Advanced Routing

1
Advanced Routing

• Chapter 9 Scaling BGP

• Route reflectors, route filtering, the Community
  attribute, and peer groups
• Redundancy and load balancing
• Route redistribution

2
Full Mesh Requirement

• BGP requires logical full mesh between all
  routers.
• To avoid long configuration with Network
  statements, Cisco introduced the Route Reflector
• The Route Reflector is recommended for large ASs
  Only.

3
Full Mesh Issue

• IBGP routers do not forward info received from
  other IBGPs.
• If the link between RTA and RTC goes down, AS300
  looses its connection to AS200.

4
Route Reflector

• A Route Reflector is a concentration point for
  the AS.
• It acts as a server delivering routes to clients

5
Route Reflector Terminology

• Three types of routers
• Servers (RR)
• Clients
• Non-Clients
• A route reflector and its clients form a cluster.

Never configure route reflector clients to peer
with IBGP speakers outside their cluster as this
can cause routing loops.
6
RR configuration

• Route Reflector configuration occurs on the
  Server

• RTC(config)router bgp 100
• RTC(config-router)neighbor 1.1.1.1 remote-as 100
• RTC(config-router)neighbor 1.1.1.1
  route-reflector-client
• RTC(config-router)neighbor 2.2.2.2 remote-as 100
• RTC(config-router)neighbor 2.2.2.2
  route-reflector-client
• RTC(config-router)neighbor 4.4.4.4 remote-as 100
• RTC(config-router)neighbor 7.7.7.7 remote-as 100
• RTC(config-router)neighbor 8.8.8.8 remote-as 200

7
Route Propagation by RRs

• The RR will propagate a route inside the AS based
  on the following rules
• If the route is received from a nonclient peer,
  reflect to clients only.
• If the route is received from a client peer,
  reflect to all nonclient peers and also to client
  peers, except the originator of the route.
• If the route is received from an EBGP peer,
  reflect to all client and nonclient peers

8
Route Filtering

• Allows a BGP speaker to choose what routes to
  exchange with any of its BGP peers.
• Can be applied between peers and between routing
  protocols running on a single router.
• Can be used to manipulate attributes

• Route Filtering uses
• Route Maps
• Distribute Lists
• Prefix Lists

9
Route Filtering Using Route Maps

• Two steps to implement routing policies
• Identify the route to filter using route maps.
• Implement the policy (restriction or
  modification)

10
Route Filtering Using Distribute Lists
11
Extended Access Lists to Control Route
Aggregation

• Extended access-lists can be used to ensure that
  route aggregation is properly advertised
• RTA(config-router)neighbor 172.16.20.1
  distribute-list 101 outRTA(config-router)EXITRT
  A(config)access-list 101 permit ip 172.16.0.0
  0.0.255.255 255.255.0.0 0.0.0.0

Restrictions on Network Address
Restrictions on subnet info
12
Prefix List

• An easy to configure alternative to distribute
  lists
• Allows a more flexible control of prefix length
• RTA(config)ip prefix-list ELMO seq 12 permit
  10.0.0.0/8 ge 16 le 24
• RTA(config)router BGP 100
• RTA(config-router)neighbor 192.168.1.1 remote-as
  200
• RTA(config-router)neighbor 192.168.1.1
  prefix-list ELMO out

Sequence number
Mask gt 16
Mask lt 24
RTA(config)show ip prefix-list
13
The BGP Community

• Group of destinations that share some common
  property
• Not restricted to ASs or Networks
• Assigned by a community attribute value sent by a
  speaker
• Type code 8, optional transitive

14
Types of Attributes

• Well Known attributes
• NO_EXPORT (not outside a confederation)
• NO_ADVERTISE (Not to any BGP peer)
• Internet (to all other routers)
• Local-as (only within AS)
• Private Community attributes
• Identified by a numeric value

15
Community Attribute Configuration
Send the community attributes to this router
Send this routes with a No-Export attribute
Sends the other routes unchanged
16
Community Attribute Configuration
Adds the community value to others
17
Peers Groups

• Group of BGP neighbors that share the same update
  policies.
• Prevents having to repetitively configure each
  BGP peer.
• The router formulates the update message once and
  then floods the same update to all the neighbors
  that fall within the group.
• Policies could be a set of IP prefix filters,
  AS_Path filters, or even filters that result in
  attribute manipulation.

18
Restriction to Peering

• The hub router (such as RTA in the figure) cannot
  be a transit router for the external autonomous
  systems.

19
Peer Group Configuration
20
Redundancy, Symmetry and Load Balancing

• Redundancy is achieved by providing multiple
  alternate paths for traffic .
• Symmetry exists if traffic leaves the AS from a
  certain exit point and returns through the same
  point.
• Load balancing results in the division of traffic
  optimally over multiple links.

21
RedundancyDefault Information in BGP

• To control the target of Default information
• RTC(config)router bgp 3
• RTC(config-router)neighbor 172.16.20.1 remote-as
  1
• RTC(config-router)neighbor 172.16.20.1
  default-originate

22
SymmetryDefault Information in BGP

• To promote symmetry outbound
• Default route with a low administrative distance
  or a high Local Preference
• To promote symmetry outbound
• Community attribute
• Route filters
• Med_attribute

23
Load Balancing

• When a BGP speaker learns two identical EBGP, it
  will choose the path with the lowest route ID as
  the best path.
• To enable BGP load balancing

Router(config-router) maximum-paths number
24
Redistribution

• IGPs can be redistibuted in BGP in two different
  ways
• Dynamically using the Redistribute command
• Semi-dynamically with the network command. A
  network command is necessary for each route
  prefix.
• The No synchronization command disables the
  verification of the existence of the IGP routes.