Introduction to eBGP
eBGP is a flavour of Border Gateway Protocol (BGP) used for communication between different autonomous systems (AS). eBGP functions as the protocol responsible for interconnection of networks from different organizations or the Internet. eBGP is used and implemented at the edge or border router that provides interconnectivity for two or more autonomous system.
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 –
Example Scenario: Directly connected eBGP neighbors
As shown in above diagram, 2 routers need to form eBGP session on directly connected Interfaces.
Now, let’s verify configuration –
Next, we issue command “show ip bgp summary” to verify BGP memory usage, BGP neighbors and the state of communication.
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 –
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 –
Since ping is not successful between directly connected interfaces, we need to verify configured Interfaces of both routers –
Further investigation reveals that Router2 interface is administratively down, so we use “no shutdown” to bring up the interface.
Below log message will confirm that eBGP is coming up after “no shutdown” command is issued.
Now, eBGP neighborship is formed and router is not showing “active” or “idle” state.