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FAQ: Questions About BGP Route Attributes local-preference and MED

Publication Date:  2013-09-03 Views:  32 Downloads:  0
Issue Description
1. Is the Multi-Exit Discrimination (MED) value available only between EBGP peers? If so, does a route learned from an EBGP peer carry the MED attribute when being advertised to an IBGP peer? If so, does the MED value affect selection of the route to the IBGP peer?
2. When IBGP peers advertise routes to each other using a routing policy, does the MED value in the apply command take effect? If an IBGP peer advertises the learned route to an EBGP peer, does the route still carry the MED value?
3. When EBGP peers advertise routes to each other, if a routing policy is applied in the outbound direction of the local end, does the local-preference value in the apply command take effect? If the routing policy is applied in the inbound direction of the peer end, does the local-preference value in the apply command take effect?
Handling Process

o better answer the preceding questions, analyze the questions according to the following topology.
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1. The MED attribute can be transmitted between IBGP peers. The route learned from an EBGP peer carries the MED attribute when being advertised to an IBGP peer. The MED value affects selection of the route to the IBGP peer. As shown in the preceding networking, when advertising a route to ASBR3, ASBR1 sets the MED value to 100; when advertising a route to ASBR4, ASBR2 changes the MED value to 200. When ASBR3 advertises the route learned from ASBR1 to ASBR4, the route carries the MED attribute. In this case, ASBR4 preferentially selects the route from ASBR3 because the MED value of this route is smaller than that learned from ASBR2.
2. When IBGP peers advertise routes to each other using a routing policy, the MED value in the apply command takes effect. When an IBGP peer advertises the learned route to an EBGP peer, the route does not carry the MED attribute. The MED value is restored to null because the MED attribute is non-transitional. As shown in the preceding networking, when advertising a route to ASBR1, SR1 sets the MED value to 100, which instructs BGP route selection. When ASBR1 advertises this route to ASBR3, the MED value is restored to null.
3. When EBGP peers advertise routes to each other, if a routing policy is applied in the outbound direction of the local end, the local-preference value in the apply command does not take effect. This is because the local-preference attribute can be transmitted only between IBGP peers. If the routing policy is applied in the inbound direction of the peer end, the local-preference value in the apply command takes effect and instructs BGP route selection. As shown in the preceding networking, when ASBR1 advertises routes to ASBR3, if the local-preference value in the apply command is 200 in a routing policy applied in the outbound direction of ASBR1, the routing policy does not take effect and the local-preference value is null in the route learned by ASBR3. If the routing policy is applied in the inbound direction of ASBR3, the local-preference value takes effect and instructs BGP route selection.
Root Cause
N/A
Solution
N/A

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