BGP disable-connected-check






  • All routers are running OSPF as IGP
  • iBGP is configured between R1 and R3
  • eBGP is configured between R1  and R2 and R3 and R2
eBGP between R1 and R2 should be configured using their loopback interfaces, one way to achieve this  is to configure eBGP multihop where TTL values of the BGP packets is increased.
Another way of doing it  is to use the  disable-connected-check   command under the BGP configuration, this will allow the two routers to become neighbours but only through direct connected links since the TTL is not touched 
let's check the configurations on R1 and R2 R1(config-router)#do sh run | sec router bgp
router bgp 100
 no synchronization
 bgp log-neighbor-changes
 neighbor 2.2.2.2 remote-as 200
 neighbor 2.2.2.2 disable-connected-check
 neighbor 2.2.2.2 update-source Loopback0
 neighbor 10.1.13.3 remote-as 100
 no auto-summary


R2(config-router)#do sh run | b router bgp
router bgp 200
 no synchronization
 bgp log-neighbor-changes
 neighbor 1.1.1.1 remote-as 100
 neighbor 1.1.1.1 disable-connected-check
 neighbor 1.1.1.1 update-source Loopback0
 no auto-summary
!
as we can see in the output of sh ip bgp summ that BGP has been established between the two neighbours

Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
1.1.1.1         4   100      32      32        1    0    0 00:28:31        0

Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
2.2.2.2         4   200      32      32        1    0    0 00:28:51        0

let's check the TTL for 2.2.2.2 on R1

R1(config-if)#do sh ip bgp ne 2.2.2.2 | in TTL
Connection is ECN Disabled, Mininum incoming TTL 0, Outgoing TTL 1


now let's see what happens if we shut down the direct link between R1 and R2
BGP went down between R1 and R2
Although R1 still has the route to 2.2.2.2
R1(config-if)#do 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, 00:01:33 ago
  Routing Descriptor Blocks:
  * 10.1.12.2, from 2.2.2.2, 00:01:33 ago, via FastEthernet0/0
      Route metric is 11, traffic share count is 1



 Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
2.2.2.2         4   200      33      37        0    0    0 00:02:21 Active 

Now let's check if we changed the configuration to eBGP multi hop while interf f0/0 is shut down on R1
on R1 and R2 under BGP configuration we use neighbor x.x.x.x ebgp-multihop 3 

Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
2.2.2.2         4   200      45      49        1    0    0 00:00:18        0


and the TTL is also changed to 3

R1(config-if)#do sh ip bgp ne 2.2.2.2 | in TTL
Connection is ECN Disabled, Mininum incoming TTL 0, Outgoing TTL 3










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