Attribute Type Codes

TABLE 8 . 2 Attribute Type Codes
Type Code Attribute Name Category
1 ORIGIN Well-known mandatory
2 AS_PATH Well-known mandatory
3 NEXT_HOP Well-known mandatory
4 MULTI_EXIT_DISC Optional non-transitive
5 LOCAL_PREF Well-known discretionary
6 ATOMIC_AGGREGATE Well-known discretionary
7 AGGREGATOR Optional transitive
8 COMMUNITY Optional transitive
9 ORIGINATOR_ID Optional non-transitive
10 CLUSTER_LIST Optional non-transitive
11 DPA Destination point attribute for BGP
12 Advertiser BGP/IDRP route server
13 RCID_PATH/CLUSTER_ID BGP/IDRP route server
14 Multiprotocol Reachable NLRI Optional non-transitive
15 Multiprotocol Unreachable NLRI Optional non-transitive
16 Extended Communities N/A
256 Reserved for development N/A

We are going to focus on only the first 10 attribute type codes. The others are beyond the
scope of the information needed for the BSCI exam. Let’s take a more in-depth look at each of
the first 10 attribute type codes:
 ORIGIN is a well-known mandatory attribute. The autonomous system that originates the
routing information creates the ORIGIN attribute. It is contained in all UPDATE messages
that propagate the routing information.
 AS_PATH is a well-known mandatory attribute that contains a list of all the autonomous
systems the routing information has transited. The AS_PATH component is composed of
a sequence of AS path segments. Each AS path segment is represented by the triplet of <path
segment type, path segment length, path segment value>. When a BGP speaker advertises
a learned route to other BGP speakers in its AS, the BGP speaker will not modify the AS_
PATH attribute. When a BGP speaker advertises a learned route to other BGP speakers outside
of its AS, it modifies the AS_PATH in the following ways:
 For AS_PATH with the first path segment of AS_SEQUENCE, the BGP speaker will
append its AS number as the last part of the sequence.
 For AS_PATH with the first path segment of AS_SET, the BGP speaker will add a new
path segment with the type of AS_SEQUENCE with its AS number in the sequence.
When a BGP speaker originates the route, it includes an empty AS_PATH attribute when
advertising to BGP speakers in its own AS, iBGP peers. The BGP speaker includes its AS
8 COMMUNITY Optional transitive
9 ORIGINATOR_ID Optional non-transitive
10 CLUSTER_LIST Optional non-transitive
11 DPA Destination point attribute for BGP
12 Advertiser BGP/IDRP route server
13 RCID_PATH/CLUSTER_ID BGP/IDRP route server
14 Multiprotocol Reachable NLRI Optional non-transitive
15 Multiprotocol Unreachable NLRI Optional non-transitive
16 Extended Communities N/A
256 Reserved for development N/A
TABLE 8 . 2 Attribute Type Codes (continued)
Type Code Attribute Name Category
246 Chapter 8  Border Gateway Protocol
number in the AS_PATH attribute when advertising to BGP speakers outside of its AS,
eBGP peers.
 NEXT_HOP is a well-known mandatory attribute that specifies the IP address of the border
router that should be used as the next hop to the destination specified. Following are
the rules for how the next hop is determined for the various cases that may arise:
 In eBGP, the next hop is the IP address of the external neighbor that announced the route.
 In iBGP:
Internal routes (from inside the AS) use as a next hop the IP address of the internal neighbor
that announced the route. Because iBGP peers will not re-advertise these routes to
each other, the next hop will be the iBGP peer that brought the route into the AS, the logical
exit point to get to the prefixes advertised with the route. Because we will have a full
mesh of TCP connections or the equivalent, all iBGP speakers will simultaneously converge
on this same next hop for the route.
External routes use the same next hop address as was originally injected into the AS by
eBGP. These next hop addresses remain unaltered, by default, within the AS.
 In a multi-access network, the next hop will generally be the IP address of the interface
on the same multi-access media that advertised the route, very much like an IGP.
 MULTI_EXIT_DISC is an optional non-transitive attribute. If multiple entry or exit points
exist to the same AS, it can be used to determine which one to use. The entry or exit point
with the lowest metric is used.
 LOCAL_PREF is a well-known discretionary attribute. This attribute is used by a BGP speaker
in setting the degree of preference of a route, which is used to indicate the preferred path to exit
the AS. The BGP speaker includes this attribute in advertisements to its iBGP peers.
 ATOMIC_AGGREGATE is a well-known discretionary attribute. When a BGP speaker
receives overlapping routes from its peers, it may set the ATOMIC_AGGREGATE attribute.
The attribute is set if the BGP speaker chooses a less specific route to a destination over a more
specific route—meaning the router chose a route with a shorter subnet mask rather than one
with a longer mask.
 AGGREGATOR is an optional transitive attribute. When a BGP speaker performs route
aggregation, it includes in the AGGREGATOR attribute its AS number and BGP identifier.
 COMMUNITY is an optional transitive attribute. The COMMUNITY attribute specifies
the communities a route belongs to. Communities are covered in more detail in Chapter 9.
 ORIGINATOR_ID is an optional non-transitive attribute. A BGP speaker performing the
role of a route reflector creates this attribute. The BGP identifier of the originating route
reflector is included in the ORIGINATOR_ID attribute. This attribute is specific only to
the local AS. Route reflectors are covered in more detail in Chapter 9.
 CLUSTER_LIST is an optional non-transitive attribute. The CLUSTER_LIST attribute
is composed of a list of CLUSTER_ID values. When a route reflector reflects a route, it
appends its CLUSTER_ID to the CLUSTER_LIST. Cluster IDs and route reflectors are
covered in more detail in Chapter 9.