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  • Table of Contents
  • Layer 2 Technologies
    • Ethernet Switching
      • L2 Switch Operations
      • Spanning Tree
        • 802.1d – STP
        • 802.1w – RSTP
        • 802.1s – MSTP
      • VTP 101
      • Private VLANs
      • VLANs
      • EtherChannel 101
    • Layer 2 WAN Protocols
      • HDLC
        • HDLC 101
      • PPP
        • PPP 101
        • PPP Authentication - PAP
        • PPP Authentication – CHAP
        • PPP Authentication – EAP
        • PPP Multilink
        • PPPoFR – PPP over Frame Relay
        • PPPoE – PPP over Ethernet
      • Frame Relay
        • Frame Relay 101
        • Frame Relay 102
        • Frame Relay Encapsulations – IETF vs Cisco
        • Multilink Frame Relay
        • Frame Relay Switching
        • Routing over Frame Relay
      • Bridging
        • Bridging on a router
        • MTU 101
    • Wireless
      • Wireless Principles
      • Wireless Implementations
      • Wireless Roaming
      • Wireless Authentication
        • WPA2 PSK
        • WPA2 802.1X
  • IPv4
    • IPv4 Addressing
      • Backup Interfaces
      • FHRP 101
      • DHCP 101
      • DNS 101
      • ARP 101
      • IPv4 101
      • Tunnel Interfaces
        • GRE Tunnels
      • BFD – Bidirectional Forwarding Detection
    • IPv4 Routing
      • How the routing table is built
        • How CEF works
        • Routing Order of Operations
        • NSF – Non Stop Forwarding
      • RIP
        • RIP 101
      • EIGRP
        • EIGRP 101
        • EIGRP Metric
        • More EIGRP Features
      • OSPF
        • OSPF 101
        • OSPF Areas
        • OSPF LSAs
        • OSPF Mechanics
      • IS-IS
        • IS-IS 101
        • IS-IS Mechanics – CLNP
      • BGP
        • BGP 101
        • BGP Attributes
        • More BGP
      • Route Redistribution
      • Policy based Routing
      • PfR 101 – Perfromance Routing
      • ODR
  • IPv6
    • IPv6-101
    • IPv6 Routing
    • Interconnecting IPv6 and IPv4
  • MPLS
    • MPLS 101
    • MPLS L3 VPN
  • Multicast
    • Multicast 101
    • PIM 101
    • IGMP 101
    • Inter Domain Multicast
    • IPv6 Multicast
    • Multicast features on switches
  • Security
    • NAT 101
    • NAT for Overlapping Networks
    • ACLs 101
    • ACLs 102
    • Cisco IOS Firewall
    • Zone Based Firewall
    • AAA 101
    • Controlling CLI Access
    • Control Plane
    • Switch Security
      • Switchport Traffic Control
      • Switchport Port Security
      • DHCP Snooping and DAI
      • 802.1x
      • Switch ACLs
    • IPSec VPN 101
      • IKE / ISAKMP 101
      • IPSEC Crypto Maps 101
      • IPSEC VTI 101
      • DMVPN 101
    • EAP 101
  • Network Services
    • NTP 101
    • HTTP 101
    • File Transfer 101 – TFTP & FTP
    • WCCP 101
  • QoS
    • QoS 101
    • Classification and Marking
    • Congestion Management
      • Legacy Congestion Management
      • SPD – Selective Packet Discard
      • CBWFQ
      • IP RTP Priority
    • Congestion Avoidance – WRED
    • Policing and Shaping
      • CAR 101
    • Compression and LFI
      • Header and Payload Compression
      • LFI for MultiLink PPP
    • Frame Relay QoS
      • Per VC Frame Relay QoS
    • RSVP 101
    • Switching QoS
  • Network Optimization
    • NetFlow 101 – TNF – Traditional NetFlow
    • NetFlow 102 – FNF – Flexible NetFlow
    • IP SLA 101
    • IP Accounting 101
    • Logging 101
    • SNMP and RMON 101
    • Cisco CLI Tips and Tricks
    • AutoInstall
    • Enhanced Object Tracking
    • Troubleshooting 101
    • SPAN, RSPAN, ERSPAN
  • Network Architecture
    • Hierarchical Network Architecture
    • SD Access
    • SD WAN
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On this page
  • Route Dampening
  • Backdoor networks
  • Fast Fallover
  • Fast External Fallover
  • Internal Fallover
  • ORF – Outbound Route Filtering
  • Local AS
  • Maximum prefixes

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  1. IPv4
  2. IPv4 Routing
  3. BGP

More BGP

Route Dampening

It is used to stop unstable routes from being forwarded throughout the network. When a route flaps, a penalty is assigned to the route (Default: 1000 per flap). A timer called Half-Life is used to reduce the penalty value to half (Default: 15 min). If the penalty value exceeds the suppress limit, the route is no longer advertised (Default: 2000). The route continues to be suppressed until the Half-Life reduces the penalty below the reuse limit (Default: 750). A route cannot be suppressed more than the Maximum Suppress Time (60 min or 5 * Half-Life). Dampening can be enabled globally:

R(config-router)# bgp dampening [PARAMS]

or only for some routes matched in a route-map: R(config-router)# set dampening PARAMS ! Must set PARAMS. It won’t use default settings When setting dampening with a route-map, define dampening parameters in the route-map. You can verify dampening with:

R# show ip bgp dampening parametrs

Dampened prefixes can be manually cleared, using:

R# clear ip bgp dampening PREFIX NETMASK

However, this will not clear the penalty value so if a new flap occurs, the route will probably be immediately dampened. To clear the penalty value, use:

R# clear ip bgp NEIGH-ADDR flap-statistics

Backdoor networks

By default, an external BGP learned route is preferred due to the lower AD (20) to an IGP learned route. If you would like to prefer the route learned via an IGP, use the Backdoor command on the network:

R(config-router)# network NETWORK-ADDR backdoor

This command will modify the AD of the BGP learned route to 200, making it less preferred over other IGP routes.

Fast Fallover

Fast External Fallover

For eBGP peers that are directly connected, the router will bring down the neighbor relationship if the interface status goes down. In case of flapping interfaces, you may want to keep the neighbor relationship up until the dead timer expires.

R(config-router)# [no] bgp fast-external-fallover
! Default: on

The command can be issued per interface, with:

R(config-router)# ip bgp fast-external-fallover {permit|deny}
! permit - enables fast-external-fallover
! deny - disables fast-external-fallover

Internal Fallover

For iBGP peers you can also enable fast fall-over with the command:

R(config-router)# neighbor NEIGH-ADDR fall-over [route-map ROUTE-MAP]
! ROUTE-MAP can be used for conditional deactivation of a session

However, this is usually not a desired behavior.

ORF – Outbound Route Filtering

Sending a lot of updates which are filtered inbound by a neighbor is unnecessary but there was no way for a router to know how its neighbors would handle the routes. With the introduction of the ORF feature, 2 ORF-capable router can exchange information regarding their inbound filters, so that the sending router can filter them in the outbound direction. To enable this feature, 2 neighbor routers must be configured with:

R(config)# neighbor NEIGH-ADDR capability orf prefix-list {both|send|receive}
! both     Capability to SEND and RECEIVE the ORF to/from this neighbor
! receive  Capability to RECEIVE the ORF from this neighbor
! send     Capability to SEND the ORF to this neighbor

After the command is enabled on both routers, they will exchange information and will filter outbound updates with the same prefix list as the filter set on the other peer in the inbound direction, therefor optimizing the bandwidth usage.

Local AS

For temporary situation when a company migrates from one AS number to another, it is necessary to be able to change the AS number used on a per-neighbor basis. This can be done using the command:

R(config-router)# neighbor NEIGH-ADDR local-as NEW-AS [no-prepend [replace-as [dual-as]]]

Now, the router will make connections to this neighbor as if it is running BGP in the NEW-AS.

When the command is entered only with the local-as keyword, then the router will do the following:

  • The AS_PATH of the routes received from this neighbor will be prepended with the NEW_AS. When they are sent out to other eBGP peers, they will also be prepended with the OLD_AS

  • The AS_PATH of the routes sent to this neighbor will be prepended with both the NEW_AS and the OLD_AS, with the NEW_AS appearing first in the AS_PATH

When the command is entered with the “no-prepend” keyword, the router will do the following:

  • The AS_PATH of the routes received from this neighbor will not be prepended with the NEW_AS. When they are sent out to other eBGP peers, they will only be prepended with the OLD_AS

  • The AS_PATH of the routes sent to this neighbor will be prepended with both the NEW_AS and the OLD_AS, with the NEW_AS appearing first in the AS_PATH

When the command is entered with the “no-prepend replace-as” keywords, the router will do the following:

  • The AS_PATH of the routes received from this neighbor will not be prepended with the NEW_AS. When they are sent out to other eBGP peers, they will only be prepended with the OLD_AS

  • The AS_PATH of the routes sent to this neighbor will be prepended only with the NEW_AS, skipping the OLD_AS

When using also the “dual-as” keyword, the router will accept peering with this neighbor on both AS-NUMBERS, making it easy to migrate from one AS-NUMBER to another.

Maximum prefixes

The internet BGP tabel size is huge and if you receive such a large number of routes from a neighbor, you’re router might not have the amount of memory to manage it. This is why you can configure a maximum number of prefixes that you are willing to receive from your neighbor. When the Maximum number of prefixes is reached, the session is shut down.

R(config-router)# neighbor NEIGH-ADDR maximum-prefix MAX [TH] [restart TIMER] [warning-only]
! MAX = maximum number of routes
! TH = When the number of received routes reaches TH% of MAX, the router generates a warning. Default: 75
! restart TIMER = The session will be reestablished after the TIMER expires. If not configured, it will remain shutdown.
! warning-only = doesn't shutdown, but issues a warnign (syslog)
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Last updated 3 years ago

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