Section 4 - BGP

address family

A major classification of type of network protocol, such as IPv4, IPv6, or VPNv4.

AS_PATH

A BGP attribute used to track the autonomous systems a network has been advertised through as a loop-prevention mechanism.

atomic aggregate

A BGP path attribute which indicates that a prefix has been summarized, and not all of the path information from component routes was included in the aggregate.

autonomous system (AS)

A set of routers running the same routing protocol under a single realm of control and authority.

eBGP session

A BGP session maintained with BGP peers from a different autonomous system.

iBGP session

A BGP session maintained with BGP peers from the same autonomous system.

optional non-transitive

A BGP path attribute that might be recognized by a BGP implementation that is not advertised between autonomous systems.

optional transitive

A BGP path attribute that might be recognized by a BGP implementation that is advertised between autonomous systems.

path vector routing protocol

A routing protocol that selects the best path based on path attributes.

well-known discretionary

A BGP path attribute recognized by all BGP implementations that may or may not be advertised to other peers.

well-known mandatory

A BGP path attribute recognized by all BGP implementations that must be advertised to other peers.

AS path access control list (ACL)

An ACL based on regex for identifying BGP routes based on the AS path and used for direct filtering or conditional matching in a route map.

BGP Multihoming

A method of providing redundancy and optimal routing that involves adding multiple links to external autonomous systems.

prefix list

A method of selecting routes based on binary patterns, specifically the high-order bit pattern, high-order bit count, and an optional prefix length parameter.

regular expression (regex)

Search patterns that use special key characters for parsing and matching.

route map

A feature used in BGP (and other IGP components) that allows for filtering or modification of routes using a variety of conditional matching ('match' and 'set' parameters).

transit routing

The act of allowing traffic to flow from one external autonomous system through your autonomous system to reach a different external autonomous system.

Multi-protocol BGP (MP-BGP)

An extension to BGPv4 that allows BGP to carry traffic for other protocols beyond just IPv4, by using address family identifiers (AFIs) and sub-address family identifiers (SAFIs).

Multiple Exit Discriminator (MED)

An optional non-transitive BGP attribute, sometimes called the metric, which provides a hint to external neighbors about the preferred path into an AS that has multiple entry points.

Local preference (LOCAL_PREF)

A well-known discretionary BGP attribute, that is exchanged between iBGP peers, and describes the preferred exit path from an AS.

ORIGIN

A well-known mandatory BGP attribute used to describe the origination of information in a BGP update, such as IGP, EGP, or incomplete.

NEXT_HOP

A well-known BGP attribute that lists the next-hop IP address used to reach an NLRI.

BGP Community

An optional transitive BGP attribute that allows tagging of routes for identification and policy decisions by other routers.

Weight attribute

A Cisco-proprietary BGP attribute, not shared with any neighbors, which allows a preferred path from a router to a specific network to be configured on a local router only.

N WLLA OMNI

A mnemonic for remembering the BGP best path selection process:

N - Next hop reachable?

W - Weight (Larger)

L - Local pref (Larger)

L - Locally injected routes

A - AS_Path (Smaller)

O - Origin (I over E over ?)

M - MED (Smaller)

N - Neighbor type (eBGP over iBGP)

I - IGP metric to next-hop (Smaller)