Huawei Quidway S5600 Series Operation Manual page 353

Operation Manual – Routing Protocol
Quidway S5600 Series Ethernet Switches-Release 1510
The MED attribute is used to determine the optimal route for traffic flows to enter an AS.
It acts the same as the metrics used in IGP. For multiple routes a BGP router receives
from different EBGP peers, if they have the same destination address but different next
hops, the route with the smallest MED value is chosen as the optimal route provided
other conditions are the same. As shown in Figure 5-7, Router B is chose as the ingress
for traffic from AS10 to AS20.
> D=9.0.0.0
Next_Hop=2.1.1.1
MED=0
RouterA
D=9.0.0.0
Next_Hop=3.1.1.1
MED=100
Figure 5-7 MED attribute
Normally, BGP only compares the MED attribute values of the routes received from the
same AS.
Note:
In VRP implementations, you can force BGP to compare MED values of routes coming
from different ASs.
5) Local_Pref
The Local_Pref attribute is only valid among IBGP peers. It is not advertised to other
ASs. It indicates the priority of a BGP router.
Local_Pref attribute is used to determine the optimal route for traffic leaving an AS. For
multiple routes a BGP receives from different IBGP peers, if they have the same
destination address but different next hops, the route with the smallest Local_Pref
value is chosen as the optimal route provided other conditions are the same. As shown
in Figure 5-8, RouterC is chose as the egress for traffic from AS20 to AS10.
MED=0
2.1.1.1
EBGP
IBGP
EBGP
3.1.1.1
AS10 MED=100
Huawei Technologies Proprietary
RouterB
9.0.0.0
IBGP
RouterD
IBGP
RouterC
AS20
5-8
Chapter 5 BGP Configuration