11-02-2022 10:44 AM
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
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11-02-2022 11:46 AM - edited 11-02-2022 11:48 AM
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.
11-02-2022 10:57 AM - last edited on 11-03-2022 12:08 AM by Translator
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.
11-02-2022 11:07 AM - edited 11-02-2022 11:14 AM
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
11-02-2022 11:12 AM
""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
11-02-2022 11:34 AM - edited 11-02-2022 11:35 AM
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?
11-02-2022 11:46 AM - edited 11-02-2022 11:48 AM
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.
11-02-2022 12:06 PM
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.
11-02-2022 11:57 AM
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.
11-02-2022 12:27 PM
Great! Thanks @MHM Cisco World
Point is that MED can be passed to ebgp, but only one time. otherwise how it can work
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