Allied Telesis x908 OSPF Configuring Route Maps for Filtering and Modifying OSPF Routes, Metric

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OSPF: Configuring Route Maps for Filtering and Modifying OSPF Routes

OSPF: Configuring Route Maps for Filtering and Modifying OSPF Routes

For information about route maps and their structure, see "Structure of a route map" on page 26.

Route maps can be applied to OSPF routes as well as BGP routes. Of course, the route maps that can be used with OSPF are rather limited in comparison with those that are used with BGP, as OSPF route updates do not carry attributes in the way that BGP route updates do.

Also, OSPF route maps can only be applied to importing:

OSPF-learnt routes into the main IP route table, or

static, BGP or RIP routes into OSPF

Filtering cannot:

remove an entry from the LSA database once the entry has been added

prevent the router from advertising an LSA database entry to interfaces in the same area that the entry is relevant to

prevent updates that OSPF learns from being put into the LSA database

change the properties of an entry in the LSA database

This is because OSPF shares LSAs between all the routers in an area. The protocol assumes that all the routers in the area have shared all the advertisements among each other, and that all agree on the state of the complete link state database for the area. If some routers in the area are learning, but not advertising, that breaks the OSPF model.

Configuring a match clause

A match clause can match on the following criteria.

Note: When configuring a match clause, make sure you are in route map mode. The prompt should look like: awplus(config-route-map)#

Metric

The entry will match all routes whose metric is equal to that specified in the clause.

To match a metric value, use the command:

match metric <value>

Interface

The entry will match all routes learnt via the specified VLAN.

To match a VLAN, use the command:

match interface <vlan>

Page 50 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 peers BGP updatesBGP Concepts and Terminology 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 awplus#show ip route Confirming the neighbor relationshipAlliedWare switch BGP Configuring Distribute Filters About ACLsFilter 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 awplusconfig-router# do show ip route BGP Configuring AS Path Filters AS path listsDiscard 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 Another example An outgoing filter that uses an AS-path listBGP Configuring AS Path Filters Mask length BGP Configuring Prefix FiltersAbout prefix lists 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 mapAn AS path list Configuring a match clauseClauses 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 Syntax neighbor unsuppress-map neighbor default-originate Other Uses of Route Maps 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 OSPF Applying Route Maps  Set the metric, by using the command set metric

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