BGP Path Manipulation: Bob is at it again...
Summer was in full swing, and it was over 105 degrees Fahrenheit outside. Bob was told it was a "dry heat", but he thought "so is my oven". Needless to say, Bob was glad to be in the data center, where the temperature and humidity controls kept it very cold. He had been asked to setup up a basic route-map with BGP, and here is the diagram he worked from.
The goal, was to modify BGP, so that all traffic going towards the 1.1.1.0 network (which is sourced from AS1), traveling either from or through AS23, would only use the 13.0.0.0/24 segment (between R3 and R1), and not use the 10.0.0.0/24 segment (between R2 and R1) as a transit path.
Bob reviewed some of the BGP topics he had recently learned. Here is the list he made of possibilities:
R1 could pre-pend to the AS path for advertisements of the 1.1.1.0/24 prefix when it is sent to R2 from R1. This way, AS23 would see a better path through R3 rather than R2. He tried this using the following on R1:
ip prefix-list JUST-1.1.1.0 seq 5 permit 1.1.1.0/24route-map PRE-PEND permit 10
match ip address prefix-list JUST-1.1.1.0
set as-path prepend 1
route-map PRE-PEND permit 20router bgp 1
neighbor 10.0.0.2 route-map PRE-PEND out
Bob cleared the BGP session, just to be sure. Unfortunately, some traffic destined to 1.1.1.0 was still flowing over the 10.0.0.0 network between R2 and R1.
Bob decided to try another approach, and instead of R1 trying to make AS23 think the path on 10.0.0.0 was worse, perhaps he would tell R3 to make the path on 13.0.0.0 look better. He considered weight, but then realized that would only work for R3, and not every other device in AS23. So Bob decided to use local-preference. On R3, he tried using local-preference, to specify that when a BGP update came in from R1 for 1.1.1.0, R3 would set the local-preference to 250 for that prefix, in hopes that this would allow traffic destined for 1.1.1.0 go exclusively through the 13.0.0.0 segment between R3 and R1. Unfortunately, even with this change, Bob noticed that traffic destined to 1.1.1.0 from our through AS23 still crossed on the link between R2 and R1.
Below are the configurations for R1, R2 and R3 along with the relevant show commands.
Can you assist Bob? What can he do? What did he do wrong, if anything?
Post your ideas and comments below!
R1:
version 12.4
hostname R1
interface Loopback0
ip address 1.1.1.1 255.255.255.0
ip ospf network point-to-pointinterface FastEthernet0/0
ip address 10.0.0.1 255.255.255.0
ip ospf 1 area 1interface FastEthernet1/0
ip address 13.0.0.1 255.255.255.0
ip ospf 1 area 1router bgp 1
no synchronization
bgp log-neighbor-changes
network 1.1.1.0 mask 255.255.255.0
neighbor 10.0.0.2 remote-as 23
neighbor 10.0.0.2 route-map PRE-PEND out
neighbor 13.0.0.3 remote-as 23
no auto-summaryip prefix-list JUST-1.1.1.0 seq 5 permit 1.1.1.0/24
route-map PRE-PEND permit 10
match ip address prefix-list JUST-1.1.1.0
set as-path prepend 1route-map PRE-PEND permit 20
R2:
version 12.4
hostname R2
interface FastEthernet0/0
ip address 10.0.0.2 255.255.255.0
ip ospf 1 area 1interface FastEthernet0/1
ip address 23.0.0.2 255.255.255.0
ip ospf 1 area 1router bgp 23
no synchronization
bgp log-neighbor-changes
neighbor 10.0.0.1 remote-as 1
neighbor 23.0.0.3 remote-as 23
no auto-summary
!
R3:
version 12.4
hostname R3
interface FastEthernet0/0
ip address 13.0.0.3 255.255.255.0
ip ospf 1 area 1interface FastEthernet0/1
ip address 23.0.0.3 255.255.255.0
ip ospf 1 area 1router bgp 23
no synchronization
bgp log-neighbor-changes
neighbor 13.0.0.1 remote-as 1
neighbor 13.0.0.1 route-map SET-LOCAL-PREF in
neighbor 23.0.0.2 remote-as 23
no auto-summaryip prefix-list LOCAL-PREF-250 seq 5 permit 1.1.1.0/24
route-map SET-LOCAL-PREF permit 10
match ip address prefix-list LOCAL-PREF-250
set local-preference 250route-map SET-LOCAL-PREF permit 20
Show commands R1:
R1#show ip bgp summary
BGP router identifier 1.1.1.1, local AS number 1
BGP table version is 2, main routing table version 2
1 network entries using 120 bytes of memory
1 path entries using 52 bytes of memory
2/1 BGP path/bestpath attribute entries using 248 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
Bitfield cache entries: current 1 (at peak 2) using 32 bytes of memory
BGP using 452 total bytes of memory
BGP activity 2/1 prefixes, 2/1 paths, scan interval 60 secsNeighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
10.0.0.2 4 23 77 73 2 0 0 00:29:01 0
13.0.0.3 4 23 74 74 2 0 0 00:29:01 0R1#show ip bgp
BGP table version is 2, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incompleteNetwork Next Hop Metric LocPrf Weight Path
*> 1.1.1.0/24 0.0.0.0 0 32768 iR1#show ip route | begin resort
Gateway of last resort is not set1.0.0.0/24 is subnetted, 1 subnets
C 1.1.1.0 is directly connected, Loopback0
23.0.0.0/24 is subnetted, 1 subnets
O 23.0.0.0 [110/2] via 13.0.0.3, 00:48:43, FastEthernet1/0
[110/2] via 10.0.0.2, 00:48:09, FastEthernet0/0
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.0.0 is directly connected, FastEthernet0/0
13.0.0.0/24 is subnetted, 1 subnets
C 13.0.0.0 is directly connected, FastEthernet1/0
Show commands R2:
R2#show ip bgp summary
BGP router identifier 2.2.2.2, local AS number 23
BGP table version is 14, main routing table version 14
1 network entries using 120 bytes of memory
2 path entries using 104 bytes of memory
3/1 BGP path/bestpath attribute entries using 372 bytes of memory
2 BGP AS-PATH entries using 48 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
Bitfield cache entries: current 1 (at peak 2) using 32 bytes of memory
BGP using 676 total bytes of memory
BGP activity 1/0 prefixes, 4/2 paths, scan interval 60 secsNeighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
10.0.0.1 4 1 73 77 14 0 0 00:29:07 1
23.0.0.3 4 23 71 73 14 0 0 01:04:54 1R2#show ip bgp
BGP table version is 14, local router ID is 2.2.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incompleteNetwork Next Hop Metric LocPrf Weight Path
*>i1.1.1.0/24 13.0.0.1 0 250 0 1 i
* 10.0.0.1 0 0 1 1 iR2#show ip route | begin resort
Gateway of last resort is not set1.0.0.0/24 is subnetted, 1 subnets
B 1.1.1.0 [200/0] via 13.0.0.1, 00:28:37
2.0.0.0/24 is subnetted, 1 subnets
C 2.2.2.0 is directly connected, Loopback0
23.0.0.0/24 is subnetted, 1 subnets
C 23.0.0.0 is directly connected, FastEthernet0/1
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.0.0 is directly connected, FastEthernet0/0
13.0.0.0/24 is subnetted, 1 subnets
O 13.0.0.0 [110/2] via 23.0.0.3, 00:48:16, FastEthernet0/1
[110/2] via 10.0.0.1, 00:49:19, FastEthernet0/0
Show commands R3:
R3#show ip bgp summary
BGP router identifier 3.3.3.3, local AS number 23
BGP table version is 6, main routing table version 6
1 network entries using 120 bytes of memory
1 path entries using 52 bytes of memory
3/1 BGP path/bestpath attribute entries using 372 bytes of memory
1 BGP AS-PATH entries using 24 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
Bitfield cache entries: current 1 (at peak 2) using 32 bytes of memory
BGP using 600 total bytes of memory
BGP activity 1/0 prefixes, 5/4 paths, scan interval 60 secsNeighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
13.0.0.1 4 1 74 74 6 0 0 00:29:09 1
23.0.0.2 4 23 73 71 6 0 0 01:04:56 0R3#show ip bgp
BGP table version is 6, local router ID is 3.3.3.3
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incompleteNetwork Next Hop Metric LocPrf Weight Path
*> 1.1.1.0/24 13.0.0.1 0 250 0 1 iR3#show ip route | begin resort
Gateway of last resort is not set1.0.0.0/24 is subnetted, 1 subnets
B 1.1.1.0 [20/0] via 13.0.0.1, 00:28:39
3.0.0.0/24 is subnetted, 1 subnets
C 3.3.3.0 is directly connected, Loopback0
23.0.0.0/24 is subnetted, 1 subnets
C 23.0.0.0 is directly connected, FastEthernet0/1
10.0.0.0/24 is subnetted, 1 subnets
O 10.0.0.0 [110/2] via 23.0.0.2, 00:48:18, FastEthernet0/1
[110/2] via 13.0.0.1, 00:48:48, FastEthernet0/0
13.0.0.0/24 is subnetted, 1 subnets
C 13.0.0.0 is directly connected, FastEthernet0/0
Best wishes.
And the answer is:
Thanks to you, and your 50+ posts, bob got his answer. By reading your responses, Bob learned the following:
For R2, the BGP next hop for the best route, is still 13.0.0.1, even though it is learned from R3. R3 doesn't bother to change the next-hop attribute when learning routes via a eBGP neighbor (R1). With R2 having 2 equal cost paths (OSPF) for the next hop of 13.0.0.1, R2 would load balance the traffic over the 10.0.0.0 and 23.0.0.0 networks for traffic going to 1.1.1.0/24
One solution would be to have R3 use next-hop-self for updates sent to R2. Then R2 would see the next hop as 23.0.0.3, and all the traffic would be forwarded to R3 as desired.
The update on R3 would look like this:
router bgp 23
neighbor 23.0.0.2 next-hop-self
This would cause R2, to have the BGP table of this:
R2#show ip bgp
BGP table version is 4, local router ID is 2.2.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incompleteNetwork Next Hop Metric LocPrf Weight Path
*>i1.1.1.0/24 23.0.0.3 0 250 0 1 i
* 10.0.0.1 0 0 1 1 i
Another option would be increasing the OSPF cost on R2's 10.0.0.0/24 interface, so that it wouldn't be considered an equal cost to get to 13.0.0.1 (the previous next hop before the change we just made).
Thanks everyone for all your assistance! You rock.
