Gunris Using a practical, example-oriented approach, this comprehensive resource provides you with real solutions for ISP connectivity issues. Internet Routing Architectures, Second Edition, explores the ins and outs of interdomain routing network designs with emphasis on BGP-4 Border Gateway Protocol Version 4 —the de facto interdomain routing protocol. Stay ahead with the jalabi most comprehensive technology and business learning platform. IP Addressing and Allocation Techniques. Benny BingPascal Lorenz Limited preview — Using a practical, example-oriented approach, this comprehensive resource provides you with real solutions for ISP connectivity issues. Halabi has held various marketing positions with leading data communications companies, including Cisco Systems, 3Com and Pluris.
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But you can use the distance command to change the default distances: distance bgp external-distance internal-distance local-distance RTA picks eBGP via RTC because of the shorter distance. If you want RTA to learn about Use BGP backdoor. Issue the network address backdoor command. The configured network is the network that you want to reach via IGP.
For BGP, this network gets the same treatment as a locally assigned network, except BGP updates do not advertise this network. RTA router eigrp 10 network RTA learns Synchronization Before the discussion of synchronization, look at this scenario.
RTC in AS sends updates about Remember that the next hop is carried via iBGP. Assume that RTA has not redistributed network At this point, RTE has no idea that Then, BGP advertises the route to external peers.
In the example in this section, RTB waits to hear about Make sure that other routers can reach Disable Synchronization In some cases, you do not need synchronization.
If you do not pass traffic from a different AS through your AS, you can disable synchronization. The disablement of this feature can allow you to carry fewer routes in your IGP and allow BGP to converge more quickly. The disablement of synchronization is not automatic.
Your router waits indefinitely for an IGP update about a certain route before the router sends the route to external peers. RTB router bgp network RTD router bgp neighbor 1. This attribute uses weight to select a best path. The weight is assigned locally to the router. The value only makes sense to the specific router. The value is not propagated or carried through any of the route updates. A weight can be a number from 0 to 65, Paths that the router originates have a weight of 32, by default, and other paths have a weight of 0.
Routes with a higher weight value have preference when multiple routes to the same destination exist. Look at the example in this section. RTA has learned about network RTB has also learned about network RTC now has two ways to reach Multiple methods achieve this weight set: Use the neighbor command. RTC router bgp neighbor 1. RTA, which has a higher weight value, has preference as the next hop. You also can achieve the same outcome with the use of route maps. A path with a higher local preference is preferred more.
The default value for local preference is Unlike the weight attribute, which is only relevant to the local router, local preference is an attribute that routers exchange in the same AS.
You set local preference with the issue of the bgp default local-preference value command. The bgp default local-preference command sets the local preference on the updates out of the router that go to peers in the same AS. In the diagram in this section, AS receives updates about Local preference helps you determine which way to exit AS in order to reach that network.
Assume that RTD is the exit point preference. This configuration sets the local preference for updates that come from AS to and for updates that come from AS to RTC router bgp neighbor 1.
The same RTD sets the local preference of all updates to There is an exchange of local preference within AS The use of route maps provides more flexibility. In the example in this section, all updates that RTD receives are tagged with local preference when the updates reach RTD. Updates that come from AS34 also are tagged with the local preference of This tag can be unnecessary.
For this reason, you can use route maps to specify the specific updates that need to be tagged with a specific local preference. Here is an example: RTD router bgp neighbor 3. Any other updates, such as updates that come from AS34, have a value of The attribute is a hint to external neighbors about the path preference into an AS.
The attribute provides a dynamic way to influence another AS in the way to reach a certain route when there are multiple entry points into that AS.
A lower metric value is preferred more. Unlike local preference, metric is exchanged between ASs. A metric is carried into an AS but does not leave the AS. When an update enters the AS with a certain metric, that metric is used to make decisions inside the AS.
When the same update passes on to a third AS, that metric returns to 0. The diagram in this section shows the set of metric. The metric default value is 0.
Unless a router receives other directions, the router compares metrics for paths from neighbors in the same AS. In order for the router to compare metrics from neighbors that come from different ASs, you need to issue the special configuration command bgp always-compare-med on the router.
The commands are the bgp deterministic-med command and the bgp always-compare-med command. An issue of the bgp deterministic-med command ensures the comparison of the MED variable at route choice when different peers advertise in the same AS.
An issue of the bgp always-compare-med command ensures the comparison of the MED for paths from neighbors in different ASs. The bgp always-compare-med command is useful when multiple service providers or enterprises agree on a uniform policy for how to set MED.
Refer to How the bgp deterministic-med Command Differs from the bgp always-compare-med Command to understand how these commands influence BGP path selection. In the diagram in this section, AS gets information about network If the command is omitted, the BGP will install route By default, a router compares metrics that come from neighbors in the same AS.
RTA must choose based on some other attributes. These configurations illustrate this process: RTA router bgp neighbor 2. RTC router bgp neighbor 2. You can also set metric during the redistribution of routes into BGP if you issue the default-metric number command. Here is the configuration: RTB router bgp redistribute static default-metric 50 ip route Community Attribute The community attribute is a transitive, optional attribute in the range of 0 to 4,,, The community attribute is a way to group destinations in a certain community and apply routing decisions according to those communities.
The routing decisions are accept, prefer, and redistribute, among others. You can use route maps to set the community attributes. The route map set command has this syntax: set community community-number [additive] [well-known-community] A few predefined, well known communities for use in this command are: no-export—Do not advertise to eBGP peers.
Keep this route within an AS. Any router belongs to this community. Here are two examples of route maps that set the community: route-map communitymap match ip address 1 set community no-advertise or route-map setcommunity match as-path 1 set community additive If you do not set the additive keyword, replaces any old community that already exits. If you use the keyword additive, an addition of to the community occurs.
Even if you set the community attribute, this attribute does not transmit to neighbors by default. In order to configure and display in AA:NN, issue the ip bgp-community new-format global configuration command. Here is an example: Without the ip bgp-community new-format command in global configuration, an issue of the show ip bgp 6. In this example, the community attribute value appears as Router show ip bgp 6.
Router configure terminal Enter configuration commands, one per line. Router config ip bgp-community new-format Router config exit With the ip bgp-community new-format global configuration command, the community value displays in AA:NN format. The value appears as in the output of the show ip bgp 6. You can filter BGP updates with route information as a basis, or with path information or communities as a basis. All methods achieve the same results.
BGP4 Case Studies/Tutorial
BGP Case Studies