CCIE examination knowledge: BGP-MED multi-export authentication attribute
The role of MED: Differentiate multiple exits and entry points to the same neighbor autonomous system.
Before the border router advertises to the eBGP peer, the MED operation in the route:
1. If it is an eBGP route, its MED will be cleared.
2. If it is an iBGP route, its MED will be cleared. (If you need to force the MED to be announced, you can use the route mapping command set-metric-type internal.)
3. If it is injected locally, carry the MED value. Its metric:
a: If the route injected into BGP through the network or redistribute command is from the IGP, then BGP MED=IGP MED;
b: If the route injected into BGP through the network or redistribute command is from a direct route, then BGP MED=0;
c: If the route is injected through the aggregate-address command, the BGP MED is not set.
The best path is selected with MED: the path with the smallest MED value wins. By default, the comparison is performed only if the first (adjacent) AS of the two paths is the same, otherwise no comparison is made. For example, the following three paths:
1. AS_Path=100 30 5, MED=100, internal, IGP metric to NEXT_HOP = 100
2. AS_Path=200 30 5, MED=200, internal, IGP metric to NEXT_HOP = 50
3. AS_Path=100 80 5, MED=300, external
Suppose the order of receipt is 3, 2, 1 (3 is the first to receive). The order of comparison is reversed: 1 2 3; that is, it will start from the latest route.
The result of the selection is: 2 is better than 1 (MED is not compared when adjacent AS is different, 2 IGP metric is lower); 3 is better than 2 (external relationship). The best path is 3.
If bgp deterministic-med is configured, all paths will be grouped based on neighboring ASs. Within each AS group, the paths are sorted according to the size of the MED, the smallest being the best path for the group. MED is not considered when comparing the best paths for different groups.
In the same example above, 1 and 3 will be grouped together in the same group, first comparing, 1 is better than 3 (MED is low), then 1 and 2 (other groups) are compared, because different neighbors AS, MED is not compared, 2 IGP metric is lower and wins. The best path is 2.
If bgp always-compare-med is also activated, the MED is compared for all paths, regardless of whether they are from the same adjacent AS. Since deterministic-med is configured, the best paths for each group are compared after grouping in the above example (when MED is considered). So 1 is the best path (MED is low).
If bgp deterministic-med is not set, only bgp always-compare-med is configured. The difference, however, is that there is no process of grouping according to neighboring ASs, but the MED will compare them in different neighboring AS paths. So 1 is better than 2 (MED is low) and 1 is better than 3 (low MED). The best path is 1.
The MED is usually used as the eBGP peer metric between the autonomous systems, and the local preference is used as the iBGP peer metric inside the autonomous system. When a border router receives an eBGP peer route, the MBGP value is transmitted to its eBGP peer by default, but it is not passed to its eBGP peer. Because of this, the configuration of bgp always-compare-med is not very large in practical production environments."
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Before the border router advertises to the eBGP peer, the MED operation in the route:
1. If it is an eBGP route, its MED will be cleared.
2. If it is an iBGP route, its MED will be cleared. (If you need to force the MED to be announced, you can use the route mapping command set-metric-type internal.)
3. If it is injected locally, carry the MED value. Its metric:
a: If the route injected into BGP through the network or redistribute command is from the IGP, then BGP MED=IGP MED;
b: If the route injected into BGP through the network or redistribute command is from a direct route, then BGP MED=0;
c: If the route is injected through the aggregate-address command, the BGP MED is not set.
The best path is selected with MED: the path with the smallest MED value wins. By default, the comparison is performed only if the first (adjacent) AS of the two paths is the same, otherwise no comparison is made. For example, the following three paths:
1. AS_Path=100 30 5, MED=100, internal, IGP metric to NEXT_HOP = 100
2. AS_Path=200 30 5, MED=200, internal, IGP metric to NEXT_HOP = 50
3. AS_Path=100 80 5, MED=300, external
Suppose the order of receipt is 3, 2, 1 (3 is the first to receive). The order of comparison is reversed: 1 2 3; that is, it will start from the latest route.
The result of the selection is: 2 is better than 1 (MED is not compared when adjacent AS is different, 2 IGP metric is lower); 3 is better than 2 (external relationship). The best path is 3.
If bgp deterministic-med is configured, all paths will be grouped based on neighboring ASs. Within each AS group, the paths are sorted according to the size of the MED, the smallest being the best path for the group. MED is not considered when comparing the best paths for different groups.
In the same example above, 1 and 3 will be grouped together in the same group, first comparing, 1 is better than 3 (MED is low), then 1 and 2 (other groups) are compared, because different neighbors AS, MED is not compared, 2 IGP metric is lower and wins. The best path is 2.
If bgp always-compare-med is also activated, the MED is compared for all paths, regardless of whether they are from the same adjacent AS. Since deterministic-med is configured, the best paths for each group are compared after grouping in the above example (when MED is considered). So 1 is the best path (MED is low).
If bgp deterministic-med is not set, only bgp always-compare-med is configured. The difference, however, is that there is no process of grouping according to neighboring ASs, but the MED will compare them in different neighboring AS paths. So 1 is better than 2 (MED is low) and 1 is better than 3 (low MED). The best path is 1.
The MED is usually used as the eBGP peer metric between the autonomous systems, and the local preference is used as the iBGP peer metric inside the autonomous system. When a border router receives an eBGP peer route, the MBGP value is transmitted to its eBGP peer by default, but it is not passed to its eBGP peer. Because of this, the configuration of bgp always-compare-med is not very large in practical production environments."
If you want to passed the exam successfully. you can review with ccde written dumps from evedumps.com that will help passed exam at first attempt. I will also recommend the ccdp 300 320 dumps too.
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