06-17-2009 08:18 PM - edited 03-04-2019 05:09 AM
Hi All,
I'm doing a lab and trying to get my head around BGP and metrics (MED).
In my lab I have 3 border routers and AS as shown below. I want R1 to be able to control which default route to use. R2 and R3 both send the default route to R1.
10.10.10.0/24
R1 (AS4854) --------------------- R2 (AS1221)
|
|
| 10.10.20.0/24
|
|
R3 (AS7474)
If for example I wanted to use R3 as the preferred default route, is it a matter of simply giving it a lower metric? So what I did was create a route-map and for routes received from R3, give it a metric of 100, and for routes received from R2, give it a metric of 200.
R1 config:
router bgp 4854
no synchronization
bgp log-neighbor-changes
network 172.16.1.0 mask 255.255.255.0
neighbor 10.10.10.2 remote-as 1221
neighbor 10.10.10.2 soft-reconfiguration inbound
neighbor 10.10.10.2 route-map BGP-IN-200 in
neighbor 10.10.20.2 remote-as 7474
neighbor 10.10.20.2 soft-reconfiguration inbound
neighbor 10.10.20.2 route-map BGP-IN-100 in
no auto-summary
!
route-map BGP-IN-100 permit 10
set metric 100
!
route-map BGP-IN-200 permit 10
set metric 200
I can see the metrics taking effect, but not sure why R3 (10.10.20.2) isn't the preferred default route when it has a lower metric.
R1#sh ip bgp 0.0.0.0 0.0.0.0 longer-prefixes
BGP table version is 5, local router ID is 172.16.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 0.0.0.0 10.10.10.2 200 0 1221 i
* 10.10.20.2 100 0 7474 i
*> 172.16.1.0/24 0.0.0.0 0 32768 i
*> 172.16.2.0/24 10.10.10.2 200 0 1221 i
*> 172.16.3.0/24 10.10.20.2 100 0 7474 i
R2 config:
router bgp 1221
no synchronization
bgp log-neighbor-changes
network 172.16.2.0 mask 255.255.255.0
neighbor 10.10.10.1 remote-as 4854
neighbor 10.10.10.1 default-originate
neighbor 10.10.10.1 soft-reconfiguration inbound
R3 config:
router bgp 7474
no synchronization
bgp log-neighbor-changes
network 172.16.3.0 mask 255.255.255.0
neighbor 10.10.20.1 remote-as 4854
neighbor 10.10.20.1 default-originate
neighbor 10.10.20.1 soft-reconfiguration inbound
Am I missing something here??? My understanding of metrics (MED) is probably totally wrong!
Thanks.
Andy
Solved! Go to Solution.
06-17-2009 10:03 PM
Andy,
MED is used to be compared routes from the same AS. What happens when you add a "bgp always-compare-med" command on.
IMHO: You are going to force R1 to use R3 as a preferred router for a default route. It's an outbound policy. What you can do is to change Weight for routes coming from R3.
Hopes you're gonna love this link : http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml
HTH,
Toshi
06-17-2009 10:03 PM
Andy,
MED is used to be compared routes from the same AS. What happens when you add a "bgp always-compare-med" command on.
IMHO: You are going to force R1 to use R3 as a preferred router for a default route. It's an outbound policy. What you can do is to change Weight for routes coming from R3.
Hopes you're gonna love this link : http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml
HTH,
Toshi
06-17-2009 10:15 PM
Hi Toshi,
That worked great. I applied the command "bgp always-compare-med" and cleared bgp and can see better preference given to the route with the lower metric now.
R0#sh ip bgp 0.0.0.0 0.0.0.0 longer-prefixes
BGP table version is 5, local router ID is 172.16.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
* 0.0.0.0 10.10.10.2 200 0 1221 i
*> 10.10.20.2 100 0 7474 i
*> 172.16.1.0/24 0.0.0.0 0 32768 i
*> 172.16.2.0/24 10.10.10.2 200 0 1221 i
*> 172.16.3.0/24 10.10.20.2 100 0 7474 i
Although it worked, I'll have a read of the doco you sent me. I've read the doco before but bgp doesn't really stick with me because I don't use it very often.
Many thanks.
Andy
06-17-2009 11:43 PM
Is there a way to control inbound traffic so that all traffic coming into R1 comes in from one AS (ISP) only??? Do we have control over this??
06-18-2009 12:35 AM
Andy,
I'm not sure that we can configure at the R1 to tell the other routers that you don't send me traffic.In your case R1 is connecting with 2 ASes.In case you've got the same routes from 2 ASes you can use "Weight" to prefer routes coming from AS you want. In case of you lose routes/connections from that AS R1 still send to the other AS.
F.e. R1 prefers R3
!
ip as-path access-list 1 permit _1221$
ip as-path access-list 2 permit _7474$
!
route-map WEIGHT1000 permit 10
match as-path 1
set weight 1000
!
route-map WEIGHT1000 permit 20
!
!
route-map WEIGHT5000 permit 10
match as-path 2
set weight 5000
!
route-map WEIGHT5000 permit 10
!
router bgp 4854
neighbor 10.10.10.2 route-map WEIGHT1000 in
neighbor 10.10.20.2 route-map WEIGHT5000 in
HTH,
Toshi
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