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    Understanding EBGP Multihop

    BGP is an exterior gateway protocol (EGP) that is used to exchange routing information among routers in different autonomous systems (ASs). The following are two ways of establishing EBGP multihop between routers:

    1. When external BGP (EBGP) peers are not directly connected to each other, they must cross one or more non-BGP routers to reach each other.

    Configuring multihop EBGP enables the peers to pass through the other routers to form peer relationships and exchange update messages. This type of configuration is typically used when a Juniper Networks routing device needs to run EBGP with a third-party router that does not allow direct connection of the two EBGP peers. EBGP multihop enables a neighbor connection between two EBGP peers that do not have a direct connection.

    2. The default behavior for an EBGP connection is to peer over a single physical hop using the physical interface address of the peer. In some cases, it is advantageous to alter this default, one-hop, physical peering EBGP behavior. One such case is when multiple physical links connect two routers that are to be EBGP peers. In this case, if one of the point-to-point links fails, reachability on the alternate link still exists.

    Figure 1: EBGP Multihop Peering

    EBGP Multihop Peering

    In figure 1, router R1 belongs to AS 1 and router R2 belongs to AS 2. The two physical links between the routers is used for load balancing. The EBGP multihop peering works with one physical link as well.

    The following configuration example helps to establish a single BGP peering session across these multiple physical links:

    1. Each router must establish the peering session with the loopback address of the remote router. You can configure this session using the local-address statement, which alters the peer address header information in the BGP packets.

    2. Use the multihop statement to alter the default use of the neighbor's physical address. In addition, you can also specify a time-to-live (TTL) value in the BGP packets to control how far they propagate. We use a TTL value of 1 to ensure that the session cannot be established across any other backdoor links in the network.

    Note: When multihop is configured, the Junos OS sets the TTL value of 64, by default.

    A TTL value of 1 is sufficient to enable an EBGP session to the loopback address of a directly connected neighbor.

    3. Each router must have IP routing capability to the remote router's loopback address. This capability is often accomplished by using a static route to map the loopback address to the interface physical addresses.

    [edit protocols bgp group ext-peers]type external;local-address 192.168.3.4;neighbor 172.16.128.1 {
    multihop ttl 1;}
    [edit routing-options]static {
    route 172.16.128.1 next-hop (10.10.1.1 | 10.10.2.1);}

    Modified: 2016-12-06