Scaling BGP

1
Scaling BGP

• Semester 5, Chapter 9
• Allan Johnson
• Modified by Sean He

2
Route Reflectors
3
Scalability and IBGP

• The IBGP Split Horizon rule
• An IBGP router will not advertise BGP routes
  received from one IBGP peer to other IBGP peers
• This rule, then, requires that all IBGP routers
  in an AS form a logical full mesh.
• Otherwise, all BGP routing updates will not be
  received by all IBGP peers.
• However, fully meshed internal BGP can become a
  burden on router resources, bandwidth usage, and
  administrative overhead.

4
Solution Route Reflectors

• We can drastically reduce the number of internal
  TCP and BGP connections by configuring router
  reflectors.
• A general rule is to configure route reflectors
  according to the physical topology.
• In other words, hub routers become reflectors and
  spoke routers become clients.
• Route reflectors must maintain a IBGP full mesh
  with other routers that are nonclients (e.g.
  other route reflectors).
• Configuring route reflectors below would reduce
  the number of TCP/BGP connections from 28 to just
  8!

5
Route Reflector Example
RTR_A(config)router bgp 100 RTR_A(config-router)
neighbor 10.0.0.3 remote-as 100 RTR_A(config-route
r)neighbor 10.0.0.3 route-reflector-client RTR_A(
config-router)neighbor 10.0.0.4 remote-as
100 RTR_A(config-router)neighbor 10.0.0.4
route-reflector-client ----------------- RTR_B(con
fig)router bgp 100 RTR_B(config-router)neighbor
10.0.0.2 remote-as 100 ----------------- RTR_C(con
fig)router bgp 100 RTR_C(config-router)neighbor
10.0.0.1 remote-as 100
6
Route Reflector Operation

• Upon receiving an UPDATE, a route reflector takes
  one of the following actions
• If received from a nonclient (e.g. another route
  reflector), the route reflector sends the update
  only to its client peers (IBGP Split Horizon
  Rule)
• If received from a client, the route reflector
  sends the update to all client and nonclient
  peers (except for the client from which it
  received the update).
• If received from an EBGP peer, the route
  reflector sends the update to all client and
  nonclient peers.

7
BGP Route Filtering
8
Extended ACL Distribute Lists

• The syntax is similar to what weve seen before
• access-list ACL permitdeny source
  source-wildcard mask mask-wildcard
• The source and source-wildcard are the same.
• However, the mask and mask-wildcard arguments
  have a different meaning.
• The mask specifies the subnet mask of the source.
• The mask-wildcard specifies what bits in the
  subnet mask are relevant

9
Distribute List Example

• Our previous example when using an extended ACL
  reduces the amount of routes advertised to
  onethe supernet only
• Router(config)access-list 101 deny 192.168.11.0
  0.0.0.255 255.255.255.0 0.0.0.0
• Router(config)access-list 101 permit 172.16.0.0
  0.0.255.255 255.255.0.0 0.0.0.0
• Router(config)router bgp 35623
• Router(config-router)neighbor 172.16.32.1
  remote-as 369
• Router(config-router)neighbor 172.16.32.1
  distribute-list 101 out
• This distribute list will stop the
  192.168.11.0/24 network from being advertised.
• It will also only allow the supernet,
  172.16.0.0/16, to be advertised.
• Notice that because of the implicit deny any,
  the first ACL statement is redundant.

10
Simple Prefix List

• Router(config)ip prefix-list CISCO deny
  192.168.11.0/24
• Router(config)ip prefix-list CISCO permit
  172.16.0.0/16
• Router(config)router bgp 35623
• Router(config-router)neighbor 172.16.32.1
  remote-as 369
• Router(config-router)neighbor 172.16.32.1
  prefix-list CISCO out

11
More Prefix Lists

• The following examples show how to use le and ge.
• To accept a mask length of up to 24 bits in
  routes with the prefix 192/8
• ip prefix-list ABC permit 192.0.0.0/8 le 24
• To deny mask lengths greater than 25 bits in
  routes with a prefix of 192/8
• ip prefix-list ABC deny 192.0.0.0/8 ge 25
• To permit mask lengths from 8 to 24 bits in all
  address space
• ip prefix-list ABC permit 0.0.0.0/0 ge 8 le 24
• To deny mask lengths greater than 25 bits in all
  address space
• ip prefix-list ABC deny 0.0.0.0/0 ge 25
• To deny all routes with a prefix of 10/8
• ip prefix-list ABC deny 10.0.0.0/8 le 32
• To deny all masks with a length greater than 25
  bits routes with a prefix of 204.70.1/24
• ip prefix-list ABC deny 204.70.1.0/24 ge 25
• To permit all routes with a prefix of 0/0
• ip prefix-list ABC permit 0.0.0.0/0 le 32

12
The COMMUNITY Attribute
13
The COMMUNITY Attribute

• The COMMUNITY attribute is used by BGP peers to
  group a list of destination networks by a common
  policy.
• For example, an ASBR may want to send all
  internal routes to its ISP. However, it may want
  to stop the ISP from further advertising some of
  those routes.

14
Well-known COMMUNITY Values

• NO_EXPORT
• Route is NOT to be advertised outside the
  receiving AS to other EBGP peers.
• NO_ADVERTISE
• Route is NOT to be advertised to any BGP peers.
• Internet
• Route is to be advertised to all BGP routers
• Local-as
• Route is to be advertised to peers within AS

15
Configuring the COMMUNITY

• The COMMUNITY attribute uses route maps to set
  the value.
• Router(config)access-list 1 permit any
• Router(config)route map SET permit 10
• Router(route-map-config)math IP address 1
• Router(route-map-config)set community no_export
• The above commands will set the no_export
  community value on all routes
• Apply the route map to desired neighbor(s).
• Router(config-router)neighbor 10.0.0.1 route-map
  SET out
• Send the attribute to the desired neighbor(s).
• Router(config-router)neighbor 10.0.0.1
  send-community

16
Setting Up Peer Groups
17
Peer Groups Example (IBGP)

• RTA(config-router)neighbor IBGP_PEERS peer-group
  
• !Names the peer group
• RTA(config-router)neighbor IBGP_PEERS remote-as
  100
• !Avoids having to use the neighbor
  remote-as command
• RTA(config-router)neighbor 192.168.1.2
  peer-group IBGP_PEERS
• RTA(config-router)neighbor 192.168.2.2
  peer-group IBGP_PEERS
• !Assigned peers to the peer group
• RTA(config-router)neighbor IBGP_PEERS route-map
  INTRA out
• !Defines an outbound update policy for all
  peers

18
Peer Groups Example (EBGP)

• RTA(config-router)neighbor EBGP_PEERS peer-group
  
• !Names the peer group
• RTA(config-router)neighbor 10.10.10.1 remote-as
  300
• RTA(config-router)neighbor 10.10.10.1 peer-group
  EBGP_PEERS
• RTA(config-router)neighbor 10.10.10.2 remote-as
  200
• RTA(config-router)neighbor 10.10.10.2 peer-group
  EBGP_PEERS
• !Assigned peers their AS and peer group
• RTA(config-router)neighbor EBGP_PEERS route-map
  EXTRA out
• !Defines an outbound update policy for all
  peers