IP Routing: BGP Configuration Guide, Cisco IOS XE Gibraltar 16.10.x
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BGP Diverse Path Using a Diverse-Path Route Reflector
The BGP Diverse Path Using a Diverse-Path Route Reflector feature allows Border Gateway Protocol (BGP) to distribute an alternative
path other than the best path between BGP speakers when route reflectors are deployed. This feature is meant to provide path
diversity within an autonomous system (AS), within a single cluster only. That is, a route reflector is allowed to advertise
the diverse path to its client peers only.
Finding Feature Information
Your software release may not support all the features documented in this module. For the latest caveats and feature information,
see the Bug Search Tool and the release notes for your platform and software release. To find information about the features
documented in this module, and to see a list of the releases in which each feature is supported, see the feature information
table at the end of this module.
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Prerequisites for BGP Diverse Path Using a Diverse-Path Route Reflector
You should understand the BGP Best External feature.
Restrictions for BGP Diverse Path Using a Diverse-Path Route Reflector
A diverse path can be configured on a route reflector only.
Only one shadow route reflector is allowed per existing route reflector, which will calculate one additional best path (the
second best path). That is, only one additional plane (topology) is configured.
Path diversity is configured within an AS, within a single route reflector cluster. That is, the route reflector will advertise
the diverse path to its route reflector client peers only.
Diverse path functionality is not supported on a route server.
Information About BGP Diverse Path Using a Diverse-Path Reflector
Limitation that a BGP Diverse Path Overcomes
As a path vector routing protocol, BGP-4 requires a router to advertise to its neighbors only the best path for a destination.
However, multiple paths to the same destination would allow mechanisms that can improve resilience, quickly recover from failures,
and load balance, for example.
The use of route reflectors is one of the main reasons for poor path diversity within an autonomous system (AS). In a network
with route reflectors, even if a prefix is learned from multiple egress routers, the route reflector reflects only the best
path to its clients. The figure below shows how deploying route reflectors might reduce path diversity in an AS, even when
the BGP Best External feature is deployed.
In the figure above, P1 and P2 are diverse paths for prefix p. Assume Path 2 (P2) has a lower MED and higher local preference
than P1. The BGP Best External feature on PE1 will make sure that P1 is propagated to the route reflectors, regardless of
P2 having a lower MED and higher local preference. The route reflectors will have path diversity; they will learn both P1
and P2 with different exit points PE1 and PE2 (assuming that PE1 and PE2 have the
setipnext-hopself command configured). However, both route reflectors select the best path as P2 due to its lower MED/higher local preference
and advertise it to PE3. PE3 will not learn P1 (that is, PE3 will not learn about existing path diversity).
The BGP Diverse Path Using a Diverse-Path Route Reflector feature is a way to resolve that limitation and achieve path diversity.
BGP Diverse Path Using a Diverse-Path Route Reflector
The BGP Diverse Path Using a Diverse-Path Route Reflector feature overcomes the lack of path diversity in an AS containing
route reflectors. This feature is meant to provide path diversity within an AS, within a single cluster only. That is, a route
reflector is allowed to advertise the diverse path to its client peers only.
For each route reflector in the AS, a
shadow route reflector is added to distribute the
second best path, also known as the
diverse path. The figure below shows the shadow route reflector for RR2. The shadow route reflector improves path diversity because PE3
can now learn both P1 (from RR1/RR2) and learn P2 from the shadow route reflector.
Note
The primary route reflector and shadow route reflector must have the exact same connections (physical/control plane) to the
rest of the routers in the network.
Shadow route reflectors can be both control plane route reflectors and data plane route reflectors.
The figure below shows a diverse path in greater detail, indicating the next hops:
BR2 announces to RR1 and shadow RR2 that R2 (BR2) is the Next Hop for those who want to reach Prefix Z. Likewise, BR3 announces
to RR1 and shadow RR2 that R3 (BR3) is the Next Hop for those who want to reach Prefix Z
RR1 sends a packet to BR1 announcing that the Next Hop is R2 if BR1 wants to reach Prefix Z. The second best path (or diverse
path) comes from shadow RR2, which sends a packet to BR1 announcing that the Next Hop is R3 if BR1 want to reach Prefix Z.
At BR1 (far right), we see there are two (diverse) paths to Prefix Z.
Triggers to Compute a BGP Diverse Path
Computation of a diverse path per address family is triggered by any of the following commands:
bgpadditional-pathsinstall
bgpadditional-pathsselect
maximum-pathsebgp
maximum-pathsibgp
The bgpadditional-pathsinstall command will install the type of path that is specified in the bgpadditional-pathsselect command. If the bgpadditional-pathsselect command specifies both keyword options (best-external and backup), the system will install a backup path.
The maximum-pathsebgp and maximum-pathsibgpcommands trigger a multipath computation, and multipaths are automatically installed as primary paths.
On the other hand, the bgpadditional-pathsinstall command triggers computation of a backup path or best-external path.
If the bgpadditional-pathsselect command is not configured, the bgpadditional-pathsinstallcommand will trigger both computation and installation of a backup path (as is done with the BGP PIC feature).
IGP Metric Check
Disabling the Interior Gateway Protocol (IGP) metric check and configuring the BGP Diverse Path feature are independent of
each other. One does not imply the other. That is, configuring
bgpbestpathigp-metricignore does not imply that the BGP Diverse Path feature is enabled. Conversely, enabling the BGP Diverse Path feature might not
require that
bgpbestpathigp-metricignore be configured (because, for example, the route reflector and shadow route reflector are co-located).
The
bgpbestpathigp-metricignore command can be configured at route reflectors and provider edges (PEs).
Note
Per-VRF functionality for the
bgpbestpathigp-metricignore command is not supported. If you use it anyway, it is at your own risk.
Route Reflector Determination
If a router’s configuration includes either one of the following commands, the router is a route reflector:
bgpcluster-id
neighborroute-reflector-client
How to Configure a BGP Diverse-Path Route Reflector
Determining Whether You Need to Disable the IGP Metric Check
Before you configure a shadow route reflector in order to get a BGP diverse path, determine whether you need to disable the
IGP metric check. The IGP metric is a configurable value indicating physical distance, and is used by an Interior Gateway
Protocol, such as Open Shortest Path First (OSPF), Enhanced Interior Gateway Routing Protocol (EIGRP), or Routing Information
Protocol (RIP). A smaller IGP metric is preferred over a larger IGP metric.
The locations of the route reflector and shadow route reflector determine whether or not you need to disable the IGP metric
check, as follows:
When the route reflector and shadow route reflector are colocated—They have the same IP subnetwork address and are connected
to the Ethernet switch with different links. Failure of such a link is equivalent to the route reflector going down. When
RRs are colocated, their IGP metrics cannot be different from each other; and therefore there is no need to disable the IGP
metric check during the best path calculation at any route reflector. Because there is no need to disable the IGP metric check,
the first plane route reflectors do not need to be upgraded to Cisco IOS XE Release 3.4S.
When the shadow route reflector is in a different IGP place from the route reflector (it is not colocated with its best path
route reflector)--In this case, the IGP metric check is ignored on both the best path route reflector and shadow route reflector
when the best path and second best path are being calculated. The IGP metric check must be disabled on the primary route reflector
by configuring the
bgpbestpathigp-metricignore command. This command is available beginning with Cisco IOS XE Release 3.4S, which means you need to upgrade to that release.
Configuring the Route Reflector for BGP Diverse Path
Perform this task to configure a route reflector for the BGP Diverse Path feature. This task specifies the IPv4 address family,
but other address families are also supported.
Enables BGP to calculate a backup path for a given address family and to install it into the routing information base (RIB)
and Cisco Express Forwarding (CEF).
(Optional) Configures a neighbor to receive the diverse path in an advertisement.
Step 13
end
Example:
Device(config-router-af)# end
(Optional) Exits address family configuration mode and returns to privileged EXEC mode.
Step 14
showipbgpneighborip-addressadvertised-routes
Example:
Device# show ip bgp neighbor 10.1.1.1 advertised-routes
(Optional) Displays the routes advertised to the specified neighbor.
Configuration Examples for BGP Diverse Path Using a Diverse-Path Route Reflector
Example: Configuring BGP Diverse Path Where Additional Path Is the Backup Path
Diverse path functionality is contained within a single cluster; that is, only the clients of a route reflector can be configured
to advertise the diverse path. A diverse path is advertised to the clients of a route reflector only if the client is configured
to get the additional path.
A shadow route reflector can be added to calculate and advertise the additional path, or an existing route reflector can
be configured to calculate and advertise the additional path. In the figure below, instead of adding a shadow route reflector,
RR2 (the existing backup RR) is configured to calculate the additional path and advertise it to a particular neighbor.
In the figure below, assume that from the route reflectors, the path to CE1 via PE1 is preferred over the path via PE2. Without
the diverse path feature, both route reflectors will advertise to PE3 that the path to CE1 is via PE1. If the connection between
RR1 and PE1 fails (or the path between PE1 and CE1 fails), there is no other path.
In the following configuration example based on the figure above, RR2 is configured with an additional path, which is a backup
path.
If RR1 and RR2 are not colocated, you must configure the
bgpbestpathigp-metricignore command before the additional path is calculated. (If RR1 and RR2 are colocated, do not configure that command.)
The
bgpadditional-pathsselectbackup command triggers calculation of the backup path at RR2, which is the path via PE2.
The
bgpadditional-pathsinstall command installs the backup path if RR2 is in the forwarding plane. (Do not configure this command if RR2 is in the control
plane.)
The address of PE3 is 10.1.1.1, and that address is used in the
neighboradvertisediverse-pathbackup command on RR2. This command triggers advertisement of the backup path to PE3. PE3 will learn the best path, (which is the
path via PE1) from RR1, and it will learn the backup path from RR2.
Example: Configuring BGP Diverse Path Where Additional Path Is the Multipath
In the following example based on the figure above, assume that paths toward CE1 via PE1 and PE2 are multipaths. The
maximum-pathsibgp command will trigger calculation of multipaths.
The address of PE3 is 10.1.1.1, and that address is used in the
neighboradvertisediverse-pathmpath command on RR2. This command will trigger advertisement of the multipath, that is, the second best path, to PE3. PE3 will
learn the best path, path via PE1 from RR1, and will learn second best path from RR2.
Example: Configuring BGP Diverse Path Where Both Multipath and Backup Path Calculations Are Triggered
The following example is based on the figure above. The
maximum-pathsibgp command will trigger calculation of multipaths. When both multipath and backup path calculations are triggered, the backup
path and the second multipath (which is the second best path) are the same paths and it will be installed as the active path,
regardless of whether the route reflector is in the control plane or forwarding plane.
The address of PE3 is 10.1.1.1, and that address is used in the
neighboradvertisediverse-pathbackupmpath command on RR2. This command causes RR2 to advertise the second best path, which is the second multipath, to PE3.
Example: Configuring Triggering Computation and Installation of a Backup Path
When the
bgpadditional-pathsinstall command is configured without configuring
bgpadditional-pathsselectbackup, the former command will trigger both computation and installation of the backup path (as it is with the existing BGP PIC
feature).
The address of PE3 is 10.1.1.1, and that address is used in the
neighboradvertisediverse-pathbackup command on RR2. This command will trigger advertisement of a backup path to PE3. PE3 will learn the best path, a path via
PE1 from RR1, and it will learn a backup path from RR2.
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Feature Information for BGP
Diverse Path Using a Diverse-Path Route Reflector
The following table provides release information about the feature or features described in this module. This table lists
only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise,
subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco
Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Table 1. Feature Information for BGP
Diverse Path Using a Diverse-Path Route Reflector
Feature Name
Releases
Feature
Information
BGP Diverse
Path Using a Diverse-Path Route Reflector
This feature
allows BGP to distribute an alternative path other than the best path between
BGP speakers when route reflectors are deployed.
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