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Allied Telesis x908, X8100 manual Router AS, Update from Peer, Update 1 to Peer, Update 2 to Peer

Models: X8100 x908

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Update from Peer 1

BGP: Configuring Route Maps

Particular mention, though, has to be made of the case where the match clause specifies prefix list or ACL as the match criterion, and the route map is being applied to outgoing route updates.

The intention of such an entry would be that the attribute values specified in the set clauses be applied to only those routes that are contained in the prefix list or ACL specified in the match clause.

What happens when there is an outgoing update message that contains several prefixes, some of which are selected by the match clause and some of which are not? Given that an update packet contains just one set of attributes, it is not possible to say “change some of the attributes in the packet, but make them apply to only some of the prefixes contained in the packet”. The attributes in an update packet must apply to all the prefixes in the packet.

The only solution is to break up the update packet. The switch creates one packet that contains the original attribute values and the prefixes that were not specified in the match clause of the route map entry. Then the switch creates another packet that contains those prefixes of the original packet which did match the prefix list or ACL in the match clause, but with modified attribute values.

The following diagram illustrates the BGP process of splitting an outgoing update:

BGP process of splitting an outgoing update when a filter changes the AS attribute for some of the prefixes in the update

Update from Peer 1

ASPathList=4,3,2,1

192.168.1.0/24

192.168.2.0/24

172.30.0.0/16

172.31.0.0/16

Router (AS 5)

Add peer Update to IP table

Router generates a new Update, from the BGP route table, to send to another peer

Take internally generated Update and add the AS “8” for matches of 192.168.0.0/16

Update 1 to Peer 2

ASPathList = 5,8,4,3,2,1 192.168.1.0/24 192.168.2.0/24

Update 2 to Peer 2

ASPathList = 5,4,3,2,1 172.30.0.0/16 172.31.0.0/16

Page 34 Use Route Maps and Other Filters to Filter and Alter BGP and OSPF Routes

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Allied Telesis x908, X8100 manual Router AS, Update from Peer, Update 1 to Peer, Update 2 to Peer

Contents

Introduction Technical GuideWhich products and software version does it apply to? ContentsIntroduction BGP Applying Route Maps to Imported RoutesRelated How To Notes Which products and software version does it apply to?BGP Concepts and Terminology BGP updatesBGP peers AS-path Update attributesOrigin Next-hopFilter types Access Control List ACL filtersBGP Overview of the Available Filter Types Distribute filtersExamples for filtering BGP Update messages Difference and Relationship in BGP Example Distribute filters on page Hierarchy of the Different FiltersBGP Example  Example AS path filters on page  Example Prefix filters on pageBasic configuration AlliedWare Plus switch Confirming the neighbor relationshipAlliedWare switch awplus#show ip routeBGP Configuring Distribute Filters About ACLsExample Distribute filters Using ACLs as filtersFilter out one particular route from a neighbor Filter out a range of prefix lengths awplusconfig# router bgp awplusconfig# neighbor 45.45.45.46 distribute-list list2 inawplusconfig-router# do show ip route 3. Check that the IP route table now includes all the routesUse a numbered ACL instead of a named ACL awplusconfig-router# do show ip route 4. Check that the IP route table no longer includes 52.0.0.0/8BGP Configuring AS Path Filters AS path listsExample AS path filters Using AS path lists as path filtersDiscard or allow routes from a neighbor a neighbor 4. Shut down the neighbor, and then bring it up again7. Shut down the neighbor, and then bring it up again Another example An outgoing filter that uses an AS-path listBGP Configuring AS Path Filters About prefix lists BGP Configuring Prefix FiltersMask length awplusconfig-router# neighbor neighbor prefix-list list-name in Using prefix lists as prefix filtersExample Prefix filters awplusconfig-router# neighbor neighbor prefix-list list-name outFilter out a range of different prefix lengths BGP Configuring Prefix Filters awplusconfig-router# do show ip routeBGP Configuring Route Maps Structure of a route mapClauses Configuring a match clauseAn AS path list Case A community listCase One or more prefixes, by using a prefix list One or more prefixes, by using an ACLAn origin Configuring a set clauseA next hop address A metric the MED attributeset community community-values additive set extcommunity rtsoo ext-comm-number set weightset atomic-aggregate set ip next-hop ipaddNo match clause and one or more set clauses The effect of different combinations of clausesOne match clause with an action A match clause and one or more set clausesUpdate 2 to Peer Update from PeerUpdate 1 to Peer Router ASFirst, enter BGP router mode for the AS. The prompt should look like awplusconfig#ip prefix-list test1 permit 52.0.0.0/8 ExamplesExample B Match on a prefix-list that denies an entry awplusconfig-route-map#set metric4. Apply this route map as the in route map on the neighbor awplusconfig-route-map#match ip address Example D Matching on a next-hop prefix-listExample E Prepending AS numbers awplusconfig-route-map#router bgpadd ip routem=com entry=1 set commmun=8989 1. Configure the AW peer to send out a community numberThe routes coming from that peer has community 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 ACLs Path filters Prefix filters Route mapsBGP Applying Route Maps to Imported Routes Syntaxneighbor unsuppress-map neighbor default-originateOther Uses of Route Maps networkRoute map configuration BGP configurationBGP Route Map Filtering Example set local-preference route-map outdef permit Interface OSPF Configuring Route Maps for Filtering and Modifying OSPF RoutesMetric Configuring a match clauseExternal route type A prefix, by using a prefix listA prefix, by using an ACL Configuring a set clauseOSPF Applying Route Maps  Set the metric, by using the command set metric