Allied Telesis X8100 Update attributes, Origin, AS-path, Next-hop, Multi-Exit-Discriminator MED

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BGP: Concepts and Terminology

Update attributes

As mentioned above, each BGP update message contains a set of attributes. These attributes describe some of the properties of the routes, and can be used in making decisions about which routes to accept and which to reject. Some of the attributes are:

Origin

How a prefix came to be routed by BGP at the origin Autonomous System (AS). Prefixes are learned from various sources such as directly connected interfaces, manually configured static routes, or dynamic internal or external routing protocols, and then put into BGP.

AS-path

The list of Autonomous Systems (ASs) through which the announcement for the prefix has passed. As prefixes pass between Autonomous Systems each one adds its Autonomous System Number (ASN).

Next-hop

The address of the next node that the router should send packets destined for the specified prefixes to, in order to get the packets closer to the destination.

Multi-Exit-Discriminator (MED)

A metric expressing the optimal path to reach a particular prefix in or behind a particular AS.

Local-preference

A metric used in IBGP so each host knows which path inside the AS it should use to reach the advertised prefix. EBGP peers do not send this value, and ignore it on receipt.

Atomic-aggregate

A non-transitive attribute that allows BGP peers to inform each other about decisions they have made regarding overlapping routes. Non-transitive means that if the attribute is received by a device that does not recognise the attribute, it is dropped and not passed on to the next router.

Aggregator

Can be attached to an aggregated prefix to specify the AS and router that performed the aggregation.

Community

Indicates where a prefix is relevant to—for example, if it is relevant to the whole Internet, or just within an AS.

Use Route Maps and Other Filters to Filter and Alter BGP and OSPF Routes Page 5

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Contents Technical Guide Introduction Introduction Contents Which products and software version does it apply to? BGP Applying Route Maps to Imported RoutesWhich products and software version does it apply to? Related How To NotesBGP peers BGP updatesBGP Concepts and Terminology Origin Update attributesAS-path Next-hopBGP Overview of the Available Filter Types Access Control List ACL filtersFilter types Distribute filtersDifference and Relationship in BGP Examples for filtering BGP Update messagesBGP Example Hierarchy of the Different Filters Example Distribute filters on page  Example AS path filters on page  Example Prefix filters on pageBasic configuration awplus#show ip route Confirming the neighbor relationshipAlliedWare switch About ACLs BGP Configuring Distribute FiltersFilter out one particular route from a neighbor Using ACLs as filtersExample Distribute filters Filter out a range of prefix lengths 6. Shut down the neighbor, and then bring it up again awplusconfig# neighbor 45.45.45.46 distribute-list list2 in3. Check that the IP route table now includes all the routes Use a numbered ACL instead of a named ACL 4. Check that the IP route table no longer includes 52.0.0.0/8 AS path lists BGP Configuring AS Path FiltersDiscard or allow routes from a neighbor Using AS path lists as path filtersExample AS path filters 7. Shut down the neighbor, and then bring it up again 4. Shut down the neighbor, and then bring it up againa neighbor An outgoing filter that uses an AS-path list Another exampleBGP Configuring AS Path Filters Mask length BGP Configuring Prefix FiltersAbout prefix lists Example Prefix filters Using prefix lists as prefix filtersawplusconfig-router# neighbor neighbor prefix-list list-name in awplusconfig-router# neighbor neighbor prefix-list list-name outFilter out a range of different prefix lengths awplusconfig-router# do show ip route Structure of a route map BGP Configuring Route MapsAn AS path list Configuring a match clauseClauses Case A community listOne or more prefixes, by using an ACL One or more prefixes, by using a prefix listA next hop address Configuring a set clauseAn origin A metric the MED attributeset community community-values additive set atomic-aggregate set weightset extcommunity rtsoo ext-comm-number set ip next-hop ipaddOne match clause with an action The effect of different combinations of clausesNo match clause and one or more set clauses A match clause and one or more set clausesUpdate 1 to Peer Update from PeerUpdate 2 to Peer Router ASFirst, enter BGP router mode for the AS. The prompt should look like Example B Match on a prefix-list that denies an entry Examplesawplusconfig#ip prefix-list test1 permit 52.0.0.0/8 awplusconfig-route-map#set metric4. Apply this route map as the in route map on the neighbor Example E Prepending AS numbers Example D Matching on a next-hop prefix-listawplusconfig-route-map#match ip address awplusconfig-route-map#router bgpThe routes coming from that peer has community 1. Configure the AW peer to send out a community numberadd ip routem=com entry=1 set commmun=8989 set bgp peer=45.45.45.45 outroutemap=com sendcommunity=yesBGP#show ip route set bgp peer=45.45.45.45 outroutemap=mixed sendcommunity=yes 8. Add that ACL as a distribute-list in-filter on the neighbor route-map marker permit Examples Route maps ACLs Path filters Prefix filtersSyntax BGP Applying Route Maps to Imported RoutesOther Uses of Route Maps neighbor default-originateneighbor unsuppress-map networkBGP Route Map Filtering Example BGP configurationRoute map configuration set local-preference route-map outdef permit Interface OSPF Configuring Route Maps for Filtering and Modifying OSPF RoutesMetric External route type A prefix, by using a prefix listA prefix, by using an ACL  Set the metric, by using the command set metric OSPF Applying Route Maps

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