2. cnp3-ebook
3. protocols/bgp
4. English
5. Browse

5 / 8

/ 8

All strings

• Untranslated strings • state:empty
• Unfinished strings • state:<translated
• Translated strings • state:>=translated
• Strings marked for edit • state:needs-editing
• Strings with suggestions • has:suggestion
• Strings with variants • has:variant
• Strings with labels • has:label
• Strings with context • has:context
• Unfinished strings without suggestions • state:<translated AND NOT has:suggestion
• Strings with comments • has:comment
• Strings with any failing checks • has:check
• Approved strings • state:approved
• Strings waiting for review • state:translated

Position and priority

• Position and priority
• Position
• Priority
• Labels
• Source string
• Target string
• String age
• Number of words
• Number of comments
• Number of failing checks
• Key

	English
	The `local-pref` attribute is often used to prefer some routes over others.
	A common utilization of `local-pref` is to support backup links. Consider the situation depicted in the figure below. `AS1` would always like to use the high bandwidth link to send and receive packets via `AS2` and only use the backup link upon failure of the primary one.
	How to create a backup link with BGP ?
	As BGP routers always prefer the routes with the highest `local-pref` attribute, this policy can be implemented using the following import filter on `R1`
	With this import filter, all the BGP routes learned from `RB` over the high bandwidth links are preferred over the routes learned over the backup link. If the primary link fails, the corresponding routes are removed from `R1`'s RIB and `R1` uses the route learned from `RA`. `R1` reuses the routes via `RB` as soon as they are advertised by `RB` once the `R1-RB` link comes back.
	The import filter above modifies the selection of the BGP routes inside `AS1`. Thus, it influences the route followed by the packets forwarded by `AS1`. In addition to using the primary link to send packets, `AS1` would like to receive its packets via the high bandwidth link. For this, `AS2` also needs to set the `local-pref` attribute in its import filter.
	Sometimes, the `local-pref` attribute is used to prefer a `cheap` link compared to a more expensive one. For example, in the network below, `AS1` could wish to send and receive packets mainly via its interdomain link with `AS4`.
	How to prefer a cheap link over an more expensive one ?
	`AS1` can install the following import filter on `R1` to ensure that it always sends packets via `R2` when it has learned a route via `AS2` and another via `AS4`.
	However, this import filter does not influence how `AS3` , for example, prefers some routes over others. If the link between `AS3` and `AS2` is less expensive than the link between `AS3` and `AS4`, `AS3` could send all its packets via `AS2` and `AS1` would receive packets over its expensive link. An important point to remember about `local-pref` is that it can be used to prefer some routes over others to send packets, but it has no influence on the routes followed by received packets.
	Another important utilization of the `local-pref` attribute is to support the `customer->provider` and `shared-cost` peering relationships. From an economic point of view, there is an important difference between these three types of peering relationships. A domain usually earns money when it sends packets over a `provider->customer` relationship. On the other hand, it must pay its provider when it sends packets over a `customer->provider` relationship. Using a `shared-cost` peering to send packets is usually neutral from an economic perspective. To take into account these economic issues, domains usually configure the import filters on their routers as follows :
	insert a high `local-pref` attribute in the routes learned from a customer
	insert a medium `local-pref` attribute in the routes learned over a shared-cost peering
	insert a low `local-pref` attribute in the routes learned from a provider
	With such an import filter, the routers of a domain always prefer to reach destinations via their customers whenever such a route exists. Otherwise, they prefer to use `shared-cost` peering relationships and they only send packets via their providers when they do not know any alternate route. A consequence of setting the `local-pref` attribute like this is that Internet paths are often asymmetrical. Consider for example the internetwork shown in the figure below.
	Asymmetry of Internet paths
	Consider in this internetwork the routes available inside `AS1` to reach `AS5`. `AS1` learns the `AS4:AS6:AS7:AS5` path from `AS4`, the `AS3:AS8:AS5` path from `AS3` and the `AS2:AS5` path from `AS2`. The first path is chosen since it was learned from a customer. `AS5` on the other hand receives three paths towards `AS1` via its providers. It may select any of these paths to reach `AS1` , depending on how it prefers one provider over the others.
	BGP convergence
	In the previous sections, we have explained the operation of BGP routers. Compared to intradomain routing protocols, a key feature of BGP is its ability to support interdomain routing policies that are defined by each domain as its import and export filters and ranking process. A domain can define its own routing policies and router vendors have implemented many configuration tweaks to support complex routing policies. However, the routing policy chosen by a domain may interfere with the routing policy chosen by another domain. To understand this issue, let us first consider the simple internetwork shown below.
	The disagree internetwork