This is used to influence return traffic from our
neighbouring AS. Lower value is better. Default is 0
RULES OF MED
1) MED is not propagated outside the receiving AS
2) MED attribute would never be used if the ASes being
compared are from different AS except we configure bgp always compare med
3) BGP best-path med missing –med-worst command is used to
represent a missing med
4) BGP best path med confederation allows us to use MED
within a confederation
5) BGP determistic med command would compare med before it
compares origin code in route-selection
Default-configs are
R1.....................
int s0/1
ip add 20.20.20.1 255.255.255.0
no shut
int s0/0
ip add 10.10.10.1 255.255.255.0
no shut
int lo 1
ip add 150.150.150.1 255.255.255.0
no shut
int lo 2
ip add 150.150.151.1 255.255.255.0
no shut
exit
router eigrp 1
network 20.20.20.0 0.0.0.255
network 10.10.10.0 0.0.0.255
no auto-summary
exit
router bgp 100
nei 10.10.10.2 remote-as 222
nei 20.20.20.2 remote-as 100
nei 20.20.20.2 next-hop-selfib
nei 30.30.30.1 remote-as 100
nei 30.30.30.1 next-hop-self
bgp router-id 150.150.150.1
no synchronization
network 150.150.150.0 mask 255.255.255.0
network 150.150.151.0 mask 255.255.255.0
R2..................
int s0/0
ip add 30.30.30.2 255.255.255.0
no shut
int s0/1
ip add 20.20.20.2 255.255.255.0
no shut
int s0/2
ip add 50.50.50.2 255.255.255.0
no shut
int lo 1
ip add 77.77.77.1 255.255.255.0
no shut
exit
router eigrp 1
network 20.20.20.0 0.0.0.255
network 30.30.30.0 0.0.0.255
network 50.50.50.0 0.0.0.255
no auto-summary
exit
router bgp 100
nei 30.30.30.1 remote-as 100
nei 30.30.30.1 next-hop-self
nei 20.20.20.1 remote-as 100
nei 20.20.20.1 next-hop-self
nei 50.50.50.1 remote-as 111
bgp router-id 77.77.77.1
no synchronization
network 77.77.77.0 mask 255.255.255.0
R3..........................
int s0/0
ip add 40.40.40.1 255.255.255.0
no shut
int s0/1
ip add 30.30.30.1 255.255.255.0
no shut
int lo 1
ip add 99.99.99.1 255.255.255.0
no shut
exit
router eigrp 1
network 30.30.30.0 0.0.0.255
network 40.40.40.0 0.0.0.255
no auto-summary
router bgp 100
nei 40.40.40.2 remote-as 222
nei 30.30.30.2 remote-as 100
nei 30.30.30.2 next-hop-self
nei 20.20.20.1 remote-as 100
nei 20.20.20.1 next-hop-self
bgp router-id 99.99.99.1
no synchronization
network 99.99.99.0 mask 255.255.255.0
ISP1..................................
int s0/0
ip add 50.50.50.1 255.255.255.0
no shut
int lo 1
ip add 200.200.200.1 255.255.255.0
no shut
exit
router bgp 111
nei 50.50.50.2 remote-as 100
bgp router-id 200.200.200.1
no synchronization
network 200.200.200.0 mask 255.255.255.0
ISP2..................
int s0/1
ip add 40.40.40.2 255.255.255.0
no shut
int
s0/0
ip add 10.10.10.2 255.255.255.0
no shut
int lo 1
ip add 100.100.100.1 255.255.255.0
no shut
int lo 2
ip add 100.100.101.1 255.255.255.0
no shut
int lo 3
ip add 100.100.102.1 255.255.255.0
no shut
int lo 4
ip add 100.100.103.1 255.255.255.0
no shut
exit
router bgp 222
nei 10.10.10.1 remote-as 100
nei 40.40.40.1 remote-as 100
bgp router-id 100.100.100.1
no synchronization
network 100.100.100.0 mask 255.255.255.0
network 100.100.101.0 mask 255.255.255.0
network 100.100.102.0 mask 255.255.255.0
network 100.100.103.0 mask 255.255.255.0
Task 1) Ensure that ISP2 receives the traffic towards
100.100.101.0/24 from R1, and the traffic towards 100.100.100.0/24 from R3. Use
MED to accomplish the goal.
On R1, bgp table is
Network Next
Hop Metric LocPrf Weight
Path
* i100.100.100.0/24 30.30.30.1 0 100 0
222 i
*> 10.10.10.2 0 0
222 i
* i100.100.101.0/24 30.30.30.1 0 100 0
222 i
*> 10.10.10.2 0 0
222 i
* i100.100.102.0/24 30.30.30.1 0 100 0
222 i
*> 10.10.10.2 0 0
222 i
* i100.100.103.0/24 30.30.30.1 0 100 0
222 i
*> 10.10.10.2
0 0
222 i
On R3, bgp table is
Network Next
Hop Metric LocPrf Weight Path
* i100.100.100.0/24 20.20.20.1 0 100 0
222 i
*> 40.40.40.2 0 0
222 i
* i100.100.101.0/24 20.20.20.1 0 100 0
222 i
*> 40.40.40.2 0 0
222 i
* i100.100.102.0/24 20.20.20.1 0 100 0
222 i
*> 40.40.40.2 0 0
222 i
* i100.100.103.0/24 20.20.20.1 0 100 0
222 i
*> 40.40.40.2 0 0
222 i
Now, to influence AS 100 to take 10.10.10.2 for
100.100.100.0 and to 40.40.40.2 for
100.100.101.0
ISP2(conf) ip
prefix-list o seq 5 permit 100.100.101.0/24
ip
prefix-list p seq 5 permit 100.100.100.0/24
route-map
o permit 10
match
ip address prefix-list o
set metric 300
exit
route-map o permit
20
exit
route-map p permit
10
match ip address prefix-list p
set metric 200
exit
route-map p permit
20
exit
router bgp 222
neighbor 10.10.10.1 route-map o out
neighbor 40.40.40.1 route-map p out
neighbor 40.40.40.1 route-map p out
Now lets check R1 bgp table
Network Next
Hop Metric LocPrf
Weight Path
*> 100.100.100.0/24 10.10.10.2 0 0
222 i
*>i100.100.101.0/24 30.30.30.1 0 100 0
222 i
* 10.10.10.2 300 0
222 i
and r3’s bgp table
Network Next
Hop Metric LocPrf
Weight Path
*>i100.100.100.0/24 20.20.20.1 0 100 0
222 i
* 40.40.40.2 200 0
222 i
*> 100.100.101.0/24 40.40.40.2 0 0
222 i
Hence we see that AS 100 chooses R1 to get to 100.100.100.0 ntwk and R2
to get to 100.100.101.0 ntwk
Task 2) cfg R1 such that routes without MED configured are set to zero
R1(conf)# router bgp
100
bgp bestpath med missing-as-worst
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