When Router is in Join State, it’s ready for a accomplished TCP connection. To do that process, the 2 neighbors should carry out the usual TCP three-way handshake and open a TCP connection to port 179.Under is a state of affairs the place 3-way handshake occurs between shopper and net server.
In the identical approach, BGP additionally makes use of TCP 3-way handshake to type neighborship.
All BGP message are unicast to the one neighbor over the TCP connection.
BGP Message Varieties
The several types of BGP message are:
- Open
- Keepalive
- Replace
- Notification
BGP Message Sort I: OPEN
Open messages are used to start out a BGP session by requesting {that a} BGP session be opened over an current TCP/IP session. As soon as two BGP routers have accomplished a TCP 3-way handshake they’ll try to ascertain a BGP session, that is achieved utilizing open messages. Within the open message details about BGP router shall be obtainable.
Routers use this message to establish itself and to specify its BGP operational parameters. Open message is all the time ship when the TCP session is established between neighbors.
Open message embrace following fields:
- Model – specifies the model (2, 3 or 4), default being model 4.
- Autonomous System – supplies AS variety of the sender. It determines whether or not the BGP session is EBGP or IBPG (if the AS quantity are the identical )
- Maintain-Time – signifies the utmost variety of seconds that may elapse with out receipt of message earlier than transmitter is assumed to be nonfunctional. The default maintain time is 180 sec. If the neighbors maintain time differ, the decrease of the 2 instances turn out to be the accepted maintain time.
- BGP Identifier – Offers the BGP identifier of the sender (an IP deal with). IOS determines identifier in precisely the identical approach as OSPF router ID. The best loopback interface deal with is used, if there isn’t a loopback the numerically highest IP deal with on a bodily interface is chosen.
- Non-compulsory Parameters Size – signifies the size or absence (with a zero worth) of the optionally available parameters filed
- Non-compulsory Parameters – incorporates an inventory of optionally available parameters as authentication, multiprotocol assist and route refresh. It consists of
- Help for MP-BGP (Multi-Protocol BGP).
- Help for Route Refresh.
- Help for 4-octet AS numbers.
BGP Message Sort II: KEEPALIVE
If a router accepts the parameters laid out in Open message, it responds Keepalive. By default Cisco sends keepalive each 60 sec or a interval equal to 1/3 the maintain time.
BGP Message Sort III: UPDATE MESSAGE
Advertises possible routes, withdrawn routes or each. Replace message incorporates 5 fields:
Discipline Size in Bytes
- Unfeasible Routes Size – Signifies the whole size of the withdrawn routes discipline or that the sector shouldn’t be current.
- Withdrawn Routes — Incorporates an inventory of IP deal with prefixes for routes being withdrawn from. These are (Size, Prefix) tuples describing locations which have turn out to be unreachable and are being withdrawn from service.
- Complete Path Attribute Size — Signifies the whole size of the trail attributes discipline or that the sector shouldn’t be current.
- Path Attributes — Describes the traits of the marketed path. The next are potential attributes for a path.
- Origin: Necessary attribute that defines the origin of the trail data
- AS Path: Necessary attribute composed of a sequence of autonomous system path segments
- Subsequent Hop: Necessary attribute that defines the IP deal with of the border router that ought to be used as the subsequent hop to locations listed within the community layer reachability data discipline
- Multi Exit Disc: Non-compulsory attribute used to discriminate between a number of exit factors to a neighboring autonomous system
- Native Pref: Discretionary attribute used to specify the diploma of desire for an marketed route
- Atomic Combination: Discretionary attribute used to reveal details about route alternatives
- Aggregator: Non-compulsory attribute that incorporates details about mixture routes
- Community Layer Reachability Data (NLRI) — Incorporates an inventory of IP deal with prefixes for the marketed routes.
BGP Message Sort IV: NOTIFICATION MESSAGE
This message is distributed every time one thing dangerous has occurred, e.g. an error is detected and causes the BGP connection to shut.
Discipline Size in Bytes
- Error Code — signifies the kind of error that occurred. The next are the error varieties outlined by the sector:
- Message Header Error: Signifies an issue with a message header, corresponding to unacceptable message size, unacceptable marker discipline worth, or unacceptable message sort.
- Open Message Error: Signifies an issue with an open message, corresponding to unsupported model quantity, unacceptable autonomous system quantity or IP deal with, or unsupported authentication code.
- Replace Message Error: Signifies an issue with an replace message, corresponding to a malformed attribute checklist, attribute checklist error, or invalid next-hop attribute.
- Maintain Time Expired: Signifies that the hold-time has expired, after which period a BGP node shall be thought of nonfunctional.
- Finite State Machine Error: Signifies an sudden occasion.
- Stop: Closes a BGP connection on the request of a BGP gadget within the absence of any deadly errors.
- Error Subcode — Offers extra particular details about the character of the reported error.
- Error Knowledge — Incorporates information primarily based on the error code and error subcode fields. This discipline is used to diagnose the rationale for the notification message.
