Administrator Guide

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Filtering Routes with Community Lists
To use an IP community list or IP extended community list to filter routes, you must apply a match community filter to a route
map and then apply that route map to a BGP neighbor or peer group.
1. Enter the ROUTE-MAP mode and assign a name to a route map.
CONFIGURATION mode
route-map map-name [permit | deny] [sequence-number]
2. Configure a match filter for all routes meeting the criteria in the IP community or IP extended community list.
CONFIG-ROUTE-MAP mode
match {community community-list-name [exact] | extcommunity extcommunity-list-name
[exact]}
3. Return to CONFIGURATION mode.
CONFIG-ROUTE-MAP mode
exit
4. Enter ROUTER BGP mode.
CONFIGURATION mode
router bgp as-number
AS-number: 0 to 65535 (2-Byte) or 1 to 4294967295 (4-Byte) or 0.1 to 65535.65535 (Dotted format)
5. Apply the route map to the neighbor or peer group’s incoming or outgoing routes.
CONFIG-ROUTER-BGP mode
neighbor {ip-address | ipv6-address | peer-group-name} route-map map-name {in | out}
DellEMC# configure terminal
DellEMC(conf)# router bgp 400
DellEMC(conf-router_bgp)# neighbor 10.10.10.1 remote-as 500
DellEMC(conf-router_bgp)# neighbor 10.10.10.1 route-map route2 in
DellEMC(conf-router_bgp)# exit
DellEMC(conf)# route-map route2 permit 10
DellEMC(conf-route-map)# match community 1
DellEMC(conf-route-map)# set community 99
DellEMC(conf-route-map)# exit
DellEMC(conf)# ip community-list 1 permit 100
DellEMC(conf)# exit
DellEMC#
In the above example, add a BGP neighbor to the AS 400 and the route-map called route2 applied to inbound routes from the
BGP neighbor at 10.10.10.1. A route map route2 is created with a permit clause and the route’s community attribute is matched
to communities in community list 1. A community list 1 that permits routes with a communities attribute of 100.
To view the BGP configuration, use the show config command in CONFIGURATION ROUTER BGP mode. To view a route
map configuration, use the show route-map command in EXEC Privilege mode.
To view which BGP routes meet an IP community or IP extended community list’s criteria, use the show ip bgp
{community-list | extcommunity-list} command in EXEC Privilege mode.
Configuring BGP Fast Fall-Over
By default, a BGP session is governed by the hold time.
BGP routers typically carry large routing tables, so frequent session resets are not desirable. The BGP fast fall-over feature
reduces the convergence time while maintaining stability. The connection to a BGP peer is immediately reset if a link to a
directly connected external peer fails.
When you enable fall-over, BGP tracks IP reachability to the peer remote address and the peer local address. Whenever either
address becomes unreachable (for example, no active route exists in the routing table for peer IPv6 destinations/local address),
BGP brings down the session with the peer.
Border Gateway Protocol (BGP)
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