BGP Peer Group Example Customer Connections

This peer group is used for all customers of the service provider because they share an almost identical routing policy. The peer group is first created as a template, which is configured with an incoming route-map named Cust_In and an outgoing route-map named Cust_Out, as well as an incoming AS-path filter-list number of 10. The peer group is also configured with a maximum limit of 50 received prefixes. Then neighbors in AS 213 and AS 314 are assigned to the peer group. These additions mean...

Neighbor advertisementinterval

To set the minimum interval in the sending of BGP routing updates, use the neighbor advertisement-interval router configuration command. neighbor ip-address peer-group-name advertisement-interval seconds To remove an entry, use the no form of this command. no neighbor ip-address peer-group-name advertisement-interval seconds Syntax Description If a BGP peer group is specified by using the peer-group-name argument, all members of the peer group will inherit the characteristic that is configured...

Multi Exit Discriminator

You can use the MED to influence path selection in neighbor autonomous systems. An AS can specify its preferred entry point using the MED in outgoing EBGP updates. The MED is not propagated outside of a receiving AS. The default value of the MED attribute is 0. The MED is called the metric in Cisco IOS software. The MED is a weak metric. A lower MED value means more preferred. The MED attribute is a hint to external neighbors about the preferred path into an AS when multiple entry points...

Connections to multiple service providers for maximum redundancy

Service provider customers have different requirements for their Internet connectivity. These different requirements result in different solutions A single permanent connection to one ISP. This solution meets the requirements for the vast majority of customers. Multiple permanent connections in which one of the lines is primary and the other line is used for backup only. This setup also provides redundancy on the links. Compared to a dial-up backup, a permanent backup link is preferred for...

Monitoring ASPath Prepending

This topic identifies the Cisco IOS commands that are required to monitor the operation of AS-path prepending. When you are monitoring AS-path prepending, the router doing the prepending is not the proper point to observe the results of the AS-path prepend operation. For instance, output from the debug ip bgp updates command does not display the prepended paths, because the routemap doing the prepending is applied afterward. The show route-map command displays the configuration details of a...

Clear ip bgp

To reset the BGP sessions with all the members of a peer group, use the clear ip bgp EXEC command. clear ip bgp * neighbor-address peer-group-name soft in out 2005, Cisco Systems, Inc. Optimizing BGP Scalability 7-59 Resets only the identified BGP neighbor. (Optional) Displays information about that specific peer group. (Optional) Initiates soft reconfiguration. (Optional) Triggers inbound or outbound soft reconfiguration. If you do not specify the in or out option, both inbound and outbound...

BGP Link Bandwidth Feature

Used to enable multipath load balancing for external links with unequal bandwidth capacity Enabled under an IPv4 or VPNv4 address family sessions by entering the bgp dmzlink-bw command Routes learned from directly connected external neighbor propagated through the IBGP network with the bandwidth of the source external link The BGP Link Bandwidth feature is used to enable multipath load balancing for external links with unequal bandwidth capacity. This feature is enabled under an IP version 4...

Example BGP Link Bandwidth Configuration

In the following example, Router A is configured to support IBGP multipath load balancing and to exchange the BGP extended community attribute with IBGP neighbors Router A(config-router) neighbor 10.10.10.2 remote-as 100 Router A(config-router) neighbor 10.10.10.2 update-source Loopback 0 Router A(config-router) neighbor 10.10.10.3 remote-as 100 Router A(config-router) neighbor 10.10.10.3 update-source Loopback 0 Router A(config-router) address-family ipv4 Router A(config-router) bgp dmzlink-bw...

Individual parameters specified in a peer group can be overridden on a neighborbyneighbor basis

On a Cisco IOS router, the peer group is created as a template. The template is configured to do the following Propagate, or not propagate, the community attribute Use the IP address of a specific interface as the source address when opening the TCP session Use, or not use, the EBGP multihop function Use, or not use, Message Digest 5 (MD5) authentication on the BGP sessions Assign a particular weight value to the routes that are received Filter out any incoming or outgoing routes based on the...

ASPath Prepending Design Considerations

There is no exact mechanism to calculate the required prepended AS-path length. If a primary and backup scenario is desired - Use a long prepended AS path over the backup link to ensure that the primary AS path will always be shorter. - A long backup AS path consumes memory on every Internet router. - Experiment with various AS-path lengths until the backup link is idle. - Add a few more AS numbers for additional security (unexpected changes in the Internet). If traffic load distribution is...

Increasing interface input queues

- Improves convergence by reducing dropped TCP ACKs BGP convergence can often be an issue in networks requiring quick propagation of routing information. Cisco IOS software provides the following performance-improvement features, which have been designed to reduce BGP convergence time and the high CPU utilization that is caused by a running BGP process Queuing to TCP peer connections Instead of queuing data once per second, BGP now queues data aggressively from the BGP output queue to the TCP...

Show ip bgp replication

To display update replication statistics for BGP update-groups, use the show ip bgp replication command in EXEC mode. show ip bgp replication index-group ip-address Syntax Description (Optional) Specifies that update replication statistics for the update-group with corresponding index number will be displayed. The range of update-group index numbers is from 1 to 4294967295. (Optional) Specifies the IP address of a single neighbor for which update-group statistics will be displayed. 2005, Cisco...

Configuring Default Local Preference

This topic describes the Cisco IOS command that is required to configure default BGP local preference on a Cisco router. Configuring Default Local Preference bgp default local-preference preference This command changes the default local preference value. The specified value is applied to all routes that do not have local preference set (EBGP routes). The default value of this parameter is 100, allowing you to specify more desirable or less desirable routers. You can use the bgp default...

Configuring BGP Route Dampening

This topic identifies the Cisco IOS commands that are required to configure BGP route dampening. bgp dampening half-life reuse suppress max-suppress-time route-map map-name Configures BGP route dampening - half-life Decay time in which the penalty is halved - suppress Value when the route starts dampening - reuse Value when the dampened route is reused - max-suppress-time Maximum time to suppress the route - route-map Name of route-map controlling dampening To enable route dampening, use the...

Dshow memory

7-86 Configuring BGP on Cisco Routers (BGP) v3.2 2005, Cisco Systems, Inc. Q5) The output of a show interfaces fastethernet 0 0 command follows Fast Ethernet0 is up, line protocol is up Hardware is DEC21140, address is 0000.0c0c.1111 (bia 0 0 02.eaa3.5a60) Internet address is 112.64.101.17 2 55.255.255.240 MTU 1460 bytes, BW 100000 Kbit, DLY 100 usec, rely 255 255, load 200 255 Encapsulation ARPA, loopback not set, keepalive not set, hdx, 100BaseTX ARP type ARPA, ARP Timeout 4 00 00 Last input...

Module Self Check

Use the questions here to review what you learned in this module. The correct answers and solutions are found in the Module Self-Check Answer Key. Q1) Which three characteristics are common to typical service provider networks (Choose three.) (Source Scaling IGP and BGP in Service Provider Networks) A) The provider network uses two IGPs, one for customer routes and one for internal provider routes. B) Service providers exchange routes with other providers using BGP. C) Service providers run...

Example Peer Group Requirements Edge Router at a Peering Point

This example illustrates an edge router at a peering point. The edge router that is located in the network where the service provider exchanges routes with other service providers is also a suitable place to use peer groups. From the edge router, the service provider AS can peer with a large number of other service providers. All peering autonomous systems should receive the same set of routes, namely the routes local to the service provider AS and the routes that are received from customer...

Define communities that signal individual goals

- Community 387 17 is used to indicate that the local preference of the route should be lowered to 50. This table lists the goals and the community values. This table lists the goals and the community values. Prepend the AS path once when sending the network to external neighbors. Prepend the AS path twice when sending the network to external neighbors. Prepend the AS path three times when sending the network to eXternal neighbors. All customers of the service provider should know this list so...

Default Return Path

Result The return traffic flows over the path with the shortest AS-path length. Result The return traffic flows over the path with the shortest AS-path length. If no BGP path selection tools are configured on the route to influence the traffic flow, AS 387 will use the shortest AS path. This action will result in unwanted behavior because the return traffic to AS 213 will be sent over the low-speed WAN link. AS 213 announces network 10.0.0.0 8 over EBGP sessions to both AS 462 and AS 387. When...

Proper Return Path Selection

Q How do you select the proper return path from AS 387 A Use local preference in AS 387. Q How do you select the proper return path from AS 387 A Use local preference in AS 387. Q Will the administrator of AS 387 configure it A Unlikely. Remember that the incoming traffic flow (from the perspective of AS 213) will be a result of the route selection for outgoing traffic in AS 387. The traffic that is going out from AS 387 will end up as incoming traffic in AS 213. If AS 387 configures some...

Primary Backup Link Selection

This topic describes how you can implement a typical backup setup between a multihomed customer and multiple service providers in a BGP environment. When using BGP on multiple links between a customer and several service provider networks, the customer is solely responsible for controlling the use of the links between them for outgoing traffic. The customer chooses whether to use these links in a primary backup or a load-sharing configuration. If one link is primary and the other is used for...

Always works

To remove incoming traffic from the backup link, the customer must influence route selection in the backup AS. The backup ISP must be forced to prefer the primary path to reach the customer network, although this choice means selecting a route with a longer AS path. One way to influence route selection is to use local preference in the network of the backup ISP. Using local preference creates an administrative scalability issue if each customer requires its use, because the ISP must maintain...

Static Route Propagation Static Routes on the Provider Edge Router

The show ip route command displays information from the routing table about subnet 11.2.3.0 24. The route is learned by static configuration and is redistributed via BGP. The router, through the use of a statically assigned tag, has assigned a tag value of 1000 to the customer route, and the route must pass through the route-map into BGP before being inserted into the BGP table. The show ip bgp command displays information from the BGP table about subnet 11.2.3.0 24. The route is local within...

IBGP Scalability Issues in a Transit AS

This topic explains the need for BGP route reflectors by describing the scalability issues of BGP transit backbones. Classic Internal Border Gateway Protocol (IBGP) split-horizon rules specify that updates that are received on an External Border Gateway Protocol (EBGP) session should be forwarded on all IBGP and EBGP sessions, but updates that are received on an IBGP session should be forwarded only to all EBGP sessions. This rule requires a BGP boundary router to be able to send routing...

Gold customers Define static route tags

In this scenario, the service provider offers two different service levels to its customers Normal and Gold. Customers are also assigned IP address blocks. Some customers have PA addresses, which the ISP does not announce as explicit routes. The large route summary block announced by the ISP covers these customers. Other customers use provider-independent PI addresses that must be explicitly announced to the Internet by the service provider. Because there are two different quality of service...