CCNP Advanced Routing BGP Part II This Presentations

CCNP – Advanced Routing BGP (Part II) This Presentations was originally created by Rick Graziane. Few modifications were made by Prof. Yousif 1

Concepts, diagrams, and examples This presentation is based partly on information from the books Routing TCP/IP Vol. II by Jeff Doyle and Jennifer Carroll and Cisco BGP Command Configuration Handbook by Parkhurst. 2

Other source, Cisco on-line n Quite a few of the examples in this presentation are taken from Cisco’s web site: – http: //www. cisco. com/univercd/cc/td/doc/cisi ntwk/icsbgp 4. htm 3

Show ip bgp To display entries in the BGP routing table, use the show ip bgp EXEC command. show ip bgp [network] [network-mask] [longer-prefixes] n Let’s look at an example, but some of the options will be discussed later. 4

Router. C#show ip bgp BGP table version is 8, local router ID is 200. 66 Status codes: s suppressed, d damped, h history, * valid, > best, i internal Origin codes: i - IGP, e - EGP, ? - incomplete Network *> 11. 0. 0. 0 *> 12. 0. 0. 0 *> 193. 10. 2. 0 Next Hop 0. 0 200. 65 Metric Loc. Prf Weight Path 0 32768 i 0 300 200 i 0 0 300 i 5

Router. C#show ip bgp BGP table version is 8, local router ID is 200. 66 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network *> 11. 0. 0. 0 *> 12. 0. 0. 0 *> 193. 10. 2. 0 Next Hop 0. 0 200. 65 Metric Loc. Prf Weight Path 0 32768 i 0 300 200 i 0 0 300 i BGP table version - Internal version number of the table. This number is incremented whenever the table changes. n local router ID - IP address of the router. n Status codes - Status of the table entry. The status is displayed at the beginning of each line in the table. It can be one of the following values: n s —The table entry is suppressed. * —The table entry is valid. > —The table entry is the best entry to use for that network. i —The table entry was learned via an internal BGP (i. BGP) session 6

Router. C#show ip bgp BGP table version is 8, local router ID is 200. 66 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network *> 11. 0. 0. 0 *> 12. 0. 0. 0 *> 193. 10. 2. 0 Next Hop 0. 0 200. 65 Metric Loc. Prf Weight Path 0 32768 i 0 300 200 i 0 0 300 i Origin codes - Origin of the entry. The origin code is placed at the end of each line in the table. It can be one of the following values: – i —Entry originated from Interior Gateway Protocol (IGP) and was advertised with a network router configuration command. – e —Entry originated from Exterior Gateway Protocol (EGP). – ? —Origin of the path is not clear. Usually, this is a router that is redistributed into BGP from an IGP. n Network - IP address of a network entity. n Next Hop - IP address of the next system that is used when forwarding a packet to the destination network. An entry of 0. 0 indicates that the router has some non-BGP routes to this network n 7

Router. C#show ip bgp BGP table version is 8, local router ID is 200. 66 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network *> 11. 0. 0. 0 *> 12. 0. 0. 0 *> 193. 10. 2. 0 Next Hop 0. 0 200. 65 Metric Loc. Prf Weight Path 0 32768 i 0 300 200 i 0 0 300 i Metric - If shown, the value of the interautonomous system metric. n Loc. Prf - Local preference value as set with the set localpreference route-map configuration command. The default value is 100. n Weight - Weight of the route as set via autonomous system filters. n Path - Autonomous system paths to the destination network. There can be one entry in this field for each autonomous system in the path. 8 n

BGP Attributes n n n n ORIGIN NEXT_HOP AS_PATH LOCAL_PREF Weight MULTI_EXIT_DISC (MED) ATOMIC_AGGREGATE NOTE: For several of these attributes, multiple options have been included in this presentation. Because of time and to make sure we first grasp the basic concepts, some of the options are added to the presentation for your own information and reference. 9

Summary of the BGP Path Selection Process n BGP selects only one path as the best path. n When the path is selected, BGP puts the selected path in its routing table and propagates the path to its neighbors. n BGP uses the following criteria, in the order presented, to select a path for a destination: 1. If the path specifies a next hop that is inaccessible, drop the update. 2. Prefer the path with the largest weight. 3. If the weights are the same, prefer the path with the largest local preference. 4. If the local preferences are the same, prefer the path that was originated by BGP running on this router. 5. If no route was originated, prefer the route that has the shortest AS_path. 6. If all paths have the same AS_path length, prefer the path with the lowest origin type (where IGP is lower than EGP, and EGP is lower than Incomplete). 7. If the origin codes are the same, prefer the path with the lowest MED attribute. 8. If the paths have the same MED, prefer the external path over the internal path. 9. If the paths are still the same, prefer the path through the closest IGP neighbor. 10. Prefer the path with the lowest IP address, as specified by the BGP router ID. 10

BGP Best Path Selection Algorithm More info: n http: //www. cisco. com/warp/public/459/25. shtml 11

The ORIGIN attribute Well-known mandatory attribute (type code 1) n Indicates the origin of the routing update – IGP: The prefix is internal to the originating AS. – EGP: The prefix was learned via some EGP, such as BGP. – INCOMPLETE: The prefix was learned by some other means, probably redistribution. n BGP considers the ORIGIN attribute in its decision-making process to establish a preference ranking among multiple routes. n Specifically, BGP prefers the path with the lowest origin type, where IGP is lower than EGP, and EGP is lower than INCOMPLETE. n 12

Path Attributes Attribute Code Type 1 -ORIGIN Well-known mandatory 2 -AS_PATH Well-known mandatory 3 -NEXT_HOP Well-known mandatory 4 -MULTI_EXIT_DISC Optional non-transitive 5 -LOCAL_PREF Well-known discretionary 6 -ATOMIC_AGGREGATE Well-known discretionary 7 -AGGREGATOR Well-known discretionary 8 -COMMUNITY Optional transitive (Cisco) 9 -ORIGINATOR_ID Optional non-transitive (Cisco) 10 -Cluster List Optional non-transitive (Cisco) 11 -Destination Preference (MCI) 12 -Advertiser (Baynet) 13 -rcid_path (Baynet) 255 -Reserved [md] 13

Origin Attribute The origin attribute provides information about the origin of the route. The origin of a route can be one of three values: n IGP—The route is interior to the originating AS. – This value is set when the network router configuration command is used to inject the route into BGP. – The IGP origin type is represented by the letter i in the i output of the show ip bgp EXEC command. n EGP—The route is learned via the Exterior Gateway Protocol (EGP). – The EGP origin type is represented by the letter e in the e output of the show ip bgp EXEC command. n Incomplete—The origin of the route is unknown or learned in some other way. – An origin of Incomplete occurs when a route is redistributed into BGP. – The Incomplete origin type is represented by the ? symbol in the output of the show ip bgp EXEC command 14

Router A router bgp 100 neighbor 190. 10. 50. 1 remote-as 100 neighbor 170. 10. 2 remote-as 300 network 150. 10. 0. 0 redistribute static ip route 190. 10. 0. 0 255. 0. 0 null 0 Router B router bgp 100 neighbor 150. 10. 30. 1 remote-as 100 network 190. 10. 50. 0 Router E router bgp 300 neighbor 170. 10. 20. 1 remote-as 100 network 170. 10. 0. 0 Given these configurations, the following is true: n From Router A, the route for reaching 170. 10. 0. 0 has an AS_path of 300 and an origin attribute of IGP. n From Router A, the route for reaching 190. 10. 50. 0 has an empty AS_path (the route is in the same AS as Router A) and an origin attribute of IGP. n From Router E, the route for reaching 150. 10. 0. 0 has an AS_path of 100 and an origin attribute of IGP. n From Router E, the route for reaching 190. 10. 0. 0 has an AS_path of 100 and an origin attribute of Incomplete (because 15 190. 10. 0. 0 is a redistributed route)

The ORIGIN attribute n Use a route map and the set origin command to manipulate the ORIGIN attribute. route-map SETORIGIN permit 10 set origin igp 16

Path Attributes Attribute Code Type 1 -ORIGIN Well-known mandatory 2 -AS_PATH Well-known mandatory 3 -NEXT_HOP Well-known mandatory 4 -MULTI_EXIT_DISC Optional non-transitive 5 -LOCAL_PREF Well-known discretionary 6 -ATOMIC_AGGREGATE Well-known discretionary 7 -AGGREGATOR Well-known discretionary 8 -COMMUNITY Optional transitive (Cisco) 9 -ORIGINATOR_ID Optional non-transitive (Cisco) 10 -Cluster List Optional non-transitive (Cisco) 11 -Destination Preference (MCI) 12 -Advertiser (Baynet) 13 -rcid_path (Baynet) 255 -Reserved [md] 17

NEXT_HOP The NEXT_HOP attribute is a well-known mandatory attribute (type code 3). n In terms of an IGP, such as RIP, the “next hop” to reach a route is the IP address of the router that has announced the route. n – Note: The abbreviation IGP (Interior Gateway Protocol) will always be in green, so not to get it confused with IBGP (Interior BGP) n The NEXT_HOP concept with BGP is slightly more elaborate. 18

NEXT_HOP For EBGP sessions, the next hop is the IP address of the neighbor that announced the route n For IBGP sessions, for routes originated inside the AS, the next-hop is the IP address of the neighbor that announced the route. n For routes injected into the AS via EBGP, the next hop learned from EBGP is carried unaltered into IBGP. n – The next hop is the IP address of the EBGP neighbor from which the route was learned. 19

n n n Router A router bgp 100 neighbor 170. 10. 2 remote-as 300 neighbor 150. 10. 50. 1 remote-as 100 network 150. 10. 0. 0 Router B router bgp 100 neighbor 150. 10. 30. 1 remote-as 100 Router C router bgp 300 neighbor 170. 10. 20. 1 remote-as 100 network 170. 10. 0. 0 n n n Router C advertises network 170. 10. 0. 0 to Router A with a next hop attribute of 170. 10. 2, and Router A advertises network 150. 10. 0. 0 to Router C with a next hop attribute of 170. 10. 20. 1. BGP specifies that the next hop of EBGPlearned routes should be carried without modification into IBGP. Because of that rule, Router A advertises 170. 10. 0. 0 to its IBGP peer (Router B) with a next hop attribute of 170. 10. 2. As a result, according to Router B, the next hop to reach 170. 10. 0. 0 is 170. 10. 2, instead of 150. 10. 30. 1. For that reason, the configuration must ensure that Router B can reach 170. 10. 2 via an IGP. Otherwise, Router B will drop packets destined for 170. 10. 0. 0 because the next hop address is inaccessible. For example, if Router B runs IGRP, Router A should run IGRP on network 170. 10. 0. 0. You might want to make IGRP passive on the link to Router C so that only BGP 20 updates are exchanged.

Summarize n Router C advertises 170. 10. 0. 0 to Router A with a next hop attribute of 170. 10. 2, and Router A advertises 170. 10. 0. 0 to Router B with a next hop attribute of 170. 10. 2. n The next hop of EBGP-learned routes is passed to the IBGP neighbor. Router A router bgp 100 neighbor 170. 10. 2 remote-as 300 neighbor 150. 10. 50. 1 remote-as 100 network 150. 10. 0. 0 Router B router bgp 100 neighbor 150. 10. 30. 1 remote-as 100 Router C router bgp 300 neighbor 170. 10. 20. 1 remote-as 100 network 170. 10. 0. 0 21

Next Hop Attribute and Multiaccess Media n n n Routers C and D are in AS 300 are running OSPF. Router C is running BGP with Router A. Router C can reach network 180. 20. 0. 0 via 170. 10. 20. 3. When Router C sends a BGP update to Router A regarding 180. 20. 0. 0, it sets the next hop attribute to 170. 10. 20. 3, instead of its own IP address (170. 10. 2). This is because Routers A, B, and C are in the same subnet, and it makes more sense for Router A to use Router D as the next hop rather than taking an extra hop via Router C. 22

Next Hop Attribute and Nonbroadcast Media Access n n Router C router bgp 300 neighbor 170. 10. 20. 1 remote-as 100 neighbor 170. 10. 20. 1 next-hop-self n n n Routers A, C, and D, use a common media such as Frame Relay (or any NBMA cloud). Router C advertises 180. 20. 0. 0 to Router A with a next hop of 170. 10. 20. 3, just as it would do if the common media were Ethernet. The problem is that Router A does not have a direct permanent virtual connection (PVC) to Router D and cannot reach the next hop, so routing will fail. To remedy this situation, use the neighbor next-hop-self router configuration command. The neighbor next-hop-self command causes Router C to advertise 180. 20. 0. 0 with the next 23 hop attribute set to 170. 10. 2.

Path Attributes Attribute Code Type 1 -ORIGIN Well-known mandatory 2 -AS_PATH Well-known mandatory 3 -NEXT_HOP Well-known mandatory 4 -MULTI_EXIT_DISC Optional non-transitive 5 -LOCAL_PREF Well-known discretionary 6 -ATOMIC_AGGREGATE Well-known discretionary 7 -AGGREGATOR Well-known discretionary 8 -COMMUNITY Optional transitive (Cisco) 9 -ORIGINATOR_ID Optional non-transitive (Cisco) 10 -Cluster List Optional non-transitive (Cisco) 11 -Destination Preference (MCI) 12 -Advertiser (Baynet) 13 -rcid_path (Baynet) 255 -Reserved [md] 24

AS_PATH n n n n An AS_PATH attribute is a well-known mandatory attribute (type code 2). It is the sequence of AS numbers a route has traversed to reach a destination. The AS that originates the route adds its own AS number when sending the route to its external BGP peers. Thereafter, each AS that receives the route and passes it on to other BGP peers will prepend its own AS number to the list. Prepending is the act of adding the AS number to the beginning of the list. The final list represents all the AS numbers that a route has traversed with the AS number of the AS that originated the route all the way at the end of the list. This type of AS_PATH list is called an AS_SEQUENCE, because all the AS numbers are ordered sequentially. 25

AS_PATH Router. C#show ip bgp BGP table version is 8, local router ID is 200. 66 Status codes: s suppressed, d damped, h history, * valid, > best, i internal Origin codes: i - IGP, e - EGP, ? - incomplete Network *> 11. 0. 0. 0 *> 12. 0. 0. 0 *> 193. 10. 2. 0 Next Hop 0. 0 200. 65 Metric Loc. Prf Weight Path 0 32768 i 0 300 200 i 0 0 300 i 26

AS_PATH – private AS numbers n n n BGP uses the AS_PATH attribute as part of the routing updates (UPDATE packet) to ensure a loopfree topology on the Internet. Each route that gets passed between BGP peers will carry a list of all AS numbers that the route has already been through. If the route is advertised to the AS that originated it, that AS will see itself as part of the AS_PATH attribute list and will not accept the route. EBGP: BGP speakers prepend their AS numbers when advertising routing updates to other autonomous systems (external peers). IBGP: When the route is passed to a BGP speaker within the same AS, the AS_PATH information is left intact. 27

AS_PATH information is one of the attributes BGP looks at to determine the best route to take to get to a destination. – In comparing two or more different routes, given that all other attributes are identical, a shorter path is always preferred. – In case of a tie in AS_PATH length, other attributes are used to make the decision. (later) n Private AS numbers cannot be leaked to the Internet because they are not unique. – Cisco has implemented a feature, remove-private -as, to strip private AS numbers out of the AS_PATH list before the routes get propagated to the Internet. n 28

AS_PATH AS 1 is providing Internet connectivity to its customer AS 65001. n Because the customer connects to only this provider and no plans to connect to an additional provider in the near future, the customer has been allocated a private AS number. n BGP will strip private AS numbers only when propagating updates to the external peers. n This means that the AS stripping would be configured on RTB as part of its neighbor connection to RTC. n 29

AS_PATH Privately numbered autonomous systems should be connected only to a single provider. n If the AS_PATH contains a mixture of private and legal AS numbers, BGP will view this as an illegal design and will not strip the private AS numbers from the list, and the update will be treated as usual. n “If the AS_PATH includes both private and public AS numbers, BGP doesn't remove the private AS numbers. This situation is considered a configuration error. ” Cisco n Only AS_PATH lists that contain private AS numbers in the 30 range 64512 to 65535 are stripped. n

AS_PATH RTB(config)#router bgp 1 RTB(config-router)#neighbor 172. 16. 20. 2 remote-as 65001 RTB(config-router)#neighbor 192. 168. 6. 3 remote-as 7 RTB(config-router)#neighbor 192. 168. 6. 3 remove-private-as Note how RTB is using the remove-private-as keyword in its neighbor connection to AS 7. n http: //www. cisco. com/warp/public/459/32. html n 31

AS_PATH - prepend AS_PATH information is manipulated to affect interdomain routing behavior. n Because BGP prefers a shorter path over a longer one, system operators are tempted to change the path information by including dummy AS path numbers that would increase the path length and influence the traffic trajectory one way or the other. n Cisco's implementation enables a user to insert AS numbers at the beginning of an AS_PATH to make the path length longer. n 32

AS_PATH – prepend Concept New “shorter path” Current “shorter path” 33

Router C AS_PATH – prepend - Example router bgp 300 network 170. 10. 0. 0 neighbor 3. 3 remote-as 200 neighbor 2. 2 remote-as 100 neighbor 2. 2 route-map SETPATH out routemap n n n route-map SETPATH permit 10 set as-path prepend 300 If you want to use the configuration of Router C to influence the choice of paths in AS 600, you can do so by prepending extra AS numbers to the AS_path attribute for routes that Router C advertises to AS 100. A common practice is to repeat the AS number, as in the above configuration. The set as-path route map configuration command with the prepend keyword causes Router C to prepend 300 twice to the value of the AS_path attribute before it sends updates to the neighbor at IP address 2. 2 (Router A). As a result, the AS_path attribute of updates for network 170. 10. 0. 0 that AS 600 receives via AS 100 will be 100, 300, which is longer than the value of the AS_path attribute of updates for network 170. 10. 0. 0 that AS 600 receives via AS 400 (400, 200, 300). 34 AS 600 will choose (400, 200, 300) as the better path.

The WEIGHT attribute AS 200 AS 100 The weight attribute is a special Cisco attribute that is used in the path selection process when there is more than one route to the same destination. n The weight attribute is local to the router on which it is assigned, and it is not propagated in routing updates. n By default, the weight attribute is 32768 for paths that the router originates and zero for other paths. n Routes with a higher weight are preferred when 35 there are multiple routes to the same destination. n

AS 200 AS 100 Router. C Router A and Router B learn about network 175. 10. 0. 0 from AS 400, and each propagates the update to Router C. n Router C has two routes for reaching 175. 10. 0. 0 and has to decide which route to use. n If, on Router C, you set the weight of the updates coming in from Router A to be higher than the updates coming in from Router B, Router C will use Router A as the next hop to reach network 175. 10. 0. 0. n 36

AS 200 AS 100 Router. C n There are three ways to set the weight for updates coming in from Router A: – Using the neighbor weight Command to Set the Weight Attribute • What we will use. • Because of time reasons, we will only discuss this option. – Using an Access List to Set the Weight Attribute • FYI – Using a Route Map to Set the Weight Attribute • FYI 37

Higher weight preferred AS 100 2000 weight 2000 AS 200 1000 weight 1000 Router. C Using the neighbor weight Command to Set the Weight Attribute n The following configuration for Router C uses the neighbor weight router configuration command: Router C router bgp 300 neighbor 1. 1 remote-as 100 neighbor 1. 1 weight 2000 neighbor 2. 2 remote-as 200 neighbor 2. 2 weight 1000 n This configuration sets the weight of all route updates from AS 100 to 2000, and the weight of all route updates coming from AS 200 to 1000. 38 n Result: The higher weight assigned to route updates from AS 100 causes Router C to send traffic through Router A.

AS 200 AS 100 weight 2000 weight 1000 Router. C Using an Access List to Set the Weight Attribute - FYI n The following commands on Router C use access lists and the value of the AS_path attribute to assign a weight to route updates: Router C router bgp 300 neighbor 1. 1 remote-as 100 neighbor 1. 1 filter-list 5 weight 2000 neighbor 2. 2 remote-as 200 neighbor 2. 2 filter-list 6 weight 1000 ip as-path access-list 5 permit ^100$ ip as-path access-list 6 permit ^200$ 39

Router C router bgp 300 neighbor 1. 1 remote-as 100 neighbor 1. 1 filter-list 5 weight 2000 neighbor 2. 2 remote-as 200 neighbor 2. 2 filter-list 6 weight 1000 ip as-path access-list 5 permit ^100$ ip as-path access-list 6 permit ^200$ n In this example, 2000 is assigned to the weight attribute of updates from the neighbor at IP address 1. 1 that are permitted by access list 5. n Access list 5 permits updates whose AS_path attribute starts with 100 (as specified by ^) and ends with 100 (as specified by $). (The ^ and $ symbols are used to form regular expressions. For a complete explanation of regular expressions, see the appendix on regular expressions in the Cisco Internetwork Operating System (Cisco IOS) software configuration guides and command references. n This example also assigns 1000 to the weight attribute of updates from the neighbor at IP address 2. 2 that are permitted by access list 6. Access list 6 permits updates whose AS_path attribute starts with 200 and ends with 200. n In effect, this configuration assigns 2000 to the weight attribute of all route updates received from AS 100 and assigns 1000 to the weight attribute of 40 all route updates from AS 200.

AS 200 AS 100 weight 2000 weight 1000 Router. C Using a Route Map to Set the Weight Attribute - FYI n The following commands on Router C use a route map to assign a weight to route updates: Router C router bgp 300 neighbor 1. 1 remote-as 100 neighbor 1. 1 route-map SETWEIGHTIN in neighbor 2. 2 remote-as 200 neighbor 2. 2 route-map SETWEIGHTIN in ip as-path access-list 5 permit ^100$ route-map SETWEIGHTIN permit 10 match as-path 5 set weight 2000 route-map SETWEIGHTIN permit 20 41 set weight 1000

AS 200 AS 100 weight 2000 weight 1000 Router C Router. C router bgp 300 neighbor 1. 1 remote-as 100 neighbor 1. 1 route-map SETWEIGHTIN in neighbor 2. 2 remote-as 200 neighbor 2. 2 route-map SETWEIGHTIN in ip as-path access-list 5 permit ^100$ route-map SETWEIGHTIN permit 10 match as-path 5 set weight 2000 route-map SETWEIGHTIN permit 20 set weight 1000 n This first instance of the setweightin route map assigns 2000 to any route update from AS 100, and the second instance of the setweightin route map assigns 1000 to route updates from any 42 other AS

Path Attributes Attribute Code Type 1 -ORIGIN Well-known mandatory 2 -AS_PATH Well-known mandatory 3 -NEXT_HOP Well-known mandatory 4 -MULTI_EXIT_DISC Optional non-transitive 5 -LOCAL_PREF Well-known discretionary 6 -ATOMIC_AGGREGATE Well-known discretionary 7 -AGGREGATOR Well-known discretionary 8 -COMMUNITY Optional transitive (Cisco) 9 -ORIGINATOR_ID Optional non-transitive (Cisco) 10 -Cluster List Optional non-transitive (Cisco) 11 -Destination Preference (MCI) 12 -Advertiser (Baynet) 13 -rcid_path (Baynet) 255 -Reserved [md] 43

The LOCAL_PREF Attribute n n n Well-known discretionary attribute (type code 5). Degree of preference given to a route to compare it with other routes for the same destination – Higher LOCAL_PREF values are preferred Local to the AS – Exchanged between IBGP peers only – It is not advertised to EBGP peers Routers within a multi-homed AS may learn that they can reach the same destination network via neighbors in two (or more) different autonomous systems. – there could be two or more exit points from the local AS to any given destination. You can use the LOCAL_PREF attribute to force your BGP routers to prefer one exit point over another when routing to a particular destination network. 44

The LOCAL_PREF Attribute ? n n n ? Which exit should all the routers within AS 256 use? Because this attribute is communicated within all BGP routers inside the AS, all BGP routers will have a common view on how to exit the AS. Although routers always prefer the lowest-route metric and administrative distance for a given destination, BGP routers prefer higher LOCAL_PREF values over lower ones. When there are multiple paths to the same destination, the local preference attribute indicates the preferred path. The path with the higher preference is preferred (the default value of the local preference attribute is 100). Unlike the weight attribute, which is only relevant to the local 45 router, the local preference attribute is part of the routing update and is exchanged among routers in the same AS.

AS 256 receives route updates for network 170. 10. 0. 0 from AS 100 and AS 300. n There are two ways to set local preference: – Using the bgp default local-preference Command – Using a Route Map to Set Local Preference - FYI n 46

Higher Local Preference is preferred! Using the bgp default local-preference Command n The following configurations use the bgp default localpreference router configuration command to set the local preference attribute on Routers C and D: Router C router bgp 256 neighbor 1. 1 remote-as 100 neighbor 128. 213. 11. 2 remote-as 256 bgp default local-preference 150 Router D router bgp 256 neighbor 3. 3. 3. 4 remote-as 300 neighbor 128. 213. 11. 1 remote-as 256 bgp default local-preference 200 47

Higher Local Preference is preferred! Local Pref = 150 All traffic in AS 256 destined for 170. 10. 0. 0 (and other ASes) Local Pref = 200 Router C router bgp 256 bgp default local-preference 150 Router D router bgp 256 bgp default local-preference 200 n The configuration for Router C causes it to set the local preference of all updates from AS 300 to 150, and the configuration for Router D causes it to set the local preference for all updates from AS 100 to 200. n Because local preference is exchanged within the AS, both Routers C and D determine that updates regarding network 170. 10. 0. 0 have a higher local preference when they come from AS 300 than when they come from AS 100. n As a result, all traffic in AS 256 destined for network 170. 10. 0. 0 is 48 sent to Router D as the exit point.

Local Pref = 150 All traffic in AS 256 destined for 170. 10. 0. 0 (and other AS’s) Local Pref = 200 Using a Route Map to Set Local Preference - FYI n Route maps provide more flexibility than the bgp default local-preference router configuration command. n When the bgp default local-preference command is used on Router D, the local preference attribute of all updates received by Router D will be set to 200, including updates from AS 34. 49

Local Pref = 150 All traffic in AS 256 destined for 170. 10. 0. 0 (and other AS’s) Local Pref = 200 The following configuration uses a route map to set the local preference attribute on Router D specifically for updates regarding AS 300: Router D router bgp 256 neighbor 3. 3. 3. 4 remote-as 300 route-map SETLOCALIN in neighbor 128. 213. 11. 1 remote-as 256 ip as-path 7 permit ^300$ route-map SETLOCALIN permit 10 match as-path 7 set local-preference 200 route-map SETLOCALIN permit 20 n With this configuration, the local preference attribute of any update 50 coming from AS 300 is set to 200. n Instance 20 of the SETLOCALIN route map accepts all other routes. n

Router D router bgp 256 neighbor 3. 3. 3. 4 remote-as 300 route-map SETLOCALIN in neighbor 128. 213. 11. 1 remote-as 256 ip as-path access-list 7 permit ^300$ route-map SETLOCALIN permit 10 match as-path 7 set local-preference 200 route-map SETLOCALIN permit 20 This command creates a special kind of access list that looks at the AS_PATH attribute. n The ip as-path access-list command is used with route maps to match part (or all) of a route’s AS PATH. – Regular expressions are used with this command 51 to provide specificity.

Router D router bgp 256 neighbor 3. 3. 3. 4 remote-as 300 route-map SETLOCALIN in neighbor 128. 213. 11. 1 remote-as 256 ip as-path access-list 7 permit ^300$ route-map SETLOCALIN permit 10 match as-path 7 set local-preference 200 route-map SETLOCALIN permit 20 n n Note that the previous example uses the ip as-path access-list command, which here matches the regular expression ^300$. Essentially, this statement matches any routes that include AS 300 in their AS_PATH attribute. With the configuration, the LOCAL_PREF attribute of any update coming from AS 300 is set to 200 by instance 10 of the route map, SETLOCALIN. 52 Instance 20 of the route map accepts all other routes.

Regular Expressions A regular expression is a pattern to match against an input string. n The input string, in the case of the ip as-path access-list command, is the AS_PATH attribute. n Once you specify a pattern (or patterns) using this command, the router tests BGP routes to see if the AS_PATH attribute matches the pattern or not. n For example, the following command will match any AS_PATH that includes 2150: n Router(config)#ip as-path access-list 1 permit 2150 Or Router# show ip bgp regexp 2150 Unfortunately, the regular expression, 2150, will match not only AS 2150, but also 12150, 21502, 21503, etc. n Because policy routing demands a certain degree of precision, you will typically use one or more these special characters when creating a regular expression. n 53

Regular Expressions Character Description ^ Matches the beginning of the input string. $ Matches the end of the input string. _ Matches a space, comma, left brace, right brace, the beginning of an input string, or the ending of an input stream . * Matches any single character Matches 0 or more single- or multiplecharacter patterns. 54

Regular Expressions n Thus, if you want to match an AS_PATH that contains AS 2150 somewhere in the string, you would use the regular expression: _2150_ If you want to match AS 2150, but only if it appears at the beginning of the AS_PATH, you would use this regular expression. n You match previous AS using ^, since it is at the leftmost side of the AS path. ^2150 – Any connected customer routes and their customers’ routes from this directly connected AS 364. n Similarly, you can match an AS_PATH that ends with 2150, which means that the route originated at AS 2150: n You match originating AS using $, since it is at the rightmost side of the AS path. _2150$ n 55

Router. C#show ip bgp Network Next Hop *> 11. 0. 0 *> 12. 0. 0. 0 200. 65 *> 192. 10. 2. 0 200. 65 Metric Loc. Prf Weight Path 0 32768 i 0 300 200 i 0 0 300 i Router. C# show ip bgp regexp ^300 n Match beginning of input string, AS_PATH, = 300 n Last prepended AS was 300: n Routes matched: 12. 0. 0. 0 and 192. 10. 2. 0 56

Router. C#show ip bgp Network Next Hop *> 11. 0. 0 *> 12. 0. 0. 0 200. 65 *> 192. 10. 2. 0 200. 65 Metric Loc. Prf Weight Path 0 32768 i 0 300 200 i 0 0 300 i Router. C# show ip bgp regexp ^200 n Match beginning of input string, AS_PATH, = 200 n Last prepended AS was 200: n Routes matched : none 57

Router. C#show ip bgp Network Next Hop *> 11. 0. 0 *> 12. 0. 0. 0 200. 65 *> 192. 10. 2. 0 200. 65 Metric Loc. Prf Weight Path 0 32768 i 0 300 200 i 0 0 300 i Router. C# show ip bgp regexp 300$ n Match end of input string, AS_PATH, = 300 n Originating AS = 300: n Routes matched : 192. 10. 2. 0 58

Router. C#show ip bgp Network Next Hop *> 11. 0. 0 *> 12. 0. 0. 0 200. 65 *> 192. 10. 2. 0 200. 65 Metric Loc. Prf Weight Path 0 32768 i 0 300 200 i 0 0 300 i Router. C# show ip bgp regexp 200$ n Match end of input string, AS_PATH, = 200 n Originating AS = 200: n Routes matched : 12. 0. 0. 0 59

AS 50#show ip bgp Network *> 5. 0. 0. 0 *> 1. 0. 0. 0 *> 2. 0. 0. 0 *> 3. 0. 0. 0 *> 4. 0. 0. 0 *> 10. 0 Path i 100 200 i 300 400 1000 I AS 50#show ip bpg regexp 100 n Match input string, AS_PATH, containing 100, including 1000 n Routes matched : 1. 0. 0. 0, 2. 0. 0. 0, 10. 0 60

AS 50#show ip bgp Network *> 5. 0. 0. 0 *> 1. 0. 0. 0 *> 2. 0. 0. 0 *> 3. 0. 0. 0 *> 4. 0. 0. 0 *> 10. 0 Path i 100 200 i 300 400 1000 I AS 50#show ip bpg regexp ^100_ n Match beginning of input string, AS_PATH, = 100 n Last prepended AS was 100: n Routes matched : 1. 0. 0. 0, 2. 0. 0. 0 61

AS 50#show ip bgp *> *> *> Network 5. 0. 0. 0 1. 0. 0. 0 2. 0. 0. 0 3. 0. 0. 0 4. 0. 0. 0 10. 0 Path i 100 200 i 300 400 1000 I AS 50# show ip bgp regexp _400$ n Match end of input string, AS_PATH, = 400 n Originating AS = 400: n Routes matched : 4. 0. 0. 0 62

AS 50#show ip bgp *> *> *> Network 5. 0. 0. 0 1. 0. 0. 0 2. 0. 0. 0 3. 0. 0. 0 4. 0. 0. 0 10. 0 Path i 100 200 i 300 400 1000 I AS 50#show ip bpg regexp _400_ n Match anywhere in input string, AS_PATH, 400 n Routes matched : 4. 0. 0. 0, 10. 0 63

AS 50#show ip bgp Network *> 5. 0. 0. 0 *> 1. 0. 0. 0 *> 2. 0. 0. 0 *> 3. 0. 0. 0 *> 4. 0. 0. 0 *> 10. 0 Path i 100 200 i 300 400 1000 I AS 50#show ip bgp regexp ^300$ n Match input string that starts and ends at 300 n Routes that originated from directly connected AS 300 customer 64 n Routes matched : 3. 0. 0. 0

Traceroute. org – www. traceroute. org 65

What is a route server? http: //www. inetdaemon. com/tools/route_servers. html n A route server provides a look into the IP routing tables of the autonomous system in which the server resides. The concept of a route server has it's origins in the old Unix-based route serves that used to be located in the Network Access Points during the early days of the Internet. These Unix machines were configured with custom routing software ('routed', pronounced 'rout-dee'), designed specifically to make best-path calculations, and distribute a routing table to the routing devices forming the backbone of the Internet at these major peering points. n As custom routing hardware became more and more powerful (and cheaper), most NAP and CIX managers started setting up Cisco routers with open logins. This reqires less manhours and less work than many other methods. You can telnet to these routers and get a direct look at another network's routing table, and test connectivity. 66

Cabrillo College: 207. 62. 184. 0 route-server. exodus. net>show ip route 207. 62. 184. 0 Routing entry for 207. 62. 0. 0/16, supernet Known via "bgp 3967", distance 200, metric 0 Tag 3561, type internal Last update from 209. 1. 40. 129 1 w 6 d ago Routing Descriptor Blocks: * 209. 1. 40. 129, from 209. 1. 40. 129, 1 w 6 d ago Route metric is 0, traffic share count is 1 AS Hops 4, BGP network version 7021683 route-server. exodus. net> 3561 – Cable and Wireless (Next AS) 3967 – Exodus. net (This AS) 67

route-server. exodus. net>show ip bgp 207. 62. 184. 0 BGP routing table entry for 207. 62. 0. 0/16, version 7021683 Paths: (15 available, best #2) Not advertised to any peer 3561 3356 11423 2150 209. 1. 220. 242 from 209. 1. 220. 242 (209. 1. 220. 242) Origin IGP, localpref 1000, valid, internal 3561 3356 11423 2150 209. 1. 40. 129 from 209. 1. 40. 129 (209. 1. 40. 129) Origin IGP, localpref 1000, valid, internal, best 3561 3356 11423 2150 209. 1. 220. 174 from 209. 1. 220. 174 (209. 1. 220. 174) Origin IGP, localpref 1000, valid, internal --More— 3561 – Cable and Wireless 3356 – Level 3 Communications 11423 – CENIC (c/o University of California) 2150 – CSUNet (4 CNet) where 207. 62. 184. 0 originated. 68

Geektools. com – www. geektools. com 69

route-server. exodus. net>show ip bgp regexp _2150_ Network Next Hop Metric Loc. Prf Weight * i 12. 96. 0/19 209. 1. 220. 134 1000 * i 209. 1. 40. 63 1000 * i 209. 1. 220. 94 1000 *>i 64. 39. 112. 0/20 209. 1. 40. 63 1000 * i 209. 1. 220. 134 1000 * i 209. 1. 220. 126 1000 * i 209. 1. 40. 141 1000 * i 209. 1. 220. 133 1000 * i 209. 1. 220. 94 1000 *>i 64. 171. 168. 0/21 209. 1. 220. 94 1000 9 i * i 209. 1. 220. 94 1000 9 i Path 0 209 0 209 0 209 0 209 2150 23483 2150 23483 2150 i 2150 i 11423 2150 i i i 2448 0 209 11423 2150 2448 2150 anywhere in the AS path 70

route-server. exodus. net>show ip bgp regexp 2150$ BGP table version is 10790603, local router ID is 209. 1. 220. 234 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network *>i 64. 39. 112. 0/20 * i * i *>i 129. 8. 0. 0 * i * i 129. 65. 0. 0 0 i Next Hop 209. 1. 40. 63 209. 1. 220. 134 209. 1. 220. 126 209. 1. 40. 141 209. 1. 220. 133 209. 1. 220. 94 209. 1. 220. 242 Metric Loc. Prf Weight Path 1000 0 209 2150 i 1000 0 209 2150 i 1000 0 209 2150 i 1000 0 209 2150 i 1000 0 3561 3356 11423 215 Routes originating at the AS 2150 71

route-server. exodus. net>show ip bgp regexp 2150$ <continued> Includes Cabrillo Best Route * i 207. 62. 0. 0/16 *>i * i * i * i 209. 1. 220. 242 209. 1. 40. 129 209. 1. 220. 174 209. 1. 220. 102 209. 1. 220. 194 209. 1. 220. 243 209. 1. 40. 148 209. 1. 220. 134 1000 1000 0 0 0 0 3561 3356 11423 2150 3561 3356 11423 2150 209 2150 <more> route-server. exodus. net>show ip route 207. 62. 0. 0 Routing entry for 207. 62. 0. 0/16, supernet Known via "bgp 3967", distance 200, metric 0 Tag 3561, type internal Last update from 209. 1. 40. 129 1 w 6 d ago Routing Descriptor Blocks: * 209. 1. 40. 129, from 209. 1. 40. 129, 1 w 6 d ago Route metric is 0, traffic share count is 1 AS Hops 4, BGP network version 7021683 72 i i i i

route-server. exodus. net> traceroute 207. 62. 184. 2 1 dcr 01 -p 0 -1. sntc 08. exodus. net (209. 1. 169. 182) 0 msec dcr 02 -p 0 -1. sntc 08. exodus. net (209. 1. 169. 178) 0 msec dcr 01 -p 0 -1. sntc 08. exodus. net (209. 1. 169. 182) 0 msec 2 bbr 01 -g 2 -0. sntc 08. exodus. net (66. 35. 194. 1) 0 msec bbr 01 -g 6 -0. sntc 08. exodus. net (66. 35. 194. 17) 0 msec bbr 01 -g 2 -0. sntc 08. exodus. net (66. 35. 194. 1) 0 msec 3 bbr 02 -p 5 -0. sntc 05. exodus. net (206. 79. 9. 26) 0 msec 4 ibr 01 -g 2 -1. sntc 05. exodus. net (64. 56. 192. 181) 0 msec ibr 01 -g 6 -0. sntc 05. exodus. net (64. 56. 192. 21) 0 msec ibr 01 -g 2 -1. sntc 05. exodus. net (64. 56. 192. 181) 0 msec 5 qwest-px. exodus. net (216. 32. 173. 98) 0 msec 4 msec 0 msec 6 svl-core-02. inet. qwest. net (205. 171. 14. 105) [AS 209] 4 msec svl-core-03. inet. qwest. net (205. 171. 14. 109) [AS 209] 0 msec svl-core-02. inet. qwest. net (205. 171. 14. 105) [AS 209] 0 msec 7 svl-core-01. inet. qwest. net (205. 171. 14. 121) [AS 209] 0 msec svl-core-01. inet. qwest. net (205. 171. 14. 117) [AS 209] 4 msec svl-core-01. inet. qwest. net (205. 171. 14. 121) [AS 209] 4 msec 8 bur-core-03. inet. qwest. net (205. 171. 8. 241) [AS 209] 12 msec 8 msec 12 msec 9 bur-core-02. inet. qwest. net (205. 171. 13. 45) [AS 209] 12 msec 8 msec 10 bur-edge-01. inet. qwest. net (205. 171. 13. 54) [AS 209] 8 msec 12 msec 11 65. 119. 29. 50 [AS 209] 132 msec 136 msec 12 QSTO-GSR-4 CNET. GE. calren 2. net (137. 164. 12. 83) [AS 2150] 140 msec 14 4 msec 13 QSV-7500 --QSTO-GSR. CSU. net (137. 145. 202. 49) [AS 2150] 16 msec 20 msec 16 mse c 14 Cabrillo-Community-College. CSU. net (137. 145. 179. 110) [AS 2150] 24 msec 137. 145. 179. 182 [AS 2150] 24 msec cab 2. CSU. net (137. 145. 179. 121) [AS 2150] 20 msec 15 seahawk. cabrillo. cc. ca. us (207. 62. 184. 2) [AS 2150] 24 msec * 20 msec route-server. exodus. net> 73

74

interface POS The Packet-Over-SONET OC 3 port adapters (PA-POSOC 3 SML, PA-POS-OC 3 SMI, and PA-POS-OC 3 MM) are available on Cisco 7000 series routers with the 7000 Series Route Switch Processor (RSP 7000) and 7000 Series Chassis Interface (RSP 7000 CI), Cisco 7500 series routers, and Cisco 7200 series routers. n The POSIP and POS OC-3 provide a single 155. 520 -Mbps, OC 3 physical layer interface for packet-based traffic. This OC-3 interface is fully compatible with SONET and Synchronous Digital Hierarchy (SDH) network facilities and is compliant with RFC 1619, "PPP over SONET/SDH, " and RFC 1662, "PPP in HDLC-like Framing. " The Packet-Over-SONET specification is primarily concerned with the use of the PPP encapsulation over SONET/SDH links. n 75

FYI In this example, access list 1 denies any update whose AS_path attribute starts with 200 (as specified by ^) and ends with 200 (as specified by $). n Because Router B sends updates about 160. 10. 0. 0 whose AS_path attributes start with 200 and end with 200, such updates will match the access list and will be denied. n By specifying that the update must also end with 200, the access list permits updates from AS 400 (whose AS_path attribute is 200, 400). n If the access list specified ^200 as the regular expression, updates 76 from AS 400 would be denied. n Router C router bgp neighbor 300 3. 3 remote-as 200 2. 2 remote-as 100 2. 2 filter-list 1 out ip as-path access-list 1 deny ^200$ ip as-path access-list 1 permit. *

In the second access-list statement, the period (. ) symbol means any character, and the asterisk (*) symbol means a repetition of that character. n Together, . * matches any value of the AS_path attribute, which in effect permits any update that has not been denied by the previous access-list statement. n Router C router bgp neighbor 300 3. 3 remote-as 200 2. 2 remote-as 100 2. 2 filter-list 1 out ip as-path access-list 1 deny ^200$ ip as-path access-list 1 permit. * 77

Path Attributes Attribute Code Type 1 -ORIGIN Well-known mandatory 2 -AS_PATH Well-known mandatory 3 -NEXT_HOP Well-known mandatory 4 -MULTI_EXIT_DISC Optional non-transitive 5 -LOCAL_PREF Well-known discretionary 6 -ATOMIC_AGGREGATE Well-known discretionary 7 -AGGREGATOR Well-known discretionary 8 -COMMUNITY Optional transitive (Cisco) 9 -ORIGINATOR_ID Optional non-transitive (Cisco) 10 -Cluster List Optional non-transitive (Cisco) 11 -Destination Preference (MCI) 12 -Advertiser (Baynet) 13 -rcid_path (Baynet) 255 -Reserved [md] 78

The MED attribute The MULTI_EXIT_DISC (Multi-Exit Discriminator) attribute is an optional non-transitive attribute (type code 4). n Informs external neighbors about the preferred path into an AS that has multiple entry points. n A lower MULTI_EXIT_DISC (or MED) is preferred over a higher MED. n 79

Multi-Exit Discriminator Attribute n n n n The multi-exit discriminator (MED) attribute is a hint to external neighbors about the preferred path into an AS when there are multiple entry points into the AS. A lower MED value is preferred over a higher MED value. The default value of the MED attribute is 0. Unlike local preference, the MED attribute is exchanged between ASes, but a MED attribute that comes into an AS does not leave the AS. When an update enters the AS with a certain MED value, that value is used for decision making within the AS. When BGP sends that update to another AS, the MED is reset to 0. Unless otherwise specified, the router compares MED attributes for paths from external neighbors that are in the same AS. If you want MED attributes from neighbors in other ASes to be compared, you must configure the bgp always-compare-med command. 80

AS 100 receives updates regarding network 180. 10. 0. 0 from Routers B, C, and D. n Routers C and D are in AS 300, and Router B is in AS 400. n Router A router bgp 100 neighbor 2. 2. 2. 1 neighbor 3. 3 neighbor 4. 4. 4. 3 Router B router bgp 400 neighbor 4. 4 SETMEDOUT neighbor 5. 5. 5. 4 route-map SETMEDOUT set metric 50 remote-as 300 remote-as 400 remote-as 100 route-map out remote-as 300 permit 10 Router C router bgp 300 neighbor 2. 2 neighbor 5. 5 neighbor 1. 1. 1. 2 route-map SETMEDOUT set metric 120 Router D router bgp 300 neighbor 3. 3. 3. 2 neighbor 1. 1 route-map SETMEDOUT set metric 200 remote-as route-map remote-as permit 10 100 SETMEDOUT out 400 300 remote-as 100 route map SETMEDOUT out remote-as 300 permit 10 81

n n By default, BGP compares the MED attributes of routes coming from neighbors in the same external AS as the route (such as AS 300). Router A can only compare the MED attribute coming from Router C (120) to the MED attribute coming from Router D (200) even though the update coming from Router B has the lowest MED value. Router A router bgp 100 neighbor 2. 2. 2. 1 neighbor 3. 3 neighbor 4. 4. 4. 3 Router B router bgp 400 neighbor 4. 4 SETMEDOUT neighbor 5. 5. 5. 4 route-map SETMEDOUT set metric 50 remote-as 300 remote-as 400 remote-as 100 route-map out remote-as 300 permit 10 Router. A can only compare MEDs from the same AS Router C router bgp 300 neighbor 2. 2 neighbor 5. 5 neighbor 1. 1. 1. 2 route-map SETMEDOUT set metric 120 Router D router bgp 300 neighbor 3. 3. 3. 2 neighbor 1. 1 route-map SETMEDOUT set metric 200 remote-as route-map remote-as permit 10 100 SETMEDOUT out 400 300 remote-as 100 route map SETMEDOUT out remote-as 300 permit 10 82

n Router A will choose Router C as the best path for reaching network 180. 10. 0. 0. Router A router bgp 100 neighbor 2. 2. 2. 1 neighbor 3. 3 neighbor 4. 4. 4. 3 Router B router bgp 400 neighbor 4. 4 SETMEDOUT neighbor 5. 5. 5. 4 route-map SETMEDOUT set metric 50 remote-as 300 remote-as 400 remote-as 100 route-map out remote-as 300 permit 10 Router C router bgp 300 neighbor 2. 2 neighbor 5. 5 neighbor 1. 1. 1. 2 route-map SETMEDOUT set metric 120 Router D router bgp 300 neighbor 3. 3. 3. 2 neighbor 1. 1 route-map SETMEDOUT set metric 200 remote-as route-map remote-as permit 10 100 SETMEDOUT out 400 300 remote-as 100 route map SETMEDOUT out remote-as 300 permit 10 83

n n To force Router A to include updates for network 180. 10. 0. 0 from Router B in the comparison, use the bgp alwayscompare-med router configuration command on Router A: Router A will choose Router B as the best next hop for reaching network 180. 10. 0. 0 (assuming that all other attributes are the same). Router A router bgp 100 neighbor 2. 2. 2. 1 remote-as neighbor 3. 3 remote-as neighbor 4. 4. 4. 3 remote-as bgp always-compare-med Router B router bgp 400 neighbor 4. 4 remote-as neighbor 4. 4 route-map SETMEDOUT out neighbor 5. 5. 5. 4 remote-as route-map SETMEDOUT permit 10 set metric 50 300 400 100 300 Router C router bgp 300 neighbor 2. 2 neighbor 5. 5 neighbor 1. 1. 1. 2 route-map SETMEDOUT set metric 120 Router D router bgp 300 neighbor 3. 3. 3. 2 neighbor 1. 1 route-map SETMEDOUT set metric 200 remote-as route-map remote-as permit 10 100 SETMEDOUT out 400 300 remote-as 100 route map SETMEDOUT out remote-as 300 permit 10 84

Path Attributes Attribute Code Type 1 -ORIGIN Well-known mandatory 2 -AS_PATH Well-known mandatory 3 -NEXT_HOP Well-known mandatory 4 -MULTI_EXIT_DISC Optional non-transitive 5 -LOCAL_PREF Well-known discretionary 6 -ATOMIC_AGGREGATE Well-known discretionary 7 -AGGREGATOR Well-known discretionary 8 -COMMUNITY Optional transitive (Cisco) 9 -ORIGINATOR_ID Optional non-transitive (Cisco) 10 -Cluster List Optional non-transitive (Cisco) 11 -Destination Preference (MCI) 12 -Advertiser (Baynet) 13 -rcid_path (Baynet) 255 -Reserved [md] 85

ATOMIC_AGGREGATE n n n n This attribute uses the aggregate-address command. A BGP speaking router can transmit overlapping routes to another BGP speaker. Overlapping routes are non-identical routes that point to the same destination. For example, 206. 25. 192. 0/19 and 206. 25. 128. 0/17 are overlapping, as the first route is included in the second route. The second route, 206. 25. 128. 0/17, points to other more specific routes besides 206. 25. 192. 0/19. When making a best path decision, a router always chooses the more-specific path. When advertising routes, however, the BGP speaker has several options with overlapping routes. 86

ATOMIC_AGGREGATE Choices: n Advertise both the more-specific and the less-specific route n Advertise only the more-specific route n Advertise only the non-overlapping part of the route n Aggregate (summarize) the two routes and advertise the aggregate n Advertise the less-specific route only n Advertise neither route. 87

ATOMIC_AGGREGATE The ATOMIC_AGGREGATE is a well-know discretionary attribute (type code 6). n The ATOMIC_AGGREGATE attribute is set to either “True” or “False. ” n If true, this attribute alerts BGP routers that multiple destinations have been grouped into a single update. – In other words, the BGP router that sent the update had a more specific route to the destination, but did not send it. – ATOMIC_AGGREGATE warns receiving routers that the information they are receiving is not necessarily the most complete route information available. n You can manually configure BGP to summarize routes by using the aggregate-address command, which has the following syntax: Router(config-router)#aggregate-address mask [as -set][summary-only] [suppress-map map-name][advertise -map map-name] [attribute-map map-name] 88

aggregate-address command n n The purpose of this command is to create an aggregate (summarized) entry in the BGP table. There are two ways to create an aggregate address under BGP: 1. 2. n Create a static entry in the routing table for the aggregate address and then advertise it with the network command. Use the aggregate-address command. An aggregate is created only if a more-specific route to the aggregate exists in the BGP table. 89

Example 1: Aggregating Local Routes RTA router bgp 1 neighbor 10. 1. 1. 2 remote-as 2 RTB router bgp 2 neighbor 10. 1. 1. 1 remote-as 1 network 172. 16. 0. 0 mask 255. 0 network 172. 16. 1. 0 mask {/24} network 172. 16. 2. 0 mask {/24} network 172. 16. 3. 0 mask {/24} Before aggregating locally sourced routes, lets configure the morespecific networks. n RTB has four loopbacks used to simulate the networks along with BGP network commands. n RTA and RTB will have all 172. 16. n. 0/24 routes in its BGP table (show ip bgp) 90

Example 1: Aggregating Local Routes RTB router bgp 2 neighbor 10. 1. 1. 1 remote-as 1 network 172. 16. 0. 0 mask 255. 0 network 172. 16. 1. 0 mask {/24} network 172. 16. 2. 0 mask {/24} network 172. 16. 3. 0 mask {/24} aggregate-address 172. 16. 0. 0 255. 252. 0 Now modify the BGP on RGB to enable the advertisement of the aggregate: n We need only one of the morespecific network commands in RTB in order to send the aggregate, but by configuring all of them the aggregate will be sent in case one of the networks goes down. n RTA and RTB will have all 172. 16. n. 0/24 routes in its BGP table (show ip bgp), and the aggregate address of 172. 16. 0. 0/2291

Example 1: Aggregating Local Routes show ip bpg 172. 16. 0. 0 will display that this route has the “atomicaggregate” attribute set. RTA#show ip bgp 172. 16. 0. 0 255. 252. 0 BGP routing table entry for 172. 16. 0. 0/22, version 18 Paths: (1 available, best #1) <text omitted> Origin IGP, localpref 100, valid, external, atomicaggregate, best 92

Summary of the BGP Path Selection Process n BGP selects only one path as the best path. n When the path is selected, BGP puts the selected path in its routing table and propagates the path to its neighbors. n BGP uses the following criteria, in the order presented, to select a path for a destination: 1. If the path specifies a next hop that is inaccessible, drop the update. 2. Prefer the path with the largest weight. 3. If the weights are the same, prefer the path with the largest local preference. 4. If the local preferences are the same, prefer the path that was originated by BGP running on this router. 5. If no route was originated, prefer the route that has the shortest AS_path. 6. If all paths have the same AS_path length, prefer the path with the lowest origin type (where IGP is lower than EGP, and EGP is lower than Incomplete). 7. If the origin codes are the same, prefer the path with the lowest MED attribute. 8. If the paths have the same MED, prefer the external path over the internal path. 9. If the paths are still the same, prefer the path through the closest IGP neighbor. 10. Prefer the path with the lowest IP address, as specified by the BGP router ID. 93

CCNP – Advanced Routing BGP (Part II) 94