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    Selecting the Correct ABR While Implementing AIGP

    When AIGP is implemented in a network where there is more than one area border router (ABR) available for the forwarding device to use, AIGP enables the device to choose the ABR that is based on the IGP distance to the BGP next hop and the AIGP value.

    Figure 1 shows the topology of this scenario. Device PE4 is advertising prefix 33.0.0.0/24 with an AIGP metric of 10, and prefix 44.0.0.0/24 with an AIGP metric of 20. Device P4 is advertising prefix 33.0.0.0/24 with an AIGP metric of 20, and prefix 44.0.0.0/24 with an AIGP metric of 10. The IGP distance for Device PE4 and Device P4 is 3.

    Device PE4 is connected to Devices P1, PE3, and PE2, and Device P4 is connected to Device P2. The router ID of Device P4 is 10.9.9.8, and the BGP next hop is 10.100.1.8. Device PE1 has two ABRs (Device P1 and Device P2) to choose from since Device PE1 receives the same prefixes from Device PE4 and Device P4. Device PE1 chooses which ABR to use by adding the IGP distance to the BGP next hop and the AIGP value:

    IGP distance to the BGP next hop + AIGP value = ABR metric

    Figure 1: Network Topology for ABR Selection

    Network Topology for ABR
Selection

    As shown in Figure 1, Device PE1 chooses ABR Device P1 for prefix 33.0.0.0, and Device P2 for prefix 44.0.0.0 based on the AIGP value, because the IGP distance is the same for both. This selection is because prefix 33.0.0.0/24 has a lower AIGP metric coming from Device PE4, and prefix 44.0.0.0/24 has a lower AIGP coming from Device P4.

    Note: In the following samples, link protection is used, which causes the bypass path to appear in the outputs. Also, the following sample outputs might not correspond to the sample outputs shown in Example: Configuring the Accumulated IGP Attribute for BGP.

    The following show command uses the extensive option and shows the route information for prefix 33.0.0.0 on Device PE1:

    user@PE1> show route 33.0.0.0 extensive
    inet.0: 66 destinations, 70 routes (63 active, 0 holddown, 1 hidden)
    33.0.0.0/24 (2 entries, 1 announced)
    TSI:
    KRT in-kernel 33.0.0.0/24 -> {indirect(1048582)}
    Page 0 idx 0 Type 1 val 91495a8
            *BGP    Preference: 170/-101
                    Next hop type: Indirect
                    Next-hop reference count: 5
                    Source: 10.9.9.4
                    Next hop type: Router, Next hop index: 1048592
                    Next hop: 20.0.0.2 via ae0.0 weight 0x1, selected
                    Label operation: Push 301664, Push 300896(top)
                    Label TTL action: prop-ttl, prop-ttl(top)
                    Next hop: 20.0.0.6 via ae1.0 weight 0x8001
                    Label operation: Push 301664, Push 300896, Push 301568(top)
                    Label TTL action: prop-ttl, prop-ttl, prop-ttl(top)
                    Protocol next hop: 10.100.1.4
                    Push 301664
                    Indirect next hop: 92740e4 1048582
                    State: <Active Int Ext>
                    Local AS: 13979 Peer AS: 13979
                    Age: 11         Metric2: 3 
                    AIGP: 10 
                    Task: BGP_13979.10.9.9.4+50932
                    Announcement bits (3): 0-KRT 6-BGP_RT_Background 7-Resolve tree 2 
                    AS path: I
                    Accepted
                    Route Label: 301664
                    Localpref: 100
                    Router ID: 10.9.9.4
                    Indirect next hops: 1
                            Protocol next hop: 10.100.1.4 Metric: 3
                            Push 301664
                            Indirect next hop: 92740e4 1048582
                            Indirect path forwarding next hops: 2
                                    Next hop type: Router
                                    Next hop: 20.0.0.2 via ae0.0 weight 0x1
                                    Next hop: 20.0.0.6 via ae1.0 weight 0x8001
                            10.100.1.4/32 Originating RIB: inet.3
                              Metric: 3                       Node path count: 1
                              Forwarding nexthops: 2
                                    Nexthop: 20.0.0.2 via ae0.0
                                    Nexthop: 20.0.0.6 via ae1.0
             BGP    Preference: 170/-101
                    Next hop type: Indirect
                    Next-hop reference count: 5
                    Source: 10.9.9.8
                    Next hop type: Router, Next hop index: 660
                    Next hop: 20.0.0.6 via ae1.0, selected
                    Label operation: Push 299776
                    Label TTL action: prop-ttl
                    Protocol next hop: 10.100.1.8
                    Push 299776
                    Indirect next hop: 92742ac 1048579
                    State: <Int Ext>
                    Inactive reason: AIGP
                    Local AS: 13979 Peer AS: 13979
                    Age: 11         Metric2: 3 
                    AIGP: 20 
                    Task: BGP_13979.10.9.9.8+179
                    AS path: I
                    Accepted
                    Route Label: 299776
                    Localpref: 100
                    Router ID: 10.9.9.8
                    Indirect next hops: 1
                            Protocol next hop: 10.100.1.8 Metric: 3
                            Push 299776
                            Indirect next hop: 92742ac 1048579
                            Indirect path forwarding next hops: 1
                                    Next hop type: Router
                                    Next hop: 20.0.0.6 via ae1.0
                            10.100.1.8/32 Originating RIB: inet.0
                              Metric: 3                       Node path count: 1
                              Forwarding nexthops: 1
                                    Nexthop: 20.0.0.6 via ae1.0
    

    The following show command uses the extensive option and shows the route information for prefix 44.0.0.0 extensive on Device PE1:

    user@PE1> show route 44.0.0.0 extensive
    inet.0: 66 destinations, 70 routes (63 active, 0 holddown, 1 hidden)
    44.0.0.0/24 (2 entries, 1 announced)
    TSI:
    KRT in-kernel 44.0.0.0/24 -> {indirect(1048579)}
    Page 0 idx 0 Type 1 val 9149880
            *BGP    Preference: 170/-101
                    Next hop type: Indirect
                    Next-hop reference count: 5
                    Source: 10.9.9.8
                    Next hop type: Router, Next hop index: 660
                    Next hop: 20.0.0.6 via ae1.0, selected
                    Label operation: Push 299776
                    Label TTL action: prop-ttl
                    Protocol next hop: 10.100.1.8
                    Push 299776
                    Indirect next hop: 92742ac 1048579
                    State: <Active Int Ext>
                    Local AS: 13979 Peer AS: 13979
                    Age: 23         Metric2: 3 
                    AIGP: 10 
                    Task: BGP_13979.10.9.9.8+179
                    Announcement bits (3): 0-KRT 6-BGP_RT_Background 7-Resolve tree 2 
                    AS path: I
                    Accepted
                    Route Label: 299776
                    Localpref: 100
                    Router ID: 10.9.9.8
                    Indirect next hops: 1
                            Protocol next hop: 10.100.1.8 Metric: 3
                            Push 299776
                            Indirect next hop: 92742ac 1048579
                            Indirect path forwarding next hops: 1
                                    Next hop type: Router
                                    Next hop: 20.0.0.6 via ae1.0
                            10.100.1.8/32 Originating RIB: inet.0
                              Metric: 3                       Node path count: 1
                              Forwarding nexthops: 1
                                    Nexthop: 20.0.0.6 via ae1.0
             BGP    Preference: 170/-101
                    Next hop type: Indirect
                    Next-hop reference count: 5
                    Source: 10.9.9.4
                    Next hop type: Router, Next hop index: 1048592
                    Next hop: 20.0.0.2 via ae0.0 weight 0x1, selected
                    Label operation: Push 301664, Push 300896(top)
                    Label TTL action: prop-ttl, prop-ttl(top)
                    Next hop: 20.0.0.6 via ae1.0 weight 0x8001
                    Label operation: Push 301664, Push 300896, Push 301568(top)
                    Label TTL action: prop-ttl, prop-ttl, prop-ttl(top)
                    Protocol next hop: 10.100.1.4
                    Push 301664
                    Indirect next hop: 92740e4 1048582
                    State: <Int Ext>
                    Inactive reason: AIGP
                    Local AS: 13979 Peer AS: 13979
                    Age: 23         Metric2: 3 
                    AIGP: 20 
                    Task: BGP_13979.10.9.9.4+50932
                    AS path: I
                    Accepted
                    Route Label: 301664
                    Localpref: 100
                    Router ID: 10.9.9.4
                    Indirect next hops: 1
                            Protocol next hop: 10.100.1.4 Metric: 3
                            Push 301664
                            Indirect next hop: 92740e4 1048582
                            Indirect path forwarding next hops: 2
                                    Next hop type: Router
                                    Next hop: 20.0.0.2 via ae0.0 weight 0x1
                                    Next hop: 20.0.0.6 via ae1.0 weight 0x8001
                            10.100.1.4/32 Originating RIB: inet.3
                              Metric: 3                       Node path count: 1
                              Forwarding nexthops: 2
                                    Nexthop: 20.0.0.2 via ae0.0
                                    Nexthop: 20.0.0.6 via ae1.0
    

    If the IGP metric on Device PE1 is changed so that the sum of the IGP distance to the BGP next hop and the AIGP metric is still lower for prefix 44.0.0.0/24 received from Device P4, then Device PE1 still selects Device P4 as the next hop for route 44.0.0.0/24, but the ABR changes from Device P2 to Device P1, as shown in the following outputs:

    user@PE1> show route 10.100.1
     inet.0: 66 destinations, 71 routes (63 active, 0 holddown, 1 hidden)
    + = Active Route, - = Last Active, * = Both
    
    10.100.1.8/32      *[OSPF/10] 00:00:02, metric 4
                        > to 20.0.0.2 via ae0.0
    
    user@PE1> show route 44.0.0.0 extensive
    inet.0: 66 destinations, 71 routes (63 active, 0 holddown, 1 hidden)
    44.0.0.0/24 (2 entries, 1 announced)
    TSI:
    KRT in-kernel 44.0.0.0/24 -> {indirect(1048579)}
    Page 0 idx 0 Type 1 val 9149880
            *BGP    Preference: 170/-101
                    Next hop type: Indirect
                    Next-hop reference count: 5
                    Source: 10.9.9.8
                    Next hop type: Router, Next hop index: 702
                    Next hop: 20.0.0.2 via ae0.0, selected
                    Label operation: Push 299776
                    Label TTL action: prop-ttl
                    Protocol next hop: 10.100.1.8
                    Push 299776
                    Indirect next hop: 92742ac 1048579
                    State: <Active Int Ext>
                    Local AS: 13979 Peer AS: 13979
                    Age: 27:58      Metric2: 4 
                    AIGP: 10 
                    Task: BGP_13979.10.9.9.8+179
                    Announcement bits (3): 0-KRT 6-BGP_RT_Background 7-Resolve tree 2 
                    AS path: I
                    Accepted
                    Route Label: 299776
                    Localpref: 100
                    Router ID: 10.9.9.8
                    Indirect next hops: 1
                            Protocol next hop: 10.100.1.8 Metric: 4
                            Push 299776
                            Indirect next hop: 92742ac 1048579
                            Indirect path forwarding next hops: 1
                                    Next hop type: Router
                                    Next hop: 20.0.0.2 via ae0.0
                            10.100.1.8/32 Originating RIB: inet.0
                              Metric: 4                       Node path count: 1
                              Forwarding nexthops: 1
                                    Nexthop: 20.0.0.2 via ae0.0
             BGP    Preference: 170/-101
                    Next hop type: Indirect
                    Next-hop reference count: 5
                    Source: 10.9.9.4
                    Next hop type: Router, Next hop index: 1048592
                    Next hop: 20.0.0.2 via ae0.0 weight 0x1, selected
                    Label operation: Push 301664, Push 300896(top)
                    Label TTL action: prop-ttl, prop-ttl(top)
                    Next hop: 20.0.0.6 via ae1.0 weight 0x8001
                    Label operation: Push 301664, Push 300896, Push 301568(top)
                    Label TTL action: prop-ttl, prop-ttl, prop-ttl(top)
                    Protocol next hop: 10.100.1.4
                    Push 301664
                    Indirect next hop: 92740e4 1048582
                    State: <Int Ext>
                    Inactive reason: AIGP
                    Local AS: 13979 Peer AS: 13979
                    Age: 27:58      Metric2: 3 
                    AIGP: 20 
                    Task: BGP_13979.10.9.9.4+50932
                    AS path: I
                    Accepted
                    Route Label: 301664
                    Localpref: 100
                    Router ID: 10.9.9.4
                    Indirect next hops: 1
                            Protocol next hop: 10.100.1.4 Metric: 3
                            Push 301664
                            Indirect next hop: 92740e4 1048582
                            Indirect path forwarding next hops: 2
                                    Next hop type: Router
                                    Next hop: 20.0.0.2 via ae0.0 weight 0x1
                                    Next hop: 20.0.0.6 via ae1.0 weight 0x8001
                            10.100.1.4/32 Originating RIB: inet.3
                              Metric: 3                       Node path count: 1
                              Forwarding nexthops: 2
                                    Nexthop: 20.0.0.2 via ae0.0
                                    Nexthop: 20.0.0.6 via ae1.0
    

    In the previous output, the IGP distance (Metric2) for ABR Device P2 changed so that the sum of the IGP distance and the AIGP metric for prefix 44.0.0.0/24 coming from Device P4 is higher than Device PE4. Table 1 shows the difference between the metric sums of Device PE4 and Device P4.

    Table 1: Metric Sums: Comparing Device PE4 and Device P4

    MetricsDevice PE4Device P4

    AIGP

    20

    10

    IGP Distance (Metric2)

    3

    53

    Total

    23 (preferred)

    63

    Although the AIGP metric is still lower for prefix 44.0.0.0/24 from Device P4, the sum of the AIGP metric and the IGP distance from Device PE4 is lower than the sum of the IGP distance and the AIGP metric from Device P4. Device PE1 selected Device PE4 for this reason, as shown in the following output:

    user@PE1> show route 44.0.0.0/24
    inet.0: 66 destinations, 71 routes (63 active, 0 holddown, 1 hidden)
    44.0.0.0/24 (2 entries, 1 announced)
    TSI:
    KRT in-kernel 44.0.0.0/24 -> {indirect(1048582)}
    Page 0 idx 0 Type 1 val 9149880
            *BGP    Preference: 170/-101
                    Next hop type: Indirect
                    Next-hop reference count: 7
                    Source: 10.9.9.4
                    Next hop type: Router, Next hop index: 1048592
                    Next hop: 20.0.0.2 via ae0.0 weight 0x1, selected
                    Label operation: Push 301664, Push 300896(top)
                    Label TTL action: prop-ttl, prop-ttl(top)
                    Next hop: 20.0.0.6 via ae1.0 weight 0x8001
                    Label operation: Push 301664, Push 300896, Push 301568(top)
                    Label TTL action: prop-ttl, prop-ttl, prop-ttl(top)
                    Protocol next hop: 10.100.1.4
                    Push 301664
                    Indirect next hop: 92740e4 1048582
                    State: <Active Int Ext>
                    Local AS: 13979 Peer AS: 13979
                    Age: 18:19      Metric2: 3 
                    AIGP: 20 
                    Task: BGP_13979.10.9.9.4+50932
                    Announcement bits (3): 0-KRT 6-BGP_RT_Background 7-Resolve tree 2 
                    AS path: I
                    Accepted
                    Route Label: 301664
                    Localpref: 100
                    Router ID: 10.9.9.4
                    Indirect next hops: 1
                            Protocol next hop: 10.100.1.4 Metric: 3
                            Push 301664
                            Indirect next hop: 92740e4 1048582
                            Indirect path forwarding next hops: 2
                                    Next hop type: Router
                                    Next hop: 20.0.0.2 via ae0.0 weight 0x1
                                    Next hop: 20.0.0.6 via ae1.0 weight 0x8001
                            10.100.1.4/32 Originating RIB: inet.3
                              Metric: 3                       Node path count: 1
                              Forwarding nexthops: 2
                                    Nexthop: 20.0.0.2 via ae0.0
                                    Nexthop: 20.0.0.6 via ae1.0
             BGP    Preference: 170/-101
                    Next hop type: Indirect
                    Next-hop reference count: 2
                    Source: 10.9.9.8
                    Next hop type: Router, Next hop index: 705
                    Next hop: 20.0.0.2 via ae0.0, selected
                    Label operation: Push 299776
                    Label TTL action: prop-ttl
                    Protocol next hop: 10.100.1.8
                    Push 299776
                    Indirect next hop: 92742ac 1048579
                    State: <Int Ext>
                    Inactive reason: AIGP
                    Local AS: 13979 Peer AS: 13979
                    Age: 18:19      Metric2: 53 
                    AIGP: 10 
                    Task: BGP_13979.10.9.9.8+179
                    AS path: I
                    Accepted
                    Route Label: 299776
                    Localpref: 100
                    Router ID: 10.9.9.8
                    Indirect next hops: 1
                            Protocol next hop: 10.100.1.8 Metric: 53
                            Push 299776
                            Indirect next hop: 92742ac 1048579
                            Indirect path forwarding next hops: 1
                                    Next hop type: Router
                                    Next hop: 20.0.0.2 via ae0.0
                            10.100.1.8/32 Originating RIB: inet.0
                              Metric: 53                      Node path count: 1
                              Forwarding nexthops: 1
                                    Nexthop: 20.0.0.2 via ae0.0
    

    Modified: 2015-05-19