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BGP Example 1

Posted by MSSK , 04 August 2012 · 1,219 views

BGP Example 1 Elements

Our network above consists of 3 Autonomous systems; R1 R2 and R3 are in AS #123, R4 in AS #4 and R5 in AS #5
We will advertise the same two networks toward AS #123 and we will trace the routes as they enter AS #123

Configuration

R1#sh run int f0/0
interface FastEthernet0/0
ip address 10.1.12.1 255.255.255.0
speed 100
full-duplex

R1#sh run int f0/1
interface FastEthernet0/1
ip address 10.1.13.1 255.255.255.0
speed 100
full-duplex

R1#sh run int lo0
interface Loopback0
ip address 1.1.1.1 255.255.255.255

R1#sh run | sec router ospf
router ospf 1
router-id 1.1.1.1
log-adjacency-changes
network 1.1.1.1 0.0.0.0 area 0
network 10.1.12.1 0.0.0.0 area 0
network 10.1.13.1 0.0.0.0 area 0

R1#sh run | sec router bgp
router bgp 123
no synchronization
bgp log-neighbor-changes
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
no auto-summary

R2#sh run int f0/0
interface FastEthernet0/0
ip address 10.1.12.2 255.255.255.0
speed 100
full-duplex

R2#sh run int s0/0
interface Serial0/0
ip address 192.168.24.2 255.255.255.0
clock rate 64000

R2#sh run int lo0
interface Loopback0
ip address 2.2.2.2 255.255.255.255

R2#sh run | sec router ospf
router ospf 1
router-id 2.2.2.2
log-adjacency-changes
network 2.2.2.2 0.0.0.0 area 0
network 10.1.12.2 0.0.0.0 area 0

R2#sh run | sec router bgp
router bgp 123
no synchronization
bgp log-neighbor-changes
neighbor 1.1.1.1 remote-as 123
neighbor 1.1.1.1 update-source Loopback0
neighbor 192.168.24.4 remote-as 4
no auto-summary

R3#sh run int f0/0
interface FastEthernet0/0
ip address 10.1.13.3 255.255.255.0
speed 100
full-duplex

R3#sh run int s0/0
interface Serial0/0
ip address 192.168.35.3 255.255.255.0
clock rate 64000

R3#sh run int lo0
interface Loopback0
ip address 3.3.3.3 255.255.255.255

R3#sh run | sec router ospf
router ospf 1
router-id 3.3.3.3
log-adjacency-changes
network 3.3.3.3 0.0.0.0 area 0
network 10.1.13.3 0.0.0.0 area 0

R3#sh run | sec router bgp
router bgp 123
no synchronization
bgp log-neighbor-changes
neighbor 1.1.1.1 remote-as 123
neighbor 1.1.1.1 update-source Loopback0
neighbor 192.168.35.5 remote-as 5
no auto-summary

R4#sh run int s0/0
interface Serial0/0
ip address 192.168.24.4 255.255.255.0
clock rate 64000

R4#sh run int lo1
interface Loopback1
ip address 197.10.1.4 255.255.255.0

R4#sh run int lo2
interface Loopback2
ip address 197.10.2.4 255.255.255.0

R4#sh run | sec router bgp
router bgp 4
no synchronization
bgp log-neighbor-changes
network 197.10.1.0
network 197.10.2.0
neighbor 192.168.24.2 remote-as 123
no auto-summary

R5#sh run int s0/0
interface Serial0/0
ip address 192.168.35.5 255.255.255.0
clock rate 64000

R5#sh run int lo1
interface Loopback1
ip address 197.10.1.5 255.255.255.0

R5#sh run int lo2
interface Loopback2
ip address 197.10.2.5 255.255.255.0

R5#sh run | sec router bgp
router bgp 5
no synchronization
bgp log-neighbor-changes
network 197.10.1.0
network 197.10.2.0
neighbor 192.168.35.3 remote-as 123
no auto-summary

Verification

Let us check the status of the BGP peering (it will show on the console via Syslog messages)

R1#sh ip bgp summary
BGP router identifier 1.1.1.1, local AS number 123
BGP table version is 1, main routing table version 1
2 network entries using 234 bytes of memory
4 path entries using 208 bytes of memory
3/0 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
BGP using 862 total bytes of memory
BGP activity 2/0 prefixes, 4/0 paths, scan interval 60 secs

Neighbor     V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
2.2.2.2 4   123   32   30     1    0    0 00:27:43     2
3.3.3.3 4   123   31   29     1    0    0 00:25:31     2

R2#sh ip bgp summary
BGP router identifier 2.2.2.2, local AS number 123
BGP table version is 3, main routing table version 3
2 network entries using 234 bytes of memory
2 path entries using 104 bytes of memory
2/1 BGP path/bestpath attribute entries using 248 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
BGP using 610 total bytes of memory
BGP activity 2/0 prefixes, 2/0 paths, scan interval 60 secs

Neighbor     V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
1.1.1.1 4   123   30   32     3    0    0 00:27:36     0
192.168.24.4    4 4   29   27     3    0    0 00:23:39     2

R3#sh ip bgp summary
BGP router identifier 3.3.3.3, local AS number 123
BGP table version is 3, main routing table version 3
2 network entries using 234 bytes of memory
2 path entries using 104 bytes of memory
2/1 BGP path/bestpath attribute entries using 248 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
BGP using 610 total bytes of memory
BGP activity 2/0 prefixes, 2/0 paths, scan interval 60 secs

Neighbor     V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
1.1.1.1 4   123   30   32     3    0    0 00:26:14     0
192.168.35.5    4 5   28   26     3    0    0 00:22:43     2

R4#sh ip bgp summary
BGP router identifier 192.168.24.2, local AS number 4
BGP table version is 3, main routing table version 3
2 network entries using 234 bytes of memory
2 path entries using 104 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
BGP using 586 total bytes of memory
BGP activity 2/0 prefixes, 2/0 paths, scan interval 60 secs

Neighbor     V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
192.168.24.2    4   123   28   30     3    0    0 00:24:37     0

R5#sh ip bgp summary
BGP router identifier 192.168.35.5, local AS number 5
BGP table version is 3, main routing table version 3
2 network entries using 234 bytes of memory
2 path entries using 104 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
BGP using 586 total bytes of memory
BGP activity 2/0 prefixes, 2/0 paths, scan interval 60 secs

Neighbor     V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
192.168.35.3    4   123   26   28     3    0    0 00:22:59     0

Now , let us check the routes on R2 and R3 (BGP table)

R2#sh ip bgp
BGP table version is 3, 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, ? - incomplete

   Network   Next Hop     Metric LocPrf Weight Path
*> 197.10.1.0    192.168.24.4 0 0 4 i
*> 197.10.2.0    192.168.24.4 0 0 4 i
R4 receives two routes from R4 (next-hop 192.168.24.4 refers to R4) , * means valid route and > means best route which means it will be installed in the routing table

R2#sh ip route bgp
B    197.10.1.0/24 [20/0] via 192.168.24.4, 00:24:08
B    197.10.2.0/24 [20/0] via 192.168.24.4, 00:23:38

And the same on R3

R3#sh ip bgp
BGP table version is 3, 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, ? - incomplete

   Network   Next Hop     Metric LocPrf Weight Path
*> 197.10.1.0       192.168.35.5 0 0 5 i
*> 197.10.2.0    192.168.35.5 0 0 5 i

R3#sh ip route bgp
B    197.10.1.0/24 [20/0] via 192.168.35.5, 00:23:56
B    197.10.2.0/24 [20/0] via 192.168.35.5, 00:23:25

We can check also from the advertising router, check on R4

R4#sh ip bgp neighbors 192.168.24.2 advertised-routes
BGP table version is 3, local router ID is 192.168.24.2
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
*> 197.10.1.0    0.0.0.0   0 32768 i
*> 197.10.2.0    0.0.0.0       0 32768 i

Total number of prefixes 2

Next Hop of 0.0.0.0 means these routes are locally advertised

Let us now check with R1
R1#sh ip bgp
BGP table version is 1, 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, ? - incomplete

   Network   Next Hop     Metric LocPrf Weight Path
* i197.10.1.0    192.168.35.5 0    100   0 5 i
* i 192.168.24.4 0    100   0 4 i
* i197.10.2.0    192.168.35.5 0    100   0 5 i
* i 192.168.24.4 0    100   0 4 i

We can see that no one of the routes regardless of the source R2 or R3 will be installed in routing table (not best) , why ?
If we checked the next-hop of the routes, they are 192.168.35.5 and 192.168.24.4, R1 does not anything about those subnets! So it will not install a route from unknown source!
So we have to do something about that, we have to use the next-hop-self command on R2 toward R1 (on R3 as well) in order for it to use R2 as a next-hop to reach BGP routes

R2(config)#router bgp 123
R2(config-router)#neighbor 1.1.1.1 next-hop-self

This command as R2 is telling R1 to use him as a next hop

R3(config)#router bgp 123
R3(config-router)#neighbor 1.1.1.1 next-hop-self

R1#sh ip bgp
BGP table version is 9, 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, ? - incomplete

   Network   Next Hop     Metric LocPrf Weight Path
* i197.10.1.0    2.2.2.2   0    100   0 4 i
*>i 3.3.3.3 0    100   0 5 i
* i197.10.2.0    2.2.2.2   0    100   0 4 i
*>i 3.3.3.3   0    100   0 5 i

Now we can see that the next hop changed and there are best routes (sure one route will be best, i.e. installed in the routing table)

Now the question arises why the routes from R3 are preferred?
We have to go through the path selection criteria, we will stop at Lowest IGP metric — this criterion prefers the path with the lowest IGP metric to the BGP next hop
That means we have to check on R1 for the metrics for the next hops: 2.2.2.2 and 3.3.3.3

R1#sh ip route 3.3.3.3
Routing entry for 3.3.3.3/32
  Known via "ospf 1", distance 110, metric 2, type intra area
  Last update from 10.1.13.3 on FastEthernet0/1, 00:42:47 ago
  Routing Descriptor Blocks:
  * 10.1.13.3, from 3.3.3.3, 00:42:47 ago, via FastEthernet0/1
  Route metric is 2, traffic share count is 1

R1#sh ip route 2.2.2.2
Routing entry for 2.2.2.2/32
  Known via "ospf 1", distance 110, metric 11, type intra area
  Last update from 10.1.12.2 on FastEthernet0/0, 00:42:51 ago
  Routing Descriptor Blocks:
  * 10.1.12.2, from 2.2.2.2, 00:42:51 ago, via FastEthernet0/0
  Route metric is 11, traffic share count is 1

Lowest metric will win, i.e. 3.3.3.3 and that what happened

Now, let us check from R1 if it advertises any routes to his neighbors: R2 and R3

R1#sh ip bgp neighbors 2.2.2.2 advertised-routes

Total number of prefixes 0
R1#sh ip bgp neighbors 3.3.3.3 advertised-routes

Total number of prefixes 0

Nothing, why? Split-horizon rule in BGP states that a route received from IBGP peer will not be advertised to another IBGP peer
The solution for this issue is either we use full mesh IBGP topology which is not scalable or we implement the route reflector concept, i.e. R1 will act as a route reflector for R2 and R3 (Its IBGP peers) which will disable split horizon

R1(config)#router bgp 123
R1(config-router)#neighbor 2.2.2.2 route-reflector-client
*Mar  1 00:56:34.667: %BGP-5-ADJCHANGE: neighbor 2.2.2.2 Down RR client config change
*Mar  1 00:56:36.683: %BGP-5-ADJCHANGE: neighbor 2.2.2.2 Up
R1(config-router)#neighbor 3.3.3.3 route-reflector-client
*Mar  1 00:56:39.795: %BGP-5-ADJCHANGE: neighbor 3.3.3.3 Down RR client config change
*Mar  1 00:56:41.911: %BGP-5-ADJCHANGE: neighbor 3.3.3.3 Up

As we can see there is no need to clear

R2#sh ip bgp
BGP table version is 3, 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, ? - incomplete

   Network   Next Hop     Metric LocPrf Weight Path
* i197.10.1.0    3.3.3.3   0    100   0 5 i
*> 192.168.24.4 0 0 4 i
* i197.10.2.0    3.3.3.3   0    100   0 5 i
*>   192.168.24.4 0 0 4 i

Now R2 see the routes from two next hops, the original one via 192.168.24.4 and the new one from R3, of course the routes from 192.168.24.4 will be preferred because they are coming from EBGP neighbor and the routes from 3.3.3.3 are IBGP neighbors

R1#sh ip bgp neighbors 2.2.2.2 advertised-routes
BGP table version is 13, 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, ? - incomplete

   Network   Next Hop     Metric LocPrf Weight Path
*>i197.10.1.0    3.3.3.3   0    100   0 5 i
*>i197.10.2.0    3.3.3.3   0    100   0 5 i

Total number of prefixes 2

As we can see from outputs that local preference value from IBGP peer is 100, weight value for locally advertised routes is 32768
Suppose we want to prefer R2 routes received on R1 using weight attribute, we want the weight value for the 197.10.1.0/24 prefix to 100 and for 197.10.2.0/24 to be 200

R1(config)#access-list 1 permit 197.10.1.0
R1(config)#access-list 2 permit 197.10.2.0

R1(config)#route-map W permit 10
R1(config-route-map)#match ip address 1
R1(config-route-map)#set weight 100
R1(config-route-map)#route-map W permit 20
R1(config-route-map)#match ip address 2  
R1(config-route-map)#set weight 200

router bgp 123
R1(config-router)#neighbor 2.2.2.2 route-map W in

R1#sh ip bgp
BGP table version is 5, 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, ? - incomplete

   Network   Next Hop     Metric LocPrf Weight Path
*>i197.10.1.0    2.2.2.2   0    100    100 4 i
* i 3.3.3.3   0    100   0 5 i
*>i197.10.2.0    2.2.2.2   0    100    200 4 i
* i 3.3.3.3   0    100   0 5 i




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