BGP does not compare the MED values, and prefers route B to route A1 because the
IGP cost (12) of route B is smaller than that of route A1 (13).
–
BGP then compares route A2 and route B. The leftmost AS number in the AS_Path of
route A2 is 65001, which is different from its counterpart in route B (65002). Therefore,
BGP does not compare the MED values, and selects route A2 as the optimal route
because its IGP cost (11) is smaller than that of route B (12).
l
Case 2: Route A2 is received first, followed by route B, and then route A1.
–
BGP then compares route A2 and route B. The leftmost AS number in the AS_Path of
route A2 is 65001, which is different from its counterpart in route B (65002). Therefore,
BGP does not compare the MED values and prefers route A2 to route B because the
IGP cost (11) of route A2 is smaller than that of route B (12).
–
BGP then compares route A1 and route A2. The leftmost AS number in the AS_Path
of route A1 is the same as its counterpart in route A2 (65001). In this situation, BGP
selects route A1 as the optimal route because its MED value (100) is smaller than that
of route A2 (150).
l
Case 3: If the
deterministic-med
command is run, BGP groups the routes that are learned
from different ASs but are destined for the same network segment based on the leftmost
AS number in the AS_Path, selects one optimal route from each group, and then compares
the optimal routes of all the groups. Detailed steps are as follows:
–
BGP first compares route A1 and route A2. The leftmost AS number in the AS_Path of
route A1 is the same as its counterpart in route A2 (65001). In this situation, BGP selects
route A1 as the optimal route because its MED value (100) is smaller than that of route
A2 (150).
–
BGP then compares route A1 and route B. The leftmost AS number in the AS_Path of
route A1 is 65001, which is different from its counterpart in route B (65002). Therefore,
BGP does not compare the MED values and selects route B as the optimal route because
the IGP cost (12) of route B is smaller than that of route A1 (13).
Case 1 and case 2 show that the route selection result is relevant to the sequence in which routes
are received if the
deterministic-med
is not configured. Case 3 shows that the route selection
result is irrelevant to the sequence in which routes are received if the
deterministic-med
is
configured.
Peer Type
When IBGP routes (routes learned from IBGP peers) and EBGP routes (routes learned from
EBGP peers) are available, BGP preferentially selects EBGP routes.
When one EBGP route and multiple IBGP routes are available, BGP selects the optimal route
based on the peer type. If no EBGP route is available or multiple EBGP routes are available,
BGP is unable to select the optimal route based on the peer type.
When multiple egress devices reside on a carrier network and receive routes from the Internet,
the egress devices select the optimal route based on the peer type in most cases. In
, all devices reside in the same AS, Router A and Router B function as egress devices and
are IBGP peers of all devices in the AS. In addition, Router A and Router B receive routes from
the Internet and advertise EBGP routes to all their IBGP peers. Therefore, Router A and
Router B have an IBGP route and an EBGP route, and the two routes have the same AS_Path.
In this situation, Router A and Router B select the EBGP route as the optimal route.
HUAWEI NetEngine80E/40E Router
Configuration Guide - IP Routing
8 BGP Configuration
Issue 02 (2014-09-30)
Huawei Proprietary and Confidential
Copyright © Huawei Technologies Co., Ltd.
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