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Thread: Border Gateway Protocol (Short Notes)

  1. #1
    Junior Member
    Join Date
    Aug 2012

    Border Gateway Protocol (Short Notes)

    Border Gateway Protocol is a protocol which runs in Internet, It can also be used in Corporate Office it it having more one ISP's for the sake of redundancy of Webserver VPN's.
    BGP is only one routing protocol which uses TCP for establishing connection, It runs on the TCP Port 179.

    BGP is a "Distance Vector Protocol" , But most call it as a "Path Vector " protocol, without tuning it just behaves like a RIP.
    BGP's Hope count is based on the AS Numbers.

    There are 2 types of BGP

    1) If the BGP is run between same AS Numbers, It is called as IBGP
    2) If it is run between different AS Numbers, It is called as EBGP

  2. #2
    Junior Member
    Join Date
    Aug 2012

    BGP Packets ,BGP Connection Establishment BGP Tables

    BGP Packets

    Open : Starts the Session
    Keepalive : Whether Neighbor router is there or not??
    Update : Network Reachability Changes
    Notification : Something Bad happens to close the session.

    BGP Connection States

    There are 6 states of BGP during BGP connection establishment
    1) IDLE State : Verifies the route to neighbor,Start the TCP connection with its configured peer.
    2) Connect State : Waits for Successful Negotiation with peer,BGP won't spend much time here.
    3) Active State : Attempting to connect with neighbor, If the neighbor cannot connect it will end up in this phase
    4) Open State : After successful establishment of connection, The BGP sends a Open message to neighbor.
    5) Open Confirm : The BGP FSM will open the OPEN MESSAGE it will check its validity, and sends keepalive messages.
    6) Established state: In this state the BGP exchanges information with peers.

    Router# show ip bgp neighbors

    BGP neighbor is, remote AS 10, external link

    Index 1, Offset 0, Mask 0x2

    Inbound soft reconfiguration allowed

    BGP version 4, remote router ID

    BGP state = Established, table version = 27, up for 00:06:12

    Last read 00:00:12, hold time is 180, keepalive interval is 60 seconds

    Minimum time between advertisement runs is 30 seconds

    Received 19 messages, 0 notifications, 0 in queue

    Sent 17 messages, 0 notifications, 0 in queue

    Inbound path policy configured

    Route map for incoming advertisements is testing

    Connections established 2; dropped 1

    Connection state is ESTAB, I/O status: 1, unread input bytes: 0

    Local host:, Local port: 11002

    Foreign host:, Foreign port: 179

    The above message is displayed after successful establishment of BGP session.

    BGP Tables:

    Neighbor Table: The connected BGP Neighbors.
    BGP Table : A list of all BGP Routes
    Routing Tables: A list of Best routes.

    R1> show ip bgp
    BGP table version is 5, local router ID is
    Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
    r RIB-failure
    Origin codes: i - IGP, e - EGP, ? - incomplete

    Network Next Hop Metric LocPrf Weight Path
    r> 0 130 0 30 i
    *> 0 125 0 30 i

  3. #3
    Junior Member
    Join Date
    Aug 2012

    BGP Trouble Shooting

    BGP Troubleshooting Simple Approach

    BGP is the most complex IP routing protocol currently deployed in the Internet.

    However, a structured approach to BGP troubleshooting, as illustrated in this and the next section can quickly lead you from initial problem diagnosis to the solution.

    Simple BGP Troubleshooting Figure

    Is it a BGP problem?

    Before jumping into BGP troubleshooting, you have to identify the source of the connectivity problem you're debugging (usually you suspect that BGP might be involved if one of your customers reports limited or no Internet connectivity beyond your network). Perform a traceroute from a workstation on the problematic LAN; if the trace reaches the first BGP-speaking router (or, even better, gets beyond the edge of your network) router, you're probably dealing with a BGP issue. Otherwise, check whether the BGP-speaking router advertises a default route into your network (without a default route, other routers in your network cannot reach the Internet destinations).

    Troubleshooting BGP adjacencies

    First check the status of TCP 3 Way handshake status before jumping into further.

    The BGP neighbors are configured manually, and the two most probable configuration errors are:

    Neighbor IP address mismatch
    AS number mismatch
    You could also have a problem with packet filters deployed on the BGP-speaking router. These filters have to allow packets to and from TCP port 179.

    Troubleshooting route propagation

    If your users want to receive traffic from the Internet, the IP prefix assigned to your network must be visible throughout the Internet. To get there, three steps are needed:

    Your BGP router must insert your IP prefix into its BGP table.
    The IP prefix must be advertised to its BGP neighbors.
    The IP prefix must be propagated throughout the Internet.

    Is the route inserted into BGP?

    Most routing protocols automatically insert directly connected IP subnets into their routing tables (or databases). Owing to security requirements, BGP is an exception; it will originate an IP prefix only if it's manually configured to do so (for example, Cisco routers use the network statement to configure advertised IP prefixes). Another option is route redistribution, which is highly discouraged in the Internet environment.

    Furthermore, to avoid attracting unroutable traffic, BGP will announce a configured IP prefix only if there's a matching route in the IP routing table. You could generate the matching IP route through route summarization, but it's usually best to configure a static route pointing to a null interface (or its equivalent).

    Is the route advertised to your neighbors?

    By default, all IP prefixes residing in the BGP table are announced to all BGP neighbors. Owing to security and routing policy requirements, the default behavior is usually modified with a set of output and input filters. If you have applied output filters toward your BGP neighbors, you have to check whether these filters allow your IP prefix to be propagated to the external BGP neighbors. The command to display routes advertised to a BGP neighbor on a Cisco router is show ip bgp neighbor ip-address advertised.

    Is the route visible throughout the Internet?

    Even if you've successfully announced your IP prefix to your BGP neighbors, it might still not be propagated throughout the Internet. It's hard to figure out exactly what's propagated beyond the boundaries of your network; the tools that can help you are called BGP looking glasses. Using these tools, you can inspect BGP tables at various points throughout the Internet and check whether your IP prefix has made it to those destinations.

    Advanced Approach

    The below are the things we need to check

    1) Have we received the prefix at distribution Edge Router.
    2) Is the the BGP prefix passing in BGP Distribution Network.
    3) Is these routes sent to external neighbors.

    BGP looking glasses canbe helpful to check the BGP Routes advertised to the network

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