributes Local Preference information with routes inside of an AS. Routers can use this number to allow the network to favor a particular exit point to reach a destination network. This information is not included with eBGP route updates.
ATOMIC_AGGREGATE Well known, Discretionary
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When a BGP router aggregates several route prefixes to simplify the routing tables that it passes to its peers, it usually sets the ATOMIC_AGGREGATE attribute to indicate that some information has been lost. It doesn't set this attribute, however, in cases in which it uses an AS_SET in its AS_PATH to show the ASNs of all of the different prefixes being summarized.
AGGREGATOR Optional, Transitive
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The AGGREGATOR attribute indicates that a router has summarized a range of prefixes. The router doing the route aggregation can include this attribute, which will include its own ASN and IP address or Router ID.
COMMUNITY Optional, Transitive
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A COMMUNITY is a logical grouping of networks. This attribute is defined in RFC 1997, and RFC 1998 describes a useful application of the concept to ISP networks.
MP_REACH_NLRI Optional, Nontransitive
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This attribute carries information about reachable multiprotocol destinations and next-hop routers. Multiprotocol in this context could refer to any foreign protocol, such as IPv6, although it is most commonly used with IP multicast, as we will discuss in Chapter 23. Multiprotocol Label Switching (MPLS) also uses MBGP for per-VPN routing tables.
MP_UNREACH_NLRIOptional, Nontransitive
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The MP_UNREACH_NLRI attribute is similar to the MP_REACH_NLRI, except that it carries information about unreachable multiprotocol destinations.
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