Community Attribute
The community attribute provides a way of grouping destinations, called communities, to which routing decisions (such as acceptance, preference, and redistribution) can be applied. Route maps are used to set the community attribute. Predefined community attributes are listed here:
- no-export—Do not advertise this route to EBGP peers.
- no-advertise—Do not advertise this route to any peer.
- internet—Advertise this route to the Internet community; all routers in the network belong to it.
Figure 39-7 illustrates the no-export community. AS 1 advertises 172.16.1.0 to AS 2 with the community attribute no-export. AS 2 will propagate the route throughout AS 2 but will not send this route to AS 3 or any other external AS.
Figure 39-7 BGP no-export Community Attribute
In Figure 39-8, AS 1 advertises 172.16.1.0 to AS 2 with the community attribute no-advertise. Router B in AS 2 will not advertise this route to any other router.
Figure 39-8 BGP no-advertise Community Attribute
Figure 39-9 demonstrates the internet community attribute. There are no limitations to the scope of the route advertisement from AS 1.
Figure 39-9 BGP internet Community Attribute
BGP Path Selection
BGP could possibly receive multiple advertisements for the same route from multiple sources. BGP selects only one path as the best path. When the path is selected, BGP puts the selected path in the IP routing table and propagates the path to its neighbors. BGP uses the following criteria, in the order presented, to select a path for a destination:
- If the path specifies a next hop that is inaccessible, drop the update.
- Prefer the path with the largest weight.
- If the weights are the same, prefer the path with the largest local preference.
- If the local preferences are the same, prefer the path that was originated by BGP running on this router.
- If no route was originated, prefer the route that has the shortest AS_path.
- 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).
- If the origin codes are the same, prefer the path with the lowest MED attribute.
- If the paths have the same MED, prefer the external path over the internal path.
- If the paths are still the same, prefer the path through the closest IGP neighbor.
- Prefer the path with the lowest IP address, as specified by the BGP router ID.
Review Questions
Q—Can IBGP be used in place of an IGP (RIP, IGRP, EIGRP, OSPF, or ISIS)?
A—Yes and no. Remember that the next-hop information from EBGP is carried into IBGP. If IBGP does not have a route to reach the next hop, then the route will be discarded. Typically an IGP needs to be used to exchange routes to the next hop, but this can be achieved by using static routes on all the routers running IBGP. So, the answer is yes if you want to use and maintain static routes. Otherwise, the answer is no.
Q—Assume that a BGP router is learning the same route from two different EBGP peers. The AS_path information from peer 1 is {2345,86,51}, and the AS_path information from peer 2 is {2346,51}. What BGP attributes could be adjusted to force the router to prefer the route advertised by peer 1?
A—Weight and local preference. Both have a higher preference than AS_path length.
Q—Can BGP be used only by Internet service providers?
A—No. BGP can be used to scale large enterprise networks. A large network can be divided into segments, with each segment running an IGP. Routing information between segments could then be exchanged using BGP.
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