Allied Telesis X8100, x908 manual Clauses, Configuring a match clause, An AS path list

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BGP: Configuring Route Maps

Clauses

There are two types of clauses that can be present in a route map entry:

match clauses, which specify attributes or prefixes to match on

set clauses, which specify the changes to be made to attribute values

A given route map entry can never have more than one match clause, but it can have multiple set clauses.

Configuring a match clause

When you configure a match clause, you can match on one of the attributes listed in the following sections.

An AS path list

For information about creating an AS path list, see "AS path lists" on page 17.

Once you have made the path list, you can apply it to the match clause of a route map entry by using the commands:

awplus(config)# route-map <map-name> {denypermit} <seq>

awplus(config-route-map)# match as-path<list-name>

This clause will cause the router to go through the entries in the AS path list. If one of the entries in the AS path list matches the AS path in the update packet, and the action on that AS path list entry is permit, then the packet is deemed to have matched this route map match clause. The fact that there are two levels of matching going on, and effectively two different uses of the permit/deny parameters in the one clause, can be a bit confusing.

When working out the correct way to configure a clause that matches on AS path list, it is important to first think about how you are configuring the match criteria within the AS path list, and then separately think about the actions of the route map entry.

In particular, it is important to note that in this context, the parameters in the AS path list do not indicate whether the matching update packet (remember that the AS Path attribute is only set once in an update packet) is being allowed or dropped; "permit" and "deny" simply indicate whether the update packet is deemed to match or not match the AS path list. This is different to the case where the AS path list itself is being used as a filter.

Just as an example, consider these configurations:

Use Route Maps and Other Filters to Filter and Alter BGP and OSPF Routes Page 27

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Contents Technical Guide IntroductionBGP Applying Route Maps to Imported Routes ContentsIntroduction Which products and software version does it apply to?Which products and software version does it apply to? Related How To NotesBGP updates BGP Concepts and TerminologyBGP peers Next-hop Update attributesOrigin AS-pathDistribute filters Access Control List ACL filtersBGP Overview of the Available Filter Types Filter typesDifference and Relationship in BGP Examples for filtering BGP Update messages Example AS path filters on page  Example Prefix filters on page Hierarchy of the Different FiltersBGP Example  Example Distribute filters on pageBasic configuration Confirming the neighbor relationship AlliedWare switchawplus#show ip route About ACLs BGP Configuring Distribute FiltersUsing ACLs as filters Example Distribute filtersFilter out one particular route from a neighbor Filter out a range of prefix lengths awplusconfig# neighbor 45.45.45.46 distribute-list list2 in 3. Check that the IP route table now includes all the routes6. Shut down the neighbor, and then bring it up again 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 FiltersUsing AS path lists as path filters Example AS path filtersDiscard or allow routes from a neighbor 4. Shut down the neighbor, and then bring it up again a neighbor7. Shut down the neighbor, and then bring it up again An outgoing filter that uses an AS-path list Another exampleBGP Configuring AS Path Filters BGP Configuring Prefix Filters About prefix listsMask length awplusconfig-router# neighbor neighbor prefix-list list-name out Using prefix lists as prefix filtersExample Prefix filters awplusconfig-router# neighbor neighbor prefix-list list-name inFilter out a range of different prefix lengths awplusconfig-router# do show ip route Structure of a route map BGP Configuring Route MapsConfiguring a match clause ClausesAn AS path list Case A community listOne or more prefixes, by using an ACL One or more prefixes, by using a prefix listA metric the MED attribute Configuring a set clauseA next hop address An originset community community-values additive set ip next-hop ipadd set weightset atomic-aggregate set extcommunity rtsoo ext-comm-numberA match clause and one or more set clauses The effect of different combinations of clausesOne match clause with an action No match clause and one or more set clausesRouter AS Update from PeerUpdate 1 to Peer Update 2 to PeerFirst, enter BGP router mode for the AS. The prompt should look like awplusconfig-route-map#set metric ExamplesExample B Match on a prefix-list that denies an entry awplusconfig#ip prefix-list test1 permit 52.0.0.0/84. Apply this route map as the in route map on the neighbor awplusconfig-route-map#router bgp Example D Matching on a next-hop prefix-listExample E Prepending AS numbers awplusconfig-route-map#match ip addressset bgp peer=45.45.45.45 outroutemap=com sendcommunity=yes 1. Configure the AW peer to send out a community numberThe routes coming from that peer has community add ip routem=com entry=1 set commmun=8989BGP#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 Routesnetwork neighbor default-originateOther Uses of Route Maps neighbor unsuppress-mapBGP configuration Route map configurationBGP Route Map Filtering Example set local-preference route-map outdef permit OSPF Configuring Route Maps for Filtering and Modifying OSPF Routes MetricInterface A prefix, by using a prefix list A prefix, by using an ACLExternal route type  Set the metric, by using the command set metric OSPF Applying Route Maps
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X8100, x908 specifications

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