Information Technology Reference
In-Depth Information
•
The next hop is manually changed through the use of a route map or
next-hop-self
command. Note that the next hop is not changed by default for a BGP session within
the same AS.
MULTI_EXIT_DISC
The MULTI_EXIT_DISC (MED) attribute is typically used on inter-AS links to discrimi-
nate among multiple exit/entry points to the same neighboring AS. Cisco IOS software also
allows you to compare MEDs among different autonomous systems using the
bgp always-
compare-med
command. MED values are expressed as metric values. In a manner consis-
tent with metrics, the path with a lower MED is preferred.
In Cisco IOS software, the following are some of the rules on MED setting and
advertisement:
•
If a route is learned from an iBGP peer, the border router removes the MED before
advertising the route to an eBGP peer. To force the border router to advertise the MED
in such a case, the route-map
set metric-type internal
command can be configured
for that eBGP peer.
•
Routes injected into BGP locally on a border router are advertised to an eBGP peer
with MED. The metric values are determined as follows:
— If the injected BGP route, using the
network
or
redistribute
command, is
from an IGP, the BGP MED is derived from the IGP metric.
— If the injected BGP route (using the
network
or
redistribute
command) is
from a connected route, the BGP MED is set to 0.
— If the route is injected by the
aggregate-address
command, MED is not set.
LOCAL_PREF
LOCAL_PREF is an attribute used by an iBGP speaker to calculate a degree of preference
for each external route. It is exchanged between iBGP peers to set a preferred exit point out
of an AS. The path with a higher LOCAL_PREF is preferred. This attribute is not included
in eBGP prefix advertisements (typically set administratively in incoming eBGP updates)
and is used only inside an AS for path selection manipulation. In comparison, MED is sent
from one AS to another neighboring AS on an eBGP link to affect the outbound policy of
the receiving AS.
In Cisco IOS software, another parameter, WEIGHT, can also influence path selection. This
parameter is Cisco-proprietary and is local to the router on which it is configured. That is,
the WEIGHT setting is not exchanged between routers.
NOTE
