Troubleshooting directly connected eBGP neighbors

In this post, we will discuss on few scenarios where eBGP neighbors are formed on directly connected links and type of issues that arise while forming eBGP neighborship between them. Below is one such instance –

As shown in above diagram, 2 routers need to form eBGP session on directly connected Interfaces.


Now, let’s verify configuration –

R1

Router1#show run | section router bgp

router bgp 65001

bgp log-neighbor-changes

neighbor 192.168.10.2 remote-as 65002

R2

Router2#show run | section router bgp

router bgp 65002

bgp log-neighbor-changes

neighbor 192.168.10.1 remote-as 65001

Next, we issue command “show ip bgp summary” to verify BGP memory usage, BGP neighbors and the state of communication.

Router1#show ip bgp summary

BGP router identifier 10.10.10.1, local AS number 65001

BGP table version is 1, main routing table version 1

 

Neighbor        V       AS       MsgRcvd   MsgSent   TblVer   InQ   OutQ Up/Down   State/PfxRcd

192.168.10.2    4      65002       0                  0              1      0         0       00:00:16      Active

As per output of Router1, the state is showing as “Active”, which mean BGP process on this router has started a new 3-way handshake.

Next, let’s see the output of Router2 –

Router2#show ip bgp summary

BGP router identifier 20.20.20.1, local AS number 65002

BGP table version is 1, main routing table version 1

 

Neighbor          V           AS     MsgRcvd   MsgSent   TblVer  InQ  OutQ  Up/Down   State/PfxRcd

192.168.10.1    4         65001         0              0              1         0        0      00:04:34        Idle

The output shows Router2 is showing state of “idle”.

Further elaborating on the above seen outputs, when 2 directly connected routers don’t form eBGP Neighbor relationship, it cloud be due to following reasons;

  • Layer 2 connectivity issue (like Interface down)
    • Use “Show ip interface brief” command to verify..
  • Layer 3 connectivity issue (like a wrong IP is configured)
    • Use Ping command to verify
  • Wrong Neighbor IP is configured in BGP configuration.
    • Both router will show idle state, you can check “show ip bgp summary”.
  • Wrong Neighbor AS is configured in BGP configuration.
    • Router will receive BGP Notification Message.
  • Access list is blocking TCP port 179 traffic
    • Verify the access list configured on directly connected physical interface.

Now let’s verify the connectivity between routers –

Router1#ping 192.168.10.2

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 192.168.10.2, timeout is 2 seconds:

…..

Success rate is 0 percent (0/5)

Since ping is not successful between directly connected interfaces, we need to verify configured Interfaces of both routers –

Router1#show ip int brief

Interface                  IP-Address         OK?     Method      Status                              Protocol

FastEthernet0/0        192.168.10.1    YES     manual        up                                   down

Router2#show ip int brief

Interface                  IP-Address         OK?     Method    Status                                 Protocol

FastEthernet0/0        192.168.10.2    YES     manual     administratively down    down

Further investigation reveals that Router2 interface is administratively down, so we use “no shutdown” to bring up the interface.

Router2(config)#int FastEthernet 0/0

Router2(config-if)#no shutdown

Below log message will confirm that eBGP is coming up after “no shutdown” command is issued.

Router1#

Apr 29 08:48:42.562: %BGP-5-ADJCHANGE: neighbor 192.168.10.2 Up

Router1#show ip bgp summary

BGP router identifier 10.10.10.1, local AS number 65001

BGP table version is 1, main routing table version 1

 

Neighbor           V           AS     MsgRcvd    MsgSent   TblVer   InQ OutQ Up/Down  State/PfxRcd

192.168.10.2    4           65002       4                 4              1        0      0     00:00:31         0

Now, eBGP neighborship is formed and router is not showing “active” or “idle” state.

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