Section B.10. BGP-4 Message Types and Parameters (Chapter 8)


B.10. BGP-4 Message Types and Parameters (Chapter 8)

Table B-23 explains the possible BGP message types in BGP message Header.

Table B-23. BGP message types

Type

Name

Description

1

OPEN

Initializes BGP connection and negotiates session parameters

2

UPDATE

Exchanges feasible and withdrawn BGP routes

3

NOTIFICATION

Reports errors or terminates BGP connections

4

KEEPALIVE

Keeps the BGP connection from expiring


At the moment, two optional parameters are specified in an OPEN message, as explained in Table B-24. The optional parameter BGP Capability is very important for IPv6 support.

Table B-24. Optional parameters

Type

Name

Description

1

Authentication

The parameter consists of two fields: Authentication Code and Authentication Data. The Authentication Code defines the authentication mechanism used and how the marker and authentication data fields are to be computed.

2

BGP Capability

The parameter consists of one or more <Code, Length, Value> triplets identifying different BGP Capabilities. It is defined in RFC 3392. The capability parameter may appear more than once in the OPEN message.

The Capability Code set to 1 indicates the Multiprotocol Extension Capability as defined in RFC 2858.


Table B-25 lists and explains some of the most common attributes. Detailed explanations should be taken directly from RFC 1771 or any RFC extending BGP (e.g., BGP Route Reflection defines attribute types 9 and 10).

Table B-25. BGP attributes

Type

Name/flags

Description

1

ORIGIN (well-known)

Defines the original source of this route.

0=IGP, 1=EGP, 2=Incomplete

2

AS_PATH (well-known)

A sequence of AS numbers that this route has crossed during its update. The rightmost AS number defines the originating AS. Each AS crossed is prepended. Prevents loops and can be used for policies.

3

NEXT_HOP (well-known)

Specifies the next hop's IPv4 address. Cannot be used for IPv6.

4

MED (optional nontransitive)

The MULTI_EXIT_DISC (MED) indicates a desired preference (4-byte) of the route to the peerthe lower the better. Designed for multiple EBGP connections between two ASes to load-share inbound traffic.

5

LOCAL_PREF (well-known)

Defines a local preference (4 byte) of the route. The higher the better. It is usually calculated on routes arriving from external peers and preserved to internal peers. Designed for multiple EBGP connections to any AS to manage outbound traffic.

6

ATOMIC_AGGREGATE (well-known)

Specifies that one of the routers has selected the less-specific route over a more-specific route.

7

AGGREGATOR (optional transitive)

The BGP Identifier of the router that aggregated routes into this route.

8

COMMUNITY (optional transitive)

Carries a 4-byte informational tag. Can be used by the route selection process. Defined in RFC 1997.

14

MP_REACH_NLRI (optional nontransitive)

Advertises multiprotocol NLRI. Used for IPv6 prefixes. See "BGP Multiprotocol Extension for IPv6."

15

MP_UNREACH_NLRI (optional nontransitive)

Withdraws multiprotocol NLRI. Used for IPv6 prefixes. See "BGP Multiprotocol Extension for IPv6."




IPv6 Essentials
IPv6 Essentials
ISBN: 0596100582
EAN: 2147483647
Year: 2004
Pages: 156
Authors: Silvia Hagen

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