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    Example: Configuring EBGP Multihop

    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);}

    Example: Configuring EBGP Multihop Sessions

    This example shows how to configure an external BGP (EBGP) peer that is more than one hop away from the local router. This type of session is called a multihop BGP session.

    Requirements

    No special configuration beyond device initialization is required before you configure this example.

    Overview

    The configuration to enable multihop EBGP sessions requires connectivity between the two EBGP peers. This example uses static routes to provide connectivity between the devices.

    Unlike directly connected EBGP sessions in which physical address are typically used in the neighbor statements, you must use loopback interface addresses for multihop EBGP by specifying the loopback interface address of the indirectly connected peer. In this way, EBGP multihop is similar to internal BGP (IBGP).

    Finally, you must add the multihop statement. Optionally, you can set a maximum time-to-live (TTL) value with the ttl statement. The TTL is carried in the IP header of BGP packets. If you do not specify a TTL value, the system’s default maximum TTL value is used. The default TTL value is 64 for multihop EBGP sessions. Another option is to retain the BGP next-hop value for route advertisements by including the no-nexthop-change statement.

    Figure 2 shows a typical EBGP multihop network.

    Device C and Device E have an established EBGP session. Device D is not a BGP-enabled device. All of the devices have connectivity via static routes.

    Figure 2: Typical Network with EBGP Multihop Sessions

    Typical Network with EBGP Multihop
Sessions

    Configuration

    CLI Quick Configuration

    To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, and then copy and paste the commands into the CLI at the [edit] hierarchy level.

    Device C

    set interfaces fe-1/2/0 unit 9 description to-Dset interfaces fe-1/2/0 unit 9 family inet address 10.10.10.9/30set interfaces lo0 unit 3 family inet address 192.168.40.4/32set protocols bgp group external-peers type externalset protocols bgp group external-peers multihop ttl 2set protocols bgp group external-peers local-address 192.168.40.4set protocols bgp group external-peers export send-staticset protocols bgp group external-peers peer-as 18set protocols bgp group external-peers neighbor 192.168.6.7set policy-options policy-statement send-static term 1 from protocol staticset policy-options policy-statement send-static term 1 then acceptset routing-options static route 10.10.10.14/32 next-hop 10.10.10.10set routing-options static route 192.168.6.7/32 next-hop 10.10.10.10set routing-options router-id 192.168.40.4set routing-options autonomous-system 17

    Device D

    set interfaces fe-1/2/0 unit 10 description to-Cset interfaces fe-1/2/0 unit 10 family inet address 10.10.10.10/30set interfaces fe-1/2/1 unit 13 description to-Eset interfaces fe-1/2/1 unit 13 family inet address 10.10.10.13/30set interfaces lo0 unit 4 family inet address 192.168.6.6/32set routing-options static route 192.168.40.4/32 next-hop 10.10.10.9set routing-options static route 192.168.6.7/32 next-hop 10.10.10.14set routing-options router-id 192.168.6.6

    Device E

    set interfaces fe-1/2/0 unit 14 description to-Dset interfaces fe-1/2/0 unit 14 family inet address 10.10.10.14/30set interfaces lo0 unit 5 family inet address 192.168.6.7/32set protocols bgp group external-peers multihop ttl 2set protocols bgp group external-peers local-address 192.168.6.7set protocols bgp group external-peers export send-staticset protocols bgp group external-peers peer-as 17set protocols bgp group external-peers neighbor 192.168.40.4set policy-options policy-statement send-static term 1 from protocol staticset policy-options policy-statement send-static term 1 then acceptset routing-options static route 10.10.10.8/30 next-hop 10.10.10.13set routing-options static route 192.168.40.4/32 next-hop 10.10.10.13set routing-options router-id 192.168.6.7set routing-options autonomous-system 18

    Device C

    Step-by-Step Procedure

    The following example requires you to navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the CLI User Guide.

    To configure Device C:

    1. Configure the interface to the directly connected device (to-D), and configure the loopback interface.
      [edit interfaces fe-1/2/0 unit 9]user@C# set description to-Duser@C# set family inet address 10.10.10.9/30
      [edit interfaces lo0 unit 3]user@C# set family inet address 192.168.40.4/32
    2. Configure an EBGP session with Device E.

      The neighbor statement points to the loopback interface on Device E.

      [edit protocols bgp group external-peers]user@C# set type externaluser@C# set local-address 192.168.40.4user@C# set export send-staticuser@C# set peer-as 18user@C# set neighbor 192.168.6.7
    3. Configure the multihop statement to enable Device C and Device E to become EBGP peers.

      Because the peers are two hops away from each other, the example uses the ttl 2 statement.

      [edit protocols bgp group external-peers]user@C# set multihop ttl 2
    4. Configure connectivity to Device E, using static routes.

      You must configure a route to both the loopback interface address and to the address on the physical interface.

      [edit routing-options]user@C# set static route 10.10.10.14/32 next-hop 10.10.10.10user@C# set static route 192.168.6.7/32 next-hop 10.10.10.10
    5. Configure the local router ID and the autonomous system (AS) number.
      [edit routing-options]user@C# set router-id 192.168.40.4user@C# set autonomous-system 17
    6. Configure a policy that accepts direct routes.

      Other useful options for this scenario might be to accept routes learned through OSPF or local routes.

      [edit policy-options policy-statement send-static term 1]user@C# set from protocol staticuser@C# set then accept

    Results

    From configuration mode, confirm your configuration by entering the show interfaces, show protocols, show policy-options, and show routing-options commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

    user@C# show interfaces
    fe-1/2/0 {unit 9 {description to-D;family inet {address 10.10.10.9/30;}}}
    lo0 {unit 3 {family inet {address 192.168.40.4/32;}}}
    user@C# show protocols
    bgp {group external-peers {type external;multihop {ttl 2;}local-address 192.168.40.4;export send-static;peer-as 18;neighbor 192.168.6.7;}}
    user@C# show policy-options
    policy-statement send-static {term 1 {from protocol static;then accept;}}
    user@C# show routing-options
    static {route 10.10.10.14/32 next-hop 10.10.10.10;route 192.168.6.7/32 next-hop 10.10.10.10;}
    router-id 192.168.40.4;autonomous-system 17;

    If you are done configuring the device, enter commit from configuration mode.
    Repeat these steps for all BFD sessions in the topology.

    Configuring Device D

    Step-by-Step Procedure

    The following example requires you to navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the CLI User Guide.

    To configure Device D:

    1. Set the CLI to Device D.
      user@host> set cli logical-system D
    2. Configure the interfaces to the directly connected devices, and configure a loopback interface.
      [edit interfaces fe-1/2/0 unit 10]user@D# set description to-Cuser@D# set family inet address 10.10.10.10/30
      [edit interfaces fe-1/2/1 unit 13]user@D# set description to-Euser@D# set family inet address 10.10.10.13/30
      [edit interfaces lo0 unit 4]user@D# set family inet address 192.168.6.6/32
    3. Configure connectivity to the other devices using static routes to the loopback interface addresses.

      On Device D, you do not need static routes to the physical addresses because Device D is directly connected to Device C and Device E.

      [edit routing-options]user@D# set static route 192.168.40.4/32 next-hop 10.10.10.9user@D# set static route 192.168.6.7/32 next-hop 10.10.10.14
    4. Configure the local router ID.
      [edit routing-options]user@D# set router-id 192.168.6.6

    Results

    From configuration mode, confirm your configuration by entering the show interfaces and show routing-options commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

    user@D# show interfaces
    fe-1/2/0 {unit 10 {description to-C;family inet {address 10.10.10.10/30;}}}
    fe-1/2/1 {unit 13 {description to-E;family inet {address 10.10.10.13/30;}}}
    lo0 {unit 4 {family inet {address 192.168.6.6/32;}}}
    user@D# show protocols
    user@D# show routing-options
    static {route 192.168.40.4/32 next-hop 10.10.10.9;route 192.168.6.7/32 next-hop 10.10.10.14;}
    router-id 192.168.6.6;

    If you are done configuring the device, enter commit from configuration mode.
    Repeat these steps for all BFD sessions in the topology.

    Configuring Device E

    Step-by-Step Procedure

    The following example requires you to navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the CLI User Guide.

    To configure Device E:

    1. Set the CLI to Device E.
      user@host> set cli logical-system E
    2. Configure the interface to the directly connected device (to-D), and configure the loopback interface.
      [edit interfaces fe-1/2/0 unit 14]user@E# set description to-Duser@E# set family inet address 10.10.10.14/30
      [edit interfaces lo0 unit 5]user@E# set family inet address 192.168.6.7/32
    3. Configure an EBGP session with Device E.

      The neighbor statement points to the loopback interface on Device C.

      [edit protocols bgp group external-peers]user@E# set local-address 192.168.6.7user@E# set export send-staticuser@E# set peer-as 17user@E# set neighbor 192.168.40.4
    4. Configure the multihop statement to enable Device C and Device E to become EBGP peers.

      Because the peers are two hops away from each other, the example uses the ttl 2 statement.

      [edit protocols bgp group external-peers]user@E# set multihop ttl 2
    5. Configure connectivity to Device E, using static routes.

      You must configure a route to both the loopback interface address and to the address on the physical interface.

      [edit routing-options]user@E# set static route 10.10.10.8/30 next-hop 10.10.10.13user@E# set static route 192.168.40.4/32 next-hop 10.10.10.13
    6. Configure the local router ID and the autonomous system (AS) number.
      [edit routing-options]user@E# set router-id 192.168.6.7user@E# set autonomous-system 18
    7. Configure a policy that accepts direct routes.

      Other useful options for this scenario might be to accept routes learned through OSPF or local routes.

      [edit policy-options policy-statement send-static term 1]user@E# set from protocol staticuser@E# set then accept

    Results

    From configuration mode, confirm your configuration by entering the show interfaces, show protocols, show policy-options, and show routing-options commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

    user@E# show interfaces
    fe-1/2/0 {unit 14 {description to-D;family inet {address 10.10.10.14/30;}}}
    lo0 {unit 5 {family inet {address 192.168.6.7/32;}}}
    user@E# show protocols
    bgp {group external-peers {multihop {ttl 2;}local-address 192.168.6.7;export send-static;peer-as 17;neighbor 192.168.40.4;}}
    user@E# show policy-options
    policy-statement send-static {term 1 {from protocol static;then accept;}}
    user@E# show routing-options
    static {route 10.10.10.8/30 next-hop 10.10.10.13;route 192.168.40.4/32 next-hop 10.10.10.13;}
    router-id 192.168.6.7;autonomous-system 18;

    If you are done configuring the device, enter commit from configuration mode.

    Verification

    Confirm that the configuration is working properly.

    Verifying Connectivity

    Purpose

    Make sure that Device C can ping Device E, specifying the loopback interface address as the source of the ping request.

    The loopback interface address is the source address that BGP will use.

    Action

    From operational mode, enter the ping 10.10.10.14 source 192.168.40.4 command from Device C, and enter the ping 10.10.10.9 source 192.168.6.7 command from Device E.

    user@C> ping 10.10.10.14 source 192.168.40.4
    PING 10.10.10.14 (10.10.10.14): 56 data bytes
    64 bytes from 10.10.10.14: icmp_seq=0 ttl=63 time=1.262 ms
    64 bytes from 10.10.10.14: icmp_seq=1 ttl=63 time=1.202 ms
    ^C
    --- 10.10.10.14 ping statistics ---
    2 packets transmitted, 2 packets received, 0% packet loss
    round-trip min/avg/max/stddev = 1.202/1.232/1.262/0.030 ms
    user@E> ping 10.10.10.9 source 192.168.6.7
    PING 10.10.10.9 (10.10.10.9): 56 data bytes
    64 bytes from 10.10.10.9: icmp_seq=0 ttl=63 time=1.255 ms
    64 bytes from 10.10.10.9: icmp_seq=1 ttl=63 time=1.158 ms
    ^C
    --- 10.10.10.9 ping statistics ---
    2 packets transmitted, 2 packets received, 0% packet loss
    round-trip min/avg/max/stddev = 1.158/1.206/1.255/0.049 ms

    Meaning

    The static routes are working if the pings work.

    Verifying That BGP Sessions Are Established

    Purpose

    Verify that the BGP sessions are up.

    Action

    From operational mode, enter the show bgp summary command.

    user@C> show bgp summary
    Groups: 1 Peers: 1 Down peers: 0
    Table          Tot Paths  Act Paths Suppressed    History Damp State    Pending
    inet.0                 2          0          0          0          0          0
    Peer                     AS      InPkt     OutPkt    OutQ   Flaps Last Up/Dwn State|#Active/Received/Accepted/Damped...
    192.168.6.7              18        147        147       0       1     1:04:27 0/2/2/0              0/0/0/0
    user@E> show bgp summary
    Groups: 1 Peers: 1 Down peers: 0
    Table          Tot Paths  Act Paths Suppressed    History Damp State    Pending
    inet.0                 2          0          0          0          0          0
    Peer                     AS      InPkt     OutPkt    OutQ   Flaps Last Up/Dwn State|#Active/Received/Accepted/Damped...
    192.168.40.4             17        202        202       0       1     1:02:18 0/2/2/0              0/0/0/0

    Meaning

    The output shows that both devices have one peer each. No peers are down.

    Viewing Advertised Routes

    Purpose

    Check to make sure that routes are being advertised by BGP.

    Action

    From operational mode, enter the show route advertising-protocol bgp neighbor command.

    user@C> show route advertising-protocol bgp 192.168.6.7
    inet.0: 5 destinations, 7 routes (5 active, 0 holddown, 0 hidden)
      Prefix                  Nexthop              MED     Lclpref    AS path
    * 10.10.10.14/32          Self                                    I
    * 192.168.6.7/32          Self                                    I
    user@E> show route advertising-protocol bgp 192.168.40.4
    inet.0: 5 destinations, 7 routes (5 active, 0 holddown, 0 hidden)
      Prefix                  Nexthop              MED     Lclpref    AS path
    * 10.10.10.8/30           Self                                    I
    * 192.168.40.4/32         Self                                    I

    Meaning

    The send-static routing policy is exporting the static routes from the routing table into BGP. BGP is advertising these routes between the peers because the BGP peer session is established.

    Modified: 2016-06-16