Influencing Inbound traffic with BGP....

[Adams A.]

Hi guys....

Here is a challenge that migh not be possible but we never know....

In the attached topology.... Let's say that R5 in AS 5 must go via R2 to reach AS 1 ... so R5 > R3 > R4 > R2 is the path to do it....

In normal situation , to influence inbound traffic to AS1 via R2 , you can use AS-Path Prepending on R1 to get the link between R1-R3 less prefered... You can also use MED the same way by increasing the MED on this link....

But lets say that there is a constraint not allowing you to change default BGP attributes in AS 2 ? (which means that any other BGP peers in AS2 must have the BGP attributes default values....

What would you now do to still have traffic from R5 to AS 1 going via R2 ?

Personally I havent been able to solved it yet.... A friend of mine just blessed me with this problem...

Thanks in advance.

[Georg Pauwen]

Hello,

so the only constraint is to not being allowed to change anything on R3 (AS 2), but everything else on any other router can be changed ?

[Adams A.]

No that's not the constraint that I mentionned... You can configure any router you want... BUT the BGP attributes of any other BGP peers in AS2 (they are not represented in the diagram , but imagine that there is iBGP peers connected to R3 in AS2) must be default value.....

So manipulating the AS-Path on R1 would break this requirement because from any devide in AS2, you wouldnt have extended AS-Path to any network in AS1...

[Jon Marshall]

So can the BGP attributes on R3 be changed as long as they are not changed on any other BGP peers in AS 2 ? 

Jon

[Julio E. Moisa]

Hi

Interesting, have you tried modifying the Administrative distance in order to prefer one with lowest AD than 20?

Something like: 

router bgp X

distance 19 <source IP> <wildcard, usually 0.0.0.0> <ACL ID> 

So every route included on the ACL and known through the source IP will modify its AD to 19.

:-)

[Adams A.]

Could you be a bit more specific Julio ? This is interesting....

[Julio E. Moisa]

Hi

As you know the administrative distance can be used to manipulate traffic, for example a router will prefer eBGP routes than iBGP routes ¿Why? because by the administrative distance 20 is lowest than 200. 

So you can manipulate the administrative distance for the received subnets, imagine the following scenario:

R1(AS100) -----eBGP ----- R2(AS200) ------eBGP-----R3(AS300) Loopback0 (3.3.3.3)

So R1 will receive the loopback with AD 20, Please let me share my config:

R1

interface FastEthernet0/0
ip address 10.12.0.1 255.255.255.0
duplex auto
speed auto


router bgp 100
no synchronization
bgp log-neighbor-changes
neighbor 10.12.0.2 remote-as 200
no auto-summary

R2

interface FastEthernet0/0
ip address 10.12.0.2 255.255.255.0
duplex auto
speed auto
!
interface FastEthernet0/1
ip address 10.23.0.2 255.255.255.0
duplex auto
speed auto
!
router bgp 200
no synchronization
bgp log-neighbor-changes
neighbor 10.12.0.1 remote-as 100
neighbor 10.23.0.3 remote-as 300
no auto-summary

R3

interface Loopback0
ip address 3.3.3.3 255.255.255.255

interface FastEthernet0/1
ip address 10.23.0.3 255.255.255.0

router bgp 300
no synchronization
bgp log-neighbor-changes
network 3.3.3.3 mask 255.255.255.255
neighbor 10.23.0.2 remote-as 200
no auto-summary

Verifying the routing table on R1

R1#SH IP ROUTE
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

3.0.0.0/32 is subnetted, 1 subnets
B 3.3.3.3 [20/0] via 10.12.0.2, 00:02:24
10.0.0.0/24 is subnetted, 1 subnets
C 10.12.0.0 is directly connected, FastEthernet0/0
R1#

So if I change the AD on R1 to receive the route 3.3.3.3 with AD 19

ip access-list standard OTHER-AD
permit host 3.3.3.3

router bgp 100
no synchronization
bgp log-neighbor-changes
neighbor 10.12.0.2 remote-as 200
distance 19 10.12.0.2 0.0.0.0 OTHER-AD
no auto-summary

Once implemented you can clear the route into the routing table:

clear ip route 3.3.3.3 

Then verify the routing table again:

R1#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

3.0.0.0/32 is subnetted, 1 subnets
B 3.3.3.3 [19/0] via 10.12.0.2, 00:03:15
10.0.0.0/24 is subnetted, 1 subnets
C 10.12.0.0 is directly connected, FastEthernet0/0
R1#

It can be configured on R3 to prefer a path over other (R4 over R1 how R3 is receiving the routes from both routers) based on the administrative distance. If one path is down the routes will be known through the other router and will have AD 20 (back up path).

Note: It will be applied to the routes explicitly configured into the ACL, the rest will keep the same AD. 

Hope it is useful  

:-)