When you're studying for the CCNP certification, especially the BSCI exam, you must gain a solid understanding of BGP. BGP isn't just one of the biggest topics on the BSCI exam, it's one of the largest. BGP has a great many details that must be mastered for BSCI success, and those of you with one eye on the CCIE must learn the fundamentals of BGP now in order to build on those fundamentals at a later time.
Path attributes are a unique feature of BGP. With interior gateway protocols such as OSPF and EIGRP, administrative distance is used as a tiebreaker when two routes to the same destination had different next-hop IP addresses but the same prefix length. BGP uses path attributes to make this choice.
The first attribute considered by BGP is weight. Weight is a Cisco-proprietary BGP attribute, so if you're working in a multivendor environment you should work with another attribute to influence path selection.
The weight attribute is significant only to the router on which it is changed. If you set a higher weight for a particular route in order to give it preference (a higher weight is preferred over a lower one), that weight is not advertised to other routers.
BGP uses categories such as "transitive", "non-transitive", "mandatory", and "optional" to classify attributes. Since weight is a locally significant Cisco-proprietary attribute, it does not all into any of these categories.
The weight can be changed on a single route via a route-map, or it can be set for a different weight for all routes received from a given neighbor. To change the weight for all incoming routes, use the "weight" option with the neighbor command after forming the BGP peer relationships.
R2(config)#router bgp 100
R2(config-router)#neighbor 100.1.1.1 remote-as 10
R2(config-router)#neighbor 100.1.1.1 weight 200
Learning all of the BGP attributes, as well as when to use them, can seem an overwhelming task when you first start studying for your BSCI and CCNP exams. Break this task down into small parts, learn one attribute at a time, and soon you'll have the BGP attributes mastered.
Showing posts with label weight. Show all posts
Showing posts with label weight. Show all posts
Thursday, December 25, 2008
Cisco CCNP / BSCI Certification: The Local Preference BGP Attribute
When studying for your BSCI exam for the CCNP, you get your first taste of BGP. One of the major differences between BGP and the other protocols you've studied to date is that BGP uses attributes to describe paths, and to influence the selection of one path over the other.
In this free tutorial, we're going to take a look at the Local Preference attribute and compare it to the Cisco-proprietary BGP attribute "weight".
The Local Preference (LOCAL_PREF) attribute is used to influence how traffic will flow from one Autonomous System (AS) to another when multiple paths exist. For example, if AS 100 has two different paths to a destination network in AS 200, the LOCAL_PREF attribute can be used to influence the path selection.
The major difference between the Weight and LOCAL_PREF attributes is that when the LOCAL_PREF attribute is changed, that change is reflected throughout the AS. The new LOCAL_PREF value will be advertised to all other routers in the AS, as compared to the Weight attribute, which is locally significant only. If you change the Weight for a path on one router in an AS, the other routers in the AS will not learn of the change.
A route-map can be used to change a local preference value. For example, if you want to change the local preference value to 200 for the path advertisement 10.2.2.0/24 coming in from neighbor 10.1.1.1, there are three steps involved. First, write an ACL matching the remote network you want to change the local preference for.
R1(config)#access-list 5 permit 10.2.2.0 0.0.0.255
Second, write a route-map setting the local preference to 200. This will double the default value of 100, and the path with the highest local preference will be the preferred path.
R1(config)#route-map PREFER_PATH permit 10
R1(config-route-map)#match ip address 5
R1(config-route-map)#set local-pref 200
Finally, apply the route-map to routes that are being received from 10.1.1.1.
R1(config)#router bgp 100
R1(config-router)#network 10.1.1.1 route-map PREFER_PATH in
R1 will then advertise this new local preference value to all other routers in AS 100 - all of its iBGP neighbors.
In this free tutorial, we're going to take a look at the Local Preference attribute and compare it to the Cisco-proprietary BGP attribute "weight".
The Local Preference (LOCAL_PREF) attribute is used to influence how traffic will flow from one Autonomous System (AS) to another when multiple paths exist. For example, if AS 100 has two different paths to a destination network in AS 200, the LOCAL_PREF attribute can be used to influence the path selection.
The major difference between the Weight and LOCAL_PREF attributes is that when the LOCAL_PREF attribute is changed, that change is reflected throughout the AS. The new LOCAL_PREF value will be advertised to all other routers in the AS, as compared to the Weight attribute, which is locally significant only. If you change the Weight for a path on one router in an AS, the other routers in the AS will not learn of the change.
A route-map can be used to change a local preference value. For example, if you want to change the local preference value to 200 for the path advertisement 10.2.2.0/24 coming in from neighbor 10.1.1.1, there are three steps involved. First, write an ACL matching the remote network you want to change the local preference for.
R1(config)#access-list 5 permit 10.2.2.0 0.0.0.255
Second, write a route-map setting the local preference to 200. This will double the default value of 100, and the path with the highest local preference will be the preferred path.
R1(config)#route-map PREFER_PATH permit 10
R1(config-route-map)#match ip address 5
R1(config-route-map)#set local-pref 200
Finally, apply the route-map to routes that are being received from 10.1.1.1.
R1(config)#router bgp 100
R1(config-router)#network 10.1.1.1 route-map PREFER_PATH in
R1 will then advertise this new local preference value to all other routers in AS 100 - all of its iBGP neighbors.
Cisco CCNP / BSCI Certification: The BGP Attribute “MED”
When you're preparing to pass the BSCI exam and earn your CCNP certification, one of the biggest challenges is learning BGP. BGP is totally different from any protocol you learned to earn your CCNA certification, and one of the differences is that BGP uses path attributes to favor one path over another when multiple paths to or from a destination exist.
Notice I said "to or from". In earlier free BGP tutorials, I discussed the BGP attributes "weight" and "local preference". These attributes are used to favor one path to a destination over another; for example, if BGP AS 100 has two paths to a destination in AS 200, these two attributes can be set in AS 100 to favor one path over another. But what if AS 100 wants to inform the routers in AS 200 as to which path it should use to reach a given destination in AS 100?
That's where the BGP attribute "Multi-Exit Discriminator", or MED, comes in. The MED value can be set in AS 100 to tell AS 200 which path it should use to reach a given network in AS 100.
As with many BGP attributes, the MED can be set with a route-map. What you need to watch is that there is no "set med" value in route maps. To change the MED of a path, you need to change the metric of that path. Let's say that there are two entry paths for AS 200 to use to reach destinations in AS 100. You want AS 200 to use the 100.1.1.0/24 path over the 100.2.2.0/24 path. First, identify the two paths with two separate ACLs.
R1(config)#access-list 22 permit 100.1.1.0 0.0.0.255
R1(config)#access-list 23 permit 100.2.2.0 0.0.0.255
Next, write a route-map that assigns a lower metric to the more-desirable path.
R1(config)#route-map PREFER_PATH permit 10
R1(config-route-map)#match ip address 22
R1(config-route-map)#set metric 100
R1(config-route-map)#route-map PREFER_PATH permit 20
R1(config-route-map)#match ip address 23
R1(config-route-map)#set metric 250
Finally, apply the route-map to the neighbor or neighbors.
R1(config-route-map)#router bgp 100
R1(config-router)#neighbor 22.2.2.2 route-map PREFER_PATH out
The key points to keep in mind is that while many BGP attributes prefer a higher value, the MED is basically an external metric - and a lower metric is preferred, just as with the protocols you've already studied to earn your CCNA certification.
Notice I said "to or from". In earlier free BGP tutorials, I discussed the BGP attributes "weight" and "local preference". These attributes are used to favor one path to a destination over another; for example, if BGP AS 100 has two paths to a destination in AS 200, these two attributes can be set in AS 100 to favor one path over another. But what if AS 100 wants to inform the routers in AS 200 as to which path it should use to reach a given destination in AS 100?
That's where the BGP attribute "Multi-Exit Discriminator", or MED, comes in. The MED value can be set in AS 100 to tell AS 200 which path it should use to reach a given network in AS 100.
As with many BGP attributes, the MED can be set with a route-map. What you need to watch is that there is no "set med" value in route maps. To change the MED of a path, you need to change the metric of that path. Let's say that there are two entry paths for AS 200 to use to reach destinations in AS 100. You want AS 200 to use the 100.1.1.0/24 path over the 100.2.2.0/24 path. First, identify the two paths with two separate ACLs.
R1(config)#access-list 22 permit 100.1.1.0 0.0.0.255
R1(config)#access-list 23 permit 100.2.2.0 0.0.0.255
Next, write a route-map that assigns a lower metric to the more-desirable path.
R1(config)#route-map PREFER_PATH permit 10
R1(config-route-map)#match ip address 22
R1(config-route-map)#set metric 100
R1(config-route-map)#route-map PREFER_PATH permit 20
R1(config-route-map)#match ip address 23
R1(config-route-map)#set metric 250
Finally, apply the route-map to the neighbor or neighbors.
R1(config-route-map)#router bgp 100
R1(config-router)#neighbor 22.2.2.2 route-map PREFER_PATH out
The key points to keep in mind is that while many BGP attributes prefer a higher value, the MED is basically an external metric - and a lower metric is preferred, just as with the protocols you've already studied to earn your CCNA certification.
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