Routing

r1----ebgp----r2------ibgp---------r3

if both r1 and r3 advertise 0/0 to r2 and I use policy to make  both of them have same AS path,r2 will conside 0/0 from r1 as best route because of - Interior > Exterior > Exterior via Interior

If I set med as 10 when r1 advertise 0/0 to r2,r2 will consider 0/0 from r3 as best route because by default med is 0 from r3

it seems even in IBGP,med will also be considered as a condition to select path

I thouoght only ebgp will consider this!!

See step 7 in bgp best-path selection for Junos.  By default. As long as the peer asn is the same, the path with the lowest MED is preferred, regardless of ibgp/ebgp.

http://www.juniper.net/techpubs/en_US/junos/topics/reference/general/routing-ptotocols-address-representation.html

Hi Rob,

By default, when the AS PATH is same in both the received routes (from R1 and R3), then MED is evaluated to select a route with the lowest one. This is irrespective of eBGP/iBGP route.

http://www.juniper.net/techpubs/en_US/junos12.2/topics/reference/configuration-statement/path-selection-edit-protocols-bgp.html

I am using one of the best example that is demonstrated in JNCIS:

If we receive a route from 3 different paths as below:

 Path 1—via EBGP; AS Path of 65000; MED of 200
 Path 2—via IBGP; AS Path of 65020; MED of 150; IGP cost of 5
 Path 3—via IBGP; AS Path of 65000; MED of 100; IGP cost of 10

then

Default:

[1] First Path-1 and Path-3 are grouped as both contains same AS PATH and evaluated. In this, Path-3 would be selected based on lowest MED value

[2] Next Path-3 and Path-2 would be then compared with other attributes to choose between one of the them. Based on IGP metric, Path 2 would be installed as active in routing-table

With always-compare-med option:

[1] In this case, irrespective of AS PATH, all routes coming upto MED evalution, are compared with MED value. Hence Path-3 would be installed as active since it has the lowest MED value.

With cisco-non-deterministic:

[1] Route would be selected based on the order they are received.

Regrads

Surya

Hi,Surya by default,it requires as path are the same,not only the latest ASn,right?

in benb's comment:

By default. As long as the peer asn is the same

suppose there 2 paths

path1: via EBGP; AS Path of 65000,20000; MED of 200

path2:via EBGP; AS Path of 65000,10000; MED of 100

whether it will compare med?they all come from 65000,but the whole AS is not euqal

Hi Rob,

As you see in my previous post for "Default", the second step would choose Path 2 over Path 3 though Path-3's MED value was lower.

However in your case, we have 2 AS in AS_PATH, then the left most would be considered for MED comparison.

path1: via EBGP; AS Path of 65000,20000; MED of 200

path2: via EBGP; AS Path of 65000,10000; MED of 100.

Path 2 would choosen because of lowest MED value.

I wouldn't call it as peer AS number, as the local administrator can always overrule this. Just like as below:

Prefix : 200/24

path1: via eBGP; Nbr: 172.168.1.2; Peer AS : 500; MED 10

path1: via eBGP; Nbr: 3.3.3.3; Peer AS : 200; MED 0

With above data, we can say MED wouldn't be compared, but look at below:

show bgp neighbor | match Peer:
Peer: 3.3.3.3+58155 AS 200     Local: 1.1.1.1+179 AS 100  
  NLRI for restart configured on peer: inet-unicast
  NLRI advertised by peer: inet-unicast
Peer: 172.168.1.2+63071 AS 500 Local: 172.168.1.1+179 AS 100  
  NLRI for restart configured on peer: inet-unicast
  NLRI advertised by peer: inet-unicast

inet.0: 50 destinations, 51 routes (49 active, 0 holddown, 1 hidden)
200.0.0.0/24 (2 entries, 1 announced)
        *BGP    Preference: 170/-101
                Next hop type: Indirect
                Address: 0x944d8b4
                Next-hop reference count: 3
                Source: 3.3.3.3
                Next hop type: Router, Next hop index: 605
                Next hop: 10.0.1.2 via ge-2/1/6.0, selected
                Session Id: 0x252
                Protocol next hop: 3.3.3.3
                Indirect next hop: 9504000 2097150 INH Session ID: 0x255
                State: <Active Ext>
                Local AS:   100 Peer AS:   200
                Age: 2:10       Metric2: 0
                Validation State: unverified
                Task: BGP_200.3.3.3.3+58155
                Announcement bits (3): 0-KRT 2-BGP_RT_Background 5-Resolve tree 1
                AS path: 4000 200 I
                Accepted
                Localpref: 100
                Router ID: 3.3.3.3
         BGP    Preference: 170/-101
                Next hop type: Router, Next hop index: 606
                Address: 0x944c8f8
                Next-hop reference count: 1
                Source: 172.168.1.2
                Next hop: 172.168.1.2 via ge-3/0/0.0, selected
                Session Id: 0x253
                State: <NotBest Ext>
                Inactive reason: Not Best in its group - Route Metric or MED comparison
                Local AS:   100 Peer AS:   500
                Age: 2:21       Metric: 10
                Validation State: unverified
                Task: BGP_500.172.168.1.2+63071
                AS path: 4000 500 I
                Accepted
                Localpref: 100
                Router ID: 10.255.178.58

Regards

Surya

thx,Surya

it is a very good example

yes,network admin can prepend AS to the orginal path and make the left most ASs equal.

By peer asn, I meant he leftmost asn in the as_path sequence.  As others pointed out, this could be changed by policy/config, to something other than the actual bgp neighbors peer asn number.