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Advanced Routing

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Identify the route to filter using route maps. ... Group of destinations that share some common property. Not restricted to ASs or Networks ... – PowerPoint PPT presentation

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Title: 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.
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