The Key Features of BGP4

Category

Description

Well Known:

Mandatory (required by all routers)

Discretionary (required by all routers and recognized by all routers)

It is not required that these attributes be present in the update messages, but if they are present, all routers running BGP-4 will recognize and act on the information contained.

These attributes are required and are therefore recognized by all BGP-4 implementations.

Table 8-5 The Four Categories of Attributes (Continued)

Category Description

Optional: The router may not recognize these attributes, but if this is the

Transitive case, it marks the update as partial and sends the update, complete with attributes, to the next router. The attributes

Nontransitive traverse the router unchanged, if they are not recognized.

Nontransitive attributes are dropped if they fall onto a router that does not understand or recognize the attribute. These attributes will not be propagated to the BGP-4 peers. Unrecognized nontransitive optional attributes must be quietly ignored and not passed along to other BGP peers. New transitive optional attributes may be attached to the path by the originator or by any other AS in the path (see RFC 1771).

The attributes are appropriately carried in the updates that inform the routers of the routes. The attributes and their attributes are listed in Table 8-6 for quick reference and comparison.

Table 8-6 The BGP-4 Attributes

Attribute Name Category Code Preference

Description

Origin

AS Path

Well known, mandatory

1 Lowest origin code

Where:

IGP < EGP < Incomplete

Well known, 2 Shortest path mandatory

This path attribute identifies the source of the routing update. The possible sources of routing information are as follows:

• The path originates from within the autonomous system. It was created with the IBGP-4 network command. The route will be marked in the routing table with an "I."

• If the source is an exterior routing protocol, it will be identified with an "e" in the routing table.

• The route could have been redistributed into BGP-4; as such, there is incomplete information. The route is marked by "?."

This is a Sequence of the autonomous systems that the packet has traversed.

continues

Table 8-6

The BGP-4 Attributes (Continued)

Attribute Name Category Code Preference

Description

Next hop

Well known, Mandatory

3 Shortest path or IGP

metric

The next hop attribute states the next hop on the path for the router to take. In EBGP-4, this will be the source address of the router that sent the update. In IBGP-4, for routes that originated outside the autonomous system, the address will still be the source address of the router that sent the update. The protocol states that the next hop advertised by EBGP-4 should be carried into the IBGP-4. Therefore, it is important that the IGP is aware of this network so that any router within the autonomous system can reach the next hop.

Multiple Exit Discriminator (MED)

Optional, nontransitive

Lowest value

Local preference Well known, 5 discretionary

Highest value

Atomic aggregate Well known, discretionary

Information not used in path selection

This attribute informs routers outside the autonomous system which path to take into the autonomous system. It is known as the external metric of a route. Therefore, it is passed between the autonomous systems, but it will not be propagated into a third autonomous system.

This attribute is used to tell routers within the autonomous system how to exit the autonomous system in the case of multiple paths. It is the opposite of the MED attribute. This value is passed solely between IBGP peers only.

This attribute states that the routes have been aggregated and that some information has been lost.

Aggregator Optional, 7 Information not used in This attribute states the BGP-4 router ID

transitive path selection and the autonomous system number of the router that was responsible for aggregating the route.

Table 8-6 The BGP-4 Attributes (Continued)

Attribute Name Category Code Preference Description

Community Optional, 8 Information not used in This is the capability to tag certain routes transitive path selection that have something in common. They are thereby made members of the same club or community. This is often used in conjunction with another attribute that will affect route selection for the community. For example, the use of the local preference and community attributes would allow the network administrators and other privileged beings to use the highspeed link to the Internet, while others shared a fractional T1. Communities have no geographical or logical limits. BGP-4 can filter on incoming or outgoing routes for filtering, redistribution, or path selection.

Originator ID Optional, 9 Information not used in The route reflector (described in the nontransitive path selection following chapter) appends this attribute.

It carries the router ID of the originating router in the local autonomous system. It is used to prevent loops.

Cluster list Optional, 10 Information not used in The cluster identifies the routers involved nontransitive path selection in the route reflection. The cluster list shows the reflection path that has been taken. This is used to prevent looping errors.

Weight Cisco-defined Highest value This is proprietary to Cisco and is used in route selection. It is local to the router and, because it is not propagated to other routers, there is no problem with compatibility. When there are multiple paths, it selects a path to a destination with different next hops to the same destination. Note that the weight attribute has no code. Because it is a local attribute and is not propagated to other routers, no code is needed.

Figure 8-12 shows the logic of the path selection used in BGP-4.

Figure 8-12 Path Selection in BGP-4

Figure 8-12 Path Selection in BGP-4

Use Figure 8-12 in association with the following list of the selection process steps.

1 If the router has a valid route to the destination, use that route.

2 If there is more than one valid route to the destination, take the route with the highest weight (Cisco proprietary).

3 If the weights are the same, then select the route with the highest local preference.

4 If the routes have the same local preference, then prefer the route that originated on that router.

5 If there are no routes that originated on the router, then examine the AS_Path and select the shortest path.

6 If the AS_Path is the same, then examine and choose the lowest origin code.

7 If the origin codes are the same, then select the path with the lowest MED (the MED values must have been sent from the same neighboring autonomous system).

8 If the MED values are the same, then choose an external BGP-4 route above an internal BGP-4 route.

9 If there is no external route, then choose the shortest path to the next-hop router for IBGP-4.

10 If all else fails, then choose the router with the lowest BGP-4 router ID. Table 8-7 summarizes the commands covered in this chapter.

Table 8-7 Summary of BGP-4 Commands

Command Function router bgp autonomous system number Starts the BGP routing process.

network network-number mask Identifies the networks to be advertised by the process. network-mask neighbor {ip-address I peer-group-name} Identifies the neighbor with whom the router is remote-as autonomous system number synchronizing its routing table, and activates a TCP

session with the neighbor.

neighbor { ip-address I peer-group-name} To avoid the problem of selecting the next-hop router on next-hop-self a NBMA network inappropriately, is used to force the router to use its own IP address as the next hop when advertising to neighbors.

no synchronization Turns off synchronization and the need for the IGP to know of a route before BGP-4 can advertise it. This is used when the IBGP-4 network is fully meshed.

continues

Table 8-7 Summary of BGP-4 Commands (Continued)

Command

Function

aggregate-address ip-address mask

Used to create an aggregate address. The summary only

[summary-only] [as-set]

advertises the summary, and the AS-set lists the

autonomous system numbers that the more specific

routes have traversed.

debug ip bgp [dampening 1 events 1

It is possible to be very specific about the debug

keepalives 1 updates]

parameters.

clear ip bgp {* I address} Resets the session between the neighbors and re-

[soft [in I out]] establishes it with the new configuration that has been entered. The soft option does not tear down the sessions, but it resends the updates. The in and out options allow the configuration of inbound or outbound soft updates. The default is for both.

clear ip bgp {* I address} Resets the session between the neighbors and re-

[soft [in I out]] establishes it with the new configuration that has been entered. The soft option does not tear down the sessions, but it resends the updates. The in and out options allow the configuration of inbound or outbound soft updates. The default is for both.

show ip bgp [summary I neighbors] Shows the BGP connections. A network can be specified to retrieve information on the lone network. The summary option will give the status of the BGP connections. The neighbor option gives both TCP and BGP connections.

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