Non-transitive MED in BGP path selection

[Leftz]

Hello, Please see below diagram. R4 send route 10.4.0.0 to R1 with MED 100. R1 can see the route with MED. MED is non transitive attribute. Why it happens? The diagram is from the link below. Thanks

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13759-37.html

[MHM Cisco World]

I am also confuse first time read about MED 
there are two metric 

one is MED 
in example below MED is 100

other is IGP metric to

next-hop 

in example below it is 74 

  192.168.30.4 (metric 74) from 3.3.3.3 (3.3.3.3)
      Origin IGP, metric 100, localpref 100, valid, internal, best

 so try change the Cost between two router you will see that IGP metric will change. 

[Leftz]

Thanks. I have the question when I review the document(see link). why R1 can see route 10.4.0.0 with MED 100, as the document discussed. MED is non transitive, why R1 can see the MED when R4 send MED to R1?

I am not sure my understanding is correct. Anyone can have some comment on this? 

Please see the below which is from the document

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13759-37.html

----------------------

R1 now sees the route over R2 as the best path for network 10.4.0.0/16 because the update received from R2 has a MED of 100 versus a MED of 200, which is what R3 advertises. Similarly, R1 uses R3 and the R3 - R4 link to access 10.5.0.0/16:

r1# show ip bgp 10.4.0.1
BGP routing table entry for 10.4.0.0/16, version 14
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x800
  Not advertised to any peer
  65502
    192.168.20.4 (metric 128) from 2.2.2.2 (2.2.2.2)
      Origin IGP, metric 100, localpref 100, valid, internal, best
r1#sh ip bgp 10.5.0.1
BGP routing table entry for 10.5.0.0/16, version 13
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x800
  Not advertised to any peer
  65502
    192.168.30.4 (metric 128) from 3.3.3.3 (3.3.3.3)
      Origin IGP, metric 100, localpref 100, valid, internal, best

[MHM Cisco World]

""When a BGP speaker learns a route from a peer, the route's MED is passed to other interior BGP (iBGP) peers, but not to exterior BGP (eBGP) peers.""

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/112965-bgpmed-attr-00.html

[Leftz]

R4's as is 65502 and R4 is exterior(eBGP) to R1, R2, and R3 whose as is 65501. why the MED can go through R2 and R3 to R1?

[MHM Cisco World · Accepted solution]

because R1,R2 & R3 same AS (65501), i.e. all router iBGP and MED is forward between router in same AS.

but if we connect another router to R1 and with different AS then the MED will not forward to new router because of MED non transitive.

for you to understand MED, the MED can pass only one AS no more.

[MHM Cisco World]

small Lab 
R1 have LO 1.1.1.1/32 and I use route-map to make MED = 1000
in R3 we see the MED is 1000 because R2 and R3 in same AS so the R2 forward MED value to R3 

Now the Lo is forward to R4 different AS but you see MED is empty becuase the R3 not forward MED value out it AS.

[rais]

Per RFC-4271:

the MULTI_EXIT_DISC attribute MAY be propagated over IBGP to
   other BGP speakers within the same AS (see also 9.1.2.2).  The
   MULTI_EXIT_DISC attribute received from a neighboring AS MUST NOT be
   propagated to other neighboring ASes.

[Leftz]

Great! Thanks @MHM Cisco World 

Point is that MED can be passed to ebgp, but only one time. otherwise how it can work