BGP - deterministic-med and always-compare-med

The inspiration for this blog was directly from here:

How the bgp deterministic-med Command Differs from the bgp always-compare-med Command

In order to get the various routes to look right in the bgp table, it took some work. Here is a picture that helps explain it. I'm not gonna put addressing on it. If you want configs, let me know.


Our focus is on R1, it has 3 bgp entries to 3.3.3.0:

R1#show ip bgp 3.3.3.0
BGP routing table entry for 3.3.3.0/24, version 24
Paths: (3 available, best #3, table Default-IP-Routing-Table)
Advertised to non peer-group peers:
172.12.12.2 172.12.14.4
400
172.12.12.2 from 172.12.12.2 (2.2.2.2)
Origin IGP, metric 100, localpref 100, valid, internal
400
172.12.14.4 from 172.12.14.4 (4.0.3.4)
Origin IGP, metric 150, localpref 100, valid, external
65003
172.12.13.3 from 172.12.13.3 (3.3.3.3)
Origin IGP, metric 200, localpref 100, valid, external, best
R1#

Tiebreaker:
1. entry1 and entry2 are compared, entry2 is picked because external > internal
2. entry2 and entry3 are compared, entry 3 picked because RID 3.3.3.3

Now let's configre always-compare-med:

R1(config)#router bgp 65000
R1(config-router)#bgp always-compare-med

This command should allow entry1 to be picked over entry2 (lower MED), then entry1 will be preferred over entry3 (also lower MED):

R1#show ip bgp 3.3.3.0
BGP routing table entry for 3.3.3.0/24, version 41
Paths: (3 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
Advertised to non peer-group peers:
172.12.13.3 172.12.14.4
400
172.12.12.2 from 172.12.12.2 (2.2.2.2)
Origin IGP, metric 100, localpref 100, valid, internal, best
400
172.12.14.4 from 172.12.14.4 (4.0.3.4)
Origin IGP, metric 150, localpref 100, valid, external
65003
172.12.13.3 from 172.12.13.3 (3.3.3.3)
Origin IGP, metric 200, localpref 100, valid, external

It works!

Notice that entries are compared in pairs. To get the pairs reordered you may have shut peers down and enable them accordingly. Example: I wanted the peers to appear in this order 4.0.3.4, 3.3.3.3, and 2.2.2.2. So I brought them up in reverse order: 2.2.2.2, 3.3.3.3, and finally 4.0.3.4. I just did a shut/no shut on the interface. Now I have:

R1#show ip bgp 3.3.3.0
BGP routing table entry for 3.3.3.0/24, version 47
Paths: (3 available, best #3, table Default-IP-Routing-Table)
Advertised to non peer-group peers:
172.12.13.3 172.12.14.4
400
172.12.14.4 from 172.12.14.4 (4.0.3.4)
Origin IGP, metric 150, localpref 100, valid, external
65003
172.12.13.3 from 172.12.13.3 (3.3.3.3)
Origin IGP, metric 200, localpref 100, valid, external
400
172.12.12.2 from 172.12.12.2 (2.2.2.2)
Origin IGP, metric 100, localpref 100, valid, internal, best

Notice the best route is still from 2.2.2.2 because always-compare-med is enabled. Let's try bgp deterministic-med, without always compare-med. First reset bgp, then continue.

R1(config)#router bgp 65000
R1(config-router)#no bgp always-compare-med
R1(config-router)#bgp deterministic-med

In this case entry2 should be compared to entry3 with entry 2 winning based on lower MED (they are in the same AS so MED is compared). Then entry2 is compared to entry1, with entry1 winning because external bgp is preferred over internal. MED is not compared between these entries.

R1#show ip bgp 3.3.3.0
BGP routing table entry for 3.3.3.0/24, version 11
Paths: (3 available, best #1, table Default-IP-Routing-Table)
Advertised to non peer-group peers:
172.12.12.2 172.12.14.4
65003
172.12.13.3 from 172.12.13.3 (3.3.3.3)
Origin IGP, metric 200, localpref 100, valid, external, best
400
172.12.12.2 from 172.12.12.2 (2.2.2.2)
Origin IGP, metric 100, localpref 100, valid, internal
400
172.12.14.4 from 172.12.14.4 (4.0.3.4)
Origin IGP, metric 150, localpref 100, valid, external

Notice in the above example that the entries are ordered in groups based on AS. I brough up 4.4.4.4 last, but it is showing up last with the other entry from AS400.

The last example uses both bgp deterministic-med and bgp always-compare-med. In this case, entry2 should win with the lowest MED. This is the same as the last example except MED is used for comparison between entry1 and entry2.

R1#show ip bgp 3.3.3.0
BGP routing table entry for 3.3.3.0/24, version 12
Paths: (3 available, best #2, table Default-IP-Routing-Table)
Advertised to non peer-group peers:
172.12.13.3 172.12.14.4
65003
172.12.13.3 from 172.12.13.3 (3.3.3.3)
Origin IGP, metric 200, localpref 100, valid, external
400
172.12.12.2 from 172.12.12.2 (2.2.2.2)
Origin IGP, metric 100, localpref 100, valid, internal, best
400
172.12.14.4 from 172.12.14.4 (4.0.3.4)
Origin IGP, metric 150, localpref 100, valid, external

Sweet!