Allied Telesis x908, X8100 manual BGP Configuring Prefix Filters, About prefix lists, Mask length

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BGP: Configuring Prefix Filters

BGP: Configuring Prefix Filters

Prefix filters use prefix lists to filter particular routes on the basis of their prefixes. Prefix filters and distribute filters both filter individual routes out of BGP update packets. They are mutually exclusive.

About prefix lists

A prefix list is a list of prefix entries. Each entry specifies a particular prefix, a mask length or range of mask lengths, and whether or not those prefixes are deemed to explicitly match or explicitly not match the prefix list.

A prefix list entry is created with the command:

awplus(config)# ip prefix-list <list-name> [seq <number>]  {denypermit} {any<ipadd>/<prefix-length>}  [ge <minimum-length>] [le <maximum-length>]

You can choose to give an entry a sequence number by using the optional seq parameter. If you do not, the switch assigns sequence numbers in steps of 5 (number 5, 10, 15 etc.) and puts the new entry at the end of the list of entries.

To see entries and their numbers, use the command:

awplus# show ip prefix-list

Mask length

You can specify a single prefix mask length, or you can use the ge and le parameters to specify a range of mask lengths for the entry to match.

If you set the mask length to:

a single mask length (by specifying neither the ge nor the le parameter), then a route matches against this entry if its prefix mask length is exactly that length.

a range of mask lengths, then a route matches against this entry if its prefix mask length is greater than or equal to ge and less than or equal to le.

For example, to deny the IP addresses between 10.0.0.0/14 (mask of 255.252.0.0) and 10.0.0.0/22 (mask of 255.255.252.0) within the 10.0.0.0/8 (mask of 255.0.0.0) addressing range, use the command:

awplus(config)# ip prefix-list mylist deny 10.0.0.0/8 le 22 ge 14

The mask length (8 in this example) must be less than the value specified for the ge parameter. This mask defines the range of subnets that the matching subnets must fall within.

Page 22 Use Route Maps and Other Filters to Filter and Alter BGP and OSPF Routes

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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 switch Confirming the neighbor relationshipawplus#show ip route BGP 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 3. Check that the IP route table now includes all the routes awplusconfig# neighbor 45.45.45.46 distribute-list list2 in6. Shut down the neighbor, and then bring it up again Use a numbered ACL instead of a named ACL awplusconfig-router# do show ip route BGP 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 again 7. 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 BGP Configuring Route Maps Structure of a route mapClauses Configuring a match clauseAn AS path list A community list CaseOne 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 Metric OSPF Configuring Route Maps for Filtering and Modifying OSPF RoutesInterface A prefix, by using an ACL A prefix, by using a prefix listExternal route type OSPF Applying Route Maps  Set the metric, by using the command set metric

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