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IBM Direct Link Routing and BGP Configuration

 For Layer 3 Direct Link, the EBGP session per gateway runs between the Tata Communications MPLS PE (ASN 4755) and IBM (ASN 13884). The service is built as an L3VPN so your remote sites reach the VPC across the Tata Communications network; you do not run a BGP session from your on-premises CPE to IBM on Layer 3. (On Layer 2, BGP is yours to manage directly with IBM – see IBM L2VPN Services) 

IBM uses ASN 13884; Tata Communications uses ASN 4755. MD5 is optional and customer-supplied. Private peering uses a /30 private subnet allocated by Tata Communications or IBM. BFD runs at 300 ms × 3, and IBM enforces a 200-prefix limit per peer. 

BGP parameters

Parameter

Value

IBM ASN

13884 (default)

Tata Communications ASN

4755 (fixed)

BGP sessions

One per gateway; a pair (primary + secondary) for redundancy

MD5 authentication

Optional; provided by the customer and shared on both sides if used

BGP transit subnet (Private peering)

/30 private, allocated by Tata Communications or IBM

BFD

300 ms hello x 3

VLAN range (direct path)

2–3967, allocated by Tata Communications

Peering modes

IBM Direct Link offers only Private peering currently. 

Private peering reaches resources in your VPC. Tata Communications or IBM allocates a /30 private transit subnet (from the acceptable private ranges, via the standard IP-allocation process).

    The service is delivered as a Layer-3 Option-A NNI configured as VRF-Lite with a dedicated /32 router ID, built using L3VPN topologies (hub-and-spoke or full mesh).

    Prefix limits

    IBM enforces a prefix limit per peer. If the limit is exceeded, the BGP session is brought down and re-establishes once advertised prefixes are back under the limit. For a private circuit, IPv4 prefixes are limited to 200.

    Keep your outbound advertisements filtered to what IBM needs, within these limits, to avoid session resets.

    Prefix policy

    • Outbound (to IBM): Advertise only the on-premises prefixes IBM needs to reach. Sending a default route to IBM is rarely correct.

    • Inbound (from IBM): Accept the VPC prefixes (Private peering).

        Path control

        Where two connections are used for redundancy, Tata Communications applies BGP Local Preference and AS-PATH prepending to make one path primary and the other secondary. The values are set by Tata Communications during provisioning. See IBM Resiliency and High Availability.

        Quality of service

        IBM does not honour QoS markings on Direct Link. On the Tata Communications backbone, a single class of service is applied to the circuit; you can select the class that suits your application, but only one class applies per circuit.

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