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Route Optimisation

Route Optimisation. Preventing Propagation of routing information. When using an on-demand WAN link To minimize, or stop entirely, the exchange of routing update information across this type of link; otherwise, the link will remain up constantly To prevent routing loops

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Route Optimisation

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  1. Route Optimisation RD-CSY3021

  2. Preventing Propagation of routing information • When using an on-demand WAN link • To minimize, or stop entirely, the exchange of routing update information across this type of link; otherwise, the link will remain up constantly • To prevent routing loops • When a path is learned to the same destination by two different routing protocols, you may want to filter the propagation of one of the paths • To preserve bandwidth • To ensure maximum bandwidth availability for data traffic by reducing unnecessary routing update traffic RD-CSY3021

  3. Controlling/Preventing Propagation • Passive interface • Prevents all routing updates from being sent through an interface • Default routes • Instructs the router that if it does not have a route for a given destination, to send the packet to the default route • Static routes • A route to a destination that is configured in the router • Route update filtering • Use access lists to filter route update traffic about specific networks • Route Maps – Policy Routing RD-CSY3021

  4. Passive Interface • With most protocols passive interface stops the router from sending updates to a particular neighbor, but continues to listen and use routing updates from that neighbor In OSPF the interface address you specify as passive appears as a stub network in the OSPF domain. • OSPF routing information is neither sent nor received through the specified router interface In EIGRP, passive interface causes the router to stop sending hello packets. • When this happens, the router can't form neighbor adjacencies on the interface or send or receive routing updates • To achieve the common effect of passive interface with EIGRP, use the distribute-listcommand RD-CSY3021

  5. Using the passive-interface Command RD-CSY3021

  6. Gateway of Last Resort • The gateway of last resort is a routing entry that the router forwards packets to when it lacks a more specific route to a network • When IP routing is Enabled 1. The ip default-network to tag a route as a gateway of last resort • Router(config)#ip default-network 171.70.24.0 • Propagated differently depending on which protocol is propagating the default route • For IGRP/EIGRP to, the network specified must be a IGRP or EIGRP derived network in the routing table or advertised into these protocols using ‘network’ command 2. Create a Static Route to quad zero (0.0.0.0 0.0.0.0) • Router (config)# ip route 0.0.0.0 0.0.0.0 • IGRP can not understand a route to 0.0.0.0 • When IP routing is disabled, • Router(config) # ip default-gateway 172.16.15.4 Can a router have multiple IP default networks entered ? 2513#show ip route Gateway of last resort is not set 161.44.0.0/24 is subnetted, 1 subnets C 161.44.192.0 is directly connected, Ethernet0 131.108.0.0/24 is subnetted, 1 subnets C131.108.99.0 is directly connected, Serial0 S 198.10.1.0/24 [1/0] via 161.44.192.2

  7. Example: Default network 2 • 2513(config)#ip default−network 198.10.1.0, • 2513#show ip route • Gateway of last resort is 161.44.192.2 to network 198.10.1.0 • 161.44.0.0/24 is subnetted, 1 subnets • C 161.44.192.0 is directly connected, Ethernet0 • 131.108.0.0/24 is subnetted, 1 subnets • C 131.108.99.0 is directly connected, Serial0 • S* 198.10.1.0/24 [1/0] via 161.44.192.2 Add another default gateway 2513(config)#ip route 171.70.24.0 255.255.255.0 131.108.99.2 2513(config)#ip default−network 171.70.24.0 1 3 2513#show ip route 2513#show ip route Gateway of last resort is not set 161.44.0.0/24 is subnetted, 1 subnets C 161.44.192.0 is directly connected, Ethernet0 131.108.0.0/24 is subnetted, 1 subnets C131.108.99.0 is directly connected, Serial0 S 198.10.1.0/24 [1/0] via 161.44.192.2 • Gateway of last resort is 161.44.192.2 to network 198.10.1.0 • 171.70.0.0/16 is variably subnetted, 2 subnets, 2 masks • S 171.70.0.0/16 [1/0] via 171.70.24.0 • S 171.70.24.0/24 [1/0] via 131.108.99.2 • 161.44.0.0/24 is subnetted, 1 subnets • C 161.44.192.0 is directly connected, Ethernet0 • 131.108.0.0/24 is subnetted, 1 subnets • C 131.108.99.0 is directly connected, Serial0 • S* 198.10.1.0/24 [1/0] via 161.44.192.2 Why is the network not flagged as default network RD-CSY3021

  8. Issues with flagging Default Routes • The ip default−network command is classful. This means that if the router has a route to the subnet indicated by this command, it installs the route to the major net. • At this point neither network has been flagged as the default network. The ip default−network command must be issued again, using the major net, in order to flag the candidate default route. • 2513(config)#ip default−network 171.70.0.0 • 2513(config)#^Z • 2513#show ip route • Gateway of last resort is 171.70.24.0 to network 171.70.0.0 • * 171.70.0.0/16 is variably subnetted, 2 subnets, 2 masks • S* 171.70.0.0/16 [1/0] via 171.70.24.0 • S 171.70.24.0/24 [1/0] via 131.108.99.2 • 161.44.0.0/24 is subnetted, 1 subnets • C 161.44.192.0 is directly connected, Ethernet0 • 131.108.0.0/24 is subnetted, 1 subnets • C 131.108.99.0 is directly connected, Serial0 • S* 198.10.1.0/24 [1/0] via 161.44.192.2 RD-CSY3021

  9. Null0 • Significant overhead can be created by packets for routes that do not exist. • the router may want to send an Internet Control Message Protocol (ICMP) host or network-unreachable message for each un-routable packet • One solution is to configure a route to the null0 interface • a legitimate interface that accepts the packets and then throws them away • No ICMP host or network-unreachable messages are sent for packets forwarded to null0 RD-CSY3021

  10. Route Map • Route maps are similar to a scripting language: • A list of statements composes a route map. • The list is processed top-down like an access list. • The first match found for a route is applied. • The sequence number is used for inserting or deleting specific route map statements. route-map my_bgp permit 10 { match statements } { match statements } { set statements } { set statements } route-map my_bgp deny 20 :: :: :: :: :: :: route-map my_bgp permit 30 :: :: :: :: :: :: RD-CSY3021

  11. route-map Commands router(config)# route-map map-tag [permit | deny] [sequence-number] • Defines the route map conditions router(config-route-map)# match {conditions} • Defines the conditions to match router(config-route-map)# set {actions} • Defines the action to be taken on a match router(config-router)# redistribute protocol [process id] route-map map-tag • Allows for detailed control of routes being redistributed into a routing protocol RD-CSY3021

  12. Route Map Operation (Cont.) • The match statement may contain multiple references. • Multiple match criteria in the same line use a logical OR. • Each vertical match uses a logical AND. • All match statements must permit the route for it to remain a candidate for redistribution. • Route map permit or deny determines if the candidate will be redistributed. RD-CSY3021

  13. Example - Policy Routing • interface Ethernet3/0 • ip address 172.16.79.3 255.255.255.0 • ip policy route-map net-10 • access-list 111 permit ip 10.0.0.0 0.255.255.255 any • route-map net-10 permit 10 • match ip address 111 • set interface Ethernet0/1 • route-map net-10 permit 20 With normal routing, all the packets from 10.0.0.0/8 network to the Internet will take the path through interface ethernet 0/0 of Cisco WAN Router (via 172.16.187.0/24 subnet) as it is the best path with least metric. Policy-based routing can be used such that these packets take the path through the Firewall to the Internet, normal routing behavior has to be overridden by configuring policy routing. The firewall translates all the packets from 10.0.0.0/8 network going to the Internet, which is however not necessary for policy routing to work.

  14. The match commands

  15. Filtering Routing Updates RD-CSY3021 • A distribute-list is used to control routing updates either coming TO router or leaving your router. • Example: we want to filter out route 100.200.100.1/32. • define an ACL that identifies that route, denies it, and allows all other traffic • Create a Distribute-List that references the ACL and defines the direction • Go into the routing process • we want to filter OSPF routes so we go into the OSPF routing process configuration • Verify that the route has been removed Router# show ip route Gateway of last resort is not set 100.0.0.0/8 is variably subnetted, 3 subnets, 2 masks O 100.200.200.1/32 [110/11] via 172.16.100.29, 00:00:10, Ethernet0 O 100.200.100.1/32 [110/11] via 172.16.100.29, 00:00:10, Ethernet0 C 100.100.250.0/24 is directly connected, Loopback0 172.16.0.0/24 is subnetted, 1 subnets C 172.16.100.0 is directly connected, Ethernet0 Define Access list Router(config)# access-list 50 deny  100.200.100.1 Router(config)# access-list 50 permit any OSPF ROUTING PROCESS CONFIGURATION Router(config)# router ospf 10 Router(config-router)# distribute-list 50 in Verify route Router# shiproute … 100.0.0.0/8 is variably subnetted, 2 subnets, 2 masks O 100.200.200.1/32 [110/11] via 172.16.100.29, 00:11:39, Ethernet0 C 100.100.250.0/24 is directly connected, Loopback0 172.16.0.0/24 is subnetted, 1 subnets C 172.16.100.0 is directly connected, Ethernet0

  16. Modifying Administrative Distance Router(config-router)# • distance administrative distance [address wildcard-mask [access-list-number | name]] • Used for all protocols except EIGRP and BGP redistribution Router(config-router)# distance eigrp internal-distance external-distance • Used for EIGRP RD-CSY3021

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