Extreme Networks ExtremeWare XOS 10.1, Manual

Page: 204 / 256

204 ExtremeWare XOS 10.1 Concepts Guide
Exterior Gateway Routing Protocols
Overview
BGP is an exterior routing protocol that was developed for use in TCP/IP networks. The primary
function of BGP is to allow different autonomous systems (ASs) to exchange network reachability
information.
An autonomous system is a set of routers that are under a single technical administration. This set of
routers uses a different routing protocol (such as OSPF) for intra-AS routing. One or more routers in the
AS are configured to be border routers, exchanging information with other border routers (in different
autonomous systems) on behalf of all of the intra-AS routers.
BGP can be used as an exterior gateway protocol (EBGP), or it can be used within an AS as an interior
gateway protocol (IBGP).
BGP Attributes
The following BGP attributes are supported by the switch:
•
Origin – Defines the origin of the route. Possible values are IGP, EGP, and incomplete.
• AS_Path – The list of ASs that are traversed for this route.
• Next_hop – The IP address of the next hop BGP router to reach the destination listed in the NLRI
field.
• Multi_Exist_Discriminator – Used to select a particular border router in another AS when multiple
border routers exist.
• Local_Preference – Used to advertise this router’s degree of preference to other routers within the
AS.
• Atomic_aggregate – Indicates that the sending border router has used a route aggregate prefix in the
route update.
•
Aggregator – Identifies the BGP router AS number and IP address that performed route aggregation.
• Community – Identifies a group of destinations that share one or more common attributes.
• Cluster_ID – Specifies a 4-byte field used by a route reflector to recognize updates from other route
reflectors in the same cluster.
• Originator_ID – Specifies the router ID of the originator of the route in the local AS.
• Multiprotocol Reachable NLRI – This is an optional attribute and is used to:
— Advertise a feasible route to a peer.
—
Permit a router to advertise the Network Layer address of the router that should be used as the
next hop to the destinations listed in the Network Layer Reachability Information field of the
MP_NLRI attribute.
— Allow a given router to report some or all of the Subnetwork Points of Attachment (SNPAs) that
exist within the local system.
• Multiprotocol Unreachable NLRI – This is an optional attribute that can be used for the purpose of
withdrawing multiple unfeasible routes from service.