Routing policies

1.5.c i Attribute manipulation

General information on “BGP Attribute manipulation”:

Inbound manipulation affects outbound traffic
Outbound manipulation affects inbound traffic
Manipulation can be done in two ways:
- Recommended: With route-maps on a per-neighbor basis inbound/outbound
- Also possible: Under the neighbor statement and defining the attributes per neighbor (weight only)
Manipulation can affect traffic flow of the whole BGP structure (all AS’es)

Attribute manipulation “WEIGHT”:

Normally used inbound
Only locally significant
Important: Significant for a whole AS when its done on a RR since he does the best path selection for all clients!

Attribute manipulation “LOCAL_PREFERENCE”:

Normally used inbound
Significant within an AS

Attribute manipulation “AS_PATH”:

Normally used outbound
Adds AS’es to the AS_PATH of a route which a defined BGP peer receives
AS-Path access-list specifies the AS which must be included in the advertised routes who you want to modify

Attribute manipulation “MED”:

Normally used outbound
Significant between two AS’es only because the attribute is optional nontransitive
Increases the metric of a route
Should be applied at the edge routers of an AS

“BGP Attribute manipulation” CLI configuration commands:

## Attaching a route map to a BGP neighbor
Router(config)# router bgp <asn>
Router(config-router)# address-family [ipv4 | ipv6 | vpnv4 | ...]
Router(config-router-af)# neighbor [IP | peer-group] route-map [ROUTE-MAP-NAME] [in | out]

## Configuring BGP attribute manipulation using WEIGHT
Router(config)# route-map [NAME] permit
Router(config-route-map)# set weight <value>

## Configuring BGP attribute manipulation using LOCAL_PREFERENCE
Router(config)# route-map [NAME] permit
Router(config-route-map)# set local-preference <value>

## Configuring BGP attribute manipulation using AS_PATH
Router(config)# ip as-path access-list <id> permit [regexp]

Router(config)# route-map [NAME] permit
Router(config-route-map)# match as-path <AS-PATH-ACL>
Router(config-route-map)# set as-path [prepend] [asn1 asn2 asn... | last-as]

## Configuring BGP attribute manipulation using MED
Router(config)# route-map [NAME] permit
Router(config-route-map)# set metric <value>

1.5.c ii Conditional advertisement

General information on “BGP Condition advertisement”:

Normally routes are advertised regardless of the existence of a different path
With conditional advertisement, the following logics can be accomplished:
- If route A exists in the local BGP table, then DO advertise route B
- If route A exists in the local BGP table, then DO NOT advertise route B
- If route A DOES NOT exist in the local BGP table, then DO advertise route B
- If route A DOES NOT exist in the local BGP table, then DO NOT advertise route B
Two components are needed:
- MANDATORY: An advertise-map (= route-map) which defines the prefixes to be advertised
- EITHER: An exist-map (= route-map) which defines the routes to be tracked if they’re existing
- OR: A non-exist-map (= route-map) which defines the routes to be tracked if they’re not existing
All components need to be linked to a route-map
Important: The networks to be conditionally advertised need to have a network statement within BGP!

“BGP Conditional advertisement” CLI configuration commands:

## Configuring BGP conditional advertisement with an exist-map
Router(config)# router bgp <asn>
Router(config-router)# address-family [ipv4 | ipv6 | vpnv4 | ...]
Router(config-router-af)# neighbor [IP | peer-group] advertise-map [ROUTE-MAP-NAME] exist-map [ROUTE-MAP-NAME]

## Configuring BGP conditional advertisement with a non-exist-map
Router(config)# router bgp <asn>
Router(config-router)# address-family [ipv4 | ipv6 | vpnv4 | ...]
Router(config-router-af)# neighbor [IP | peer-group] advertise-map [ROUTE-MAP-NAME] non-exist-map [ROUTE-MAP-NAME]

1.5.c Conditional Route Injection (Not on blueprint)

General information on “BGP Conditional Route Injection”:

Conditional route injection can be used to add (more specific) routes to the local BGP table
This is useful if neighbors only advertise a summary (aggregate) route without the more specific prefixes
Problem: BGP neighbors only advertise a summary (aggregate) route of 192.168.0.0/23. For traffic engineering the more specific prefixes 192.168.0.0/24 and 192.168.1.0/24 are needed.
Solution: Conditional route injection can be used for this:
1. An exist-map (= route-map) setting the unaggregated prefixes is created (linking to an ip prefix-list).
Important: Injected routes will be advertised to peers. This should be filtered out outbound if it’s unwanted!

“BGP Route Injection” CLI configuration commands:

## Configuring conditional route injection
Router(config)# router bgp <asn>
Router(config-router)# address-family [ipv4 | ipv6 | vpnv4 | ...]
Router(config-router-af)# bgp inject-map [ROUTE-MAP-NAME] exist-map [ROUTE-MAP-NAME] copy-attributes

1.5.c iii Outbound Route Filtering

General information about “BGP Outbound Route Filtering”:

Route filtering can be done outbound and inbound
Problem: With inbound route filtering the router still has to process the routers before discarding it which results in unnecessary waste of CPU and memory power.
Solution: Outbound route filtering which means that unnecessary routes aren’t even sent out to a peer who doesn’t need it.
Prefix-lists are configured locally and “sent” to the remote peer.
Important: After changing a prefix-list, it needs to be re-sent by the router!

Configuration information:

ORF needs to be enabled on both sides of the link (Example: one side send-only and one side receive-only)
When enabling ORF after a peering established, the peering needs to be flapped

“BGP Outbound Route Filtering” CLI configuration commands:

## Enabling BGP Outbound Route Filtering on a per-peer basis
Router(config)# router bgp <asn>
Router(config-router)# address-family [ipv4 | ipv6 | vpnv4 | ...]
Router(config-router-af)# neighbor [IP | peer-group] capability orf prefix-list [send | receive | both]

## Defining the prefix-list which gets sent to the BGP neighbor
Router(config-router-af)# neighbor [IP | peer-group] prefix-list [PREFIX-LIST-NAME] in

## Re-sending the prefix-list to the BGP neighbor after a change
Router# clear ip bgp [IP | *] in prefix-filter

1.5.c iv Standard and extended communities

General information on “BGP Standard and extended communities”:

Communities are basically tags that are attached to BGP routes
Communities are optional and transitive
Communities need to be explicitly enabled
Can be used for local route-manipulation based on pre-defined communities
Example: A BGP router can pre-define a route-map with different match/set statements (eg. match ASN:123, set localpref 123). This route-map can be configured inbound on every neighbor so that the BGP router automatically sets the LOCAL_PREF to what the BGP neighbor requests.

Well-known standard communities:

NO-EXPORT: Don’t advertise to eBGP neighbors
NO-ADVERTISE: Don’t advertise to any peers
LOCAL-AS: Don’t advertise to confederation eBGP neighbors
INTERNET: Advertise the prefix to all BGP neighbors

Configuration considerations:

Setting a community is done directly in the route-map
Matching a community is done with a route-map linked to an ip community-list
Within a RR cluster all clients need to have the the send-community enabled or else the communities get stripped
With the additive keyword all other communities will get preserved, otherwise they will be overwritten

“BGP Standard and extended communities” CLI configuration commands:

## Attaching a route map to a BGP neighbor
Router(config)# router bgp <asn>
Router(config-router)# address-family [ipv4 | ipv6 | vpnv4 | ...]
Router(config-router-af)# neighbor [IP | peer-group] route-map [ROUTE-MAP-NAME] [in | out]

## Changing the CLI display format of communities into two 16-bit values separated by a colon
Router(config)# ip bgp-community new-format

## Enabling sending of communities on a per-peer basis
Router(config)# router bgp <asn>
Router(config-router)# address-family [ipv4 | ipv6 | vpnv4 | ...]
Router(config-router-af)# neighbor [IP | peer-group] send-community

## Adding a community to a route (and optionally preserving the already existing ones)
Router(config)# route-map [NAME] permit
Router(config-route-map)# set community <community> [additive]

## Deleting all communities of a route
Router(config)# route-map [NAME] permit
Router(config-route-map)# set community none

## Deleting a subset of communities of a route (must be linked to an ip community-list)
Router(config)# route-map [NAME] permit
Router(config-route-map)# set comm-list delete [NAME]

## Configuring an IP community list (to be used standalone or with route-maps)
Router(config)# ip community-list [standard | extended] [NAME] [permit | deny] <community-string>

## Configuring a route-map (to be configured inbound) to change parameter x based on a received community
Router(config)# route-map [NAME] permit
Router(config-route-map)# match community [NAME]
Router(config-route-map)# set [command]

1.5.c v Multi-homing

General information on “BGP Multi-homing”:

There are four possible “Homing” constellations:
- Single Homing: Single link to a single ISP
- Dual Homing: Multiple links to a single ISP
- Single Multi-Homing: Single link to multiple ISPs
- Dual Multi-Homing: Multiple links to multiple ISPs