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ME60 V800R010C10SPC500 Feature Description - WAN Access 01

This is ME60 V800R010C10SPC500 Feature Description - WAN Access

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BGP Best External

BGP Best External

Background

If multiple routes to the same destination are available, a BGP device selects one optimal route based on BGP route selection policies and advertises the route to its BGP peers.

NOTE:

For details about BGP route selection policies, see BGP Principles.

However, in scenarios with master and backup provider edges (PEs) or route reflectors (RRs), if routes are selected based on the preceding policies and the primary link fails, the BGP route convergence takes a long time because no backup route is available. To address this problem, the BGP Best External feature was introduced.

Related Concepts

BGP Best External: A mechanism that enables a backup device to select a sub-optimal route and send the route to its BGP peers if the route preferentially selected based on BGP route selection policies is an Internal Border Gateway Protocol (IBGP) route advertised by the master device. Therefore, BGP Best External speeds up BGP route convergence if the primary link fails.

Best External route: The sub-optimal route selected after BGP Best External is enabled.

Networking with Master and Backup PEs

In the networking shown in Figure 9-39, CE1 is dual-homed to PE1 and PE2. PE1 has a greater Local_Pref value than PE2 and CE1. EBGP peer relationships are established between CE1 and PE1, and between CE1 and PE1. In addition, IBGP peer relationships are established among PE1, PE2, and PE3. PE1 and PE2 receive the same route to 1.1.1.1/32 from CE1. After receiving this route, PE1 advertises it to PE2 and PE3. Therefore, PE2 has two routes to 1.1.1.1/32. Of the two routes, PE2 preferentially selects the route from PE1 because PE1 has a larger Local_Pref value than CE1. PE2 does not advertise the selected route to PE3. Therefore, PE3 has only one route to 1.1.1.1/32, which is advertised by PE1. If the link between CE1 and PE1 fails, a new route must be selected to take over traffic after routes are converged, which takes a long time.

Figure 9-39 Networking with master and backup PEs

BGP Best External can be enabled on PE2 to address this problem. With BGP Best External, PE2 selects the EBGP route from CE1 and advertises it to PE3. Therefore, PE3 has two routes to 1.1.1.1/32, in which the route CE1 -> PE2 -> PE3 backs up CE1 -> PE1 -> PE3. Table 9-4 lists the differences with and without BGP Best External.

Table 9-4 Differences with and without BGP Best External

BGP Best External

Route Available on PE3

Optimal Route

If the Link Between CE1 and PE1 Fails

Not enabled

CE1 -> PE1 -> PE3

CE1 -> PE1 -> PE3

A new route must be selected to take over traffic after routes are converged.

Enabled

CE1 -> PE1 -> PE3

CE1 -> PE2 -> PE3

CE1 -> PE1 -> PE3

Traffic is switched to CE1 -> PE2 -> PE3 immediately.

Networking with Master and Backup RRs

In the networking shown in Figure 9-40, an EBGP peer relationship is established between Device A and Device B. An IBGP peer relationship is established between each two devices among RR1, RR2, Device B, and Device C except between Device B and Device C. Device B is a client of RR1 and RR2. RR1 has a greater Local_Pref value than RR2, and therefore RR1 is the master device whereas RR2 is the backup device. RR1 and RR2 receive the same route to 1.1.1.1/32 from Device B. After receiving this route, RR1 advertises it to RR2 and Device C. Therefore, RR2 has two routes to 1.1.1.1/32. Of the two routes, RR2 preferentially selects the route from RR1 because RR1 has a greater Local_Pref value. RR2 does not advertise the selected route to Device C. Therefore, Device C has only one route to 1.1.1.1/32, which is advertised by RR1. If the link between Device B and RR1 fails, a new route must be selected to take over traffic after routes are converged, which takes a long time.

Figure 9-40 Networking with master and backup RRs

BGP Best External can be enabled on RR2 to address this problem. With BGP Best External, RR2 selects the EBGP route from Device B and advertises it to Device C. Therefore, Device C has two routes to 1.1.1.1/32, in which the route Device A -> Device B -> RR2 -> Device C backs up Device A -> Device B -> RR1 -> Device C. Table 9-5 lists the differences with and without BGP Best External.

Table 9-5 Differences with and without BGP Best External

BGP Best External

Route Available on Device C

Optimal Route

If the Link Between Device B and RR1 Fails

Not enabled

Device A -> Device B -> RR1 -> Device C

Device A -> Device B -> RR1 -> Device C

A new route must be selected to take over traffic after routes are converged.

Enabled

Device A -> Device B -> RR1 -> Device C

Device A -> Device B -> RR2 -> Device C

Device A -> Device B -> RR1 -> Device C

Traffic is switched to Device A -> Device B -> RR2 -> Device C immediately.

Usage Scenario

The BGP Best External feature applies to scenarios in which master and backup PEs or RRs are deployed and the backup PE or RR needs to advertise the sub-optimal route (Best External route) to its BGP peers to speed up BGP route convergence.

Advantages

As networks develop, services, such as voice over IP (VoIP), online video, and financial services, pose higher requirements for real-time transmission. With BGP Best External, the backup device selects the sub-optimal route and advertises the route to its BGP peers, which speeds up BGP route convergence and minimizes service interruptions.

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Updated: 2019-01-04

Document ID: EDOC1100059473

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