Monday 8 August 2016

BGP Basics



The basic BGP config on each router looks like this:
R1#sh run | s router
router bgp 1
 no synchronization
 bgp log-neighbor-changes
 neighbor 2.2.2.2 remote-as 23
 neighbor 2.2.2.2 ebgp-multihop 2
 neighbor 2.2.2.2 update-source Loopback0
 neighbor 3.3.3.3 remote-as 23
 neighbor 3.3.3.3 ebgp-multihop 2
 neighbor 3.3.3.3 update-source Loopback0
 no auto-summary

R2#sh run | s router
router bgp 23
 no synchronization
 bgp log-neighbor-changes
 neighbor 1.1.1.1 remote-as 1
 neighbor 1.1.1.1 ebgp-multihop 2
 neighbor 1.1.1.1 update-source Loopback0
 neighbor 3.3.3.3 remote-as 23
 neighbor 3.3.3.3 ebgp-multihop 2
 neighbor 3.3.3.3 update-source Loopback0
 no auto-summary

R3#sh run | s router
router bgp 23
 no synchronization
 bgp log-neighbor-changes
 neighbor 1.1.1.1 remote-as 1
 neighbor 1.1.1.1 ebgp-multihop 2
 neighbor 1.1.1.1 update-source Loopback0
 neighbor 2.2.2.2 remote-as 23
 neighbor 2.2.2.2 ebgp-multihop 2
 neighbor 2.2.2.2 update-source Loopback0
 no auto-summary
We’ve now got peering between all three routers.
R1#sh ip bgp neighbors | i BGP
BGP neighbor is 2.2.2.2,  remote AS 23 external link
  BGP version 4, remote router ID 2.2.2.2
  BGP state = Established, up for 05:17:02
  BGP table version 1, neighbor version 1/0
  External BGP neighbor may be up to 2 hops away.
BGP neighbor is 3.3.3.3,  remote AS 23 external link
  BGP version 4, remote router ID 3.3.3.3
  BGP state = Established, up for 05:16:57
  BGP table version 1, neighbor version 1/0
  External BGP neighbor may be up to 2 hops away.

R1#sh ip route
{output omitted}

Gateway of last resort is not set

     1.0.0.0/32 is subnetted, 1 subnets
C       1.1.1.1 is directly connected, Loopback0
     2.0.0.0/32 is subnetted, 1 subnets
S       2.2.2.2 [1/0] via 10.0.12.2
     3.0.0.0/32 is subnetted, 1 subnets
S       3.3.3.3 [1/0] via 10.0.13.3
     10.0.0.0/29 is subnetted, 2 subnets
C       10.0.12.0 is directly connected, FastEthernet0/0
C       10.0.13.0 is directly connected, FastEthernet1/0
There are two main methods of advertising routes into BGP – network statements and redistribution.

Network Statements

On R2 we will use a network statement to advertise the 192.168.2.0 /24 network into BGP (note that bgp network statements use subnet masks rather than wildcard masks):
R2(config)#router bgp 23
R2(config-router)#network 192.168.2.0 mask 255.255.255.0
If we check R1′s BGP table we see that it has learned of 192.168.2.0/24 route, also that the best route is via R2, and that the path is through AS23:
R1#sh ip bgp             
BGP table version is 36, 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
*  192.168.2.0      3.3.3.3                                0 23 i
*>                  2.2.2.2                  0             0 23 i
The best route is now also in R1′s routing table (note the administrative distance of 20 for external BGP):
R1#sh ip route bgp
B    192.168.2.0/24 [20/0] via 2.2.2.2, 00:25:55

Redistribution

On R3 we will redistribute connected networks so that we advertise the 192.168.3.0 /24 network into BGP:
R3(config)#router bgp 23         
R3(config-router)#redistribute connected
This works but is a little clumsy. We can now see the 192.168.3.0 /24 network in R1′s routing table, but we also see the 10.0.23.0 /29 transit network between R2 and R3:
R1#show ip route bgp
     10.0.0.0/29 is subnetted, 3 subnets
B       10.0.23.0 [20/0] via 3.3.3.3, 00:01:22
B    192.168.2.0/24 [20/0] via 2.2.2.2, 00:20:27
B    192.168.3.0/24 [20/0] via 3.3.3.3, 00:01:22
Plus we also get some RIB failure messages relating to R3′s other connected networks:
R1#sh ip bgp                                                       
BGP table version is 8, 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
r  3.3.3.3/32       2.2.2.2                                0 23 ?
r>                  3.3.3.3                  0             0 23 ?
r  10.0.13.0/29     2.2.2.2                                0 23 ?
r>                  3.3.3.3                  0             0 23 ?
*  10.0.23.0/29     2.2.2.2                                0 23 ?
*>                  3.3.3.3                  0             0 23 ?
*  192.168.2.0      3.3.3.3                                0 23 i
*>                  2.2.2.2                  0             0 23 i
*  192.168.3.0      2.2.2.2                                0 23 ?
*>                  3.3.3.3                  0             0 23 ?
This is because R1 already has a static route to 3.3.3.3/32 and is directly connected to 10.0.13.0/29.
To tidy this up we’ll filter routes using a route map that references an access list to match only the 192.168.3.0/24 network:
R3(config)#route-map R3RouteMap permit 10
R3(config-route-map)#match ip address 10

R3(config)#access-list 10 permit 192.168.3.0 0.0.0.255

R3(config)#router bgp 23 
R3(config-router)#redistribute connected route-map R3RouteMap
Now R1 only learns the route to 192.168.3.0/24 from R3, as the route map is preventing the other networks from being advertised:
R1#sh ip bgp
BGP table version is 17, 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
*  192.168.2.0      3.3.3.3                                0 23 i
*>                  2.2.2.2                  0             0 23 i
*  192.168.3.0      2.2.2.2                                0 23 ?
*>                  3.3.3.3                  0             0 23 ?
 

R1#sh ip route bgp
B    192.168.2.0/24 [20/0] via 2.2.2.2, 02:20:54
B    192.168.3.0/24 [20/0] via 3.3.3.3, 02:16:40