What Is eBGP Multi-hop?
eBGP (External Border Gateway Protocol) Multi-hop is a networking method in the field of BGP (Border Gateway Protocol) routing, which is crucial in the internet’s framework.
BGP, as a path vector protocol, is responsible for exchanging routing information between autonomous systems (ASes), which are large networks or groupings of networks under a common administration, often an Internet Service Provider (ISP).
Standard eBGP sessions typically occur directly between border routers of neighboring ASes using a single-hop (direct connection).
However, in some network architectures, eBGP must function over multiple hops, hence the term “eBGP Multi-hop.”
Reasons For eBGP Multi-hop:
Purpose: eBGP Multi-hop is used when BGP routers need to establish BGP sessions with routers that are not directly connected to them. This can be useful in situations where routers are separated by multiple hops in the network.
Configuration: To configure eBGP Multi-hop, administrators specify a TTL (Time to Live) value in the BGP session setup. This TTL value determines the maximum number of hops (routers) that can exist between the two BGP peers. If the TTL value allows for the required number of hops, the BGP session is established.
Use Cases: eBGP Multi-hop is commonly used in scenarios such as connecting to BGP peers in a different AS through an intermediate router or establishing BGP sessions in complex network topologies where direct connections are not feasible.
Security Considerations: Since eBGP Multi-hop allows BGP sessions to traverse multiple hops, it’s important to ensure that security measures, such as access control lists (ACLs) and authentication, are in place to prevent unauthorized access to BGP sessions.
Unlike standard eBGP which requires direct physical connectivity, eBGP Multi-hop can establish BGP sessions over multiple IP hops. This allows eBGP peers to be several routers apart, extending the protocol’s reach beyond adjacent ASes.
To connect to an external BGP network or peers not directly connected to your network, you need to use the neighbor ebgp-multihop command in the BGP router configuration mode.
The multihop command is used only when connecting to an external neighbor, peers or a third-party router. The example topology and configuration below show how this works.
RouterHQ Configuration
RouterHQ(config)#router bgp 255
RouterHQ(config-router)#neighbor 10.10.1.2 remote-as 556
RouterHQ(config-router)#neighbor 10.10.1.2 ebgp-multihop
RouterHQ(config-router)#end
Branch1 Configuration
Branch1(config)#router bgp 556
Branch1(config-router)#neighbor 10.10.1.1 remote-as 255
The above eBGP configuration on RouterHQ indicates the connection to an external neighbor or peer not directly connected. Branch1 has a direct link now and doesn’t have to use the BGP-multihop command but the neighbor remote-as command.
In summary, eBGP Multi-hop is a BGP configuration option that enables BGP routers to establish sessions with peers that are not directly connected, making it a valuable tool for routing in complex network environments and when connecting to BGP peers in different ASes.
iBGP Configuration eBGP Configuration
Configuring eBGP with Loopback Address
How To Configure BGP Peers with Authentication
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