Information Technology Reference
In-Depth Information
in the list is between 1 and 255. Prepending the same AS number to the list is a common
method of influencing inbound path selection, because the path with the shortest list is
preferred. Four types of AS segments within the AS_PATH are supported in Cisco IOS
software:
•
AS_SET
•
AS_SEQUENCE
•
AS_CONFED_SET
•
AS_CONFED_SEQUENCE
The difference between SET and SEQUENCE is that the list of autonomous systems is
unordered (with regard to the autonomous systems traversed in the path) in a SET and is
ordered in a SEQUENCE. The latter two apply only to paths originated within the local
confederation. Additionally, they are counted differently in path selection, as discussed in
the section “Path Decision Process.”
NEXT_HOP
This attribute defines the next-hop IP address to reach a prefix from the BGP point of view.
This does not necessarily mean that the next hop is directly connected. If the BGP next hop
is not the immediate next hop, a recursive route lookup in the IP RIB is needed. A prefix
must have a reachable next hop before BGP considers it in the best path selection. In other
words, the next hop must be under a prefix in the routing table, including the 0.0.0.0/0.
There are three points at which the next-hop attribute for a BGP path is commonly set:
•
When the prefix is first injected into BGP, the next hop is set by the BGP speaker that
injects the prefix. The next hop's value depends on how the prefix is injected. If the
prefix is injected by the
aggregate-address
command, the prefix's BGP next hop is
the BGP speaker doing the aggregation. If the prefix is injected by the
network
command or redistribution, the IGP next hop before the injection becomes the BGP
next hop. For example, if an OSPF prefix is redistributed into BGP, the BGP next hop
is not necessarily the BGP speaker doing the redistribution, but rather is the original
next hop of the OSPF prefix. Thus, in this case, it is advisable to reset the next hop at
the redistribution point to the BGP speaker itself. If the IGP next hop does not exist
(such as in the case of a route pointing to the Null0 interface), the next hop is the
BGP speaker itself. If the local BGP speaker becomes the next hop, the next-hop field
in the BGP RIB is 0.0.0.0. The next hop in the outgoing updates is set to the local BGP
peering address.
•
When the prefix is advertised via eBGP, the next hop is automatically set to the IP
address of the eBGP peer that is sending the prefix. If three or more peers are sharing
the same multiaccess network, however, the advertising speaker sets the original
speaker on the same segment as the next hop, rather than itself. This is called
third-
party next hop
.
