BGP Route map and MED – GNS3 Lab
In this lab we will learn how to use Route map to set MED attribute to tell which path the neighbor should choose to a specific network.
Topology:
IOS used in this lab: c3640-jk9s-mz.124-16.bin
Tasks in this lab:
1) Set up basic IBGP & EBGP
2) Advertise loopback 1 & loopback 10 on R1
3) Use route-maps on R2 & R3 to set MED for only 192.168.1.0, verify on R6 and will R6 see network 192.168.10.0?
Basic Configuration – IP Assignments
| R1: interface f0/0 ip address 12.12.12.1 255.255.255.0 no shut interface f1/0 ip address 13.13.13.1 255.255.255.0 no shut interface loopback0 ip address 1.1.1.1 255.255.255.255 interface loopback1 ip address 192.168.1.1 255.255.255.0 interface loopback10 ip address 192.168.10.1 255.255.255.0 |
R4: interface f1/0 ip address 46.46.46.4 255.255.255.0 no shut interface s0/0 ip address 24.24.24.4 255.255.255.0 no shut |
| R2: interface f0/0 ip address 12.12.12.2 255.255.255.0 no shut interface s1/0 ip address 24.24.24.2 255.255.255.0 no shut interface loopback0 ip address 2.2.2.2 255.255.255.255 interface loopback1 ip address 192.168.2.1 255.255.255.0 |
R5: interface f1/0 ip address 56.56.56.5 255.255.255.0 no shut interface s0/0 ip address 35.35.35.5 255.255.255.0 no shut |
| R3: interface f0/0 ip address 13.13.13.3 255.255.255.0 no shut interface s1/0 ip address 35.35.35.3 255.255.255.0 no shut interface loopback0 ip address 3.3.3.3 255.255.255.255 interface loopback1 ip address 192.168.3.1 255.255.255.0 |
R6: interface f0/0 ip address 46.46.46.6 255.255.255.0 no shut interface f1/0 ip address 56.56.56.6 255.255.255.0 no shut |
1) Set up IBGP & EBGP connections
| R1: router bgp 123 neighbor 2.2.2.2 remote-as 123 neighbor 2.2.2.2 update-source loopback0 neighbor 3.3.3.3 remote-as 123 neighbor 3.3.3.3 update-source loopback0 exit ip route 2.2.2.2 255.255.255.255 12.12.12.2 ip route 3.3.3.3 255.255.255.255 13.13.13.3 —————————————————- Note: R1 establishes BGP neighbor relationships with R2 & R3 via their loopback interfaces but R1 does not know how to reach there (and neighbor relationship can not be established) so the most simple way is to use a static route. |
R4: router bgp 456 neighbor 24.24.24.2 remote-as 123 neighbor 46.46.46.6 remote-as 456 neighbor 46.46.46.6 next-hop-self neighbor 56.56.56.5 remote-as 456 —————————————— Note: the “next-hop-self” feature is used so that networks learned from R2 will be advertised to R6 with R5’s next-hop IP address. If not the networks learned from R2 would be marked as “inaccessible” on R6 (because R4 would advertise with R2’s IP address but R6 can not reach the next-hop 24.24.24.2) |
| R2: router bgp 123 neighbor 1.1.1.1 remote-as 123 neighbor 1.1.1.1 update-source loopback0 neighbor 3.3.3.3 remote-as 123 neighbor 3.3.3.3 update-source loopback0 neighbor 24.24.24.4 remote-as 456 exit ip route 1.1.1.1 255.255.255.255 12.12.12.1 ip route 3.3.3.3 255.255.255.255 12.12.12.1 |
R5: router bgp 456 neighbor 46.46.46.4 remote-as 456 neighbor 56.56.56.6 remote-as 456 neighbor 56.56.56.6 next-hop-self neighbor 35.35.35.3 remote-as 123 |
| R3: router bgp 123 neighbor 1.1.1.1 remote-as 123 neighbor 1.1.1.1 update-source loopback0 neighbor 2.2.2.2 remote-as 123 neighbor 2.2.2.2 update-source loopback0 neighbor 35.35.35.5 remote-as 456 exit ip route 2.2.2.2 255.255.255.255 13.13.13.1 ip route 1.1.1.1 255.255.255.255 13.13.13.1 |
R6: router bgp 456 neighbor 46.46.46.4 remote-as 456 neighbor 56.56.56.5 remote-as 456 |
Note: The use of a loopback interface to define neighbors is common with iBGP, but is not common with eBGP. Normally, you use the loopback interface to make sure that the IP address of the neighbor stays up and is independent of hardware that functions properly. In the case of eBGP, peer routers frequently have direct connection, and loopback does not apply.
2) Advertise networks 192.168.1.0 & 192.168.10.0 on R1:
On R1 we advertise two networks 192.168.1.0/24 & 192.168.10.0/24:
| R1: router bgp 123 network 192.168.1.0 mask 255.255.255.0 network 192.168.10.0 mask 255.255.255.0 ———————————————————————— In fact we don’t need the “mask 255.255.255.0” because it is the default mask value for class C IP address. |
Now check on both R2 & R3 to see both networks are advertised
Also on EBGP neighbors, these networks are also advertised (you have to wait a moment for BGP updates)
On R6 networks 192.168.1.0 & 192.168.10.0 are learned via both R4 & R5:
Note: the path via R4 (46.46.46.4) are chosen for both networks as the best path because of lower router-ID on R4)
3) Use route-maps on R2 & R3 to set MED for 192.168.1.0 to 150 & 170, respectively:
| R2: router bgp 123 neighbor 24.24.24.4 route-map MED_to_456 out exit access-list 1 permit 192.168.1.0 0.0.0.255 route-map MED_to_456 permit 10 match ip address 1 set metric 170 |
R3: router bgp 123 neighbor 35.35.35.5 route-map MED_to_456 out exit access-list 1 permit 192.168.1.0 0.0.0.255 route-map MED_to_456 permit 10 match ip address 1 set metric 150 |
Now R6 only receives updates about 192.168.1.0. The metric advertised by R3 is lower so it is chosen as the best path to 192.168.1.0 (marked with “>”).
Notice that updates for 192.168.10.0 have been filtered out because there is an implicit “deny all” statement at the end of each access-list.
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